Download Electric conversion notes:

Electric Conversion Notes:
By Franny Brodigan - 4/2011
Email: [email protected]
Website: http://www.frannybrodigan.com
Introduction: Have you ever wondered about converting an old gas/nitro IC powered airplane to electric? Are you
thinking of electric power for that next sport or scale build? Electric power has evolved and improved so much in
the past several years that it rivals and even surpasses traditional gas and alcohol powered engines in power,
reliability, ease of use, noise levels, and mess. Below are some thoughts on converting to electric power. It is a bit
different than specing an engine, but there is also much more flexibility allowing you to really dial in performance
and get the most out of your power plant! This write-up will deal with “brushless” three phase motors only. There
are two basic types: “inrunners” and “outrunners”. The important difference is that the outrunners are higher
torque, slower rotation speed and slightly less efficient, but more economical. Inrunners are mostly for electric
ducted-fan, boat and maybe helicopter applications. What follows best applies to outrunners.
Advantages:
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No Mess!
Quiet.
Reliable.
Highly configurable – anything from high torque to high speed.
Max torque at zero RPM!
Batteries are transferable from model to model and can be combined for different voltages and currents.
You can test at your workbench.
Indoor flying is possible.
No dangerous emissions.
Refueling can be free (if charged at the field’s solar charging station) or very cheap compared to liquid
fuel.
Wide range of sizes from 1oz to full scale.
No “pickling” needed for long term storage.
No large cylinder(s) to try and hide under a cowl – A BIG help in scale modeling.
Constant weight distribution from take-off to landing.
Much smoother running allowing for a lighter airframe.
Motor output is not affected by altitude changes (the propeller still is of course).
Disadvantages:
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Application is different than IC engines and thus there is a learning curve.
Requires new equipment (chargers, batteries, etc.)
For sizes over .75 equivalent (>1000W) it can be more expensive than IC.
Lack of “realistic” sound.
Not nostalgic or historically accurate.
LiPo batteries can be dangerous if mistreated (this includes bad crashes).
Weight distribution is different – two weight components motor and battery. Also, the battery does not
loose weight during the flight as a fuel tank does (good and bad).
Usually, it is best to start with a new airframe.
Initial notes on Wattage requirements per pound from about 1/2A (12oz, 80W) on up:
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50W/lb – Minimum power for flight. May be good for slow floaty models
75W/lb – Light to medium performance. Simple aerobatics but no sustained vertical
100W/lb – Good all around flying. Perform most aerobatic maneuvers and limited vertical
125-150W/lb – high performance flying – Speed and aerobatics. Possibly unlimited vertical depending on
torque vs. Speed setup
Above 150W/lb – very high speed and performance – unlimited vertical
Conversion steps:
New or existing model?
New model: Significant weight savings can be achieved due to the reduced stress of the electric motor. Try to
make most weight reductions behind the CG 9center of gravity), but also eliminate overkill of doubled ply and thick
firewalls. Remember, your easiest performance increase comes from reducing the weight.
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Drill holes
Build up surfaces instead of using solid, flat plates of balsa for stabilizers, elevators and rudders.
Use lighter components from servos to wheels.
Use “BEC”s to eliminate the use of an additional radio battery pack.
Try to use lighter weight coverings (Ultracoat Light, etc.).
No need for fuel proofing.
Use epoxy sparingly.
Use spar webs and other structural elegance to eliminate heavy glass and plywood.
Existing model: There are still several things that you can do to lighten it up.
 Consider a re-covering job and take the opportunity to remove material through lighting holes and such.
 Remove engine mounts.
 Replace/update wheels and servos and other heavy components
Determine dry weight. A good starting point is the current weight with the engine or with projected weight on
the plans. You can fine-tune this when you have a group of motors/batteries to choose from.
Decide on performance level: Is it a slow floating model, or a high-speed aerobatic ship or something in between.
This will give you your rough Wattage-per-pound rating.
Measure your maximum ground clearance on take-off and landing. You will need this to arrive at the largest
propeller you can use. Remember that you can increase the number of blades if need be. A comparable three blade
propeller will be about 1” less in diameter than a two blade, but a bit less efficient. Larger propellers can also
cause more aerodynamic torque to be applied to the fuselage during take-off. The larger the propeller, the more
rudder you will need on take-off to counter the vortex force. Finally, the larger and slower turning the propeller,
the more efficient it is generally (all else being equal).
