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Speed stability
As we explained in Chapter 4, in level flight, the contributions to drag from surface friction and normal pressure rise roughly as the square of the speed. The trailing vortex drag, however, decreases with speed, because the circulation, and lift coefficient required, decrease. In Fig. 4.21 we showed how the contributions to drag vary with speed. It was shown that the resulting total drag has a minimum value. The curve of resulting drag is repeated in Fig. 11.18. If we try to fly at a speed less than the minimum drag speed whilst trying to maintain a steady flight path then a decrease in speed will cause increased drag. The thrust of turbo-jet engines is not very sensitive to speed changes, so on jet – propelled aircraft the increase in drag will slow the aircraft down further. Similarly, a small increase in speed will result in less drag, so the aircraft will tend to fly even faster. Therefore, at speeds less than that for minimum drag, a turbo-jet aircraft suffers an instability of speed.
On piston-engined aircraft where the power is not greatly affected by the speed, a reduction in speed is usually accompanied by an increase in thrust, since power = thrust x speed. Up to a point, therefore, the increase in thrust
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Fig. 11.18 Speed instability and the effect of air brakes, etc When an aircraft is flying slower than the minimum drag speed, as at A, then any increase in speed results in a reduction in drag if the pilot maintains a steady flight path. The aircraft will therefore accelerate until point B is reached where the thrust and drag are once again in balance Conversely, if the speed falls, then the drag will rise, and the aircraft will slow producing more drag. The vicious circle continues until the aircraft stalls. In the landing configuration, the deployment of flaps, landing gear and if necessary, air-brakes increases the boundary layer (profile) drag. This lowers the minimum drag speed, and consequently reduces the speed at which the onset of speed instability occurs |
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SPEED STABILITY 317 Fig. 11.19 Air brakes not only slow the aircraft down, but may be useful in preventing speed-instability (Photo courtesy of Alistair Copeland) |
tends to compensate for the increase in drag, so piston-engined aircraft are less prone to speed instability.
There are also other reasons why turbo-jet aircraft are more prone to speed instability. When we looked at aircraft performance, we saw that the most economical flying speed is above the minimum-drag speed. For piston-engined aircraft, where the equivalent air speed (EAS) at cruise is only about two or three times as fast as the landing speed, the landing speed is normally fairly close to this minimum point. Any tendency to speed instability is, therefore, slight, and can be easily controlled by the pilot. For high speed turbo-jet aircraft, the cruising (EAS) speed may be many times greater than the landing speed. Thus if the cruise is to be efficient, the landing speed will be well below the minimum drag speed, and speed instability becomes a more serious problem.
The problem of speed instability on turbo-jet aircraft is made worse by the fact that the response to throttle changes is much slower than for a piston – engined type. If the pilot of a turbo-jet propelled aircraft tried to flatten out and float down to a three-point landing, as was the custom in the piston-engine era, he might find himself taking-off again instead.
To solve the speed-instability problem, air brakes may be fitted as shown in Fig. 11.19. These devices increase the drag, and have the effect of pulling the minimum drag position point further to the left on the curve, as shown in Fig. 11.18. Flaps also help to increase the drag, and are normally deployed more fully for landing than for take-off. On Concorde an automatic throttle control system was used to help iron out the inherent speed instability at low speeds.
