Aircraft Stalls and Recovery Techniques

1. Theory of Flight 6.07 Stalls References: FTGU pages 18, 35-38

2. 6.07 Stalls • Laminar Flow • Definition of a Stall • Centre of Pressure • Critical Angle of Attack • Stalls • Factors affecting stalls

3. 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

4. Stall What is a stall?

5. 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

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

7. 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

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

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

10. 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

11. Stall Symptoms of a Stall Buffeting

12. 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

13. 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

14. 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

15. 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)

16. Factors affecting a Stall Turns • As angle of bankincreases the load factoralsoincreases • 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

17. Turns and Stall Speeds

18. 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

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

20. Confirmation Check

21. Confirmation • Draw the movement of the C of P leading up to the stall. • What are some factors that increase the stall speed?

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

23. Stall Recovery