Variation of CL with flight conditions

In steady level flight, the lift force must always be equal and opposite to the aircraft weight. In landing and take-off where the speed, and thus dynamic pressure, are low, a large CL value is required. As the flight speed increases, the lift coefficient required reduces.

The pilot controls the lift coefficient value primarily by altering the angle of attack of the aircraft. The angle of attack must be gradually reduced as the flight speed increases. Most aircraft are designed to fly in a near level attitude at cruise, and must therefore adopt a nose-up attitude on landing and take-off. An extreme example was Concorde, as illustrated in Fig. 1.20. On landing, the angle of attack of this aircraft was so large, that the nose had to be hinged downwards, otherwise the pilot would not have been able to see the runway.

Airliners cruise at high altitude, where the air density is much lower than at sea level. The reduction of density p, which reduces the dynamic pressure, partly compensates for the difference between the cruising and landing speeds. The maximum CL required at take-off, however, may still be many times greater than the minimum cruise value.

Very early aircraft such as that shown in Fig. 1.7(a) could only just stagger into the air, and their maximum speed was little greater than their take-off speed. As seen in Fig. 1.7(a), such aircraft therefore had a highly cambered wing section that produced a large CL in order to minimise the wing area and hence keep the weight down. Most modern aircraft have a less cambered wing section that is optimised to produce low drag at cruising speed. The high lift coefficient required for landing is normally produced by means of some form of flap which effectively increases the camber and sometimes the area of the wing (see Fig. 3.13). Flaps and other high lift devices are described in Chapter 3.