Chapter

8

High Lift Devices

Purpose of High Lift Devices . . . . . . . . . . .	. .	. .	. .	. .	. .	. . . . 209
Take-off and Landing Speeds							209
CLMAX Augmentation . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. . .	209
Flaps . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. . .	209
Trailing Edge Flaps . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. . . .	209
Plain Flap . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. . .	209
Split Flap . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. . .	210
Slotted and Multiple Slotted Flaps							210
The Fowler Flap . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. . .	211
Comparison of Trailing Edge Flaps							211
CLMAX and Stalling Angle . . . . . . . . . . . .	. .	. .	. .	. .	. .	. . . .	212
Drag . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. . .	213
Lift / Drag Ratio							214
Pitching Moment . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. . . . 214
Centre of Pressure Movement . . . . . . . . . .	. .	. .	. .	. .	. .	. . . .	214
Change of Downwash . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. . . . 214
Overall Pitch Change . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. . .	215
Aircraft Attitude with Flaps Lowered . . . . . . . .	. .	. .	. .	. .	. .	. . .	215
Leading Edge High Lift Devices . . . . . . . . . .	. .	. .	. .	. .	. .	. . .	216
Leading Edge Flaps . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. . . .	216
Effect of Leading Edge Flaps on Lift . . . . . . . .	. .	. .	. .	. .	. .	. . . .	217
Leading Edge Slots . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. . . .	218
Leading Edge Slat							218
Automatic Slots . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. . .	220
Disadvantages of the Slot . . . . . . . . . . . .	. .	. .	. .	. .	. .	. . .	220
Drag and Pitching Moment of Leading Edge Devices . .	. .	. .	. .	. .	. .	. . .	220
Trailing Edge Plus Leading Edge Devices . . . . . .	. .	. .	. .	. .	. .	. . . .	220

Continued Overleaf

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8		High Lift Devices
		Sequence of Operation				221
		Asymmetry of High Lift Devices . . . . . . . . . .	. .	. . . . . . . .	. .	221
		Flap Load Relief System				222
		Choice of Flap Setting for Take-off, Climb and Landing .	. .	. . . . . . . . .	.	222
		Management of High Lift Devices . . . . . . . . .	. .	. . . . . . . . .	.	224
		Flap Extension Prior to Landing . . . . . . . . . .	. .	. . . . . . . .	. .	226
		Questions . . . . . . . . . . . . . . . . . .	. .	. . . . . . . .	. .	227
		Annexes . . . . . . . . . . . . . . . . . .	. .	. . . . . . . . .	.	. 229
		Answers . . . . . . . . . . . . . . . . . .	. .	. . . . . . . . .	.	. 230

Devices Lift High 8

208

High Lift Devices		8

Purpose of High Lift Devices

Aircraft are fitted with high lift devices to reduce the take-off and landing distances. This permits operation at greater weights from given runway lengths and enables greater payloads to be carried.

Take-off and Landing Speeds

The take-off and landing distances depend on the speeds required at the screen, and these are laid down in the performance regulations. For both take-off and landing, one of the requirements is for a safe margin above the stalling speed (1.2VS1 for take-off and 1.3VS0 for landing). The stalling speed is determined by the CLMAX of the wing, and so to obtain the lowest possible distances, the CLMAX , must be as high as possible.

CLMAX Augmentation

One of the main factors which determine the CLMAX of an aerofoil section is the camber. It has been shown earlier that increasing the camber of an aerofoil section increases the CL at a given

angle of attack and increases CLMAX. For take-off and landing a cambered section is desirable, but this would give high drag at cruising speeds and require a very nose-down attitude. It is usual to select a less cambered aerofoil section to optimise cruise and modify the section for take-off and landing by the use of flaps.

Flaps

A flap is a hinged portion of the trailing or leading edge which can be deflected downwards and so produce an increase of camber. For low speed aerofoils the flaps will be on the trailing edge only, but on high speed aerofoils where the leading edge may be symmetrical or have a negative camber, there will usually be flaps on both the leading edge and the trailing edge.

Trailing Edge Flaps

The basic principle of the flap has been adapted in many ways. The more commonly used types of trailing edge flap are considered below.

Plain Flap

The plain flap, illustrated in Figure 8.1, has a simple construction and gives a good increase in

CLMAX, although with fairly high drag. It is used mainly on low speed aircraft and where very short take-off and landing is not required.

High Lift Devices 8

Figure 8.1 Plain flap

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8 High Lift Devices

Split Flap

The flap forms part of the lower surface of the wing trailing edge, the upper surface contour being unaffected when the flap is lowered.

Devices Lift High 8

Figure 8.2 Split flap

The split flap gives about the same increase in lift as the plain flap at low angles of attack but gives slightly more at higher angles as the upper surface camber is not increased, and so separation is delayed. The drag, however, is higher than for the plain flap due to the increased depth of the wake.

Slotted and Multiple Slotted Flaps

When the slotted flap is lowered, a slot or gap is opened between the flap and the wing.

Figure 8.3 Slotted flap

The purpose of the slot is to direct higher pressure air from the lower surface over the flap and re-energize the boundary layer. This delays the separation of the airflow on the upper surface

of the flap. The slotted flap gives a bigger increase in CLMAX than the plain or split flap and much less drag, but it has a more complex construction.

210