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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
The Gibbs Guide to
Electric Power Systems
Part 2
More Than
Motors
by
Andrew Gibbs
gibbsguides.com
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© Copyright Andrew Gibbs 2013
Gibbs Guides.com
Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
The Gibbs Guide to
Electric Power Systems
Part 2
More Than Motors
A Gibbs Guides e-book
By
Andrew Gibbs
gibbsguides.com
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© Copyright Andrew Gibbs 2013
Gibbs Guides.com
Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
Messages from Andrew Gibbs
Thank you for purchasing this e-book to Electric Power Systems. I sincerely hope
it will be an enjoyable read, and that it will be a great deal of help to you. If you
like the guide, please tell your modelling friends. If you have suggestions for
improvements or alterations, please feel free to contact me.
Copyright matters
Several months of full-time work plus a great deal of effort went into creating this
guide. As the author of this e-book, I hold the copyright to it. I make my living
from helping my fellow modellers by writing about modelling matters, so I do have
to charge for some products such as this e-book. There’s lots of high quality freeto-access information on my website at www.gibbsguides.com which you are
welcome to access and share. There’s also a completely free, high quality e-zine
which you are most welcome to sign up for at this site.
I make sure my products offer excellent value by providing high quality
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copyright and do not share this publication with others without my permission. If
you did this, you would deprive me of the chance of a sale. And if that happened I
might have to stop writing Gibbs Guides and get a proper job!
Andrew Gibbs
Cover photograph
The cover photograph shows a 65 inch span Hangar 9 P51D in Miss America
colours. The careful pilot is busy working through his Before Take Off checklist.
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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
DISCLAIMER
Electric power system components such as motors, batteries, electronic speed
controllers, propellers, chargers and so on are supplied with instructions. Additional
information may also be available on the supplier’s and/or manufacturer’s website. You
are strongly encouraged to read and implement any such instructions, especially those
instructions relating to safety. Such instructions must be accurately followed without
deviation. None of the information within this guide is intended to overrule such
instructions, and where any disagreement exists, always follow the instructions of the
manufacturer and contact them or the supplier of your equipment for advice about the
particular circumstances of your application.
All possible care has been taken with this guide and the information within it is offered
in good faith; nevertheless, in using this guide you do so at your own risk and absolve
the author of any liability whatsoever in respect of death, personal injury, damage to
property or any other kind of accident. The Gibbs Guides terms and conditions state
that by purchasing this guide, you have already indicated that you accept these terms.
Copyright notice
This guide is copyright Andrew Gibbs. All rights strictly
reserved. Storage on a retrieval system, reproduction
or translation of any part of this work by any means
electronic or mechanical, including photocopying,
beyond that permitted by Copyright Law, without the
written permission of Andrew Gibbs is unlawful.
Printing your guide
By buying this e-book, you have bought a license to
print a single copy single copy for your own use.
If you choose to print your copy, I suggest using good
quality 90 gsm paper or heavier for the pages, and
thin card for the front and rear covers.
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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
Contents
Preface and Acknowledgements
6
Chapter 1: Propeller Primer
7
Chapter 2: Propeller Power, Efficiency & Thrust
12
Chapter 3: Reduction Gearing
19
Chapter 4: Heat, Efficiency and Cooling
22
Chapter 5: Introduction to Electronic Speed Controllers
26
Chapter 6: Electric Motor Safety
29
Chapter 7: BEC and PCO Functions of the ESC
32
Chapter 8: Opto Isolating ESCs
34
Chapter 9: Electronic Speed Controller Limitations
35
Chapter 10: Setting up Electronic Speed Controllers
37
Chapter 11: Battery Basics
41
Chapter 12: Battery Voltage Characteristics & Charging
44
Chapter 13: Wiring and Connectors
48
Chapter 14: Wiring Diagram for Single Motor Models
53
Chapter 15: Example Power Systems in Detail
54
Index
66
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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
Preface
This guide is the second in the Electric Power Systems series. While part 1 examined
the operation of electric motors, this part follows on and discusses the remaining
components of the power system such as the battery, ESC, propeller and so on.
For modellers used to dealing with internal combustion (i.c.) engines, electric flight
can seem like a mysterious black art. The aim of this guide is to bridge the gap and
provide an enjoyable grounding in the principles of electric powered model aircraft.
Development of model power systems. Shown here are (L to R) Webra 0.40 (circa
1980), OS35 FP (1987), brushed 600 motor with gearbox, brushless Jeti Phasor
45-3 and brushless Mega 16/15/3.
The principles governing the operation of electric power systems are completely different
to those of i.c. models. For modelers used to dealing only with glow, diesel or petrol
fuelled engines, this new world of Volts, Amps, Watts can be rather unsettling. The aim
of this series of guides is to demystify electric flight and to allow the reader to develop a
sense of familiarity and comfort with the concepts involved. As with part 1, I’ve tried to
make the presentation of the material as easy as possible to get to grips with.
Andrew Gibbs
Southampton, UK, Mar 2013
Acknowlegements
No work of technical complexity is ever produced in isolation, and this guide is no
exception. I would like to thank Toni Reynaud in particular for producing most of the
many wonderful diagrams and graphs. Grateful thanks are also extended to Toni, Chris
Golds, Bruce Smith and my father John Gibbs, all of whom kindly reviewed the rough
drafts and offered their valuable suggestions.
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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
Chapter 1
Propeller Primer
The propeller is one of the most important parts of an electric power system, but it is
often given only scant attention. The choice of propeller has a major effect on the way a
model flies, so propellers deserve to be carefully considered as the motor that drives it,
when thinking about electric power systems.
The way in which propellers operate is fundamentally straightforward, although propeller
theory becomes very complex indeed when it is analysed in detail. My objective here is
to present a simple, concise and easy to understand introduction to propellers which can
be applied to the real-life practical needs of aeromodellers.
Propeller basics
The propeller converts the mechanical power supplied by the electric motor into forward
thrust. The main parts of a propeller, or prop for short, are shown in the diagram below:
The various parts of a propeller. The blade cross-sections show how the blades
are twisted as they extend towards the tip: the blade angle flattens as it reaches
the tip.
The blades of the propeller pull the model through the air by generating a force in a
forwards direction, called thrust. The thrust developed by the propeller is generated in
exactly the same way that a wing develops its lift, so in essence, the propeller is really
little more than a rotating wing. A variety of propeller airfoils (aerofoils) are used for
model aircraft. Propellers which are designed to turn relatively slowly, such as those for
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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
slow-fly models, may use undercambered blades (i.e. the blades with a concave
rearmost surface), similar to the undercambered wing of an aircraft designed for slow
flight. Conversely, propellers designed for higher speed use have a flatter, less
cambered airfoil profile.
Left: This propeller blade has been cut in half to reveal its airfoil section. The airfoil is
just like that found on a wing. Right: This undercambered slow-fly prop is a good match
for this slow flying vintage-style model. Although the term ‘undercambered’ is technically
meaningless, it nevertheless serves as a convenient term of reference for aeromodelers.
Pitch and diameter
The two propeller characteristics of most interest to us are its diameter, and its pitch.
Propellers are in fact defined by their diameter and pitch measurements. For example, a
12 x 8 prop has a diameter of 12 inches and a pitch of 8 inches.
In the early days of flight, propellers were called airscrews. Although a propeller
does not actually move like a mechanical screw through a solid material, this
concept can help to visualise the forward helical movement of propellers with
various pitches.
The diameter is the diameter of the circle which the propeller tips describe as they rotate.
The pitch is the distance the propeller would move forward in one revolution, if it did not
have the drag of a model attached to it. The propeller pitch is determined by the angle at
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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
which the propeller blades are set; a propeller with a relatively fine pitch will move
forward a short distance with one revolution, while a prop with a relatively high, or coarse
pitch will move further. Thus, a high speed model will prefer a relatively course pitch
prop, while a slow speed model will use a finer pitch prop.
Left: Differences in pitch are not always easy to see by eye. These two 9 inch diameter
props of the same propeller ‘family’ are very different in pitch, but at first glance they look
very similar. Right: When clamped together, the differences in pitch are obvious. The
left hand prop has 7.5 inches of pitch, while the right hand one has 4 inches. The
dimensions of a prop should always be marked on at least one blade root.
Pitch speed
The pitch speed of a prop is the forward speed the propeller would achieve, if no
airframe drag existed. Although this is clearly a theoretical speed, a knowledge of the
propeller’s pitch speed is of practical use when working with power systems. Pitch speed
is found by multiplying the propeller pitch and the rpm it is turning at. For example, if a
prop has a pitch of 6 inches, and is rotating at 100 revolutions per second (= 6,000 rpm),
it will try to move forward 600 inches per second, equivalent to 50 feet/sec or 34 mph.
The table below shows the pitch speed for a variety of propeller pitch and rpm
combinations (applies to electric and i.c):
Propeller pitch speeds (mph)
RPM in thousands
Pitch in
inches 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000
15
19
23
27
30
34
38
42
45
49
53
57
61
4
17
21
26
30
34
38
43
47
51
55
60
64
68
4.5
19
24
28
33
38
43
47
52
57
62
66
71
76
5
21
26
31
36
42
47
52
57
63
68
73
78
83
5.5
6
23
28
34
40
45
51
57
63
68
74
80
85
91
7
27
33
40
46
53
60
66
73
80
86
93
99
106
8
30
38
45
53
61
68
76
83
91
98
106
114
121
9
34
43
51
60
68
77
85
94
102
111
119
128
136
10
38
47
57
66
76
85
95
104
114
123
133
142
152
45
57
68
80
91
102
114
125
136
148
159
170
182
12
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Model aircraft propellers are available in a wide variety of blade shape and airfoil types.
From left to right: A wooden electric propeller, a GWS slow fly prop, APC electric prop,
another style of GWS prop, an APC slow fly prop, two more props and finally a Günther
5 x 4.3 prop, commonly used with brushed ‘400’ size motors. All the props shown here
are tractor props, i.e. designed to rotate anticlockwise when viewed from in front of a
model.
Usefully, a reasonable approximation of pitch speed can be made by multiplying
propeller pitch in inches by rpm in thousands. For example, an 8 inch prop at 5,000 rpm
has a pitch speed of about 8 x 5 = 40 mph. This is very close to the actual pitch speed of
38 mph, and quite accurate enough for most modelling purposes.
Slippage
The drag of the model will ensure that the actual distance the propeller moves forward
will be less than its pitch speed. This difference is termed ‘slippage’. A good rule of
thumb is that provided a model uses an appropriate propeller, the pitch speed will be
approximately 25% greater than the flying speed.
These two views of the same propeller illustrate the twisted nature of the propeller blade.
This twist helps to ensure that all parts of the propeller blade work at a similar angle of
attack in flight, keeping the propeller operating as efficiently as possible.
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Pitch to diameter ratio
The pitch to diameter ratio (p/d ratio) is the ratio of a propeller’s pitch to its diameter. For
example, a 12 x 6 prop has a pitch of 6 inches and a diameter of 12 inches, so it has a
p/d ratio of 0.5 (because 6 ÷ 12 = 0.5). Similarly, a 12 x 9 has a p/d ratio of 0.75
(because 9 ÷ 12 = 0.75), while a 12 x 12 has a p/d ratio of 1.0 (12 ÷ 12 = 1). A propeller
should be chosen with a suitable p/d ratio for the model in question. Most model aircraft
props are in the range 0.4 to 1.0. Slow flying models perform best using a low p/d ratio,
somewhere around 0.5, average club sport models require a p/d ratio around 0.7 – 0.8
and very fast models up to 1.0. The table below outlines a suitable choice of p/d ratio for
various model types.
Model type
Highly aerobatic (3D) models, prophanging (hovering flight) etc
Slow models e.g. WW1 biplane
Aerobatic models where good vertical
performance (high rate of climb) is req’d.
Slow-medium speed model e.g. vintage
types
Medium speed models e.g. trainer
Moderately fast sports models
Fast models e.g. WW2 fighter
Very fast high-performance models e.g.
high-power sport or racing models
Typical suitable
propeller p/d ratio
Example propeller
& its p/d ratio
0.3 - 0.4
12 x 4 (0.3)
0.4 - 0.6
12 x 6 (0.5)
0.5 – 0.7
11 x 7 (0.63)
0.5 - 0.6
11 x 6 (0.54)
0.5 – 0.7
0.7 – 0.8
0.8 – 0.9
8 x 6 (0.75)
9 x 7 (0.77)
10 x 8 (0.8)
4.7 x 4.7 (1.0)
7 x 7 (1.0)
0.9 – 1.0
Left: This large Flair Fokker Dr1 triplane by Alan Gorham has plenty of drag, and so is a
slow flying model. A suitable motor and prop combination will produce a relatively low
pitch speed. Right: The prop of this Balsacraft Hawker Hurricane built by Stuart Warne
has a relatively high pitch speed. The blue swastika markings on this model identify it as
a Hurricane of the Finnish Air Force which operated the type 1940-44. The blue
swastika, the ancient symbol of the sun and good luck, was used by the Finnish Air
Force between 1918 and 1945. The Finnish swastika had nothing to do with the Nazi
swastika, which came into being in 1935.
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Chapter 2
Propeller Power, Efficiency & Thrust
Power Requirements of Propellers
The amount of power required to turn a prop depends on its diameter, its pitch and the
rpm we wish to turn it at. The power required is proportional to rpm³ x diameter4 x pitch.
We can summarise in a non-mathematical way what this formula tells us about the way
propeller changes will affect the required power:
Relationship between pitch and required power
Small increases in propeller pitch require a proportionate increase in power. For
example, if the pitch is increased by 10 % (e.g. an increase in pitch from 5 inches to
5.5 inches), the power required to maintain the same rpm also rises by 10 %.
Relationship between rpm and required power
Small increases in rpm result in a disproportionately large increase in power. This
means that if for example, the rpm is increased by 10 % (e.g. an increase in rpm from
7,000 rpm to 7,700 rpm), the power required to maintain the same propeller rpm will
rise by 33 %.
Relationship between diameter and required power
Small increases in diameter result in a disproportionately large increase in power. For
example, if the diameter is increased by 10 % (e.g. an increase in diameter from 10
inches to 11 inches), the power required to maintain the same rpm will rise by 46 %.
Each propeller blade of this Hurricane taking off is absorbing 1/3 of its engine’s
1,185 hp. Conditions of high humidity are causing the vortices present at each
prop tip to condense as they are left behind, resulting in continuous trail of visible
moisture. The helical pattern that each blade makes through the air can clearly be
seen. The Hurricane’s designer, Sidney Camm, was an aeromodeller in his youth.
