Theory of Flight

6.07 StallsReferences: FTGU pages 18, 35-38

6.07 Stalls

• Laminar Flow

• Definition of a Stall

• Centre of Pressure

• Critical Angle of Attack

• 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?

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 Most aerofoil or wing designs have a stall angle designs have a stall angle of 15of 15°° to 20 to 20°°

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

Symptoms of a StallBuffetingBuffeting

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 levelAttitude to fly straight and level

2500 lbs2500 lbs, at 90 kt (closer to the , at 90 kt (closer to the critical A of A)critical A of A)

1000 lbs1000 lbs, at 90 kt, at 90 kt

Start of a tripStart of a trip End of a tripEnd 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 VsForward CG

Increased weightTurbulence

Greater angle of bank

Decrease VsAft 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?

Confirmation

3. When can an aircraft stall?

4. What are the symptoms of a stall?

Stall Recovery