basic aerodynamics dartmouth flying club october 10, 2002 andreas bentz
TRANSCRIPT
Basic AerodynamicsBasic Aerodynamics
Basic AerodynamicsBasic Aerodynamics
Dartmouth Flying ClubOctober 10, 2002
Andreas Bentz
Basic AerodynamicsBasic Aerodynamics
LiftLift
Bernoulli’s Principle
3
EnergyEnergy
Definition: Energy is the ability to do work. Energy cannot be created or destroyed. We
can only change its form. A fluid in motion has (mainly) two forms of
energy: kinetic energy (velocity), potential energy (pressure).
4
The Venturi Tube and Bernoulli’s PrincipleThe Venturi Tube and Bernoulli’s Principle
kinetic energy(velocity)
potential energy(pressure)
velocity increases
pressure decreases
5
Lift: Wing SectionLift: Wing Section
Air flows toward the low pressure area above the wing: upwash and downwash.
Newton’s third law of motion: to every action there is an equal and opposite reaction. “The reaction to downwash is, in fact, that misunderstood
force called lift.” Schiff p. 8
upwash downwash
relative low pressure
6
Angle of AttackAngle of Attack
chord line
average relative wind
The angle of attack is the angle between the chord line and the average relative wind.
Greater angle of attack creates more lift (up to a point).
total lift
7
Lift and Induced DragLift and Induced Drag
Lift acts through the center of pressure, and perpendicular to the relative wind.
This creates induced drag.
chord line
average relative wind
total lift
effective lift
induced drag
8
Got Lift? FlapsGot Lift? Flaps
Flaps increase the wing’s camber. Some also
increase the wing area (fowler flap).
Almost all jet transports also have leading edge flaps.
9
Too Much Lift? SpoilersToo Much Lift? Spoilers
Spoilers destroy lift: to slow down in flight (flight spoilers); for roll control in flight (flight spoilers); to slow down on the ground (ground spoilers).
Basic AerodynamicsBasic Aerodynamics
Side EffectsSide Effects
There is no such things as a free lunch.
11
Drag: Total Drag (Power Required) CurveDrag: Total Drag (Power Required) Curve
50 100 150 200
Indicated Airspeed (knots)
Dra
g (
lbs)
1,400
1,200
1,000
800
600
400
200
induced drag parasite drag
resistance
total drag
max. lift/drag
best glide
12
Wingtip Vortices and Wake TurbulenceWingtip Vortices and Wake Turbulence
Wingtip vortices create drag: “ground effect”; tip tanks, drooped wings, “winglets”.
relative low pressure
Basic AerodynamicsBasic Aerodynamics
StabilityStability
Longitudinal: Static, DynamicLateral
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Longitudinal StabilityLongitudinal Stability
Static stability (tendency to return after control input) up elevator increases downward lift, angle of attack increases; lift increases, drag increases, aircraft slows; less downward lift, angle of attack decreases (nose drops).
we
igh
tdow
n lif
t
lift
15
Aside: CG and Center of Pressure LocationAside: CG and Center of Pressure Location
we
igh
tdow
n lif
t
lift
Aft CG increases speed: the tail creates less lift (less drag); the tail creates less down force (wings need to create less lift). This also decreases stall speed (lower angle of attack req’d).
16
Lateral StabilityLateral Stability
If one wing is lowered (e.g. by turbulence), the airplane sideslips. The lower wing has a greater angle of attack (more
lift). This raises the lower wing.
relative
wind
relative
wind
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Directional StabilityDirectional Stability
As the airplane turns to the left (e.g. in turbulence), the vertical stabilizer creates lift toward the left. The airplane turns to the right.
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Speed Stability v. Reverse CommandSpeed Stability v. Reverse Command
Power curve: Power is work
performed by the engine. (Thrust is force created by the propeller.)
Suppose airspeed decreases. “Front Side”: Power is
greater than required: aircraft accelerates.
“Back Side”: Power is less than required: aircraft decelerates.
50 100 150 200
Indicated Airspeed (knots)
Dra
g (
thru
st r
equ
ired
)
1,400
1,200
1,000
800
600
400
200
Per
cen
t h
ors
epo
we
r
100%
50%
max. endurance
ca. 75% of max.
lift/drag
Basic AerodynamicsBasic Aerodynamics
Turning FlightTurning Flight
Differential Lift
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Turning FlightTurning Flight
More lift on one wing than on the other results in roll around the longitudinal axis (bank). Lowering the aileron on one
wing results in greater lift and raises that wing.