Photograph copyright Randy Coniam & used with his kind permission
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Propeller efficiency and size
The propeller provides thrust for a model by accelerating a mass of air. There are two
ways to provide a given quantity of thrust; either by accelerating a small volume of air to
high speed, or by accelerating a larger volume of air by a smaller amount.
Small volume of air
accelerated to a
high velocity
Equal thrust and
forward velocity
Relatively low efficiency
Larger volume of
air accelerated to a
less high velocity
Relatively high efficiency
EFM025
The first method uses a relatively small prop turning at high rpm, while the second uses
a larger propeller turning at a lower rpm. For the same thrust, less energy is required to
accelerate the larger volume of air by the smaller amount. For this reason, generally
speaking, the larger the prop is the more efficient it will be. One of the advantages of
electric power systems is that where appropriate, they allow us to design power systems
with relatively large, efficient props compared to those used for i.c models.
This large prop of this
Pilatus PC21 driven by a
low Kv motor at relatively
low rpm. This offers a
higher level of efficiency
than the alternative of a
smaller prop driven at
higher rpm by a higher Kv
motor.
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The two large propellers of the Wright brothers’ 1905 Flyer III were geared to
rotate slowly, enabling them to achieve the maximum of thrust from the mere 20 hp
at their disposal. This quite remarkable machine remains one of the lowest
powered successful flying machines in the world. The talented American modeler
Wayne Ulery designed and built this impressive, accurate 1/10 electric powered
scale RC model of the Flyer III. The model dates from 2003, and features wing
warping and a pilot whose articulated limbs appear to actually move the controls.
The propellers are 12 x 6 APC E cut down to 10 inches, with tips shaped to better
represent the full size propeller blade. The 48.5 inch span model weighs 2 lbs.
Photograph by Wayne Ulery and used with his kind permission
Airflow considerations
Since a propeller is really a rotating wing, it is subject to the same aerodynamic effects
as a wing. As the propeller rotates, the blades meet the incoming air at an angle. The
angle which the prop blades meet the air at (called the angle of attack) depends on the
propeller pitch, the forward speed of the model and the motor rpm. Like a wing, if this
angle is too low or too high, the blades will not work efficiently and may even be stalled.
For a given propeller, the most efficient performance will occur with a particular
combination of airspeed and rpm.
Static & Dynamic thrust
It can be said that there are two types of thrust produced by a propeller; static thrust, and
dynamic thrust. Static thrust is the thrust produced by the propeller with the model in a
zero airspeed condition, while dynamic thrust is the thrust developed by the propeller
when it has some forward airspeed.
Static thrust is sometimes measured by modelers with the assumption that it will be a
useful indicator of flying performance. However, static thrust is only of relevance in zero
airspeed situations such as a) during the first instant of take off; b) during prop hanging
manoeuvres and c) when considering the rotor of a helicopter in hovering flight.
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Left: Models in flight are propelled by dynamic thrust. This ARF DH Chipmunk is the
work of Andrew Weight. Right: It is static thrust that is keeping this prop hanging model
airborne. A relatively large diameter, low pitch prop is fitted to this Extra 330 Shock Flyer,
the model that began a whole new flying genre. This photograph was taken in 2001, and
the pilot is a young Derk Van der Vecht, who now represents his native Holland in
competitions.
A large diameter, low pitch prop may produce an impressive amount of static thrust, but
this will not necessarily translate into good flight performance. The reason for this is that
as soon as a model is moving, it is the dynamic thrust that counts. The amount of
dynamic thrust developed will vary depending on the chosen propeller, the propeller rpm
and the airspeed of the model. Unfortunately, without access to a wind tunnel, dynamic
thrust is impossible for modelers to measure.
Spinners
The spinner is the streamlined cone found at the centre of the propeller. The purpose of
the spinner is to reduce drag by streamlining the airflow around the propeller hub and
nose area. Spinners also add greatly to the appearance of a model, and are of course
an essential part of most scale models. It is important to make sure that no part of a
spinner rubs on the fuselage. The location of spinners means that it is often the case
that its presence prevents cooling air from reaching the cooling holes in the front of the
motor. In this case, an alternative means of delivering cooling air must be found such as
by means of a ‘chin’ intake and suitable ducting.
Spinner size
The inner 1/3 or so of the propeller blades produce very little thrust, so large spinners
cause very little loss of thrust. In fact, what appears to be an excessively large spinner
may actually be beneficial, since it will help to smooth the flow of air over the model.
Propeller Balancing
An out of balance propeller on an electric model can cause a lot of vibration, which will
impose a high rate of wear on motor bearings and everything else in the model. An
unbalanced propeller can also waste a surprisingly high amount of power, causing a
significant loss of performance. Fortunately, propellers can quite easily be balanced
using a commercial balancer. It is worth purchasing a good quality balancer, as cheaper
ones may not work very well.
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For best performance, reduced noise, and increased motor life, all propellers
should be balanced before use. Spinners may also need to be balanced. The chin
intake on this P51 Mustang provides cooling air for the power system components.
Folding propellers
Folding propellers have their blades hinged so that when the motor is stopped, the
blades can fold against the fuselage. On restarting, the blades are flung out by
centrifugal force to their normal position and the prop then works as normal. The
aerodynamic drag of a stationary propeller is surprisingly high, but it falls to a negligible
amount if the blades are folded. A ‘windmilling’ propeller (i.e. an unpowered prop, made
to turn by the model’s movement through the air) is a very high source of drag.
The primary application of folding props is electric gliders, which enjoy a significantly
improved gliding performance once stationary prop blades are folded. They are also
appropriate for hand launched models that are landed on their bellies; in this situation
conventional props are often vulnerable to damage, unlike a folding prop.
Left: My hand-launched, belly landed small P51 has broken quite a number of 7 x 6
propellers in its lifetime. A folding propeller would be a much better solution for this
otherwise delightful model.
Right: The propeller on this competition model is in the
folded, low drag position. The drag from the externally mounted ESC will probably be of
some significance, but there is no denying it is well cooled.
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Choosing a prop for a model
In broad terms, we can say that propeller diameter is chosen to give the required thrust
by absorbing the required power, while propeller pitch is chosen to give a suitable pitch
speed. Low speed models such as biplanes tend to use props with a low p/d ratio, sport
models use a medium ratio, and fast models tend to use props with a high p/d ratio. We
can think of the choice of propeller as being a bit like choosing which gear to drive a car
in – a low pitch prop can be thought of as being like first gear, whereas a high pitch prop
is more like a higher gear.
Experimentation
Choosing the optimum prop for a model is an inexact process, and some
experimentation is the only way to be sure of the best choice. Note that nominally similar
props from different manufacturers can vary widely in airfoil and blade shape. These
variations mean that the load they present to the motor will also very significantly. For
example, a 7 x 6 prop from manufacturer ‘A’ can present a significantly different load to a
motor compared to a 7 x 6 prop from manufacturer ‘B’. Also, propeller pitch is not
always accurately specified, so a prop that is marked with a pitch of 6 inches might
actually be somewhere between 5 and 7 inches in pitch. It is always worth checking the
current draw of your power system any time the prop is changed.
Tractor and pusher installations
A conventional installation in which the propeller is at the front of a model is known as a
‘tractor’ propeller. This is because like a farm tractor, the propeller pulls the model along
behind it. Conversely, models in which the propeller is behind the motor driving it are
said to be ‘pusher’ installations. Conventional propellers for model aircraft are all made
to turn anti-clockwise when viewed from the front of the model. In a pusher installation a
tractor propeller must still be fitted so that the blades face the same way. This requires
the motor to operate in the opposite direction to a tractor installation, i.e. clockwise.
Alternatively, a special ‘pusher’ propeller can be used, for which the motor direction need
not be changed. In contrast, i.c. engines in pusher installations must use a pusher
propeller as their direction of rotation cannot usually be changed.
Left: This prop has been correctly fitted to the motor in this pusher installation. In one
sense, it could be said the prop has been fitted ‘backwards’ to the motor shaft. The
motor is now required to turn clockwise when viewed from the propeller. Right: This
electrically assisted glider is gliding with its prop blades folded against the fuselage. This
greatly reduces the drag of the stationary blades.
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Propeller safety
All propellers are potentially dangerous, even small ones. It is a wise policy to consider
the propeller to be ‘live’ any time that the battery is connected to the model. There exists
a possibility of the prop turning if the battery is connected, even if the transmitter is set to
motor ‘off’. Inadvertent operation of the throttle, equipment failure and interference are all
possibilities.
The propeller of this Scottish Aviation Bulldog has been painted in the same way
that the RAF paint full size propellers. Although both blades are striped, each
blade is painted differently so that when rotating, the propeller is more visible.
Photograph by kind permission of Colin Low
The consequences of increasing the voltage (cell count) using the same propeller
It may be that higher performance is desired for a model, and so consideration is given
to changing the battery for one with an additional cell. In the previous chapter we
discussed how a mere 10 % increase in propeller rpm results in a 33 % increase in the
power required to maintain the same rpm.
Clearly, this means that if the system voltage is increased while the propeller remains
unchanged, we can expect a very substantial increase in the current that will be
demanded by the motor. The table below illustrates the approximate consequences of
increasing the cell count while leaving the propeller unchanged:
Increasing the cell count with an existing propeller
Change
Change from
Approximate change in current
in voltage
2 Cell LiPo to 3 cell LiPo
+ 50 %
+ 80 % e.g. 10 A to 18 A
3 Cell LiPo to 4 cell LiPo
+ 33 %
+ 60 % e.g. 10 A to 16 A
4 Cell LiPo to 5 cell LiPo
+ 25 %
+ 40 % e.g. 10 A to 14 A
5 Cell LiPo to 6 cell LiPo
+ 20 %
+ 30% e.g. 10 A to 13 A
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Chapter 3
Reduction Gearing
Gearboxes were quite commonly seen on models in the days when brushed motors
were a popular choice of motor. This was because many of the brushed motors in use
were ferrite types, which tend to be inefficient sources of power for a model aircraft in
direct drive format, but rather better once they are geared.
However, these days the relatively inexpensive brushless outrunner motor is by far the
most popular choice for the majority of sport and scale models. These are generally
relatively low-revving (i.e. low rpm per Volt / low Kv) and are therefore able to drive a
relatively large and efficient propeller without the necessity for gearing. For this reason,
gearing is no longer much used for most sport and scale models. However, for certain
types of application, gearing is still useful. These applications include:
(i) Some high performance models which may use a geared inrunner
(ii) Some small models powered by small inrunners or perhaps brushed motors
(iii) The majority of helicopters
Why use reduction gearing?
The benefit of reduction gearing is that it allows a motor to turn a larger propeller than
would otherwise be the case. Larger propellers are more efficient than smaller ones, so
the use of a larger prop yields gains in propeller efficiency. Propeller efficiency is difficult
to measure, so it is often overlooked by modelers. Whether or not we are conscious of
this fact, propeller efficiency has an important influence on a model’s performance.
Reduction gearing is only worth considering for slow to medium speed models. Fast
models are best fitted with a small high pitch prop turned at high rpm, a requirement
which can only be supplied by a direct drive motor.
Both of these models are fine fliers. Left: This reduced size Brigadier by Alan Whipp
spans 36 inches and uses a small, brushed geared motor from GWS to good effect. The
model weighs around 200 g and achieves 20 min. flights from a 300 mAh nicad. Right:
This 1.5 times enlarged Keil Kraft Senator was built from a scaled-up plan by Louis
Louth-Davis, and is powered by a geared 400 size brushed motor.
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Methods of gearing
The simplest method of reducing the speed of a propeller is by using a single pair of
gears. The motor drives a small pinion gear, and this drives a larger gear attached to the
propeller shaft. This arrangement results in an offset between the motor and gearbox
shafts. More complex epicyclic gearboxes are sometimes used, and these produce an
inline gearbox in which no offset is present.
Belt drives are another way of reducing propeller speed. These used to be quite
common and were inherently quiet. However, perhaps because they may incur greater
losses than gears, it is now very unusual to see belt drives.
Gearing losses
Mechanical devices are never 100 % efficient, so the use of a gearbox (or other speed
reduction system such as a belt drive) will always incur losses of energy. A typical
gearbox might operate with an efficiency of perhaps 95 - 97 % so the losses are small.
Since these mechanical losses cost perhaps only 3 – 5 % of the input power, they are
easily more than made up for by the significant efficiency gains of using a larger prop.
Left: This superb scale model of a BAM Swallow was designed and by Jeremy Collins.
The original machine’s Pobjoy engine was geared 3:1 to allow it to use a larger, more
efficient propeller. Right: Jeremy’s model uses a large, slow revving outrunner so he
had no need to gear the motor output down. However, the model does use an unusual
1:1 ratio gearbox, built into the dummy engine to achieve the offset thrust line of the fullsize Pobjoy. The 200 Kv outrunner is fed by a 9S 2P LiPo pack and consumes 2,100
Watts. For this power input, the 22 x 12 prop is turned at around 5,300 rpm.
Advantages and disadvantages of gearing
The main advantage of reduction gearing is that it allows the use of a larger propeller
than would otherwise be possible, increasing propulsion efficiency. The main
disadvantages of gearing are the cost, weight and complexity of the gearbox.
Gear ratios
A gearbox allows us to exchange rpm for torque. For example a gearbox with a 2:1 ratio
will cause the prop shaft to rotate at half the motor’s rpm, with twice the torque. Similarly
a 3:1 ratio will cause the prop shaft to rotate at one third the motor’s rpm, and with three
times the torque. Ratios up to about 6:1 are commonly found in modeling applications.
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Gearing down Inrunners
Inrunners are relatively high speed, low torque motors, and are therefore always
potential candidates for gearing. An inrunner coupled to a high ratio gearbox can make
an effective and efficient power plant for electric gliders.
Gearing and Outrunners
Outrunner motors are inherently low rpm, high torque motors, and therefore are only
very rarely found used with gearboxes in fixed wing models.
Left: This Jeti Phasor 45-3 brushless inrunner motor is connected to a gearbox,
coloured gold. The gearbox shaft is offset slightly and the ratio is 2:1. This gearing allows
the motor to turn a substantially larger propeller than it could in direct drive format. The
efficiency gains from a larger prop more than outweigh the slight losses from the
gearbox. This motor has no apertures in the case for cooling, so careful attention must
be given to supplying the case with a flow of cooling air. The metal cased gearbox
actually helps in this respect, since it conducts heat away from the motor and adds
considerably to the surface area available for dissipating heat. Right: This impressive
electric helicopter was built by talented Dutch modeler Appie van Moorst.
Helicopters
Helicopters have a large rotor which is turned at low rpm. Generally helicopter rotors turn
at approximately 1,000 – 2,000 rpm. This low rotor speed requires the use of a highly
geared motor. Both inrunners and outrunners are used for helicopter models.
