Aerodynamics of Powered High-Lift Systems

Mechanical high-lift systems of various degrees of sophistication are used on most production aircraft today. These systems of flaps located at the leading and trailing edges of wings are known to generate remarkable gains in lift, leading to lower direct operating costs, increased payload capacity, and higher revenues. Although lift coefficients of the order of 3 have been achieved, boundary-layer separation at large flap deflections and angles of attack limits further increase. Today's need for and increasing emphasis on vertical and short takeoff and landing aircraft, however, demand additional lift and higher wing loadings at the lowest possible thrust-to-weight ratios, in order to reduce takeoff and landing distances below 2500 ft, and through high values of eLm" to achieve low approach speeds. This additional lift requirement can be best satisfied by so-called "powered high-lift systems." The flight trajectory of a conventional takeoff and landing aircraft (CTOL) is, in Figure 1, compared with the trajectories for short (STOL) and vertical (VTOL) takeoff and landing aircra,ft. The greater demand for lift and power of V jSTOL aircraft is self-evident and demonstrated furthermore by some characteristic values of the ratio of engine thrust to aircraft gross weight of TjW = OJ, 0.6, and 1.2 for CTOL, STOL, and VTOL aircraft, respectively. There are, however, designs that significantly deviate from these ratios. Whereas the CTOL aircraft for approach, or getting off the ground, depends on flap-augmented wing lift alone, the STOL aircraft requires that its basic wing lift be augmented by engine power. This wing-lift augmentation can fundamentally be obtained by various degrees of integration of the lifting and the propulsive systems, whereby the lifting effectiveness of the wing can be substantially increased. This may be achieved, e.g., by means of propeller slipstream deflectors, externally blown flaps, boundary-layer control (BLC), supercirculation, jet flap or jet-augment or wing, etc, or by vectoring part of the engine power into the lift direction by means of jet deflector flaps, slotted flaps, shrouded fans, etc. Tilt wings, helicopter rotors, wing fans, and other devices, which generate the still higher lift values required