Theory of Flight

1
Theory of Flight

• 6.07 Stalls
• References
• FTGU pages 18, 35-38

2
Review

• What are the two main types of drag?
• 2. Is it possible to eliminate induced drag? Why
  or why not?
• What is one way to increase lift?
• What are some ways to minimize drag?

3
6.07 Stalls

• MTPs
• Laminar Flow
• Definition of a Stall
• Critical Angle of Attack
• Centre of Pressure
• Stalls
• Factors affecting stalls

4
Laminar Flow

• Boundary Layer The thin layer of airflow over
  the wing
• Laminar Layer Smooth portion of the boundary
  layer nearest the leading edge of the wing
• Transition/Separation Point Point on wing where
  the boundary layer becomes turbulent
• Turbulent layer Turbulent portion of the
  boundary layer at the trailing edge of the wing

5
Stall

• What is a stall?
• When a wing is no longer capable of producing
  enough lift to counteract the weight of the
  aircraft
• As a result, can no longer be maintained

6
Centre of Pressure

• Point on a wing where total aerodynamic pressure
  acts

7
Centre of Pressure

• Centre of pressure moves forward as angle of
  attack increases to the point of a stall
• After a stall the centre of pressure moves
  rapidly back
• The movement of the centre of pressure causes an
  aircraft to be unstable

8
Critical Angle of Attack

• Angle of attack above which air will cease to
  flow over the wings and a stall will occur

9
Critical Angle of Attack

• Most aircraft have a stall angle of positive
• 15 to 20 degrees

10
Stall

• Centre of pressure and separation point move
  forward to point of stall and lift production is
  increased
• Angle of attack is increased beyond critical
  angle of attack
• Wing stops producing lift and stalls
• Centre of pressure moves rapidly backward

11
Stall

An aircraft can stall 1) When the critical
angle of attack is exceeded 2) At any airspeed
if the critical angle of attack is exceeded 3)
At any altitude if the critical angle of attack
is exceeded
12
Stall

• Symptoms of a Stall

Buffeting
13
Factors affecting a Stall

• Centre of Gravity
• Weight
• Turbulence
• Turns (and load factor)
• Snow, Frost, Ice

14
Factors affecting a Stall

• Centre of Gravity (C of G)
• C of G too far forward
• Loading on the horizontal tail surfaces increase
• Overall weight of aircraft increases
• Stall speed increases
• C of G too far aft
• Decreased longitudinal stability
• Violent stall characteristics
• Poor stall recovery (very dangerous!)
• Stall speed decreases

15
Factors affecting a Stall

• Weight
• The more weight on an aircraft means that it must
  fly at a higher angle of attack
• Therefore the critical angle of attack will be
  reached at a higher airspeed

Attitude to fly straight and level 2500 lbs,
90kts (closer to the critical A of A) 1000
lbs, 90 kts
Start of a trip
End of a trip
16
Factors affecting a Stall

• Turbulence
• Upward vertical currents cause the airplane to
  increase its angle of attack
• Could result in the aircraft stalling if flying
  near the stall speed

17
Factors affecting a Stall

• Turns
• As angle of bank increases the load factor also
  increases
• Therefore, an increased angle of attack is
  required to maintain level flight in the turn
• Subsequently, the stall speed in a turn increases
• Just like adding more weight to an aircraft

18
Turns and Stall Speeds
19
Factors affecting a Stall

• Snow, Frost, and Ice
• Accumulation of snow, frost, and ice affects a
  wings ability to produce lift
• Increase in stall speed

20
Factors affecting a Stall

• Factors affecting a stall
• Increase stall speed
• Forward C of G
• More weight
• Turbulence
• Greater angle of bank

• Decrease in stall speed
• Aft C of G

21
Confirmation Check
22
Confirmation

• Draw the movement of the C of P leading up to the
  stall.
• What are some factors that increase the stall
  speed.

23
Confirmation

• 3. When can an aircraft stall?
• 4. What are the symptoms of a stall?

24
Stall Recovery