Download Aerodynamics_Mel 2009 v 3

Teaching
Basic Aerodynamics
Mel Burkart, MCFI
2008 FAA Flight Instructor of the Year
St. Louis Flight Standards Region
Lecture Content
• Glider Aerodynamics
• Effect of Controls
– Low and High speed flight
– Thermaling
– Approach and Landing
• Stall, Spin and Spiral Dives
• Limit Load Considerations
Analysis of Accident Data
6 Year Summary
Fatal
Accidents
Year
Other
Takeoff
In-flight
Landing
9
31
2003
0
20.7
24.1
55.2
8
29
2004
6.9
20.7
10.3
58.6
6
32
2005
12.1
9.1
15.2
60.6
3
35
2006
8.3
19.4
13.9
61.1
7
36
2007
6.9
20.5
11.4
65.9
3
30
2008
3.3
23.3
20
53.3
Average Fatalities = 6 - Average Accidents = 32
Analysis of Accident Data
6 Year Summary
Fatal
Accidents
Year
Other
Takeoff
In-flight
Landing
9
31
2003
0
21
24
55
8
29
2004
7
21
10
59
6
32
2005
12
9
15
61
3
35
2006
8
19
14
61
7
36
2007
7
21
11
66
3
30
2008
3
23
20
53
6
32
19
16
59
How to Correct/Reverse Trend
• Teach your student a basic,
simplified way to understand and
apply basic aerodynamics in a high
stress environments.
– Takeoff
– Approach and Landing
Basic Aerodynamics
Lift / Drag Formula
L = CL S
P/ V2
2
D = CD S
P/ V2
2
Lift / Drag Formula
P
2
L = CL S /2 V
P
2
D = CD S /2 V
Lift & Airspeed Relationship
• With constant Angle of Attack:
– Doubling the airspeed quadruples wing lift
– Increasing airspeed by 1.4 doubles wing lift
– Increasing airspeed by 1.2 gives 40% increase
in wing lift
– Conclusion: Relatively small increase in
airspeed yields substantial increase in lift.
Speed, Lift, & Turning Flight:
• Which wingtip is faster in turning flight the outer wingtip or the inner wingtip?
• What effect does this speed differential
have on the lift the inner wingtip and the
outer wingtip produce?
• Is this effect related to overbanking
tendency in a glider?
Airspeed, Lift, & Load Factor
• If the stall airspeed in a 1 ‘G’ wings-level is 30
knots (at Maximum Gross Weight)…
• Then what is the maximum load factor that can
be produced at 60 knots?
• Doubling airspeed quadruples wing lift
potential, and so:
• 4 ‘G’s is the maximum possible before stall if
flying at 60 knots’ airspeed in this glider.
What Does This Mean?
• In any glider, if you cannot remember the roughair speed or the maneuvering speed, then:
• Limiting airspeed to no more than 2 times the
wings-level 1 ‘G’ stall speed will keep flight loads at
or below 4 ‘G’s.
• On the other hand, flying at 4 times stall airspeed
means that you have the power to induce 16 ‘G’s!
• 16 ‘G’s is well beyond the capability of the glider to
withstand.
• These are practical aerodynamic considerations.
Practical Drag Considerations
Drag Versus Airspeed
[With Illustrations From The
FAA Glider Flying Handbook]
Parasite Drag
Parasite Drag – any surface which deflects or
interferes with the smooth airflow around the glider.
Is Parasite Drag...
• Intuitive?
• Or is it ...
• Counter-Intuitive?
Parasite Drag & Airbrakes
• What type of drag do airbrakes generate?
– Induced drag?
– Or Parasite drag?
• What happens to airbrake parasite drag if
airspeed is doubled?
– Airbrake parasite drag quadruples!
(Reference V squared)
With Airbrakes Full Open:
• What happens to airbrake drag if airspeed is
doubled?
– Airbrake drag quadruples!
• What happens to airbrake drag if airspeed is
increased by 40%? [e.g. from 50 to 70 knots]
– Airbrake drag doubles!
• What happens to airbrake drag if airspeed is
increased by only 20%? [e.g from 50 to 60 knots]
– Airbrake drag increases a whopping 40%!
Airbrake/Parasite Drag
Questions To Ask...
• With airbrakes full open, what is the sink
rate of your training glider at 50 knots?
• With airbrakes full open, what is the sink
rate of your training glider at 65 knots?
• With airbrakes full open, what is the sink
rate of your training glider at 80 knots?
• So… If you find yourself too high on final,
what can you do to get down?
• Deploy full airbrakes and increase airspeed!
Induced Drag & Parasite Drag
Induced Drag – airflow circulation around the wing.
Is Induced Drag...
• Intuitive?
• Does it make sense to the aviation
newcomer that induced drag is very low at
high speed, and very high at slow speed?
• Or is Induced Drag Counter-Intuitive?
• Most newcomers choose counter-intuitive.
• So, what simile or metaphor can we use to
model the concept of induced drag?