Gearbox noise
All gearboxes will produce some degree of noise, although the amount of noise is
usually not great and is not often considered to be a nuisance. I remember seeing a
geared P51 Mustang fly some years ago which emitted quite a growl from its gearbox,
which I thought actually added to the model’s character!
The design of the airframe will have a significant effect on the apparent volume of noise.
For example, a foam-winged model with a lot of mass will tend to deaden any noise,
while a very light built-up open structure with a taut covering will tend to amplify noise. A
reduction in noise can often be accomplished by ensuring the propeller is properly
balanced.
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Chapter 4
Heat, Efficiency and Cooling
The cause of heat
All electrically conductive materials have some electrical resistance, and when a current
is passed through a resistance, heat is generated. From experience, we know that all
electrical items generate heat in use. An example of this is the old-fashioned one-bar
electric fire; the heating element is simply a length of special wire with a suitable
resistance, wound around a former. The current flowing through this resistance causes
heat to be generated. Virtually all of the energy passing through the element creates
heat, and since heat is the required output, it can be said that an electric heater is almost
100% efficient.
Another example of electrical resistance is the old-fashioned household incandescent
light bulb. The bulb’s filament is also a length of resistive wire which is heated by the
passage of current such that it glows white hot. Virtually all of the energy the bulb
consumes goes to produce heat, and only a tiny proportion of the energy is emitted as
light. Since light is the required output, the bulb is therefore considered to have a very
low efficiency. In contrast, modern LED (light emitting diode) lights are much more
efficient than this, as they do not get very hot, and emit a far higher proportion of the
input energy as light.
It is inescapable that the motor, battery and speed controller of a model will each
generate some heat, even in components of the very highest quality. The model’s power
system wires and the associated connectors will also generate a tiny amount of heat,
although if the wires are adequately sized, the amount of heat generated will be
insignificant. The energy to create heat in a model’s power system comes from the
battery, and is therefore unavailable for driving the propeller. Heat therefore represents a
loss of efficiency in an electric power system.
Small increases in current will produce disproportionately large increases in the
amount of heat generated. So, if a model’s propeller is changed for a larger one
which draws more current, this may have unexpected consequences in terms of
the amount of heat that is generated. This electrified ‘Panic’ biplane built by
Andrew Weight uses a 5S 3,200 LiPo supplying an Axi 4120/14 which draws
around 50 Amps turning its 14x7 APC thin electric prop. The motor is somewhat
over propped, but careful throttle management gives 10 – 15 minute flights.
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The amount of heat generated depends on the resistance of the motor’s windings, the
resistance of the battery, the resistance of the ESC, and on the amount of current
flowing:
● The higher the current, the greater the amount of heat generated.
● The higher the resistance, the greater the amount of heat generated.
How heat varies with current
It might be supposed that the amount of heat generated is proportional to the current
flowing. However, this is not the case. In fact, the amount of heat generated is
proportional to the square of the current. Heat generation follows the ‘square law’
To illustrate what this means, let us consider an example - suppose a motor draws a
current of 10 Amps at full power with a particular propeller. The propeller is then
changed for a larger one; the higher load causes the current drawn to increase to about
14 Amps. The consequence of the higher current is that the heat generated by the
battery, the motor and the ESC will each double, even though the current has only risen
by about 40 %. If a still larger prop is fitted so that the current becomes 20 Amps, the
heat generated will be four times what it was at 10 Amps. The graph below illustrates
this principle:
Relationship of Heat to Current
50
40 units of heat
generated at 20A
45
Units of heat generated
(Watts)
40
35
20 units of heat
generated at about 14A
30
25
20
10 units of heat
generated at 10A
15
10
5
0
0
2
4
6
8
10
12
14
16
18
20
22
Current in Amps
From the graph, we can see that small reductions in current can result in
disproportionately greater gains in efficiency. However, if we want to have a high
performance model, we will need to draw a lot of power, so it may appear that there is no
alterative to a system consuming a high current.
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A note for the mathematically minded
The amount of power generated as heat can be found using an electrical
engineering formula which says:
Dissipated power = I² R
(Confusingly, the letter ‘I’ is often used in electrical calculations to
represent current, and not the letter A as we might expect. This stems
from the early days of electricity, when the letter ‘I’ was used to denote
intensity of current.)
For the non-mathematicians, we can rewrite this formula in plain English,
like this:
Amount of Heat produced = Amps x Amps x Resistance.
However, in fact, there is an alternative - there are two basic ways of getting same
amount of power in a system; a high voltage and a low current may be used, or
alternatively a low voltage and a high current.
For example, suppose we need a 500 Watt power system. Practically, we could achieve
this using either a 3-cell battery or a 4-cell battery. The table illustrates this:
Factor
System voltage
Current
Total power consumption
3-cell
system
11.1 Volts
45 Amps
500 Watts
4-cell
system
14.8 Volts
34 Amps
500 Watts
If our system has a total electrical resistance of say 0.075 Ohms we can now work out
the power dissipated as heat:
3-cell system
Dissipated power = I² R = 45 x 45 x 0.075 = 151 Watts
Power remaining to drive the prop = 349 Watts
4-cell system
Dissipated power = I² R = 34 x 34 x 0.075 = 86 Watts
Power remaining to drive the prop = 414 Watts
Both power systems are consuming the same 500 Watts, yet one generates 151 Watts
of power as wasted heat due to resistance, while the other only generates 86 Watts of
wasted heat.
Note that heat is also generated in the motor for other reasons such as the effect of the
changing magnetic flux on the iron-rich components, but we can ignore this for the
purposes of this discussion.
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Clearly, the higher voltage system is the more efficient option. This example illustrates
an important principle of electric flight – high voltage systems are inherently more
efficient than low voltage ones.
Both systems will allow us to enjoy a model, however all else being equal, the 4-cell
system will give the model more performance and/or flight time. The table illustrates the
two alternatives in mode detail:
Factor
Total power consumption
Current
Resistance of system
Power wasted as heat
Power remaining to drive the prop
Percentage of 500 W available to drive
the prop
3-cell
system
500 Watts
45 Amps
0.075
Ohms
151 Watts
349 Watts
70 %
4-cell
system
500 Watts
34 Amps
0.075
Ohms
86 Watts
414 Watts
83 %
All electric power systems will generate some heat, no matter how efficient the
components of the power system are. Any heat produced by the power system
represents wasted energy which could have been used to turn the propeller. The
overall system efficiency of this Long EZ - or any other model – may be found by
multiplying the efficiency of each of the components together. For example, if the
battery is 80 % efficient, and the motor is 85 % efficient, the ESC 95 % efficient
and the prop 70 % efficient, the overall efficiency will be 80 x 85 x 95 x 70 = 45 %
overall efficiency. This may seem low, but it is still a great deal better than an i.c
engine can achieve!
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Chapter 5
Introduction to Electronic Speed Controllers
In this chapter we discuss various topics relating to Electronic Speed Controllers (ESCs)
for both brushed and brushless motors. As modelers, we don’t need to be
knowledgeable about ESC electronics, so this section will provide only a brief overview
of the electronic side of matters. We will however discuss in detail those aspects of ESC
operation which will be of practical use to aeromodellers.
ESC electronics
Electronic speed controllers would not be possible without the invention of two types of
electronic components. These are the microprocessor and the transistor. The
microprocessor is the ‘brain’ of the ESC, and it is programmed during manufacture to tell
the ESC how to carry out its many functions. Microprocessors are also found in other
modeling devices such as ‘intelligent’ battery chargers, as well as being commonplace in
everyday life. Transistors are essentially nothing more than solid state (i.e. no moving
parts) electrical switches. ESCs use a metal oxide semiconductor field-effect transistor,
commonly abbreviated to MOSFET or just FET. These transistors can both switch very
quickly and also handle a high power. The combination of the microprocessor and the
high power FET (switch) are the foundation of all ESCs in modeling use.
Brushed motor ESC operation
The brushed ESC is in principle very simple; control of a brushed motor is achieved by
continuously switching the battery supply voltage on or off. Although the motor is
supplied with either zero Volts or the full battery Voltage, the switching happens so
quickly that the effect is indistinguishable from fully proportional control.
Brushed ESC operation
Motor
off
1/3
power
2/3
power
Full
power
Battery ON
Battery OFF
Time
EFM015
The diagram above illustrates the basic principle of operation of the brushed motor
ESC. The brushless motor ESC carries out this same function to control the speed
of a motor, as well as performing many other functions to make the motor operate.
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Brushless motor ESC operation
For brushless motors, the operation of the ESC is a great deal more complex. In addition
to providing precisely timed voltage pulses to each of the three sets of windings, the
brushless ESC must also carry out the commutation function and much more besides in
order to successfully operate the motor and control its speed.
Left: A brushed ESC. The red and black wires connect to the battery positive and
negative respectively, while the two yellow wires connect to the motor. The battery
connections must never be reversed, as this will cause immediate and irreversible
damage to the ESC. Right: A brushless ESC can always be identified by the presence
of three wires to connect to the motor. The colour of the ESC to motor wires is of no
significance. In this case, the wires happen to be blue.
Heat generation in ESCs
The FETs used to perform the switching have a resistance, and so just as with motors
and batteries, the current passing through the ESC will cause it to become warm. The
characteristics of FETs are such that they dissipate very little power, except when
switching takes place. This means that they will create the most heat when the motor is
operated at part throttle.
In general, the higher the capacity of the controller, the less resistance it will offer to the
flow of current, and the less heat it will generate for any given current. This means that
ESCs with some spare capacity should repay their additional weight and cost by being
slightly more efficient.
It is worth remembering the ‘square law’ which determines how much heat is generated;
to recap, small increases in current will result in disproportionately large increases in
generated heat. This is the reason why it is an extremely bad idea to try and use an ESC
(or any other electrical component) beyond its rated current capability.
This square law is the reason that small ESCs will generate much less heat than larger
ones. To illustrate this, the heat generated by a 60 Amp ESC operating at its rated
current will be sixteen times more than a 15 Amp ESC operating at its rated current.
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Cooling and mounting the ESC
It is absolutely essential to make sure the ESC is sufficiently cooled, particularly if the
BEC function is in use – if the controller overheats, the BEC function may shut down or
fail, and you will lose control of your model.
To prevent overheating, always make sure the ESC is supplied with a generous supply
of cooling air. The ESC cannot be over cooled. It should never be mounted in protective
foam, as this will prevent air from reaching its surface. Instead, allow it to hang, or for
larger controllers, the ESC can be secured by attaching to the wires at each end. Make
sure the ESC cannot become accidentally disconnected in flight.
Cool ESCs provide more power
The resistance of a power MOSFET rises with temperature, meaning that ESCs that are
kept cool will present less resistance to the flow of current, and will be more efficient and
allow more power to reach the motor. ESCs that are allowed to become very hot are at
risk of thermal runaway, which can lead to the destruction of the ESC.
Left: The heat shrink wrapping over ESCs does tend to reduce the effect of cooling
air. Right: this specialist, high power ESC has no heat-shrink covering. It is mounted
on the outside of a model, and the exposed heat sink allows the airflow to dissipate
the maximum possible quantity of heat.
Improving ESC Cooling
The electronics of the ESC is invariably covered in a protective wrapping of heat-shrink
tube. This serves very well to support the wires as they leave the circuit boards and to
protect the components from damage.
However, heat shrink tubing is an excellent insulator of heat, so it greatly reduces the
ability of the ESC to shed its heat. For this reason, high-power ESCs tend to have an
aluminium heat sink in contact with the FETs that is not covered by the heat-shrink tube.
This allows the heat from the FETs to be dissipated much more easily.
To increase the ability of standard, heat-shrink wrapped ESC to dissipate heat, it is
possible to carefully cut away a portion of the heat shrink tube to allow the FETs to be
better cooled. Great care must be taken to avoid damage to the ESC during this
operation, which would of course invalidate the ESCs guarantee.
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Chapter 6
Electric Motor Safety
Electric Motor Safety and the role of the ESC
In modern times, the issue of safety is perhaps emphasized rather more than is
necessary or sensible in many areas of life. However, when considering electric flight,
safety is an issue which most definitely needs to be taken very seriously. The lack of
noise, smoke and vibration from electric motors can lead to the impression of an overall
lack of significant danger from this type of power source. This is an illusion.
Electric motors are no safer than i.c. engines, and in fact, in my opinion, electric power
actually poses more potential danger to modelers than i.c. engines. This is for the
following three primary reasons:
1. An i.c engine cannot spontaneously burst into life. In contrast, this possibility is
always present any time the battery is connected to an electric model’s ESC.
2. If the propeller of an i.c engine accidentally contacts your body, the tendency will
be for the engine to slow down and stop. In contrast, an electric motor will not
only keep going, it will do so with even more power - as we have seen when
discussing propellers; the greater the load on the motor, the more power it will
develop. Thus, the injuries from a given propeller and rpm combination are likely
to be worse from an electric motor than a comparable i.c. engine.
3. The noise of an operating i.c. engine provides an alert that its propeller is turning.
Electric motors emit almost no such audible warning.
For the above reasons, perhaps the most important sentence in this entire guide is this
one:
TO AVOID INJURY, YOU MUST TAKE ELECTRIC MOTOR SAFETY SERIOUSLY
ESC arming
Electronic Speed Controllers won't ‘arm’ until they recognize a low throttle command
from the receiver. This is an important and useful safety precaution. So, if the battery is
plugged in with the throttle stick set anywhere above the closed (‘stop’) position, the ESC
should not arm, and in theory, the motor should not turn. However, there are a number of
reasons why a motor could start up unexpectedly. These include the following:
1. Electronic devices, including ESCs, do occasionally malfunction. Cases have
occurred where electric motors started up even with the throttle set to ‘stop’
2. The transmitter’s throttle stick may be accidentally moved, either by you or by
someone else.
3. Interference may result in a high throttle command reaching your receiver, even
with your transmitter broadcasting a throttle closed (‘stop’) command.
The only safe policy for electric motors is to consider that the motor is liable to start up at
any time when the battery is connected to the model. With electric power systems, the
bottom line is this: the only time a ‘live’ electric motor should be considered unlikely to be
able to injure you is when it does not have a propeller attached to its shaft.
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An important safety note on testing a new power system
Virtually all transmitters include a servo reverse function, which allows the sense of a
transmitter command to be reversed to suit the modeler’s needs. This is a very helpful
facility when setting up control surfaces. This facility is also present on the throttle
channel, where it is equally handy when setting up an i.c. engine installation. However,
for electric models, this reversing facility poses a very significant potential danger.
The throttle channel should of course be set up so that with the stick fully back, a ‘stop’
command (throttle closed) signal is received by the ESC. However, let us consider what
could happen if the throttle channel were inadvertently set up in the incorrect ‘sense’.