Determine where on the Torque vs. Speed continuum you want to be. This is related to the performance level.
A big WWI biplane with rigging and such will need lots of torque will little speed. A pylon racer will be all speed
without the need for torque. High speed will mean a smaller propeller turning faster and high torque will be a
larger propeller turning slower. If you run out of propeller clearance, you can sacrifice a little efficiency with a
three or four bladed prop. Also, you can increase the prop pitch to increase power output and wattage draw.
Start looking at motors in your power range. This can be a little daunting as the specs for motors can vary by
manufactures. There are several website listed at the end of this paper where you can visit to get some specs.
There are also many different brands to choose from.
A note about motor specs: AXI motors for example are designated by their physical size in mm. In contrast, the
E-Flight motors are labeled by their glow equivalent so there is no real labeling standard as there is with IC
engines. Most do, however, publish specs that look like stereo or electronic specs. What to look for:
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KV – The RPM per Volt. This is important in determining the voltage the motor requires to turn your
propeller at your desired RPM. Use lower KV for high torque and higher KV for speed.
Voltage Range or Max Voltage: This determines the Max battery Voltage the motor can handle.
Resistance (Ri): This will give you an indication of efficiency.
Idle Current (Io):This is the no-load current draw. This also speaks to efficiency by showing what is
required just to spin the motor.
Shaft Diameter: Important for special Propeller collets.
Overall Length: Nice to know when trying to fit into small cowls.
Weight: Very important for overall aircraft weight and balance. Don’t forget the collet, propeller and
spinner.
Overall Diameter: This may surprise you. This is usually the limiting dimension as opposed to the length.
Continuous Current: This is the Max continuous current the motor can sustain with adequate cooling.
Maximum Burst Current: xA (for x seconds) This is what can the motor tolerate for the given time.
Cells: This is how some give the input voltage range. Ni-Mh/Nicad are 1.2V/cell and LiPos are 3.7V/Cell.
Speed Control: A recommended “brushless” Speed Control amperage and voltage range.
Recommended Prop Range: The range of props that have been tested with the motor.
Probably the most important specs are the KV (rpm/volt), max voltage, max current, weight, and size. One of the
easiest ways to make a decision on brand/size, etc. is to ask others what they are using and how they rate it. I
have used AXI, E-Flight, Turnigy, and HiMax motors and have found them all to work quite well. I did buy one
Chinese small “bell” outrunner, but the long shaft bent on the first rough landing. I have learned to avoid long thin
propeller shafts.
Pick out a Speed control. Once you have a few motors in mind that will fill your needs, you will need to pick out an
ESC or electronic speed control that will handle the current and voltage of your motor and battery pack. It may
also include an on-board “battery elimination circuit” or BEC. This will allow you to power your receiver and servos
from the single flight battery eliminating the need and weight of a flight pack battery. This can be a good weight
savings, but be aware that there are different BEC circuits. The older controllers used “linear voltage regulators”
that are only good up to about 11.5V (so a three cell or 3S Lipo). Past that they can’t deliver ample power without
over heating and you risk radio failure. The newer onboard BECs are of the “switching” type that can tolerate
much input voltages and deliver higher currents. There are also external BECs (Castel Creations, Dimensional
Engineering, etc.) that you can add to the older controllers while disabling the onboard one. Many of these have
programmable output voltages allowing you to run your servos at 6V or higher which will give a boost in power and
speed. Be sure to check to be sure your servos can handle the higher voltage.
The ESC you choose must support the type of batteries you will be using. LiPos can’t be discharged below a certain
amount or they will not re-charge. Almost all current ESCs have a low voltage cut-off for LiPos and other battery
types that cut power to the motor, but still allow enough power for the BEC to power the radio and servos in the
event that the flight time has exceeded the capacity of the battery.
I have used Castle Creations ESCs and BECs, Turnigy ESCs and Jeti ESCs controllers and they have worked well
for me.