The Water-Ski
Provides A Nice Metaphor
The Water-Ski
Provides A Nice Metaphor
The Water-Ski
Provides A Nice Metaphor
The Water-Ski
Provides A Nice Metaphor
The Water-Ski
Provides A Nice Metaphor
The Water-Ski
In Utah
F18 Water Wake
F18 Air Wake
FAA GFH Pix
The Wing’s Magic
With Illustrations From The
FAA Glider Flying Handbook
And The Web
Angle Of Attack 101...
Tufted Cherokee Wing
Wind Tunnel Classic
The Turbulent Wake
Of The Stalled Wing
Acceleration & Bernouilli
Smoke Pulse Wind Tunnel
Acceleration & Rarefaction
CP Versus AOA
(Actual Marianne Glider Wing)
As the
Angle Of Attack Increases
• The CL (Center of Lift, a.k.a. the Center of
Pressure) moves forward on the wing
• At high angle of attack, the main lifting force
is far forward on the wing
• The higher the angle of attack, the further
forward the CL moves.
CP migration as AOA increases
Summary: CP Versus AOA
Electronics in the Cockpit...
Plank & Pivot Point
Add One Left Weight
Set the weight on the plank
The plank reacts
Add two weights right
Set them on the plank
The plank reacts,
rotating clockwise about the
pivot point
The CG has shifted ...
CG
...To the right.
CG
But the pivot point - the CLremains in the same place
CL
CG
Adding the glider
CG is forward of the pivot point
(the CL is the glider’s pivot point)
Tail-down force counters
the nose-heavy tendency
CG/CL Summary
• The aircraft is supported equally on either
side of the CL (By definition, half the lift is
forward of the CL, and half the lift is aft of
the CL).
• The CG is forward of the CL in a properly
loaded aircraft
• The aircraft pitches about its pivot point - the
CL, in other words - not about the CG, as
many aviation texts claim.
Flutter
And A Movie About Flutter:
Does Flutter Correlate with...
• Indicated airspeed?
• Or does flutter correlate with...
• True airspeed? (the particle velocity of the
airstream moving past the airframe in flight)
• The harmonica (or its evil cousin, the
accordion!) provides the answer
• E flat is E flat is E flat, regardless of altitude
• True airspeed is correlated with flutter.
DG 100 Flutter Test
Summary
• Relationship of Lift and Drag
• Parasite Drag and Induced Drag
• Center of Pressure (Lift) and Center of
Gravity
– CL = Pivot Point
• Flutter
Questions ?
Aerodynamics/Spin
Training
This Presentation Is Based
On A Chapter In:
Why Is Spin Training Important?
• Spins have been with us since the Wright
Brothers, and probably always will be
• Spin accidents are usually fatal
• Ignorance of spins contributes to spin accidents
• Fear of spins causes many pilots to avoid spin
training
Why Do Pilots
Avoid Spin Training?
• Pilots avoid maneuvers that they understand
poorly or not at all
• Few fixed-wing pilots understand what drives
auto-rotation (in other words, what makes an
aircraft spin)
• Some pilots often believe spins are chaotic
and unpredictable, and avoid spin training as
a result
Three Spin Questions
• At spin entry, why does the aircraft nose pitch
down?
• At spin entry, why does the aircraft roll left or
right?
• At spin entry, why does the aircraft yaw develop
into continuing rotation?
At Spin Entry, Why Does
The Aircraft Nose Pitch Down?
• The aircraft nose pitches down because the
spin is a variety of stall. In a properly loaded
aircraft, this nose-heavy tendency at the stall is
caused by the CG being placed forward of the
center of lift.
At Spin Entry, Why Does The
Aircraft Roll Left Or Right?
• The aircraft rolls left or right because one
wing is more stalled than the other wing.
• The aircraft rolls toward the more deeply
stalled wing because it produces less lift than
the other wing.
At Spin Entry, Why Does Yaw
Develop Into Continuing Rotation?
• A spinning aircraft auto-rotates, or spins,
because the more stalled wing is producing
considerably more induced drag (or rearward
acting force) than the other wing.
• Auto-rotation generally continues until this
inequality is reduced or eliminated.
Does The Induced Drag Curve
Explain Yaw Behavior In The
Spin?
Large AOA =
High Induced Drag
Small AOA =
Low Induced Drag
Lift vs. Drag
Three Strikes ... And You're Out!
• Imagine a glider in the following flight
condition:
– A shallow bank (15 degrees or so), slow flight turn
to the left, with excess left rudder applied.
– Let's take a look at the aerodynamic configuration of
the wing in this condition of flight and analyze any
stall-spin risk factors present.
Strike One:
• During turning flight, the inner wing has a
higher AOA than the outer wing.
– In a shallow bank turn to the left, the left wing is
flying at greater AOA than the right wing.
Strike Two:
• During shallow bank turns, over-banking tendency
is pronounced.
– The pilot puts the stick to the high side of the cabin,
increasing the AOA of the lower wingtip/aileron
combination and decreasing the AOA of the upper
wingtip/aileron combination.
– In a shallow bank turn to the left, the left wingtip is
flying at higher AOA than the right wingtip.
Strike Three:
• Skidding the turn with the rudder increases
the AOA of the lower wingtip and reduces
the AOA of the upper wing.