The transmitter is switched on, and we check carefully that the throttle stick is set to
‘stop’. We then connect the battery to the model. The ESC appears not to operate. We
move the throttle forward a little, but nothing happens. The ESC makes noises, but no
propeller rotation occurs. Except for a confusing noise, the power system appears dead.
We then move the stick fully forward, hoping for a response but still the motor does not
turn. The ESC’s noise changes, but we do not understand why (In fact, the ESC has just
armed the motor, since it believes it has just received a ‘stop’ command). So we decide
to switch the system off and investigate.
Before switching off, we decide to move the stick back to the safe ‘stop’ position,
whereupon the motor bursts into life. We move the stick all the way to ‘stop’ but the
motor accelerates to full power. This may be very confusing, since the throttle is set
closed. Now, we have a motor spinning a sharp edged propeller which is effectively out
of control. Clearly, this scenario could lead to a very serious accident involving
lacerations to arms, the loss of digits or even an eye.
Left: The author, holding a brushless powered Cougar 2000 fun-fly model, back in 1999.
Although heavy by modern standards, the model would prop-hang on its geared
brushless inrunner motor which sucked more than 500 Watts from its 16 Nicad cells.
This would be a dangerous way to hold a model, if the battery was connected to the
model.
Right: Even a small model such as this 300 Watt Acromaster could cause
serious injury if its propeller was given an opportunity to come into contact with the body.
The tips of the propeller blades are sharp, and reach a very high speed when the motor
is rotating.
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Bearing the above in mind, it is very wise to ensure that ALWAYS and without exception
the model is securely restrained on the ground before connecting the battery. This
precaution applies most especially when testing a new installation. However, it also
applies to established installations, since it is quite possible for transmitter programming
to inadvertently become altered. This is known to have occurred, and at least one
example was caused by a cellphone (mobile telephone) being placed near to a
computerized transmitter.
Testing without a prop
It is also worth considering testing a new system without a propeller fitted in order to
remove the danger of a rotating propeller. Clearly without a propeller fitted, the potential
exists for the motor to over-speed, so care must be taken to avoid full throttle operation
with no load, especially if the moor is an inrunner type.
Electric power is safe – provided appropriate precautions are taken
Please don’t let the above discussion scare you into feeling that electric motors are too
dangerous to use! As with many things, electric motors are only as safe as the user
makes them. When the proper precautions are applied, electric motors represent a very
safe, clean and enjoyable form of power for model aircraft.
Left: The battery of John Ranson’s Hawker Tempest, built from the Sepp Uberlacher
plan is housed under this hatch. As soon as the battery is connected to the model, John
takes care to keep well behind the prop. Whatever the size of the model, whenever the
battery is connected, care should be taken to restrain the model and to keep body parts
out of the propeller arc. This ensures that even if the motor of a model starts
unexpectedly, no injury will be caused. Right: One excellent idea to increase safety is
to arrange the ESC to motor wires such that one of them can be left disconnected right
up until the pilot is fully ready to fly. The motor cannot run unless this connection is made
so this idea allows the battery to be installed and for the flight controls to be tested
without risk that the motor will burst into life. A good way of arranging this is to route one
motor wire out through the side of the cowling, such as seen here with George Worley’s
impressive, weathered Hangar 9 P47 Thunderbolt.
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Chapter 7
BEC and PCO Functions of the ESC
The primary purpose of the ESC is of course to control the speed of the motor. However,
all types of ESC can do a lot more for us than just this. The two most important
additional ESC functions are the BEC (Battery Eliminator Circuit) which provides power
for the model’s receiver and servos, and the PCO (Power Cut Off) circuit which monitors
the voltage of the flight battery and guards against it becoming exhausted:
The Battery Eliminator Circuit (BEC)
Many ESCs, especially smaller ones, incorporate a Battery Eliminator Circuit, or BEC.
The BEC provides a constant voltage output, usually of 5 Volts, to supply the receiver
and servos. The BEC therefore eliminates the need for a separate 4.8 Volt Hydride or
Nicad receiver battery to supply the receiver and servos, saving weight and complexity.
BEC current limit
BEC circuits are limited in the current they can supply. The instructions should detail this
aspect of an ESCs specification, and advise as to the number and type of servos that
may be used. The characteristics of the electric motors used inside servos are the same
as for any other electric motors; if they are required to work hard, they will draw more
current. For this reason, it is important that there is no excessive friction in the linkages
operating the control surfaces such as the rudder, elevator and ailerons. If these
linkages are in any way sticky or have significant friction, the current demand of the
servo will increase, and the limit of the BEC may be exceeded, even if the allowable
number of servos is not exceeded.
The Power Cut Off function (PCO)
If a model was flown until the flight battery became completely exhausted, at some point
the battery voltage could easily become so low that the BEC would cease to function.
Left: A BEC circuit requires an input voltage at least a Volt or two higher than 5 Volts.
This requirement is easily satisfied since the flight battery of most models is at least a 2cell LiPo, nominally rated at 7.4 Volts. Right: It’s good practice to land a model while at
least 20 % of battery power remains in the battery. LiPo battery life will be extended by
this means which also avoids the danger of having the motor stop unexpectedly in flight.
The subject of this photograph is John Ranson’s very fine DH Hornet.
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With no power to the RC system, control would be lost of the model. To deal with this
problem, all ESCs incorporate a Power Cut-Off (PCO) function. This is sometimes
known instead as the Low Voltage Cut-Off, or LVC for short. The PCO circuitry monitors
the flight battery voltage, and if it falls to a particular (sometimes user set) threshold, the
PCO will cut the power to the motor. This voltage at which this happens is of course set
to be higher than the voltage at which the battery would be totally flat. For example, for a
3 cell LiPo, the PCO might operate at 9.0 Volts. By this means, the ESC protects the
supply of power to the BEC.
Power can be restored to the motor by shutting the throttle and gently reopening it again,
to less than full power. This causes the battery to suffer less of an on-load voltage drop,
so operating at reduced power should prevent the PCO from operating again for a while
whilst the model is positioned for a landing. The PCO also prevents the battery from
becoming discharged to the point that it may sustain damage. Note that although the
PCO exists to preserve voltage for the BEC, the two functions are entirely separate –
motor power is cut by the PCO, and not by the BEC as is often incorrectly believed.
Disconnecting the ESCs BEC function
For some models, it is desirable to disconnect the BEC function, and to power the RC
system by an alternative means. The BEC’s output can be disconnected by cutting the
power supply wire (generally coloured red) of the lead connecting the receiver to the
ESC. Of course, cutting this wire does not stop the ESC’s BEC circuit from working, but
it does prevent its output from being used. The two remaining wires connecting the
receiver to the ESC allow it to still be controlled.
Remove the battery after flight
Note that it is important to make sure the battery of a BEC-equipped model is removed
after flight. If the ESC is left connected to the battery, it will still cause a drain of a few
milliamps even if switched off. For this reason, if the battery is left connected to a model
for an extended period after flying there is a good chance that the LiPo battery would
become deeply discharged, causing damage and an increased risk of fire.
Left: The positive wire of this ESC-to-receiver wire has been prised out of its plastic
receptacle, disabling the BEC output. The loose wire will be insulted with heat-shrink
tube or insulating tape. The wire can be replaced when if needed. Right: An alternative
to altering the wire of the ESC itself is to add a short extension lead between ESC and
receiver, and to alter the power supply wire in this lead instead. This leaves the ESC
wiring intact. Here, a section of the red (positive) wire has been removed.
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Chapter 8
Opto Isolating ESCs
The power system of an electric model unavoidably generates some radio frequency
(RF) interference. The constant switching taking place inside an ESC means that it too
creates RF interference. Some of this interference will inevitably find its way to the
receiver along the lead connecting the ESC to the receiver. Clearly this situation is not
ideal, although for most models it does not usually pose a problem. However, for large,
fast or expensive models, this factor may present a risk that is considered unacceptable,
especially as large and fast models may well be flown at some distance away from the
transmitter, when the received transmitter signal will be relatively weak.
In such cases, an opto isolating ESC is worth considering. These are a variation on the
standard ESC, and are the same as a normal ESC except for the way they connect to
the receiver; inside the ESC, the receiver to ESC connection is made optically, not
electrically. Since opto ESCs are electrically isolated from the receiver, there is no direct
electrical connection between the two units, which ensures that no RF (radio frequency)
interference fed to the receiver by the ESC.
This is John Ranson’s atmospheric electric powered Brian Taylor Me109, showing
off its brutally handsome lines. Installing a high quality receiver and an opto
isolating ESC is a particularly wise precaution for a large scale model like this one,
the construction of which involved a great deal of work. Opto isolating ESCs may
be used for any model which can carry the weight of a separate ESC battery.
Opto ESCs have no BEC function
Because there is no direct electrical connection between the receiver and the ESC, opto
isolating ESCs cannot offer a BEC function. The use of a separate power source for
receiver and servo power is therefore required. High current ESCs tend to be of the opto
type; this reflects the fact that (a) the more current the ESC handles, the more intense
any RF emissions are likely to be and also that (b) larger models are more able to carry
the weight of a separate receiver battery. Since the whole point of an opto ESC is to
isolate the RC system from the power system, there is little point in considering the use
of a UBEC fed from the flight battery to provide RC system power.
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Chapter 9
Electronic Speed Controller Limitations
Like all things, the ESC must be used within its current and voltage limitations. The ESC
can only be relied upon to perform reliably if the cooling arrangements are good enough
– if insufficient cooling air reaches the ESC, it may very well overheat and become
damaged and/or shut down. Cooling is an especially important consideration if the ESC
is also used to supply power to the RC equipment. It is impossible to over cool an ESC,
so do not be hesitant to supply it generously with cooling air.
ESC Voltage limitation
In addition to a current limitation, ESCs are also designed to operate with an upper
voltage limit. For smaller ESCs, usually 3 or perhaps 4 LiPo cells is the upper limit. If an
ability to handle a higher voltage is needed, an appropriately rated ESC must be used.
ESCs are available to handle 6, 8 or even 10 or 12 LiPo cells. Don’t ever be tempted to
exceed the allowable cell count; you will run a great risk of permanently damaging the
controller.
ESC Current limitation
All ESCs are rated to handle a particular maximum current. If an ESC is made to operate
at a higher current than it is rated for, it may well overheat and become damaged and/or
shut down, the same as if it were not cooled properly. Do not be tempted to use an ESC
at a higher current than it is designed to handle.
Left: This 70 Amp ESC is mounted on the outside of a model. The propeller wash
ensures a healthy supply of cooling air. The air scoop was made from a white plastic
spoon, and supplies the battery with cooling air. Right: This small brushless ESC is
rated for 17 Amps. The plastic heat shrink sleeve around this ESC protects it from
physical damage, but does nothing to assist cooling. If even a small quantity of cooling
air can be made to flow through the ESC, so much the better.
Over specifying an ESC
Over specifying an ESC (e.g. selecting a 30 Amp model when the required current
capability is only 20 Amps) is a very good idea. Some spare current capacity has a
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number of benefits; it will allow for propeller changes which may increase the current
draw of the motor and it will operate cooler. Also, a unit used at less than its full current
capacity will be more reliable, and less likely to overheat and/or shut down. For most
models, the additional weight of the larger ESC is of no consequence, and the cost
increment is usually only small. There is no benefit over specifying an ESC in terms of its
voltage capability.
Properties of wires
Wires carrying electric current have a number of properties which are not necessarily
obvious to the modeler. As well as electrical resistance, the wires also have a property
called inductance. The switching action of the ESC unavoidably creates voltage ‘spikes’
(momentary periods of high Voltage) in the wires linking the battery to the ESC. The
longer the battery-to-ESC wires are, the larger these spikes will be. To absorb the
spikes, and thus prevent the voltage spikes from reaching and damaging the ESC, one
or more capacitors are soldered across the input wires to the ESC. These capacitors can
easily be seen on many ESCs, and without them, the ESC would be easily damaged by
the spikes.
Length of wires between Battery and ESC
The capacitors built in to ESCs are sufficient for a total battery to ESC lead length of
approximately 20 cm. Extending these leads by just a few centimeters (perhaps 5 or 6
cm) should present no problems. However, if the battery to ESC lead length needs to be
significantly increased, then one of more additional capacitors will be needed to absorb
the inevitable voltage spikes and prevent ESC damage. Additional capacitors must be
installed as close to the ESC as possible, soldered across the input wires.
Length of wires between ESC and Motor
There are no restrictions on the length of wires linking the ESC to the motor; inductance
is not a factor here. However, the resistance of these wires is worth considering. If the
ESC-to-motor wires need to be extended, make sure they are the same size (or larger)
as those of the ESC itself. It is always worth gently twisting any power system wires
together to reduce the amount of RF emitted. One turn per 2.5 cm (1 inch) is sufficient.
Left: The two round objects situated at the battery end of this 4-Max 80 Amp ESC are
capacitors. These are necessary to absorb the voltage spikes caused by the inductance
of the battery to ESC wire. These are rated at 35 V, 470 μF. Right: All ESCs employ
capacitors which are located between the wires connecting the ESC to the battery.
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Chapter 10
Setting up Electronic Speed Controllers
Most modellers use their ESCs on a ‘plug and play’ basis, using the manufacturer’s
default settings. The default settings are usually quite suitable for the average sport
model, so no set up at all is required. You may however wish to adjust the way some of
the ESCs features work to suit your exact requirements. This is easily accomplished.
ESC sounds
Modern ESCs incorporate the ability to make sounds to inform the user of its status, and
to guide the user through the set up process. One example is when connected to a
battery the ESC will announce that it is ‘live’ with a few tones or even a short tune. The
ESC’s sounds are actually produced by the motor windings, to which the ESC sends an
appropriate signal in order to create the sound. For this reason, the motor must be
attached to the ESC before setting up, unless using a programming card.
Left: Setting up an ESC is a simple matter. It can either be done using the throttle stick
of the transmitter, or by using a simple programming card like this one. Right: The
sounds emitted during ESC set up actually come from the windings of the motor.
Programming, or setting up the ESC
If required, set up is generally very easy. There are three possible methods of setting up
an ESC. The most basic method, which is entirely satisfactory for most models, is to
programme the ESC simply by moving the throttle stick of the transmitter to a high or low
position as required, responding to the ESCs various questioning beeps.
A second, easier method is to use a programming card, which is an inexpensive device
temporarily connected to the receiver. Lastly, sophisticated ESCs may sometimes be set
up by connecting them to a personal computer (PC) with the appropriate software. The
list of user-definable ESC parameters may include any or all of the following:
(a) BEC voltage selection
The usual output voltage of a BEC is 5.0 V. Some ESCs allow the user to specify a
different voltage, for example 5.5 V or 6.0 V.
