# Theory of Flight Stalls References FTGU pages 19

• Slides: 23

Theory of Flight Stalls References: FTGU pages 19, 29 -32

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 changes from laminar to turbulent • Turbulent layer – Turbulent portion of the boundary layer from the centre to 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 • When the wing is 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 Co. P moves forward of the Co. G it causes an aeroplane to become unstable, nose of the aeroplane does not drop at the stall

Critical Angle of Attack • The Ao. A 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 – the more forward the CG, the higher the stall speed (Vs) • Weight - increase in weight = increase in Vs • Turbulence - changes the load factor as well as sudden changes in Ao. A (greater then the critical Ao. A) • 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 Ao. A (for a given speed) • Therefore the critical Ao. A 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 Ao. A to increase • Could result in the aeroplane stalling of the critical Ao. A 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?

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

Stall Recovery