– This inequality in AOA produces inequality in
induced drag, causing the yawing moment of the
glider and developing into auto-rotation.
Summary
– In a shallow bank, skidding turn to the left, the
left wingtip is flying at higher angle of attack
than the right wingtip due to the combination of
three factors:
• curving flight path to the left
• aileron positions at left wingtip and right wingtip to
resist over-banking tendency
• skidding turn to the left reduces left wingtip forward
speed and increases right wingtip forward speed.
Three Strikes...
...You're O-U-T.
Why Are Many Two-Seat Gliders
Good Spin Trainers?
• Many gliders are excellent stall/spin trainers
because they are free of engine noise &
vibration, propeller noise & vibration, torque,
and P-factor
• Gliders are silent enough that you can actually
hear the wings stall
– “I t g e t s
q u i e t!”
Spin Training Methods
• Wings level, nose high, haul back & kick the
rudder
• Snap roll
• Shallow bank skidding turn (Nose on or near the
horizon)
Wings Level, Nose High,
Haul Back & Kick
• This is the long-established method of teaching
spins
• Emphasis has been on precision recovery after
multiple turn spins
• Three turns, roll out on a pre-selected heading
Advantages of the
Wings Level, Nose High Method
• Teaches ability to remain oriented during the
spin rotation
• Allows candidate to learn the characteristics of
the fully developed spin
Disadvantages of the
Wings Level, Nose High Method
• Nose high attitude at spin entry reinforces the
impression that pitch attitude is extremely nosehigh for all spin entries
• Pilots trained solely by this method often conclude
that all spins are preceded by extreme nose-high pitch
attitude
Snap Roll Spin Entry
• Spin entry in the horizontal plane
• Useful teaching method in aerobatic
airplanes
Advantages of the
Snap Roll Spin Entry Method
• Teaches that spins can occur from any pitch
attitude
• Teaches precision recovery from snap roll/spin
• Teaches advanced orientation skills
Disadvantages of the
Snap Roll Spin Entry Method
• Not often taught in gliders due to substantial
structural stresses that snap rolls impart to
the long, massive wings of gliders
• Violent control application to initiate snap
rolls may lead pilots to conclude that all spins are
preceded by violent control inputs
Shallow Bank Skidding Turn Spin
Entry
• This is the inadvertent spin entry from low
altitude that takes lives every year
• Encountered in the traffic pattern when
turning from downwind to base or from base
to final approach
• Few pilots receive training in this type of spin
entry!
Shallow Bank Skidding Turn Spin
Entry Teaching Method
• Practiced at safe altitude
• Simulates spin out of turn in the traffic
pattern
• Wings are banked, not level, during spin
entry
• Spin warning signs are emphasized
• Gradual control inputs
• Recovery is initiated immediately!
Advantages of the Shallow Bank
Skidding Turn Spin Entry
• Spin entry occurs from relatively flat pitch
attitude, not from extreme nose-high pitch
attitude
• Realistic simulation (at safe altitude) of the
type of spin entry that claims lives every
year
• Cross control of ailerons and rudder is
applied gradually and realistically, not
violently
When Can I Get Spin Training?
•
•
•
•
During primary flight training
During recurrent training (BFR)
During recurrent training (seasonal checkout)
Anytime you can fly with a spin-qualified CFIG!
(do you really need to be required to seek
additional training?!?)
What Type Of Spin Training
Is Best For Me?
• Training that emphasizes the warning signs
that a spin is imminent
• Entry from shallow bank, skidding turn
• Training with gradual control inputs
• Training that emphasizes immediate and
correct application of controls to exit from
the incipient spin
What Type Of Spin Training
Is Best For Me?
• Spin training in a two-seat glider that has
stall/spin characteristics similar to the
stall/spin characteristics of the single seat
glider(s) that you fly
• Spin training given to you by a CFIG with
experience in providing spin instruction
What Types Of Spin Training
Are Worst For Me?
• The spin training that is worst for you . . . is
to have no spin training at all!
• Spin training at unsafe, low altitude
• Spin training that you conduct all on your own
without dual instruction in spins from a
qualified CFIG
Who Doesn’t Need
Spin Training?
• Ercoupe pilots don’t need spin training!
Ercoupes Are Incapable
Of Spinning
Ercoupes Were The Only
Spin-Proof Airplanes
• If you are flying a fixed-wing aircraft, and it is
not an Ercoupe, then it is capable of spinning!
• Spin training is your best defense against
inadvertent spin at low altitude
Causes Of Inadvertent Spins
In The Approach To Landing
• Delayed entry into the pattern
• Belief that pulling back on the stick will make
the glider continue to gain altitude
• Distraction
• Unexpected fundamental development(s)
The Cure For Inadvertent Spins
• Have the self-discipline to arrive in the vicinity of
the intended landing area with plenty of altitude
and time to do the job right the first time!
• Familiarity with warning signs of approaching
stall or spin
• Situational awareness
• Seek out recurrent spin training!
This Presentation Was Based
On A Chapter In:
Spin Training
Mel Burkart, MCFI
E-Mail: [email protected]