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(b) PCO Voltage threshold
Most ESCs for use with LiPo batteries will automatically detect the number of cells in
uses, and automatically initiate PCO function when a suitable Voltage threshold is
reached. For example, an ESC used with a 3-cell LiPo would typically cut motor power
when the on-load battery Voltage reaches 3.0 Volts. This Voltage threshold may often be
adjusted if required.
(c) PCO function operation
The way the PCO function operates may be varied. For example, when the threshold
voltage is reached, the voltage supply to the motor may be cut off or alternatively just
reduced.
(d) Switching frequency
Some ESCs allow the switching frequency to be varied. The switching frequency may
need to be increased to work with high speed motors, and/or those with a high number
of poles.
Left: The threshold Voltage of PCO operation is usually automatic. This large Shiebe
Falke FS28 motor glider, built by Martin Tremlett from the Cliff Charlesworth plan & now
owned by Bob Mahoney uses a 21-cell nicad battery and a high quality brushed motor. If
it were to be converted to LiPo power, and the same ESC and motor was retained, it
would be necessary to confirm that the cut-off voltage was still suitable for the LiPo, as it
would be dangerous to allow the battery to become over-discharged. A much better idea
would be to purchase a complete new power system. Right: Four tiny EDF units power
this modestly sized Vickers Valiant, designed and built by Chris Golds. Each of the KP
fans turns at an incredible 80,000 rpm. This high rotational speed requires the ESCs to
carry out their duties with a very high speed of switching. Not all ESCs could cope with
this demanding requirement.
(e) Soft start function
At the instant of start up, the motor will have no rotational speed, and so would not be
generating any back EMF. If full battery voltage was applied to a stationary motor, the
current would momentarily be extremely high. This would waste energy, and for brushed
motors will also risk damage to the brushes and commutator. To address this problem,
most ESCs offer a ‘soft start’ capability. The soft start function allows full power to be
developed only gradually, perhaps over a period of one second, in response to a rapid
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full throttle command. This allows the motor to develop some rpm before full battery
voltage is applied, thus avoiding the problem of full voltage being applied to a stationary
motor. This prevents the very high current of an instant full power start. This feature is
especially useful for gearbox equipped models, since gearbox teeth would be at risk of
damage from the excessive torque of a rapidly applied application of full power. The high
inertia of a large propeller also means the motor will take a relatively long time to speed
up extending the high current period resulting from a zero or low back EMF.
(f) Brake function
If a motor is spun up, and the battery is then disconnected, the speed of the motor will
decay over a period of several seconds. However, if the terminals of a brushed or
brushless motor are connected together after power is removed, the motor will stop
almost immediately. This is the principle by which the ‘brake’ function on an ESC works;
when the brake function is enabled, the ESC electronically connects the terminals of the
motor together. This prevents the motor from rotating freely, braking it. The model’s
forward airspeed causes the now stationary blades to fold backwards. The braking effect
from a direct connection between the motor’s terminals is very powerful, so many ESCs
offer a ‘soft’ or ‘hard’ braking option, allowing the user to select the degree of motor
braking that is desired.
Left: An EDF Skyhawk. Adjusting the ESC’s timing of a motor can make a big difference
to the current it draws. The cause of excessive current consumption may be incorrectly
set motor timing. Right: With the brake function set to off (disabled) the motor will
freewheel when the throttle is set closed. A freewheeling prop generates a lot of drag;
even more than a stationary prop. Increased drag can be useful on approach to land.
This lovely electric Bristol Freighter is seen here about to touch down.
(g) Motor timing
For brushless motors, the commutation and its timing is carried out by the ESC. It is
sometimes useful to be able to adjust this timing – this can significantly alter the
characteristics of the motor to suit the needs of the particular set up. For example,
advancing the timing will result in the motor drawing more current and turning at a higher
rpm with the same prop, whereas retarding it will have the opposite effect. Thus by
adjusting the timing, it is possible to choose a suitable balance of power and economy
for any given propeller. For correct operation, some motor types will require that the
timing is adjusted from the ESC’s default settings.
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Setting up the sense of an ESC
Virtually all transmitters have a servo reverse function, which allows the sense of a
transmitter command to be reversed to suit the model’s needs. This is a very helpful
facility when setting up control surfaces. This facility is also present on the throttle
channel, where of course it would be equally handy if setting up an i.c. engine
installation. However, for electric installations, this reversing facility poses a very
significant potential danger. The throttle channel should of course be set up so that with
the stick fully back, a ‘stop’ command (throttle closed) signal is received by the ESC.
It is a wise precaution to make a preliminary check on a new system without a propeller
fitted. This creates an opportunity to check the system for correct transmitter sense and
correct rotational direction with a much higher degree of safety. This precaution means
that a sense of familiarity can be developed with a new system before adding the
element of a rotating propeller. There is no need to operate the motor at full throttle
during such a preliminary test. This type of testing is wise for all motors, and makes
particular sense with outrunners, which generally use relatively large props. Since these
are relatively low Kv motors, they are unlikely to over-speed with no load connected.
Propeller shaft direction
One possible difficulty is that with no prop fitted, it can be difficult to see which way the
propeller shaft is turning. A solution to this is to make a simple disc about three of four
inches in diameter, divided into four quadrants, two of which are painted black, and two
white. The disc is attached to the prop shaft, making the direction in which the shaft turns
easy to see. The disc should be accurately made, preferably from a light weight material,
such as balsa or Depron so it is not out of balance.
Measuring Kv
The simple tool mentioned above may also be useful at a later date, when it can be used
in conjunction with a rev counter for determining the off load speed of a motor. By
comparing this rpm with the battery voltage, the motor’s Kv may easily be calculated. For
example, if the motor achieves 9,760 rpm off-load with a battery supply Voltage of 9.76
Volts, the motor has a Kv of 1,000 (9,760 ÷ 9.76 = 1,000).
Left: John Ranson’s superbly realistic Bristol Beaufighter. In the excitement to test a
new model, it is easy to forget the danger which a rotating propeller represents. Always
make sure a model is securely restrained when testing its motor. Right: A useful tool for
enabling motor Kv to be measured is a disc such as this one.
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Chapter 11
Battery Basics
This chapter is intended to give you an overview of the battery types used in RC
modelling. For more detailed information on each of the battery types, a separate Gibbs
Guide is available. The types of batteries used by RC modellers include the following:
Lithium Polymer
Lithium Polymer (LiPo) batteries offer a light weight, energy dense source of power at a
reasonable price. For these reasons, they are the usual battery of choice for model
aircraft. However, they are relatively fragile, and do not tolerate overcharging or
excessive discharging. LiPo batteries are housed in a flexible Mylar (plastic) case, so
they are much less tolerant than other types of physical abuse such as being dropped or
if involved in a crash.
LiPo batteries have a nominal voltage of 3.7 Volts per cell (Vpc). Common battery cell
counts are 2 or 3 cells (7.4 Volts) for indoor and small park flyer models, 3 or 4 cells
(11.1 V or 14.8 V) for many smaller sized models, and perhaps 6 cells (22.2 V) for
medium to large models such as a 60 inch span P51 Mustang. Really large and/or high
power models can use up to 12 or even 14 LiPo (51.8 V) cells.
LiPo batteries are available in a wide range of capacities and cell counts. Used
with care, LiPo batteries are an excellent source of power for model aircraft
Used with care and knowledge, LiPo batteries are an excellent, reliable source of power
for model aircraft. However, the safety of lithium polymer batteries is a subject that
needs serious attention if accidents are to be avoided.
I strongly recommend the purchase of the Gibbs Guide to Lithium Polymer Batteries if
you intend to use this battery type. Care must be taken to avoid both over charging and
excessive discharging. The batteries should be stored with care, and only charged in
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such a place that if fire breaks out it will not cause a disaster. Modellers who have not
taken sufficient care have lost workshops, garages and cars to LiPo battery fires. The
cells of LiPo batteries must also be kept in a balanced state, which requires the use of a
balancer when charging.
Left: This small LiPo battery was deliberately overcharged to the point where it caught
fire. If a LiPo battery breaks out into fire, the flame cannot be put out. Right: Although
the risk of a LiPo battery fire is slight, the consequences can be very serious. For this
reason, it is an extremely good idea to use a protective charging sack like this one as a
precaution when charging LiPo batteries.
LiPo battery fire image by Simon Sheldon and used with his kind permission.
Lead acid
The principal use of lead acid batteries for electric RC aircraft modellers is as the power
source for chargers. They are also commonly used for i.c. modeller’s flight boxes. Lead
acid batteries are heavy, but represent an economical method of storing a relatively
large quantity of electrical energy. Lead acid batteries are not able to provide a high
discharge current, so they are impractical for use in model aircraft. However, they are
widely used in modelling applications.
A single lead acid cell provides a nominal 2 Volts. Usually, batteries contain six such
cells, providing 12 Volts. Some modellers use their car battery for supplying their
charger. Provided the demands on the battery are reasonable, this can work well
although it should be noted that car batteries are not designed for this type of duty.
However care must be taken not to over discharge the car battery, otherwise the car will
be unable to start. For a small car, with a battery capacity of perhaps 20 Ah, the battery
could be flattened in as few as perhaps 6 charges of a 3-cell 2,200 mAh LiPo. In
contrast, a typical 12 Volt, 60 Ah leisure battery can provide somewhere around 20
charges of a 3-cell 2,200 mAh LiPo. Lead acid batteries should always be recharged
immediately after use to avoid damage.
Lead acid batteries require careful handling if injury is to be avoided. Care must be taken
to ventilate the area in which lead acid batteries are kept and charged. Also when
transporting batteries great care must be taken, especially if they are to be transported in
vehicles – if the vehicle is involved in an accident the extremely high weight of a lead
acid battery may cause immense injury and damage.
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Nickel Cadmium
Nickel Cadmium (NiCd, or Nicad) batteries used to be in very common use for modelling
applications. They used to be more or less the only choice for flight batteries, and also
for receiver batteries. Nicad batteries are no longer available for modeling purposes due
to their cadmium content, which gives rise to environmental concerns. At the end of their
life, nicads should be disposed of responsibly as their cadmium content is very toxic and
is best kept out of landfill sites.
Nicad batteries have a nominal voltage of 1.2 Volts. They will tolerate some
overcharging without significant damage, and will also recover following deep
discharging. Nicad batteries tend also to be very durable, and there are still plenty of
examples still in use.
Except for avoiding short circuits and excessive overcharging, there is little of concern in
terms of safety.
Left: Hydride batteries are still in common use as a power source for transmitters and
receivers. Shown here is a 4-cell receiver battery.
Right: Lead acid batteries are in
common use in electric flight modelling, but not for flight batteries as their weight is
prohibitive. Leisure batteries are the best choice as a charger power source as they are
more tolerant of repeated discharge and charge cycles.
Nickel Metal Hydride
Nickel Metal Hydride (NiMH, or just Hydride for short) batteries are in very common use,
mostly as a power supply for transmitters. Before LiPo batteries Hydrides were also
commonly used as flight batteries.
In many ways, Hydride batteries are similar to nicads. Hydrides also have a nominal
voltage of 1.2 Volts. However they are nowhere near as durable as nicads. They will
tolerate limited overcharging, but some types are damaged by deep discharging.
Traditional hydrides have a rapid self discharge rate, however the latest ‘Eneloop’ or
similar hydride batteries have a low self discharge rate making them more suitable for
transmitter and receiver use.
As with nicads, except for avoiding short circuits and excessive overcharging, there is
little of concern in terms of safety.
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Chapter 12
Battery Voltage Characteristics & Charging
All varieties of battery share certain basic characteristics. For example, all types will
respond in fundamentally the same way when discharged, and when charged. The
battery voltage should be considered to be a baseline or ‘nominal’ voltage. For example,
an 11.1 V LiPo battery is nominally 11.1 V, however its actual voltage may be either
more or less than this, depending upon the circumstances. For example we instinctively
know that a battery in a low state of charge has a low voltage, while a fully charged
battery has a relatively high voltage. The voltage of a battery will of course change more
than that of a single cell. A battery will always be in one of three states:
● Off-load, or ‘open circuit’ i.e. disconnected from any electrical load.
● On-load, i.e. under discharge
● On-charge, i.e. while being charged
Batteries of all types share certain fundamental characteristics. Left: Various LiPo
batteries. Right: Three Lead Acid batteries.
Off-Load (or resting) Voltage
Let us first consider a cell or battery (or single cell) in an ‘off load’ condition. Its voltage
will vary according to its state of charge, and we would correctly expect a fully charged
cell to have a high voltage, and a discharged cell to have a low voltage.
On-Load Cell Voltage
When a battery is on-load, in addition to its state of charge, a second factor will affect its
voltage. This factor is the electrical resistance of the cell itself, which we call its internal
resistance. When a current starts flowing the effect of internal resistance is that the cell’s
voltage will be slightly reduced compared to it’s off-load voltage. The cell is then said to
have suffered a voltage drop. The amount of voltage drop will depend both on the
internal resistance of the cell and the amount of current flowing:
The higher the electrical current (load), the greater this voltage drop will be.
The higher the cell’s internal resistance, the greater the voltage drop will be.
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Relationship between
battery voltage and load
8
Battery Voltage
7
6
Half Power selected
5
4
Full Power selected
Motor turned off
3
2
1
0
0
2
4
6
8
Time in Seconds
The voltage of a battery depends on the state of charge as well as the load placed
on it. This diagram shows how the battery voltage will vary depending on the load
placed on it.
To clarify this important concept, let us now consider some examples:
Low loads
A cell of low internal resistance under a very low load suffers only a very small voltage
drop. A cell of higher internal resistance under the same load will show a higher voltage
drop. In both cases the drop will be very small if the current is very low.
Higher loads
A battery of low internal resistance under a heavy load will suffer a larger voltage drop
compared to being under a lighter load. A battery of higher internal resistance under a
heavy load will suffer the greatest voltage drop of all. The larger a battery is in terms of
its physical size and/or its capacity the lower its internal resistance will tend to be.
Batteries with a relatively low internal resistance are legitimately given a high C rating.
All else being equal, the lower the internal resistance of a particular battery is, the more
suitable that battery is for high current applications such as electrical powered models.
Thus, larger cells are generally better suited to higher currents than small ones are.
Note that once we stop taking current from a battery, its voltage will begin to rise back to
the voltage determined by its state of charge. An important point to remember is that a
battery’s internal resistance will only affect its measured voltage when it is on load.
