Basic Aerodynamics Dartmouth Flying Club October 10, 2002 Andreas Bentz

Basic AerodynamicsBasic Aerodynamics


Dartmouth Flying ClubOctober 10, 2002

Andreas Bentz


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).


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


StabilityStability

Longitudinal: Static, DynamicLateral

14

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

17

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.

18

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


Turning FlightTurning Flight

Differential Lift

20

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.

21

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.


StallsStalls

Too Much of a Good Thing

24

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).

25

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


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

27

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.

28


Stall strips make the wing stall sooner.

29


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.

30

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).

31

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)

32

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:

33

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.


High and FastHigh and Fast

In the Flight Levels

35

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).

36

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

37


Vortex generators delay boundary layer separation.

38


With altitude: indicated

stall speed (low speed buffet) increases;

indicated airspeed that results in critical Mcrit decreases.

coffin corner

39

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

Documents

Basic Aerodynamics Dartmouth Flying Club October 10, 2002 Andreas Bentz