Theory of Flight
6.07 Stalls
References:
FTGU pages 18, 35-38
Review
1. What are the two main types of drag?
2. Is it possible to eliminate induced
drag? Why or why not?
3. What is one way to increase lift?
4. What are some ways to minimize
drag?
6.07 Stalls
MTPs:
• Laminar Flow
• Definition of a Stall
• Critical Angle of Attack
• Centre of Pressure
• Stalls
• Factors affecting stalls
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
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
Centre of Pressure
Point on a wing where total aerodynamic pressure acts
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
Critical Angle of Attack
• Angle of attack
above which air
will cease to flow
over the wings
and a stall will
occur
Critical Angle of Attack
Most aircraft
have a stall
angle of
positive
15 to 20
degrees
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
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
Stall
Symptoms of a Stall
Buffeting
Factors affecting a Stall
– Centre of Gravity
– Weight
– Turbulence
– Turns (and load factor)
– Snow, Frost, Ice
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
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
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
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
Turns and Stall Speeds
Factors affecting a Stall
Snow, Frost, and Ice
– Accumulation of snow, frost, and ice affects a wing’s
ability to produce lift
– Increase in stall speed
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
Confirmation Check
Confirmation
1. Draw the movement of the C of P
leading up to the stall.
2. What are some factors that increase
the stall speed.
Confirmation
3. When can an aircraft stall?
4. What are the symptoms of a stall?
Stall Recovery