Some practical examples of voltage drop are: (a) When a lead-acid battery is used as
the power source for a LiPo charger, its voltage drop under load (caused, remember, by
its own internal resistance) means the charger will have to operate on a slightly reduced
battery voltage. This effect can easily be seen on chargers which display the input
voltage. (b) In the case of an electric LiPo powered model, the voltage drop within its
drive battery (again caused by the battery’s own internal resistance) will mean a reduced
voltage is available at the motor.
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Models in the moonlight! Friends Louis Louth-Davis and Alan Whipp hold their
models for the camera after a relaxing summer evening flying session. Vintage
style models like this pair of electric powered Mercury Matadors require only a little
power to fly well. This places a relatively low load on the battery.
Voltage recovery
As soon as a load is removed from a cell it will immediately start to ‘recover’ back to the
off load voltage determined by its state of charge. The graph here illustrates both the
voltage drop and voltage recovery effects that we have just discussed. In this example (a
2-cell LiPo in a part discharged condition) the off load battery voltage is 7.4 Volts, and it
falls to about 6.7 Volts when the motor is set to half power after 2 seconds. The voltage
falls further to about 6.3 Volts when full power is selected at 4 seconds. At 6 seconds the
motor is switched off and the battery voltage then starts to rise again towards the off-load
voltage of 7.4 Volts.
Battery Voltage under charge
The voltage of a cell under charge will appear to rise; the higher the charge current is,
the higher the cell’s apparent voltage will be. This characteristic is easily observed on
any charger displaying battery voltage. Cell voltage under charge is a somewhat artificial
figure because what is really being measured is the output voltage of the charger
necessary to drive the required charge current - the higher the charge current required,
and the higher the internal resistance of the cell under charge, the greater the charger
voltage needs to be.
Battery capacity
Battery capacity is stated in terms of the current it can provide for one hour. For
example, a 1,000 mAh battery should deliver a current of 1 Amp (1,000 mAh) for a
period of 1 hour. Similarly, a 2,200 mAh should provide 2.2 Amps for one hour.
Alternatively it should be able to provide 10 times this current for 1/10 as long; in this
case 22 Amps for 6 minutes. In practice, the battery is less efficient at high currents and
only a proportion of the stated capacity will be available at high currents. The reason is
of course the battery’s own internal resistance; at high currents this will cause a
significant amount of heat to be generated.
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Battery chargers like this one are frequently sophisticated microprocessorcontrolled devices. Recharging batteries requires the voltage of the charger to be
somewhat higher than that of the battery, in order to force current through the
battery in a ‘backwards’ direction.
Charging Principles
When charging a battery, the charger forces current ‘backwards’ through the battery i.e.
in the opposite direction to normal. This is accomplished by connecting the positive of
the battery charger to the positive of the battery, and of course negative to negative. The
battery’s own voltage will oppose that of the charger, so the charger’s voltage must be
higher than that of the battery. Thus to recharge a 12 Volt battery requires a charger
voltage of perhaps 15 Volts. Recharging causes the chemical changes that occurred as
the battery gave up its charge to be reversed.
This vintage Lanzo Stik flown by Mike Burke was originally built for rubber power,
but was later converted to electric power by Mike. This model has no
undercarriage, so the wise choice of a folding propeller greatly reduces the
chances of propeller damage on landing. A 2-cell 1,300 mAh LiPo gives the pilot at
least 15 minutes of enjoyable flying. The low power requirement means the power
system for such a model is relatively inexpensive.
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Chapter 13
Wiring and Connectors
Size and quality
The quality and size of wiring and connectors used in RC electric models is clearly of
importance to the performance of the model. Wires must be of adequate cross sectional
area in order to carry the required current. Connectors must be of low resistance,
otherwise they will impede the flow of current and will become warm.
Wire size
The size of wire used by ESCs provides an excellent guide as to the correct diameter to
use - a 15 Amp ESC will use wire with a relatively small cross sectional area, while
larger wires are used by ESCs that can handle more current. Another useful guide is the
rating of ordinary household mains wire. In the UK this is usually rated for 13 Amps. In
the USA, most household wiring uses 14 AWG (15 Amp) or 12 AWG (20 Amp) wire.
Left: Comparing the current rating of an ESC with its wires is an excellent guide as to
the appropriate size of wire to use. The ESC above is rated at 40 Amps and uses wire
with a relatively large cross-sectional area. Right: This ESC with its significantly thinner
wires is rated for the lower current of 17 Amps.
Choice of connectors
Choosing a connector system is an important decision to make when starting out in
electric modelling. The quantity of batteries and chargers, charge leads etc can quickly
mount up, and so if you change your mind about your chosen system you are in for
rather a lot of work! There is no perfect connector; all types have advantages and
disadvantages. Some batteries are supplied with pre-fitted connectors. It is a good idea
to standardize on one connector type for your batteries, but this does not mean you
should necessarily standardise on that type. All connectors will require soldering, which
is an essential skill for the electric modeller. There are a multitude of connector types,
from which I suggest you choose from this short list of three:
Gold, or Bullet connectors
These connectors are so called because they resemble bullets. They may also be known
by other names such as G2 (for 2mm gold), G4, round, bunch and Corally. Many motors
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are commonly supplied with 3.5 mm gold/bullet connectors as standard. Gold/bullet
connectors have remained popular over a long period, and are often favoured by
experienced modelers. Gold/bullet connectors are available in several sizes including 2,
3.5, 4 and 5 mm sizes. The 2 mm size will handle up to 20 Amps, and the 4 mm size will
handle up to about 50 amps. The 2mm size is particularly small and so is ideal for small
models. All sizes present no difficulty to connect or separate.
They are manufactured as individual connectors rather than as a moulded pair, and so
are not polarized. This means that multiple batteries can easily be connected in series.
This is handy if you need, for example, to connect two 3S batteries to make a 6S battery.
Other connector types will require a short 3-connector harness to permit this.
Gold/bullet connectors are easy to solder and there is no risk of melting a plastic
connector body as they are of all metal construction. They are probably the best
connector type for reliability, however unless care is taken, there is a greater risk of misconnection and short circuits. Short circuit protection takes the form of a length of tubing
fitted to exposed male connectors, or else adding a shield made from a length of large
diameter heat shrink tube. Either method works well. Different makes of gold/bullet
connector are available. Compatibility issues can occur but these are rare and can be
easily avoided by sticking with one make.
Left: A pair of 4 mm gold/bullet connectors fitted to a battery. Note that the plug is fitted
to the positive red wire. The connectors have deliberately been spaced to minimise the
risk of a short circuit. If no shield is fitted, the plug must be protected by a length of
suitable insulating tubing. Right: The plugs from two separate 4 mm gold/bullet battery
connectors are shown here alongside each other. The left hand example has been fitted
with a safety shield made from a length of 8 mm heat shrink tubing. Note that for
maximum protection the safety shield must be slightly longer than the plug body.
XT60 connectors.
XT60 connectors are a relatively new connector, and seem to be increasing in
popularity. They are available in one size only, and are rated for 60 Amps. XT60
connectors comprise a pair of 3.5mm gold/bullet connectors encased in a moulded
housing. The female part is used for the battery, and the male part is soldered to the
ESC leads. XT60 connectors are therefore polarized, and are helpfully marked with +
(red wire) and – (black wire) leaving no doubt as to how they should be wired up and
ensuring compatibility with others using this system. XT60 connectors are a little easier
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than Deans connectors to pull apart. They are larger than gold/bullet connectors, and
also slightly larger than Deans connectors. Original XT60 connectors and inferior quality
clones are both available.
Left: A pair of XT60 connectors. The ridged logo provides fingers with a better grip.
Right: Deans connectors. This original example does not include the ridged surface of
copies, making them harder to pull apart.
Deans connectors
Deans connectors are commonly supplied with batteries, but seem to be in decline. They
will handle up to about 40 Amps. The connector body is smooth, making them hard to
pull apart, although some connector bodies have a moulded-in ridge to make this easier.
As with XT 60 connectors, the female part is fitted to the battery.
Some modelers have reported that once they have become well used, the spring plates
of these connectors gradually flatten out, so that contact between the male/female parts
of the connector pair eventually becomes intermittent. This is important because any
connector failures will result in a dangerous loss of control if you are relying on the BEC
of an ESC to supply RC system power. Deans connectors are moulded as a pair
meaning that they are polarized. They are not difficult to solder, but are perhaps a little
less easy than Gold/Bullet or XT60 types.
Factor
Availability
Ease of assembly
Current rating
Series connection of
multiple batteries?
Popularity
Polarised?
Shielded?
Durability & reliability
Ease of connection
Ease of disconnection
Comparison of connector systems
Gold/Bullet
XT60
Good
Good
Easy
Easy
2mm: 20A, 4mm: 50A
60 Amps
Just plug together
Requires
additional lead
Consistently popular
Increasing
No
Yes
Yes, if shield added
Yes
Excellent
Excellent
Easy
Easy
Easy
Quite easy
50
Deans
Good
OK
40 Amps
Requires
additional lead
Decreasing
Yes
Yes
Less good
Easy
Can be hard
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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
Author’s choice
My personal choices
supplied with 3.5mm
convenience I do not
gold/bullet connectors
for 20 Amps or more.
concerning connectors are as follows: Motors are frequently
gold/bullet connectors for ESC to motor connections, so for
change these. For the battery to ESC connection I use 2 mm
for applications below 20 Amps and 4 mm gold/bullet connectors
If making my connector choices afresh, I would again select 3.5mm gold/bullet
connectors for ESC to motor connections, but I might choose XT60 connectors for my
batteries.
Tamiya connectors
Some batteries for electric power models are supplied with Tamiya connectors. These
were supplied with the electric powered cars that this company pioneered in starting in
the late 1970s. The connector since found its way into the electric flight arena, although
it is now not much seen. The Tamiya brand is synonymous with high quality, but this
connector is a rare exception - the contacts are of high resistance and very quickly
become distorted in use, leading to an unreliable and even higher resistance connection.
Tamiya connectors are completely unsuitable for model aircraft and should not be used.
Left: A Tamiya style connector - notice that one of the sockets has opened out and the
other has become distorted. Right: This interesting variant on the Gold/Bullet system
uses a plastic housing to accommodate a pair of these connectors.
Wiring up of Gold connectors – make the battery plug positive
Sooner or later, most modellers will need to operate their charger from their car’s battery.
It is possible that the charger’s output lead could accidentally come into contact with the
vehicle’s bodywork if using the Gold/Bullet connector system. The problem here is that
vehicle bodywork is made to be the negative part of the car’s electrical system, so if it
was the positive lead from the charger that touched the vehicle’s bodywork or engine
parts, a short circuit could occur. To minimise the consequences of such an accident, I
recommend you wire up your connectors so that the negative (black) wire of the
charger’s output lead is fitted with a plug (the ‘male’ part), the exposed part of the
connector system. This means that any short circuit between the output lead plug (wired
to be negative) and your car’s bodywork (also negative) will be much less serious than if
it were a positive to negative accidental short circuit. Following this convention will of
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course mean that the charger positive (red) lead will be a socket. Consequently, the
battery positive lead will be wired with a plug and the battery negative will be wired with a
socket; pleasingly this ‘plug positive’ policy also accords with standard electrical
engineering practice. A second and very useful advantage of using this convention is
that batteries and charging equipment may be easily shared with other modellers.
This convention may seem contrary to the ‘common-sense’ approach of keeping a
positive or ‘live’ (mains electricity) connection covered up. However, in the context of low
voltage DC, this approach has no value; if the positive and negative wires touch there
will be a short circuit irrespective of which one was wired with the unprotected plug.
It is also a good idea to make the lead fitted with the plug about 10mm shorter than its
adjacent lead. This reduces the chance of a short circuit between connectors because
this difference in length puts distance between the two connector ends.
Using fuel tube with Gold connectors.
The unshielded plug of the commonly used Gold connector system presents a risk of
short circuits occurring. To guard against this, ensure that you always fit a 25mm (1 inch)
length of clean brightly coloured I.C. modeller’s fuel tube over the battery’s exposed plug
as a conspicuous protective sleeve whenever the battery is not in use.
Left: If you choose Gold/Bullet connectors, and you choose not to fit a shield made of
heat shrink tubing then an alternative precaution should be made to guard against
accidental short circuits. This unshielded connector is protected with a 25mm (1 inch)
length of clean, brightly coloured i.c. fuel tube over the battery’s exposed plug whenever
it is not in use. Note that a plug is fitted to the battery positive (red) connector, and the
socket to the negative one. Right: Sooner or later, many modellers will need to power
their charger from a car battery which is still in-situ. Adopting the connector policy
described here will minimise the consequences of an accidental short circuit between the
charger’s lead and the vehicle’s metal bodywork.
Additional wire
For most models, the length of motor, ESC and battery wires will be adequate and no
additional wire will be needed. If you do need additional wire for a model, this should be
sized to match the current demand of the model, using the ESC size as a guide.
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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
Chapter 14
Wiring Diagram for Single Motor Models
The diagram shown below illustrates how to connect the motor, ESC and battery for a
single motor model. The diagram assumes the ESC’s BEC function is used to supply
power for the receiver and servos. This is the simplest possible method of providing RC
system power, and is suitable for the majority of small to medium size models. Any
suitable connector system can be used, but if Gold/bullet connectors are chosen, as
shown here, they should be used as detailed here for reasons described in the previous
chapter.
The RC system (receiver, servos and their associated wires and aerial or antenna)
should be kept as physically distant as is practically possible from the power system
(ESC, motor and power system wiring). This minimizes the chance of the power
system’s radiated RF interference affecting the RC system. This precaution is especially
important with non 2.4 GHz RC systems. If the motor runs in the wrong direction (it
should be anticlockwise when viewed from the front of the model) then this can be
corrected by changing over any two of the three motor wires.
One of the three ESC to motor wires
on this brushless model has been
routed outside the cowling to allow it
to serve as a safety device. Unless
the wire is connected, the motor
cannot run.
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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
Chapter 15
Example Power Systems in Detail
This chapter consists of a number of example models, all chosen for their successful
power systems. My definition of ‘successful’ means in its broadest sense a power system
which works well, and is well matched to the model. A power system will ideally be set up
so that the motor is operating at a current which returns a reasonable efficiency. It will
also use a propeller that is large enough to offer reasonable efficiency.
Each page of this chapter is devoted to a single model. Each entry starts with a short
description of the model and highlights any particularly noteworthy points about its power
system. A photograph accompanies the description, and finally a comprehensive set of
data for the model and its power system completes the entry. The data includes model
type and brief constructional details, its wingspan, wing area, weight, wing loading, details
of the motor used including its rpm, Kv and no-load current, details of the propeller and
associated data such as prop make and dimensions, p/d ratio and pitch speed. The full
throttle current and power is also provided for each model. Additional details include the
ESC, the important power loading in Watts per pound and an overview of how the RC
system is powered.
