Basic Aerodynamics

1
Basic Aerodynamics

• Dartmouth Flying Club
• October 10, 2002
• Andreas Bentz

2
Lift

• Bernoullis Principle

3
Energy

• 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 Bernoullis Principle
kinetic energy(velocity) potential
energy(pressure)
velocity increases pressure decreases
5
Lift Wing Section

• Air flows toward the low pressure area above the
  wing upwash and downwash.
• Newtons 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

relative low pressure
upwash
downwash
6
Angle of Attack

• 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).

7
Lift and Induced Drag

• Lift acts through the center of pressure, and
  perpendicular to the relative wind.
• This creates induced drag.

8
Got Lift? Flaps

• Flaps increase the wings camber.
• Some also increase the wing area (fowler flap).
• Almost all jet transports also have leading edge
  flaps.

9
Too 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).

10
Side Effects

• There is no such things as a free lunch.

11
Drag Total Drag (Power Required) Curve

• induced drag

• parasite drag
• resistance

• total drag

12
Wingtip Vortices and Wake Turbulence
relative low pressure

• Wingtip vortices create drag
• ground effect
• tip tanks, drooped wings, winglets.

13
Stability

• Longitudinal Static, Dynamic
• Lateral

14
Longitudinal 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).

15
Aside CG and Center of Pressure Location

• 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 reqd).

16
Lateral 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.

17
Directional 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 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.

1,400 1,200 1,000 800 600 400 200
100 50
Percent horsepower
Drag (thrust required)
50 100 150 200 Indicated Airspeed (knots)
19
Turning Flight

• Differential Lift

20
Turning 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, contd

• 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 reqd)

Centrifugal Force
22
Adverse 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.

23
Stalls

• Too Much of a Good Thing

24
Stalls

• 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, contd

• 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.

26
Stalls, contd

• 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, contd (Stalls with one Engine Inop.)

• Stalls in a twin with one engine inoperative lead
  to roll or spin entry
• Propeller slipstream delays stall.

28
Stalls, contd

• Stall strips make the wing stall sooner.

29
Stalls, contd

• 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 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 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, contd

• Load factor (multiple of aircraft gross weight
  the wings support) increases with bank angle.

limit load factor

• Stall speed increases accordingly.

33
Turns

• 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.

34
High and Fast

• In the Flight Levels

35
High 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, contd

• 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
High and Fast, contd

• Vortex generators delay boundary layer separation.

38
High and Fast, contd

• With altitude
• indicated stall speed (low speed buffet)
  increases
• indicated airspeed that results in critical Mcrit
  decreases.
• coffin corner

39
References

• De Remer D (1992) Aircraft Systems for Pilots
  Casper IAP
• FAA (1997) Pilots 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