High Lift Devices

Question 1

what is the effect of lowering trailiing edge flaps on camber, effective alpha, CL, CLmax and critical alpha?

Answer 1

Deploying flaps effectively increases the camber of the wing, and the change in the chord line gives an increase in local alpha. There is increased upwash ahead of the wing, and increased down wash behind it

Together, these changes give an increase to both CL and CLmax but remember, when a wing is given increased camber, CLmax comes at a lower critical alpha

Question 2

what is the effect on CP and pitching moment when lowering flaps?

Answer 2

Lowering T/E flaps will alter the pressure pattern over the wing, moving CP aft, which will tend to create nose-down aerodynamics on the wing, which will give a nose-down pitching moment on most aircraft

Question 3

what effect do all leading edge devices have on the value of critical alpha?

Answer 3

Unlike trailing edge flaps, all leading-edge devices INCREASE the value of critical alpha

Question 4

what effect does leading edge flaps have on camber and effective alpha?

Answer 4

camber is increased but the effective chord decreases alpha

The flaps will send airflow that would have flowed under the wing over the top instead. The mass flow over the top surface is therefore both increased and energised, which increases CL and CLmax and delays forward movement of the separation point. The stall will not occur until a higher alpha

Question 5

what are the most effective high lift devices and why?

Answer 5

slats

a slot is opened up behind it creating a venturi effect to increase the mass flow and energy even more, making slats the most effective of all high lift devices

As with trailing edge flaps, the extension of leading edge devices reduces the lift/drag ratio (more lift but also more drag) but the increase in drag is less marked, so they are more efficient high lift devices