The tables include a figure for the model’s average in-flight power consumption. This
figure depends of course to a considerable extent on the pilot’s flying style; pilots who like
to throttle back frequently will yield a lower figure than one who uses high power for most
of the time. The average in flight power loading figure in Watts per pound is nevertheless
an interesting and useful reference to have available.
The average in-flight power consumption depends to a large extent on
the pilot’s flying style.
There is a great deal of information on each page, so this chapter should contain much to
interest a modeller wanting to learn more about electric power systems. This chapter is
intended to be a useful resource of examples which will help you learn more about power
systems, and will I hope help to guide you in making your own power system decisions.
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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
Multiplex Acromaster
By Andrew Gibbs
This model was the subject of a magazine review, and used the manufacturer’s
recommended system, which under test consumed 30 Amps and 290 Watts. It’s always
worth paying attention to how a major manufacturer such as Multiplex specifies a power
system. You can be sure the power system was carefully selected, tested and developed,
the result being a system which is an excellent match for the model and its intended
mission profile. Another reviewer running the exact same system except for a higher
battery C rating (lower internal resistance) reported significantly more power - 37 Amps
and 395 Watts. However, much of this difference can be accounted for by the fact that my
figures were taken with a part discharged battery, while the other reviewer used a fully
charged battery to find peak power values.
The installed power system provides for a very capable model which has the ability to
hover as well as achieve a good turn of speed in level and climbing flight.
Multiplex Acromaster
Model type and construction
Quick build kit, all foam construction
Wingspan & wing area
43 inches (1,095 mm), 567 sq in / 3.93 sq ft
Weight and wing loading
1,063 g (2.35 lb), 9.5 oz / sq ft
Motor
HiMax HC 3516-1130 brushless outrunner
Motor current for max efficiency
Optimum working range: 10 – 34 Amps
Motor data: Max Amps, Kv & Io
Max 48A/15 sec, Kv: 1.130, Io: 1.8 Amps
Propeller, pitch speed & p/d ratio 11 x 5.5 APC E, p/speed 46 mph, p/d ratio 0.5
Battery
3S 2,500 mAh LiPo
Propeller rpm at full throttle
8,900 rpm
Full throttle current & power
30 Amps / 290 Watts
ESC
45 Amp
Power loading
123 Watts / lb
Flight time vs charge quantity
8 minutes / 1,800 mAh
Average in flight power
19 Amps / 210 Watts equivalent to 89 W / lb
Performance description
Good speed, good rate of climb, ability to hover
RC power source & no of servos ESC integral BEC, 4 mini servos.
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Hangar 9 P51D Mustang
By Andrew Gibbs
This Hangar 9 P51D was the subject of a review for RC Scale magazine. The power
system consists of a 6S LiPo and a relatively low Kv motor. This is a common method of
powering ‘0.60 = 0.90 size’ warbirds, and it works well. The combination of components
seems to be especially efficient; on a test/photographic mission involving a mix of some
aerobatics with some lower throttle camera-passes the average in-flight current was
equivalent to only 34 Watts per pound. The model has a nice, scale performance. The
model is no slouch, but more performance would make the model more enjoyable. The
performance could be significantly enhanced if desired simply by changing the propeller
for one which drew more power from the battery such as a 16 x 10.
The RC system is powered by two separate LiPo batteries, each feeding a separate
UBEC; a 3S 800mAh pack supplies the receiver and 4 flying control standard servos,
while a 3S 350 mAh example powers the single retract servo.
Hangar 9 P51 D Mustang
Model type and construction
ARF semi scale, all wood
Wingspan & wing area
65 inches (1,650 mm), 743 sq in / 5.15 sq ft
Weight and wing loading
3,998 g (8.8 lb), 27.3 oz / sq ft
Motor
E Flite Power 60
Motor current for max efficiency
No data available
Motor data: Max Amps, Kv & Io
Max 55 A continuous, Kv: 400, Io: 2.7 A @ 10 V.
Propeller, pitch speed & p/d ratio 15 x 8 APC E, p/speed 57 mph, p/d ratio 0.53
Battery
6S 5,000 mAh LiPo
Propeller rpm at full throttle
7,500 rpm
Full throttle current & power
48 Amps / 1,050 Watts
ESC
70 Amp
Power loading
120 Watts / lb
Average in flight power
14 Amps / 300 Watts equivalent to 34 W / lb
Flight time vs charge quantity
6.5 minutes / 1,500 mAh
Performance description
Reasonable speed and rate of climb
RC power source & no of servos 2 x LiPo & UBEC systems. 5 servos total
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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
Lockheed C130 Hercules
By Toni Reynaud
This wonderful model epitomises Toni’s philosophy of maximum fun with minimum
financial outlay. The four Speed 400 7.2 V brushed motors are fed by a 3 cell LiPo. They
are somewhat overloaded at full throttle but are lasting well since full throttle is used only
rarely. Toni has enjoyed many flights with this model which is still on its original motors. A
single brushed ESC controls all four Speed 400 brushed motors. The model is easy to fly
and lands quite slowly. A GPS was installed in the model for a few flights, which revealed
that its average flight speed was around 35 mph. It is interesting to compare this with the
propeller pitch speed of 49 mph, suggesting the motor/prop combination results in a pitch
speed which is a good match for the airframe. More information about this model can be
found at the Gibbs Guides website. Incidentally, Toni created many of the excellent
diagrams in this guide.
C130 Lockheed Hercules
Model type and construction
Built from a plan using foam & brown paper
Wingspan & wing area
71 in (1,800 mm), 558 sq in / 3.87 sq ft
Weight and wing loading
1,660 g / 58.5 oz (3 lb 10.5 oz), 15 oz / sq ft
Motors
4 x Speed 400 7.2V brushed can motors
Motor current for max efficiency Approx 3.3 Amps
Motor data: Kv and Io
Kv = 2,277 Io = 0.5 A
Propeller, pitch speed & p/d ratio 4 x Günther 5 x 4.3, p/speed 49 mph, p/d ratio 0.86
Battery
2 x 3S 1,500 mAh LiPo in parallel
Propeller rpm at full throttle
12,000 rpm (estimated)
Full throttle current & power
50 Amps (12.5 A per motor) / 490 Watts
ESC
50 Amp brushed
Power loading
134 Watts / lb
Average in flight power
12 Amps / 175 Watts equivalent to 48 W / lb
Flight time vs charge quantity
6 minutes / 1,560 mAh
Performance
Fairly fast with good rate of climb
RC power source & no of servos From ESC via integral BEC. 4 servos.
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ElectRick
By Rick Morris
This charming vintage style model was designed and built by Rick Morris, a retired airline
pilot. No plan was drawn; the model was built by eye! It was initially powered by a small
diesel engine, and later converted to quiet clean electric power. The model received its
name only after it received its new power system. Rick made sure the wing spars were
beefed up considerably compared to those of a comparable free flight model, so they
could accommodate the higher forces that result from being able to control the model.
The power system employs a small outrunner operating on a 3S (3-cell) battery. The
propeller is a 10 x 3.8 APC-E slow fly and this is about as large as is safe for use with this
motor and a 3S battery. Alternatively, the model could have used a 2S LiPo which would
have allowed the use of a significantly larger prop. The battery is located right up in the
nose, slotted in vertically just behind the motor. The propeller develops approximately 2
lbs of static thrust, although this is of course of little relevance to the flying performance.
Model type and construction
Wingspan & wing area
Weight and wing loading
Motor
Motor current for max efficiency
Motor data: Max Amps, Kv & Io
Propeller, pitch speed & p/d ratio
Battery
Propeller rpm at full throttle
Full throttle volts, current & power
ESC
Power loading
Flight time vs charge quantity
Average in flight power
Performance description
RC power source & no of servos
ElectRick
Vintage style, balsa and ply construction
49 in (1,246 mm), 367 sq in / 2.55 sq ft
1,105 g / 39 oz (2 lbs 7 oz), 15.3 oz / sq ft
Hacker A20 - 20L outrunner
Approx 8 Amps (estimated)
Max 20 A, continuous, Kv = 1,022, Io = 0.85 A
APC 10x3.8, p/speed 27 mph (est), p/d ratio 0.38
3S 2,100 mAh LiPo rated at 25 C
7,400 rpm (estimated)
19.3 Amps / 205 Watts
20 Amp
85 Watts / lb
18 minutes / 1,500 mAh
5 Amps / 56 Watts equivalent to 24 W / lb
Moderate speed, good rate of climb
ESC integral BEC. 2 mini servos.
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Extreme Flight Edge 540T-E ARF
By Andrew Gibbs
This model is included as it makes for an interesting comparison with the Mutliplex
Acromaster. The two models are very similar in size and capability. This model has a
slightly lower Kv motor, which drives a bigger prop. Both models demand a very similar
level of power from the battery, and the propeller rpm is the same.
Extreme Flight Edge 540T- E ARF
Model type and construction
ARF, traditional balsa and ply construction
Wingspan & wing area
45 inches (1,143 mm), 402 sq in / 2.79 sq ft
Weight and wing loading
1,172 g (2.58 lb), 14.8 oz / sq ft
Motor
Torque 2818T brushless outrunner
Motor current for max efficiency
n/a
Motor data: Max Amps, Kv & Io
Max / sec, Kv: 900, Io: n/a
Propeller, pitch speed & p/d ratio 12 x 6 APC E, p/speed 43 mph, p/d ratio 0.5
Battery
3S 2,500 mAh LiPo
Propeller rpm at full throttle
7,500 rpm (fresh battery)
Full throttle current & power
31 Amps / 315 Watts
ESC
Airboss 35 Amp
Power loading
122 Watts / lb
Flight time vs charge quantity
8 minutes / 1,800 mAh
Average in flight power
19 Amps / 210 Watts equivalent to 82 W / lb
Performance description
Good speed, good rate of climb, 3D manoeuvres
RC power source & no of servos ESC integral BEC, 4 mini servos.
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Cermark Banchee E3D ARF
By Andrew Gibbs
The power system of this model comprises Cermark’s recommended low Kv motor
coupled to a large diameter, low pitch prop. This makes it a suitable choice for the low
speed 3D manoeuvres which this model is designed to excel at.
On paper, this combination of motor, battery and propeller would be expected to turn
significantly faster than the measured 8,500 rpm, and should consume considerably more
power than the actual consumption of 450 Watts. The battery used had a relatively high
internal resistance, and this was a significant factor limiting the current and thus the motor
rpm. This power system illustrates how significant the internal resistance of the battery is.
Left: The battery is located underneath the wing. Right: The Banchee in flight. This
particular model proved to be a little delicate around the nose and landing gear areas,
but was very entertaining to fly.
Cermark Banchee E3D ARF
Model type and construction
ARF, traditional balsa and ply construction
Wingspan & wing area
54 inches (1,370 mm), 710 sq in / 4.93 sq ft
Weight and wing loading
1,812 g (3.94 lb), 12.8 oz / sq ft
Motor (manfr’s recommended)
CEM-4220-770 brushless outrunner
Motor current for max efficiency
n/a
Motor data: Max Amps, Kv & Io
Max Amps: n/a, Kv: 770, Io: n/a
Propeller, pitch speed & p/d ratio 15 x 8 APC E, p/speed 64 mph, p/d ratio 0.53
Battery
4S 4,000 mAh LiPo
Propeller rpm at full throttle
8,500 rpm
Full throttle current & power
40 Amps / 450 Watts
ESC
60 Amp
Power loading
114 Watts / lb
Flight time vs charge quantity
6 minutes / 2,000 mAh
Average in flight power
20 Amps / 288 Watts equivalent to 73 W / lb
Performance description
Good speed, good rate of climb, 3D manoeuvres
RC power source & no of servos UBEC supplied by flight battery, 4 mini servos.
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Zephyr ARF
By Andrew Gibbs
This vintage style model was purchased used in 2002, for the purpose of testing power
systems. Its original system comprised a Speed 600 and 7 x 2,400 mAh Nicad cells which
made for a heavy model (about 3.3 lbs) with very poor performance. A brushed, geared
480 motor and ten 1,100 mAh Nicad cells was installed instead. This transformed the
model, which then flew beautifully. The present brushless system is even lighter and
consists of a small low Kv outrunner driving a fine pitch prop. This makes for a reasonably
efficient system and the model easily achieves long flight times. Component weights for
the model are: fuselage: 700 g (including bomb drop module, 2 standard servos and a
large 35 MHz receiver); wings: 265 g; battery 150 g, giving a total flying weight of 1,115 g.
Left: The Zephtyr is not a pretty model, but it has proven to be a useful workhorse.
Right: The business end. ESC and battery live in the nose area. The box underneath
houses a servo operated bomb drop mechanism!
Model type and construction
Wingspan & wing area
Weight and wing loading
Motor (manfr’s recommended)
Motor current for max efficiency
Motor data: Max Amps, Kv & Io
Propeller, pitch speed & p/d ratio
Battery
Propeller rpm at full throttle
Full throttle volts, current & power
ESC
Power loading
Flight time vs charge quantity
Average in flight power
Performance description
RC power source & servos
Zephyr ARF
ARF, traditional balsa and ply construction
62 inches (1,570 mm), 520 sq in / 3.60 sq ft
1,115 g (2.45 lb / 39.3 oz), 10.9 oz / sq ft
MP Jet AC28 / 7-35 D outrunner.
Up to 8 Amps
Max Amps: 9 (estimated), Kv: 950, Io: n/a
10x4.7 GWS slow fly, p/s 54 mph, p/d ratio 0.47
3S 2,200 mAh LiPo
6,540 rpm
9.8 Volts, 12 Amps, 115 Watts (half charged)
30 Amp (large excess capacity; hardly gets warm)
47 Watts / lb
18 minutes / 1,800 mAh
6 Amps / 67 Watts equivalent to 28 W / lb
Moderate speed, good rate of climb.
ESC integral BEC, 2 standard servos.
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Avicraft Panic
By Andrew Weight
This kit built model, a British classic model design, is intended for i.c. power, but builder
Andrew Weight gave it an electric power system derived from a previous project. The
motor is propped for a full throttle current of 50 Amps. This compares with the
manufacturer’s maximum burst current of 55 Amps for 60 seconds. This figure suggests
the highest safe continuous current would be around 45 Amps. The chosen propeller, a
14 x 7, therefore presents a load which is very close to the maximum for this motor. The
propeller’s pitch speed is 64 mph, which is perhaps a little high for this type of model
which, being a strutted biplane is not inherently a high speed model. It may well be that a
lower pitch prop such as a 14 x 5 would be a better match for the model’s strengths of low
and medium speed aerobatics. This propeller choice would also usefully reduce the
current draw.