Pick out your batteries. There are many considerations while picking out your battery, but the best to start with
is voltage. Once you know what RPM you want to turn and the KV of your motor, you can determine your needed
input voltage. Sometimes, this will help eliminate a few motors you are considering as the number of cells in the
pack sets battery pack voltage. Since Lipo cells are 3.7V a piece, a three cell or 3S (three in Series) pack will be
11.1V. A 4S pack will be 14.7V which is a big jump. The next thing to consider is the current delivery ability of the
pack. The pack will have an Amp (or milliamp) per Hour rating (mAh). This is the “C” valve of the pack and it is
what the pack can deliver consistently for an entire hour. All the battery technologies will allow you to draw
current at greater than the “C” amount and will have a published spec to that affect as a multiple of “C”. A 10C
pack that is rated at 2000mAHr will allow you to draw 20A (2000mA X 10 = 20000mA or 20A), but will discharge
itself in 1/10 hour or six minutes. Use this to determine the smallest pack the will get you your desired flight time
at the current draw your performance level requires. I like to then move up to the next Amp-Hr size pack (same
voltage) to give myself a little added headroom. We don’t want to take along too much extra battery, as it is one
of the heavier components. NmHi and NiCad packs are made up of 1.2V cells, but the same process is used to arrive
at a pack. Other considerations are weight and size. Make sure you have enough room in your fuselage. Finally, it
is a good idea to have a couple or more battery packs so you can charge while you are flying.
At this point you will just need to pick out connectors for your battery if it doesn’t come with them. The motor
will most likely come with connectors to solder on to mate with your ESC. Also, you may need to purchase a
propeller collet or firewall mount (AXI motors do not ship with a mount, but most others do).
A note about cooling… It is very tempting to close up the nose of your airplane now that you don’t have a gigantic
cylinder protruding out. That is fine, but remember that the motor, ESC, AND the battery need to have airflow to
cool. Even at 75% efficiency, 14.7V @ 45Amps is over 660W and a 25% loss is over 165W of heat. LiPos need to
be kept under about 120 deg F. They should be no more than warm to the touch or else they will “puff” up which
can reduce their lifespan. I usually add cooling holes in the fuselage and a scoop or some air inlet up front to allow
air to pass over the motor and battery. Several large holes in the firewall can be a big help too. Of course the
harder and longer you push it, the more cooling is required. Powered gliders that are only under power for a minute
or so are fine closed up, but a hotliner or pattern airplane needs very good flow.
A note about propellers… There are several propeller manufactures that sell “electric only” propellers that are
quite different from the IC variety. They are also a bit more fragile as they do not have to contend with the
hammering that an IC engine dishes out. They come in slow and high speed versions and are designed specifically
for a motor’s unique torque curve. They will perform better than existing IC propellers. Both can be used, but the
electric version CAN NOT be used on IC engines. One of the most popular makers is APC, but Master Aeroscrew
also has an electric line and are a bit more durable but not as efficient. A word of CAUTION: When working on a
motor, remove the propeller before making any electrical tests. The blades can be quite sharp and can cause
severe lacerations. I lightly sand the leading edges of new props as the can be sharp enough to slice skin with very
little force.
Tuning and optimizing performance. For most “torquey” applications, ground clearance will be your limiting factor.
The largest possible diameter propeller may not pull enough wattage at lower pitches and performance will suffer.
In this case, buy a few propellers of increasing pitch and, using your Wattmeter, add pitch until your wattage gets
close to where you want it to be. Realize that a propeller turning in static air (airplane not moving) will cavitate a
bit (more as the pitch increases) than when the airplane is actually flying, but it will get you close. Also watch
cooling as well, but it won’t take long to get your measurements. Take the best two or three props and try each one
at the flying field and you will know quickly which one is best for your application. I highly recommend you
purchase a Wattmeter. There are several available and are an absolute necessity to determine if you are overstressing your components. Most will read Voltage, Current, and Wattage so you can really dial in your entire
system. I also suggest a propeller RPM meter to be certain you are getting the RPM you need. Your airplane’s
maximum forward speed will be limited by the tip speed of the propeller.