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Turning Flight, cont’dTurning Flight, cont’d
More lift on one wing than on the other results in roll around the longitudinal axis (bank). Lowering the aileron on one
wing results in greater lift and raises that wing.
This tilts lift sideways. The horizontal component of
lift makes the airplane turn. (To maintain altitude, more
total lift needs to be created: higher angle of attack req’d)
Centrifugal Force
22
Adverse Yaw and Frise AileronAdverse Yaw and Frise Aileron
However, more lift on one wing creates more induced drag on that wing: adverse yaw.
Adverse yaw is corrected by rudder application.
Frise ailerons counter adverse yaw: They create parasite drag
on the up aileron.
Basic AerodynamicsBasic Aerodynamics
StallsStalls
Too Much of a Good Thing
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StallsStalls
A wing section stalls when its critical angle of attack is exceeded. Indicated stall speed depends on how much lift the
wing needs to create (weight, G loading).
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Stalls, cont’dStalls, cont’d
The disturbed airflow over the wing hits the tail and the horizontal stabilizer. This is the “buffet”.
Eventually, there will not be enough airflow over the horizontal stabilizer, and it loses its downward lift. The nose drops: the stall “breaks”.
wei
ght
lift
26
Stalls, cont’dStalls, cont’d
The whole wing never stalls at the same time. Power-on stalls in
most light singles allow the wing to stall more fully. Why?
Where do you want the wing to stall last? Ailerons
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Stalls, cont’d (Stalls with one Engine Inop.)Stalls, cont’d (Stalls with one Engine Inop.)
Stalls in a twin with one engine inoperative lead to roll or spin entry: Propeller
slipstream delays stall.
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Stalls, cont’dStalls, cont’d
Stall strips make the wing stall sooner.
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Stalls, cont’dStalls, cont’d
Definition: The angle of incidence is the acute angle between the longitudinal axis of the airplane and the chord line of the wing.
Twist in the wing makes the wing root stall first: The angle of incidence decreases away from the wing root.
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Preventing StallsPreventing Stalls
Slats direct airflow over the wing to avoid boundary layer separation.
Slots are similar but fixed, near the wingtips. Delays stall near the wingtip (aileron effectiveness).
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Stalls and TurnsStalls and Turns
Greater angles of bank require greater lift so that: the vertical component of lift equals weight (to
maintain altitude), the horizontal component of lift equals centrifugal
force (constant radius, coordinated, turn)
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Stalls and Turns, cont’dStalls and Turns, cont’d
Load factor (multiple of aircraft gross weight the wings support) increases with bank angle.
acrobatic 6G
Normal 3.8G
Stall speed increases accordingly.
limit load factor:
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TurnsTurns
As bank increases, load factor increases. But: as airspeed increases, rate of turn
decreases. In order to make a 3 degree per second turn, at 500
Kts the airplane would have to bank more than 50 degrees.
Uncomfortable (unsafe?) load factor. This is why for jet-powered airplanes, a
standard rate turn is 1.5 degrees per second.
Basic AerodynamicsBasic Aerodynamics
High and FastHigh and Fast
In the Flight Levels
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High and FastHigh and Fast
Mach is the ratio of the true airspeed to the speed of sound. Speed of sound decreases with temperature. Temperature decreases with altitude. At higher altitudes, the same indicated airspeed
leads to higher Mach numbers. Conversely: at higher altitudes, a certain Mach
number can be achieved at a lower indicated airspeed.
The indicated stall speed increases with altitude (compressibility).
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High and Fast, cont’dHigh and Fast, cont’d
At high subsonic speeds, portions of the wing can induce supersonic airflow (critical Mach number Mcrit).
Where the airflow slows to subsonic speeds, a shockwave forms.
The shockwave causes boundary layer separation. High-speed buffet, “aileron snatch”, “Mach tuck”.
velocity increases
velocity decreases, shockwave forms
boundary layer separates
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High and Fast, cont’dHigh and Fast, cont’d
Vortex generators delay boundary layer separation.
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High and Fast, cont’dHigh and Fast, cont’d
With altitude: indicated
stall speed (low speed buffet) increases;
indicated airspeed that results in critical Mcrit decreases.
coffin corner
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ReferencesReferences
De Remer D (1992) Aircraft Systems for Pilots Casper: IAP
FAA (1997) Pilot’s Handbook of Aeronautical Knowledge AC61-23C Newcastle: ASA
Lowery J (2001) Professional Pilot Ames: Iowa State Univ. Press
Schiff B (1985) The Proficient Pilot vol. 1 New York: Macmillan
U.S. Navy (1965) Aerodynamics for Naval Aviators Newcastle: ASA