Category Aircraft Flight

Thrust and momentum

The propeller, the jet, and indeed all conventional aircraft propulsion systems involve changes in momentum of the air. When a change of momentum occurs, there must be a corresponding force, but it should not be thought that thrust is caused directly by the change of momentum, with no other mechanism being involved. As we have seen in the above examples, the force is produced and transmitted to the structure by pressure differences acting across the various sur­faces of the device. It is perhaps best not to think of rate of momentum change and force as cause and effect, but as two consequences of one process. In mak­ing practical measurements, or even theoretical estimates, we normally have to consider a combination of pressure-related forces and momentum changes.

Comparison between jet and propeller for thrust production

Figure 6.3 shows a jet aircraft and a propeller-driven one producing equal amounts of thrust at zero forward speed. In the case illustrated, the jet engine is transferring energy to the slipstream or jet five times as fast as the propeller. Since this energy must ultimately have come from the fuel, it indicates that the propeller-driven aircraft is producing the thrust more economically.

When the aircraft are in motion, the jet engine will still transfer energy to the air at a faster rate than the propeller at any given thrust and forward speed, but the difference in energy transfer rate becomes less marked as the speed increases.

Pure rocket propulsion

The pure rocket will work at very high altitude and in the vacuum of space. The high speed of the exhaust gases and the added weight of the oxidant that must be carried, however, mean that it is extremely inefficient in comparison with air-breathing engines at low altitude.

The thrust of a rocket motor comes from the high pressure on the walls of the combustion chamber and exhaust nozzle. The same high pressure produces the acceleration and momentum change of the exhaust gases.

Rockets have been used to assist the take-off, and for experimental high altitude high speed research aircraft, but one production rocket aircraft was the Second World War swept tailless Messerschmitt Me 163. The motor used two chemicals, one of which was highly reactive and, if it did not explode during a

Fig. 6.40 Turbo-ramjet propulsion for very high speed flight

The Lockheed SR-71 was capable of flight at Mach 3+

Note the central shock-generating movable spike in the axi-symmetrical engine intakes, and the exhaust nozzles fully open for operation with reheat The photograph was taken as the aircraft was manoeuvring at a high angle of attack. The strong conical vortices generated by the fuselage strakes and the wing have been made visible by the clouds of water vapour produced (not smoke). The engines have flamed-out leaving spectacular fireballs. The engine has a very complex internal variable geometry, and any mismatch is liable to produce a failure of the combustion process, leading to flame-out (Photo from Duncan Cubitt, Key Publishing)

heavy landing, was liable to dissolve the occupant. It was reportedly unpopular with pilots!

The swing-wing

One of the most obvious ways in which to satisfy the conflicting requirements imposed by a large speed range is to provide some mechanism to vary the sweep angle of the wing. Although this seems an attractive solution the mechanical problems faced in such a design are considerable. The hinge mechanism must clearly be at the root of the wing and this is the very position at which bending moment and structural demands will be greatest. Other important mechanical problems may be encountered such as the requirement to keep underwing stores, such as missiles or fuel tanks, aligned with the free stream direction as the sweep angle is changed on a military aircraft. It will also place restrictions on the positioning of the engines since wing mounting will clearly lead to severe complications.

In spite of these difficulties this solution has been employed on a number of aircraft, including the Tornado (Fig. 11.12), which was designed to fulfil a variety of roles from strike aircraft to high speed interceptor, and on the F-14 (Fig. 8.2). Both these aircraft are required to operate at high speed at low alti­tude. If the wing is operating at a relatively high loading then the increase in angle of attack due to an upwards gust will be less than that for a wing with a lower loading per unit area. This is because the more highly loaded wing will be operating at a greater angle of attack. A gust at a given flight speed will thus produce a smaller percentage change in angle of attack than it would for a wing operating at a reduced loading. This is a particularly important consideration for high speed low altitude operation and a swing-wing produces a suitable compromise.

Another method of sweep variation which has been proposed is to simply yaw the whole wing in flight as on the experimental NASA AD-1 shown in Fig. 8.16. This solution is not without its own complications, though, and some mechanical hinges may still be required (e. g. for any wing-mounted com­ponents, such as vertical stabilisers or at the wing fuselage junction). More­over the configuration is inherently asymmetrical in the swept configuration, and this is likely to lead to drag penalties because of the need for aerodynamic trim.

The properly executed turn

Unlike a car, an aircraft cannot be turned satisfactorily by means of the yaw control alone. This is because there is no road to provide a reaction to produce the cornering forces. In an aircraft, the cornering (centripetal) force must be provided by aerodynamic means. When the rudder is deflected so as to yaw the aircraft, the force that it produces is actually outwards; the opposite direction to that required.

As illustrated in Fig. 10.12, to execute a level turn properly, the aircraft must be banked, and the lift increased so that the horizontal component of lift is exactly the right size to provide the centripetal force required for the turn, and the vertical component exactly balances the weight. Normally a certain amount of rudder control is necessary in order to keep the aircraft pointing in the intended direction. Excessive use of the rudder, however, produces a skidding turn, with an uncomfortable sideways acceleration, and a potentially danger­ous sideslip.

vertical component of lift

Horizontal component of lift


Fig. 10.12 Turning flight

For a correctly banked turn, the lift force must be increased so that its vertical component exactly balances the weight. The horizontal component can then provide the required centripetal acceleration

The precise coupling between roll and yaw varies from one aircraft design to another. In general, a combination of aileron and rudder movement is required, but most aircraft can be turned smoothly using ailerons alone. Some early Farman aircraft had no rudder at all. The balance between rudder and aileron control also depends on whether the aircraft is climbing, descending, or flying level. A more detailed description will be found in practical flying manuals such as Birch and Bramson (1981).

Note, that once a properly executed turn has been initiated, the control stick or handlebars are returned to somewhere near the neutral or mid-position, and the aircraft keeps turning. Holding the stick over would cause the aircraft to continue rolling. This is quite different from steering a car, where the steering wheel must be held in the turned position.

One very special case where flat turns were necessary was in the man – powered Gossamer Albatross shown in Fig. 10.13. Because of its exceptionally low power, this aircraft required a high-aspect-ratio wing with a span similar to that of a large airliner, and could only fly close to the ground. In a banked turn the wing tip would be likely to hit the ground. The aircraft was therefore turned by means of the canard foreplane, which could be canted over so as to

produce a sideforce component to pull the nose round. Note that no fin or rud­der was provided.

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

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


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.