Our heavyweight helicopter equal in the world does not have
In Rostov started production of the most load-lifting rotary-wing car The Russian holding «Helicopt[...]
Optimum economy with the jet engine
The fuel flow rate in a gas-turbine engine depends only on the throttle setting and is approximately proportional to the thrust produced by the engine rather than the power. Unlike the piston engine the efficiency of the turbo-jet engine (Chapter 6) improves with increasing dynamic pressure and reducing temperature. For optimum engine efficiency we therefore need to fly fast and high.
Because the engine efficiency increases with speed, the best speed to fly at is a compromise between the requirements of the airframe and the engine. Thus, unlike the piston engined aircraft, the best cruising speed will be somewhat higher than the minimum drag speed (Fig. 7.4).
Because of the way in which the engine behaves, we now need the dynamic pressure (and hence operating speed) to be as high as possible. Thus we need to design and operate the aircraft so that the best airframe performance is obtained at as high a speed as possible. The requirement for high speed is good news for the commercial operator, as we will see shortly. The aircraft should also be operated at high altitude so that the temperature of the air is low, to further improve engine performance.
As we have seen, reducing the wing area enables us to increase the dynamic pressure to compensate. In order to fly at high speed we therefore need an aircraft with the smallest possible wing area, consistent with acceptable low speed performance.
Flying high, too, has its limitations. The lower the air density the higher the stalling speed of the aircraft (Chapter 2). The maximum speed, for a conventional transonic airliner, will be dictated by the onset of problems associated with high Mach number (the buffet boundary Chapter 9), and so flight becomes possible over an increasingly restricted speed range as height is increased (Fig. 7.8). From an operational viewpoint a safety margin must be allowed to allow for accidental speed changes and for manoeuvres
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Fig. 7.8 Effect of weight on stall and buffet boundary A reduction in aircraft weight as fuel is used means that the intersection between stall and buffet boundaries occurs at a greater height |
such as making turns which make extra demands on the wing lift as will be seen later.
