# Theory of Flight 6.07 Stalls References: FTGU pages

Theory of Flight 6.07 Stalls References: 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 designs have a stall angle of 15 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 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 aeroplanes 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 wings 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?

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

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