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Devices Lift High 8
Automatic Slots
On some aircraft the slots are not controlled by the pilot, but operate automatically. Their movement is caused by the changes of pressure which occur around the leading edge as the angle of attack increases. At low angles of attack the high pressures around the stagnation point keep the slat in the closed position. At high angles of attack the stagnation point has moved underneath the leading edge and ‘suction’ pressures occur on the upper surface of the slat. These pressures cause the slat to move forward and create the slot.
This system is used mainly on small aircraft as a stall protection system. On larger aircraft, the position of the slats is selected when required by the pilot, their movement being controlled electrically or hydraulically.
Disadvantages of the Slot
The slot can give increases in CLMAX of the same magnitude as the trailing edge flap, but whereas the trailing edge flap gives its CLMAX at slightly less than the normal stalling angle, the slot requires a much increased angle of attack to give its CLMAX. In flight this means that the aircraft will have a very nose-up attitude at low speeds, and on the approach to land, visibility of the
landing area could be restricted.
Drag and Pitching Moment of Leading Edge Devices
Compared to trailing edge flaps, the changes of drag and pitching moment resulting from the operation of leading edge devices are small.
Trailing Edge Plus Leading Edge Devices
Most large transport aircraft employ both trailing edge and leading edge devices. Figure 8.17 shows the effect on the lift curve of both types of device.
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CL |
FLAP + SLOT |
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TRAILING EDGE |
BASIC + SLOT |
FLAP |
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SECTION |
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High Lift Devices |
Figure 8.17
Sequence of Operation
For some aerofoils the sequence of flap operation is critical. Lowering a trailing edge flap increases both the downwash and the upwash. For a high speed aerofoil, an increase of upwash at the leading edge when the angle of attack is already fairly high could cause the wing to stall. The leading edge device must therefore be deployed before the trailing edge flap is lowered. When the flaps are retracted, the trailing edge flap must be retracted before the leading edge device is raised.
Asymmetry of High Lift Devices
Deployment of high lift devices can produce large changes of lift, drag and pitching moment. If the movement of the devices is not symmetrical on the two wings, the unbalanced forces could cause severe roll control problems. On many flap control systems the deflection on the two sides is compared while the flaps are moving, and if one side should fail, movement on the other side is automatically stopped. However, on less sophisticated systems a failure of the operating mechanism could lead to an asymmetric situation. The difference in lift will cause a rolling moment which must be opposed by the ailerons, and the difference in drag will cause a yawing moment which must be opposed by the rudder. Whether the controls will be adequate to maintain straight and level flight will depend on the degree of asymmetry and the control power available.
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Flap Load Relief System |
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On large high speed jet transport aircraft, a device is fitted in the flap operating system to |
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prevent the flaps deploying if the aircraft speed is too high. The pilot can select the flaps, but |
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they will not extend until the airspeed is below the flap extend speed (VFE). If a selection is |
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made and the flaps do not run because the speed is too high, they will extend as soon as the |
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airspeed decreases to an appropriate value. |
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Choice of Flap Setting for Take-off, Climb and Landing |
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Take-off |
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Take-off distance depends upon unstick speed and rate of acceleration to that speed. |
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Lowest unstick speed will be possible at the highest CLMAX and this will be achieved at a |
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large flap angle, Figure 8.18. |
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But large flap angles also give high drag, Figure 8.19, which will reduce acceleration and |
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increase the distance required to accelerate to unstick speed. |
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A lower flap angle will give a higher unstick speed but better acceleration, and so give |
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a shorter distance to unstick. |
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Thus there will be some optimum setting which will give the shortest possible take-off distance.
If leading edge devices are fitted, they will be used for take-off as they increase the CLMAX for any trailing edge flap setting.
Climb
After take-off, a minimum climb gradient is required in the take-off configuration. Climb gradient is reduced by flap, so if climb gradient is limiting, a lesser flap angle may be selected even though it gives a longer take-off distance.
Landing
Landing distance will depend on touchdown speed and deceleration. The lowest touchdown
speed will be given by the highest CLMAX, obtained at a large flap angle, Figure 8.18. Large flap angle will also give high drag, Figure 8.19, and so good deceleration. For landing, a large
flap angle will be used. Leading edge devices will also be used to obtain the highest possible
CLMAX.
20°
FLAPS UP