An Example:
(Using the .40 size “Taube” - IC to electric conversion @5lbs AUW; slow flying):
Using the rule of thumb of 100W/pound (flying weight). That will put the 4.5-5lb Taube around 450-500W. Since
the Taube is a slower flying airplane, we want more thrust than speed, so that means a slower, but larger diameter
and higher pitch prop. With an electric motor you can dial in the RPM and the wattage easily by selecting a motor
with the right specs. The KV of a motor will tell you the RPM of unloaded motor for 1V. So, a 750KV motor will
turn 750rpm at 1V and 7500rpm at 10V. For the Taube, you want somewhere between 7500 and 10,000RPM
depending on the required prop tip speed. For a 750KV motor, that would be between 10V and 13.3V. LiPoly packs
come in voltages based on the number of cells (3.7V) per pack, so the three cell pack (3S) is 3*3.7V= 11.1V and the
four cell pack (4S) is 14.7V. In addition to voltage the other important battery spec is the amount of current you
will draw to keep the motor spinning at its KV value while you load it (with drag from the prop). The more the load
(i.e. larger prop/more pitch) the more current the motor will draw to keep up with demand. If the demand is too
high for the battery or speed control, then you can damage those components. If the motor is asked to deliver
more power than it’s rating, it too can be damaged... So, we want to spec out a power system to be able to safely
handle the power required; in our case the 450-500W.
If we say 500W and a 14.7V (4S) battery that we will say runs at 14V under load, that will translate to a current of
A=P/V or 500/14 = 35.7A. We will need a motor that can handle close to 40A and the same for the speed control.
Armed with all that info, we have determined that we need is a 4S lipo that can deliver 35A.
Batteries are rated in the amount of current you can draw over a period of time – usually in milliamp hours or AmpHours. A review from above: In addition to the capacity rating, they are also rated at how much current you can
safely pull as a multiple of the capacity ("C"). You'll see 20C, 30C, 40C, etc. This means that if a battery has a
capacity of 1000mAH, you can, theoretically, pull 1000mA or 1A continuously for one hour. If the battery is also
rated at 20C, then you can pull up to 20A without damaging the battery. Now, if you pull at that rate, the battery
will be exhausted in 1/20 of an hour or 3 minutes.
Another factor is weight. Keeping the weight down is the cheapest form of a performance increase. As an
example: if an airplane flies full throttle at 500W, it probably only needs 200W to cruise around the sky. If we
want 12 minutes of flight time at an average power draw of, say 275 watts (a bit of full throttle and more cruising)
then, at 14V, that is about 275W/14V = 20A. Our 1000ma 20C battery will deliver the 20A, but only for 3 min and
we won't get to 500W for max power - we need 35A for that. So, if our battery was say 30C, then we would need
at least 35/30 = 1166mAH. But this would only give us a run time of 3.5 min @ 20A (our 275W average draw) ->
60min/(20A/1.166A). We would like a comfortable 12 minutes of flying time, so, working through the algebra,
capacity in Amps = (20A*12min)/60min = 4A or 4000mA. A 4 cell 4000mA Lipo will give us our 12min run time and
deliver the power we need. And, at a 4000mAh capacity would only need to be 35A/4A or 8.75C so a 20C pack
would be just fine. If we would be fine with, say an 8 min flight time at our average 275W, then we could go with a
(20A*8min)/60min = 2600mA pack which would need to be rated at 35A/2.6A = 13.5C so a 20C pack would still
work and the 2600mAh pack will shave off about 3-4oz of weight.
Summary Recap:
So, to recap, for the Taube, biased on the weight, we choose a motor rated between 600 and 750W; a 750KV
(RPM/Volt) rating that can handle at least a 4S lipo pack (14.7V) and over 35A or current. The speed control needs
to handle 4S (14.7V) and over 35A as well.
Next we pick out a propeller range. We start with a 14" prop with 6-8 pitch - 14X6 or 14X8. We’ll up the diameter
(watching the ground clearance) and/or the pitch, and measure the draw with a Watt meter until we get to 500W
at full throttle and we are set.
Don’t forget to verify that your max current draw is below the spec of the motor, battery, and speed control and
you'll get many hours of flying fun. Just be sure the motor has sufficient cooling so as not to over heat.
