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Aircraft control at low speed
Control problems at low speed stem from three major factors; weak aerodynamic control forces, the danger of provoking a stall, and the immersion of control surfaces in a slow-moving separated or wake flow, which may be highly turbulent.
The weakness of the aerodynamic control forces is one of the major factors limiting the minimum speed of short take-off and landing (STOL) aircraft. Very large surfaces must be installed. Note the very large fin in the photograph of the C-17 shown in Fig. 10.20. Such large surfaces, however, represent a source of drag and weight, and detract from the cruise performance. In gliders, where cruise performance is all important, the tail surfaces are normally very small.
At low speeds, the wing is fairly close to its stall angle of attack. Downward deflection of an aileron could cause the wing tip to stall, and drop instead of rising, thus giving rise to control reversal, as well as the possibility of provoking a spin (described in Chapter 12). The geared aileron in which the downgoing surface moves less than the rising one can help to overcome this problem, but the use of spoilers for low speed roll control is sometimes a better solution.
For very low speed flight, and vertical take-off and landing (VTOL), the aerodynamic forces are too small to be used for control purposes. Some form
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Fig. 10.20 The C-17 has a STOL (short take-off and landing) capability The large vertical tail surface is necessary to provide stability and control at low speeds. The very high mounting position of the horizontal tail surface helps to keep it out of the wake of the wing at high angles of attack |
of reaction control has to be used, as on the Harrier, where ‘puffer’ jets are located at each wing tip, and on the nose and tail, as illustrated in Fig. 10.21. The jets are fed by compressed air from the engines. A reaction control arrangement is also used for controlling and stabilising spacecraft.
When an aircraft is flown under reaction control, its safety is totally dependent on an uninterrupted supply of compressed air, which makes the idea of civil VTOL aircraft, other than helicopters, unattractive to civil airworthiness authorities. However, operational experience with the Harrier indicates that because of the low speeds involved, serious accidents on landing and take-off are, if anything, less frequent than with conventional aircraft.
