Aircraft Stall Characteristics

Question 1

What is the boundary layer of an airfoil? Draw a diagram

Answer 1

• Layer of air closes to the surface of the aerofoil that has a velocity gradient from the surface until free stream air.

Question 2

What are the three distinct boundary layer flow types over an aerofoil?

Answer 2

• Laminar
• Turbulent
• Separated

Question 3

What are the points between the zones called?

Answer 3

• Transition point

- Separation point

Question 4

Where is the transition point normally located on the wing?

Answer 4

At the point of max camber.

Question 5

What will move the separation and transition point in the aerofoil?

Answer 5

Change in AoA

Question 6

Will aerofoil surface roughness or icing influence the position of the transition and separation points?

Answer 6

Yes

Question 7

Where does the positive pressure gradient extend from and to?

Answer 7

The LE to the point of max thickness/camber at the transition point.

Question 8

Where does the negative pressure gradient extend from and to?

Answer 8

The point of max camber (transition point) to the TE.

Question 9

What does an increase in flow turbulence do to the boundary layer?

Answer 9

Thickens it.

Question 10

What does the kinetic energy gained by the turbulence allow the boundary layer to do?

Answer 10

Penetrate the adverse pressure gradient further

Question 11

What does the boundary layer look like at the separation point? Draw a diagram

Answer 11

Boundary layer will reach the free stream velocity at the separation point.

Question 12

What does the boundary layer look like at the TE? Draw a diagram

Answer 12

Large vortices are formed and reverse flow occurs.

Question 13

How does the CoP change with increasing AoA?

Answer 13

Will move it forward.

Question 14

What can the pilot feel as the boundary layer starts to separate from the wing?

Answer 14

Light buffet

Question 15

What occurs when the aircraft has achieved the critical angle of attack?

Answer 15

• Flow no longer remains attached
• Flow separates
• Cp moves rearwards, separation point moves rapidly forward
• Pressure drag increases rapidly
• Heavy buffet on the wing
• Lift is significantly reduced and the aircraft is stalled.

Question 16

Is the tailplane stalled when the main wing stalls? Why or Why not?

Answer 16

• No
• Because the tailplane is set at a lower angle of attack than the wings, so it will continue to provide an upward force at the tail, pitching the nose up.

Question 17

What does the rearward moving Cp cause the aircraft to do?

Answer 17

Pitch forward, aiding stall recovery.

Question 18

What should the pilot do with the stick to recover from the stall?

Answer 18

Pitch the nose forward.

Question 19

Why do low aspect ratio wings stall at a much higher AoA than large aspect ratio wings?

Answer 19

Increased wingtip vortices at the tips which induce a downwash that causes relative airflow at the tips to reduce.

Question 20

What effect does surface roughness have on Cl and angle of attack?

Answer 20

Increased surface roughness will cause Clmax to decrease and AoAcrit to decrease.

Question 21

What effect does camber have on Cl and AoAcrit?

Answer 21

Increased camber will increase the Clmax of the wing but will reduce the critical angle of attack.

Question 22

What is the effect of Leading edge radius on Cl?

Answer 22

A larger leading edge radius will cause a more gradual reduction in Cl at the AoA crit while the sharper leading edge causes a more abrupt collapse

Question 23

Does aircraft attitude, flight path and airspeed effect the stalling AoA?

Answer 23

No, it will always stall at the same angle of attack.

Question 24

What can be said about light and heavy buffet relating to stall?

Answer 24

Light buffet = Approaching stall

Heavy buffet = Aircraft is stalled

Question 25

How is stall speed affected by load factor?

Answer 25

Stall speed = Basic stall speed * sqrt(load factor)

Question 26

How does stall speed increase with weight?

Answer 26

Increases by the square root of the proportion of weight increase (load factor).

Question 27

How does load factor vary with angle of bank?

Answer 27

Load factor in a turn is applied to the previous formula.

Question 28

What does the effect of power have stall?

Answer 28

• Nose attitude will be higher, speed slower in a power on stall.
• Stall will be more sudden
• Pre-stall buffet is masked, making it harder to determine that you are going into a stall.
• Rudder and elevator will remain powerful but the ailerons will become less responsive as wing tip stalls.

Question 29

Why do rectangular wings naturally stall at the wing root first?

Answer 29

High wingtip vortices reduce AoA of relative airflow, which results in less loss of lift compared to at the wing root and will reach a stalling AoA at a later point.

Question 30

Why do swept wings stall at the wing tip before wing root?

Answer 30

Because increased spanwise flow can’t get past strong wingtip vortices and will create an area of low energy air at the wingtip, resulting in a stall at the wingtip before the wing root.

Question 31

What are some design methods to reduce the tendency of wing tip stalling?

Answer 31

1) Change of aerofoil section towards the wing tip
2) Root spoilers
3) Washout
4) Wing spoilers or vortex generators
5) Wing fences
6) Wing twist
7) Turbulators

Question 32

What are the buffet corners on the V-n diagram?

Answer 32

Areas of high g loadings and speed, whereby the airframe may be overloaded when encountering buffet or turbulent flow striking the tailplane when approaching the stalling critical angle.

Question 33

What is the rolling g limit?

Answer 33

Additional twisting and lifting force on the wing with downgoing aileron will take that wing past max g limit.

Question 34

What is the load factor limit?

Answer 34

Area around the operational limits whereby elastic changes can occur.

Question 35

What is the red area?

Answer 35

Where catastrophic failure could occur.