In my Taube I installed the Eflight Power46 motor with a 4S4000 pack and a 60A speed control. Probably a 45A
would work fine too. It seems to fly well on that, but the whole setup is a bit heavy. A 4S3000mAh pack would be
better and a lighter motor would help... Check out HobbyKing (www.hobbyking.com) for great deals on motors,
batteries and speed controls. They are in HK (they just opened a US shipping warehouse!), but every order of mine
has been spot on and the prices are 1/3 what you will find anywhere else – albeit, shipping can take up to a month.
General Notes:
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LiPo batteries can be dangerous! Make sure you use a charger that is designed for the chemistry of your
batteries. LiPos MUST be charged with a charger designed specifically for LiPos or they can explode!
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Buy cell balancer. They are easy to use and can greatly extend the life of your battery by ensuring each cell
is at the same voltage.
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Power your receiver off the flight battery. This is a neat feature of electrics and can save the weight of a
separate flight pack. You’ll always know if you battery is charged!
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Remember to unplug your battery after each flight. Even with just the BEC drawing power, you batteries
could be discharged below their threshold eventually.
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Electric motors are sort of backwards from glow engines; going to a bigger propeller increases power.
What this means is that a motor is rated to turn a certain number of RPM based on the input voltage. As
the drag increases, the motor will pull more amperage to maintain that RPM. Since increasing the diameter
and/or pitch of the propeller increases the drag, the motor will compensate by pulling more amperage and
delivering more power.
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If you increase the voltage, the motor will also increase the current draw. Once again, this is the motor
trying it’s best to maintain it’s RPM/Volt (KV) rating. Always double-check every component change with
your Wattmeter.
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Motors produce max torque at stall (like a steam engine). An IC engine torque curve is quite different. An
Electric motor will accelerate much quicker and can stall high pitch propellers until forward speed is
achieved.
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Invest in a Watt Meter. They will read real-time Wattage, current, and Voltage as well as battery capacity
(mAh). It is an invaluable tool in tuning your performance by giving you “real-world” numbers. It can also be
an important safety tool to let you know if you are exceeding the specifications of your power train.
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Invest in an RPM meter as well. If you truly want to duplicate your IC performance, you will need RPM
readings for each system.
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Think of Voltage as the potential to perform a task and Amperage as the muscle to carry it out. So,
Voltage is the potential to do work (spin a prop at a certain RPM and Amperage is the force to get the job
done without much drop in potential (voltage)
Calculation formulas:
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Power (in Watts) = Volts X Amps
Motor KV is the RPM per Volt
Battery “C” rating is the amperage delivery it can maintain for one hour.
1 horsepower = 746 watts
Battery discharge time = =60*(((mAh/1000)*V)/W)
Types of batteries:
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LiPo (lithium-poly) – the highest power per weight. They are compact, light and very powerful, but can burn
or explode if mishandled. Most cannot be charged any faster than 1 ”C” or 1 times the charge capacity.
Therefore, all 1C charge LiPos take one hour to charge from exhausted to fully charged regardless of
actual capacity. Also, LiPos can’t be discharged past a certain voltage threshold are they will be destroyed.
Finally, they should be “balanced” when charged to ensure that all cells in a pack are close in voltage so that
one cell does not reach the point of no return on discharge (while flying).
A123 Cells – These are the Li-Ion cells found in the new generation of high power cordless tools. Cells and
packs can be purchased at on line retailers or can be constructed from replacement tool battery packs.
These are a little heaver than Lipo (about 125% the weight) but can be charged four times as fast (in a
little as 15 min). They are also much safer than LiPos in that they will not catch fire or explode in all but
the most extreme conditions. These also require a special charger.
NiMHi – Nickel Metal Hydride are cylindrical cells that can be built into multi-cell packs. This is an older
technology and their use is declining. They have good power delivery and are quite safe. Another plus is
the lack of any “charging memory” making them a bit less maintenance. – Very heavy compared to
Lipo/A123.
NiCad – Nickel-Cadmium batteries are the oldest of the group and deliver high current. They are also the
heaviest and can develop a charging “memory” that must be cleared through deep charge-discharge cycling.