The large prop causes a high current draw. However, the motor could hardly be better
cooled, and this factor combined with good throttle management prevents overheating.
Model type and construction
Wingspan & wing area
Weight and wing loading
Motor (manfr’s recommended)
Motor current for max efficiency
Motor data: Max Amps, Kv & Io
Propeller, pitch speed & p/d ratio
Battery
Propeller rpm at full throttle
Full throttle volts, current & power
ESC
Power loading
Flight time vs charge quantity
Average in flight power
Performance description
RC power source & servos
Avicraft Panic
Traditional kit, balsa and ply construction
48 inches (1,220 mm), 960 sq in / 6.66 sq ft
2,100 g (4.63 lb), 11.2 oz / sq ft
AXI 4120/14 Gold Line brushless outrunner
20 – 40 Amps (efficiency 82 % or greater)
Max Amps: 55A / 60 sec, Kv: 660, Io: 2A @ 10 V
14x7 APC thin E, p/speed 64 mph, p/d ratio 0.50
5S 3,200 mAh LiPo
9,700 rpm (estimated)
50 Amps / 790 Watts
70 Amp Jeti Advance
170 Watts / lb
7 minutes / 2,400 mAh
21 Amps / 390 Watts equivalent to 84 W / lb
Good, prop hanging possible.
Separate receiver battery; 2,000 mAh, 4.8 Volt
62
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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
Electrified Gentle Lady
By Andrew Gibbs
I converted this glider to electric power around the turn of the century. The use of an
ancient, high rpm brushed ferrite Mabuchi RS540 motor coupled to a small direct drive
prop, all installed in a very low speed model is just about the least efficient way possible
to power this model. However, since powered gliders need very little power, this
combination is workable. The performance data given below is for a 7 cell Nicad, this
providing the lowest power the model can fly on. The model has also flown with other
batteries including 2 and 3 cell 2,200 mAh LiPo packs. However even with a 3-cell LiPo,
the performance is not startling; although the motor consumes more than twice the power
with this battery, it is operating well past its point of maximum efficiency and so the
performance increment is only modest. The small margin between the power loading (25
W/ lb) and the average in flight power (21 W / lb) illustrates how marginally the model is
powered on 7 cells. Component weights: Fuselage 645 g, wings 405 g, battery 370 g,
total 1,420 g.
Model type and construction
Wingspan & wing area
Weight and wing loading
Motor
Motor current for max efficiency
Motor data: Max Amps, Kv & Io
Propeller, pitch speed & p/d ratio
Battery
Propeller rpm at full throttle
Full throttle volts, current & power
ESC
Power loading
Flight time vs charge quantity
Average in flight power
Performance description
RC power source & servos
Gentle Lady
Traditional kit, balsa and ply construction
78.25 inches (1,987 mm), 663 sq in / 4.60 sq ft
1,420 g (3.15 lb / 50 oz), 10.9 oz / sq ft
RS 540 (600 size, circa 1980) brushed can motor
Approximately 5 Amps (estimated)
Max 12 A cont, Kv: 1,500, Io: 2 A (all estimated)
8 x 4 folder, p/speed 31 mph, p/d ratio 0.50
7 cell 2,400 mAh NiCd
8,200 rpm (measured)
6.8 V, 11.7 Amps, 78 Watts
30 Amp Jeti brushed
25 Watts / lb
14 minutes / 2,200 mAh
9.4 Amps / 66 Watts equivalent to 21 W / lb
Very sedate; modest speed & very low rate of climb
Separate 280 mAh receiver battery, 2 x mini servos
63
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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
Rumpler Taube
By Chris Golds
The Taube (Dove) was used by Imperial Germany as a fighter, bomber, surveillance
aircraft and trainer from 1910 until the start of World War I in August 1914. In the early
days of aviation, the pilot was considered to be ‘just’ a chauffer and often had a low rank.
The observer was considered to be the one doing the real work!
The low wing loading and high drag of this beautiful model suggests its flying speed in
scale-like cruise is probably no more than 20 -25 mph. This is a great deal lower than the
estimated pitch speed of 57 mph, suggesting that the motor and prop combination is not
well matched to the airframe. The model employs a small inrunner driving a relatively
small prop. While the model flies very well, a more efficient power system would be
slower revving motor (lower kv) turning a larger prop with a lower p/d ratio. The model
does serve as an excellent illustration that a less than ideal power system can still allow
us to enjoy a model.
Rumpler Taube (Dove)
Model type and construction
Balsa & light ply.
Wingspan & wing area
44 inches (1,117 mm), 324 sq in / 2.25 sq ft
Flying weight & wing loading
820 g (1.80 lb / 29 oz), 12.8 oz / sq ft
Motor
Mega 16-15-6
Amps for max efficiency & max eff. Approx 3-12 Amps (max eff. approx. 88 %)
Motor data: Max Amps, Kv & Io
Max approx 18 A, Kv: 1,500, Io: 0.65 A
Propeller, pitch speed & p/d ratio
APC-E 7 x 5, p/speed 57 mph, p/d ratio 0.71
Battery
3-cell 1,300 mAh LiPo
Propeller rpm at full throttle
12,000 rpm (estimated)
Full throttle current & power
14-16 Amps, 165 Watts (estimated)
ESC
Jeti Master 18-3p B.E.C.
Power loading
60 Watts / lb
Flight time vs charge quantity
6 minutes / 800 mAh (estimated)
Average in flight power
Approx 8 A / 88 W equivalent to approx 49 W / lb
Performance description
Brisk rate of climb and forward speed.
RC power source & servos
B.E.C. 2 x mini servos
64
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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
DH Venom
By Chris Golds
This successful small hand launched model was designed and built by the prolific British
electric scale modeller Chris Golds. The model dates from a time when brushed motors
were still dominant. At the time, the Speed 400 6 V motor in direct drive format, turning a
Günther 125 mm x 110 mm (5x4.5 inches) prop was in widespread use for small models.
This motor and prop combination also found popularity for multi engine models. Chris
deliberately pushed the motor very hard by using nine NiMH cells to gain the maximum
possible performance. This resulted in a current of 15 Amps at full throttle and would have
resulted in a limited motor life. However, the motors were cheap to replace, and this
informed choice did result in a higher performance than would otherwise be possible. A
brushless 400 size motor and a LiPo battery would usually be chosen today instead. This
combination would supply at least twice the power to the prop, with longer duration.
Model type and construction
Wingspan & wing area
Flying weight & wing loading
Motor
Motor current for max efficiency
Motor data: Max Amps, Kv & Io
Propeller, pitch speed & p/d ratio
Battery
Propeller rpm at full throttle
Full throttle current & power
ESC
Power loading
Flight time vs charge quantity
Average in flight power
Performance description
RC power source & servos
DH Venom
Balsa and foam semi scale
36 inches (914 mm), 230 sq in / 1.60 sq ft
700 g (1.54 lb / 24.7 oz), 15.4 oz / sq ft
Speed 400 6 V brushed
Approx 4 Amps
Max approx 12A, Kv: 3,000, Io: 0.4 A
Günther 5 x 4.5, p/speed 64 mph, p/d ratio 0.9
9 cell 1,600 mAh NiMH
15,000 rpm (estimated)
Approximately 15 Amps, 150 Watts
Unknown, but probably 20 A or higher
93 Watts / lb
Approx 6.5 minutes / 1,400 mAh
Approx 13 A / 117 W equivalent to approx 76 W / lb
Sufficient, simple aerobatics possible
4.8 V receiver battery
65
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Gibbs Guides.com
Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
Index
2.4 GHz
3.5 mm gold/bullet connectors
4 mm gold/bullet connectors
53
51
49
Acromaster
Airflow
Airframe
Arming (ESC)
30
13
21
29
BAM Swallow
Balsacraft
Balancing
Banchee
Battery
- capacity
- charging
- discharging
- lithium polymer
- lead acid
- nickel cadmium
- nickel metal hydride
- voltage characteristics
BEC
- current limit
- input voltage
- disconnecting BEC function
- voltage selection
Belt drive
Brigadier
Bristol Freighter
Bristol Beaufighter
Brushless motor
Bulldog
Bullet (gold) connector
Burke, Mike
20
11
15
60
33
46
46, 47
45
41
42
42
43
44 - 46
32
32
32
33
37
19
19
39
40
20
18
48, 49
47
C130 Hercules
Camm, Sidney
Capacitor
Capacity (battery)
Cellphone
Charging principles
Charing from a vehicle
Charlesworth, Cliff
Collins, Jeremy
Commutator
Coniam, Randy
Connectors
- author’s choice
- Deans
- fuel tube
- gold (bullet)
- Tamiya
57
12
36
46
31
47
51, 52
38
20
38
12
48
51
50
52
48, 49
51
- wiring up
- XT60
Cooling
Cougar 2000
Current
Deans connector
DH Chipmunk
DH Hornet
DH Venom
Disclaimer
Dissipated power
Dynamic thrust
51
49, 50
15, 16, 21, 22
30
23
50
15
32
65
4
24
14, 15
EDF
38
Edge 540
54, 59
13, 19, 22, 25
Efficiency
25
- electrical
Electric glider
16, 18
Electric motors
- AXI 4120/14 Gold Line
62
- Cermark CEM-4220-770
60
- EFlite Power 60 motor
56
- Hacker A20 - 20L
58
- Mabuchi RS540
63
- Mega 16-15-6
64
- MP Jet AC28 / 7-35 D
61
- Multiplex HiMax HC 3516-1130
55
- Speed 400 6 V
65
- Speed 400 7.2 V
57
- Speed 480
61
- Speed 600
61
- Torque 2818T
59
Electrical resistance
22
ElectRick
58
ESC
16, 26, 32, 37, 48
- arming
29
- BEC
32
- brake function
39
- brushed
27
- capacitor
36
- cooling
28
- heat generation
27
- limitations
35
- LVC
33
- operation
26
- PCO
32
- power
28
- programming
37
- set up
37, 40
- soft start
38
- switching speed
38
- sounds
37
66
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Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
Extra 330
Extreme Flight Edge 540T
FET
Finnish Air Force
Fires (LiPo)
Fokker Dr1
Folding propeller
Flair
Flyer III
Fuel tube
Gearbox, gearing
- losses
- methods
- noise
- offset
Gentle Lady
Gold (bullet) connectors
- safety shield
- wiring up
Golds, Chris
Günther propeller
GWS
Hacker A20-20L
Hawker Hurricane
Hawker Tempest
Heat
- causes
- ESC heat
Helical pattern
Helicopters
HiMax HC 3516-1130
Humidity
Inrunner
Inductance
i.c. engine
15
59
25
11
42
11
16, 17, 63
11
13
52
Mahoney, Bob
Me109
Microprocessor
Mobile phone
Mosfet
Motor
- cooling
- Kv
- measuring
- outrunner
- safety
- timing
- windings
Multiplex Acromaster
19
20
20
21
21
63
48, 49
49
51
38
57, 65
19, 61
58
11, 12
31
22, 23, 24
22
27
12
21
55
12
Opto isolating ESC
Outrunner
38
34
25
31
25, 28
21
13
40
20
29
39
23, 37
30, 55
34
20, 40, 55, 56, 58
P47 Thunderbolt
P51 Mustang
Panic biplane
PCO (Power cut off)
- voltage threshold
Pilatus PC21
Pitch speed
Pitch speed table
Pitch to diameter (p/d) ratio
Power
Power systems
- Avicraft Panic
- Acromaster
- C130 Hercules
- Cermark Banchee E3D
- DH Venom
- ElectRick
- Extreme Flight Edge 540T
- Gentle Lady
- Hangar 9 P51 Mustang
- Taube
- Zephyr
Power system testing
21
19
38
13, 20
Lanzo Stik
Lead acid battery
- safety
- voltage
LED
Lift
Light bulb
LiPo, lithium polymer 20, 41, 44, 47
- fires
- guide to LiPo batteries
42
41
41
17
45
25
20
19
18
33
Next week’s winning lottery numbers 85
43, 61
Nickel cadmium, nicad
Nickel metal hydride
43
21, 30
36
29
Jeti Phasor 45-3
Keil Kraft Senator
KP
Kv
- sack
- safety
- voltage
Load (propeller)
Load (electrical)
Long EZ
Losses from gearing
Louth-Davis, Louis
Low, Colin
Low voltage cut off (LVC)
47
42
42
42
22
7
22
42
41
67
31
16, 56
22, 62
32
38
13
9
9
10, 11, 17
28
54-65
62
55
57
60
65
58
59
63
56
64
61
29
© Copyright Andrew Gibbs 2013
Gibbs Guides.com
Gibbs Guide to Electric Power Systems: Part 2 – More Than Motors
Power 60 motor (EFlite)
Programming card
Propeller
- balancing
- blade cross section
- blade shape
- diameter
- direction
- efficiency
- folding
- GWS
- load
- painting
- parts
- pitch
- power requirements
- safety
- size
Pusher propeller
56
37
7 – 19
15
7, 8
10
8, 12, 17
17
13, 19
16
61
17
18
7
8, 9, 12, 17
12
18, 29- 31
13
17
18
18
19
37
42
38
15
39
10
38
37
15
61
61
23, 27
14, 15
20
11
25
38
Taube
Tempest
Thrust
- dynamic
- static
Timing
Torque 2818T
Tractor propeller
64
31
7, 13
13
13
39
59
17
38
Twist in prop blades
10
Uberlacher, Sepp
Ulery, Wayne
Undercambered
31
13
8
Van der Vecht, Dirk
Van Moorst, Appie
Vibration
Voltage
- drop
- off-load
- on load
- recovery
- spikes
Vortices
31, 32, 34, 40
Ranson, John
RC system wiring
53
Reduction gearing
19
Resistance (electrical) 22, 27, 44, 45, 48
Resting voltage
44
RPM
9, 12
Safety of propellers
Scottish Aviation Bulldog
Senator (Keil Kraft)
Setting up an ESC
Sheldon, Simon
Shiebe Falke FS28
Shock flyer
Skyhawk
Slippage
Soft start
Sound from ESC
Spinner
Speed 480
Speed 600
Square law
Static thrust
Swallow, BAM
Swastika
Switching
- frequency
Tremlett, Martin
Weight, Andrew
Whipp, Alan
Windings
Wing warping
Wire, wiring
- additional wire
- battery to ESC wires
- ESC to motor wires
- size
Wiring diagram
Worley, George
Wright brothers
XT 60 connector
- clone, copy
Zephyr
68
15
21
15, 29
24, 25
44
44
44
46
36
12
15, 22, 62
19
23, 37
13
36, 48
52
36
36
48
53
31
13
49, 50
50
61
© Copyright Andrew Gibbs 2013