Devices Lift High 8

Management of High Lift Devices

To take full advantage of the capabilities of flaps, the flight crew must properly manage their retraction and extension.

Flap Retraction afterTake-off

With reference to Figure 8.20, assume the aircraft has just taken off with flaps extended and is at point ‘A’ on the lift curve. If the flaps are retracted, with no change made to either angle of attack or IAS, the coefficient of lift will reduce to point ‘C’ and the aircraft will sink.

1.From point ‘A’ on the lift curve the aircraft should be accelerated to point ‘B’.

2.From point ‘B’, as the flaps are retracted the angle of attack should be increased to point ‘C’ to maintain the coefficient of lift constant.

The pilot should not retract the flaps until the aircraft has sufficient IAS. Of course, this same factor must be considered for any intermediate flap position between extended and retracted. (Refer to Page 76 for a review of the Interpretation of the Lift Curve if necessary.)

As the configuration is altered from the flaps down to the flaps up or “clean” configuration, three important changes take place:

•Changes of pressure distribution on the wing generate a nose-up pitching moment. But reduced wing downwash increasing the tailplane effective angle of attack generates a nosedown pitching moment. The resultant, actual, pitching moment experienced by the aircraft will depend upon which of these two pitching moments is dominant.

•With reference to Figure 8.21, the retraction of flaps (‘B’ to ‘C’) causes a reduction of drag coefficient. This drag reduction improves the acceleration of the aircraft.

•Flap retraction usually takes place in stages, and movement of the flaps between stages will take a finite period of time. It has been stated that as flaps are retracted, an increase in angle of attack is required to maintain the same lift coefficient.

If aircraft acceleration is low throughout the flap retraction speed range, the angle of attack must be increased an appreciable amount to prevent the aircraft from sinking. This situation is typical after take-off when gross weight and density altitude are high.

However, most modern jet transport aircraft have enough acceleration throughout the flap retraction speed range that the resultant rapid gain in airspeed requires a much less noticeable increase in angle of attack.

ANGLE OF ATTACK

Figure 8.20

Figure 8.21

High Lift Devices 8

Questions

1.With the flaps lowered, the stalling speed will:

a.increase.

b.decrease.

c.increase, but occur at a higher angle of attack.

d.remain the same.

2.When flaps are lowered the stalling angle of attack of the wing:

a.remains the same, but CLMAX increases.

b.increases and CLMAX increases.

c.decreases, but CLMAX increases.

d.decreases, but CLMAX remains the same.

3.With full flap, the maximum lift/drag ratio:

a.increases and the stalling angle increases.

b.decreases and the stalling speed decreases.

c.remains the same and the stalling angle remains the same.

d.remains the same and the stalling angle decreases.

4.When a leading edge slot is opened, the stalling speed will:

a.increase.

b.decrease.

c.remain the same but will occur at a higher angle of attack.

d.remain the same but will occur at a lower angle of attack.

5.Lowering the flaps during a landing approach:

a.increases the angle of descent without increasing the airspeed.

b.decreases the angle of descent without increasing power.

c.eliminates floating.

d.permits approaches at a higher indicated airspeed.

Questions 8