Theory of Flight
References: FTGU pages 18, 35-38

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

• 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
Laminar Flow

Stall

Stall
• When a wing or aerofoil has air flow separation increasing the drag and reducing the lift
• No longer capable of producing enough lift to counteract the weight of the aircraft
• As a result, can no longer maintain level flight

Centre of Pressure
• Point on a wing where total aerodynamic pressure acts

Centre of Pressure
• Centre of pressure moves forward as the angle of attack increases to the point of a stall
• After a stall the centre of pressure moves rapidly back
• If the CoP moves forward of the CG it causes an aeroplane to become unstable , nose of the aeroplane does not drop at the stall

Critical Angle of Attack
• The AoA above which airflow will separate and become turbulent
• The wing stall will occur at any speed

Critical Angle of Attack
• Most aerofoil or wing designs have a stall angle of 15 ° to 20°

• 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

Stall
Buffeting

Factors affecting a Stall
• Position of the Centre of Gravity, more forward CG the higher the Vs
• Weight, increase in wt = increase in Vs
• Turbulence, changes the load factor as well as sudden changes in AoA (greater then the critical AoA)
• Turns, increases the load factor which increases the Vs
• Snow, Frost, Ice cause early airfoil separation causing an increase in Vs

Factors affecting a Stall
Centre of Gravity (CG)
• CG forward
– Loading on the horizontal tail surfaces increases
– Overall weight of aircraft increases
– Vs increases
• CG aft
– Decreased longitudinal stability
– Violent stall characteristics
– Poor or NO stall recovery (very dangerous!)
– Vs decreases

Factors affecting a Stall
Weight
• The more weight on an aircraft means that it must fly at a higher AoA (for a given speed)
• Therefore the critical AoA will be reached at a higher airspeed (instead of stalling at 40 kt stalls at 50 kt)
Attitude to fly straight and level
2500 lbs , at 90 kt (closer to the critical A of A)
1000 lbs , at 90 kt
Start of a trip End of a trip

Factors affecting a Stall
Turbulence
• Upward vertical currents cause the aeroplane’s AoA to increase
• Could result in the aeroplane stalling of the critical AoA is reached, more likely at reduced speeds (approach)

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 a turn
• Subsequently, the stall speed in a turn increases , just like adding more weight to the aircraft

Turns and Stall Speeds

Factors affecting a Stall
Snow, Frost, and Ice
• Accumulation of snow, frost, and ice reduce a wing’s ability to produce lift
• Increase in Vs

Factors affecting a Stall
Increase Vs
Forward CG
Increased weight
Turbulence
Greater angle of bank
Decrease Vs
Aft CG
Decreased weight

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?

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

Stall Recovery