Types of motors:
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Brushless “Outrunner” – These are currently the most popular motors available. They have efficiencies in
the ~80% range and can deliver a wide range from high torque to high speed. They are three phase motors
and require a special speed controller, but there are many to chose from. The term “outrunner” is due to
their rotating bell. They are the “Rotary” version of electric motors in that the central shaft with the
windings is stationary and the outer “bell” containing the magnets rotates. This configuration reduces their
efficiency slightly, but allows for better cooling. Most are reversible mounting and can be mounted on
either side of the firewall.
Brushless “Inrunner” – These are also three phase motors requiring the brushless speed controllers, but
are the reverse of the outrunner in that the fixed pole magnets are mounted on a center rotating shaft
with the windings arranged in a ring around them. They are long and cylindrical in shape and are generally
constructed to have very high KV (RPM/Volt) values. They are usually paired with a high quality gear box
reduction unit to allow very large propeller diameters achieving efficiencies in the high 80% range. They
are used on very large powered sailplanes, very high-speed pylon racers (without the gear box) and electricducted-fan jets.
Brushed with gear reduction – These are the standard older technology DC carbon brushed motors
sometimes mated to a gear box for increased efficiency. They do not require the more advanced brushless
controllers and can in fact, be flown with out any controller at all. They do require a break-in period and
have a limited life span due to the wear of the brushes. They also are heavy for their power output and can
only hope to deliver efficiencies in the mid 50% range.
IC comparison:
An OS .46 cost about $150, and you’ll need a tank ~ $5 and a throttle servo ~ $25 for a total ~ $180. It puts
out (at our altitude) about an equivalent 978W. (746W/HP)
An E-Flight Power46 motor cost about $110, and you’ll need a speed control ~$50 and a battery ~$50 bringing
it up to $210
An OS .60 cost about $200, and you’ll need a tank ~ $5 and a throttle servo ~ $25 for a total ~ $230. It puts
out (at our altitude) about an equivalent 1162W.
An E-Flight Power60 motor cost about $130, and you’ll need a speed control ~$50 and a battery ~$50 bringing
it up to $230
Now these are rather expensive… From Hobby King, you will pay about $25-$30 for an equivalent 1000W1200W motor. You can also find a good all-around speed control for about $25- $30 and batteries below $40
Let’s look at fuel. A gallon of fuel is $22-$25. If a usual flight consumes 8oz that means there are 16 flights
per gallon. Dividing that out, we get ~$1.50 per flight.
Then there is the weight. An OS .46 weighs about 17oz plus 8oz of fuel ~ 25oz total. The Power46 motor
weighs 10oz (on the heavy side) and the battery and esc is about 12oz for a total of 22oz. – a savings of 3oz.
This will not decrease during flight as will happen with the IC setup keeping the balance constant. You also
save the weight of a throttle servo and linkage!
An OS .61 weights almost 26oz and with fuel, that comes to 34oz! A Power60 motor runs 13oz and with
battery and esc (another 16oz) that comes to 29oz. A savings of 7oz.
There are cheaper engines and there are cheaper motors… The important thing is how close they really are in
price. Sure you need a charger and you may want a second battery, but you also need gear like a starter and
big battery for the IC world as well.
Websites:
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Hobby King out of Hong Kong – Best prices. Good quality: www.hobbyking.com
Glow to Electric conversions: http://www.csd.net/~cgadd/eflight/calcs_gloconvert.htm
Power calculators: http://brantuas.com/ezcalc/dma1.asp
Hobby Lobby AXI motor selection tool: http://www.modelmotors.cz/axisetuphobby-lobby/
Discussion of different motor technologies (brushed, outrunner, etc.): http://www.hooked-on-rcairplanes.com/brushless-rc-motors.html
Many Electric power calculators: http://www.rcgroups.com/forums/showthread.php?t=606703
A PDF of many motors an their specs: http://www.flyingmodels.org/motortest/pdf/TheMotortest_5.pdf
and a web page: http://www.flyingmodels.org/index_en.htm
Examples of successful power systems: http://www.rcgroups.com/forums/showthread.php?t=203529
A dissertation on designing a power system:
http://www.ampaviators.com/index.php?option=com_content&task=view&id=41&Itemid=27
And another: http://www.wattflyer.com/forums/showthread.php?t=18521
MotoCalc, the premier motor/performance calculator: http://www.motocalc.com/