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[...]
Lift generation using engine thrust
Gas turbine engines are capable of producing a maximum thrust of more than twenty times their own weight. It is therefore possible to dispense with winggenerated lift, and use engine thrust instead, by directing the jet downwards. This method of lift production was successfully demonstrated on a skeletal rig aptly known as the ‘Flying Bedstead’, shown in Fig. 1.24.
|
Fig. 1.24 Lift from downward deflected engine thrust The practicality of jet lift was demonstrated by the Rolls-Royce ‘Flying Bedstead’ rig. Stability and control were provided by subsidiary jets which can be seen clearly. The same basic control system is used on the VTOL Harrier (Photo courtesy of Rolls-Royce pic) |
The British Aerospace Harrier (manufactured under licence in the USA as the McDonnell Douglas AV-8) shown in Fig. 7.12, was the first operational aircraft to use this method of generating lift, and employs rotatable nozzles to direct or ‘vector’ the engine jets. Two sets of nozzles are used; one pair for the exhaust jet, and the other for a jet of air bled from the compressor. For vertical take-off and landing (VTOL), the jets are directed vertically downwards. For forward flight, the nozzles are rotated to direct the jets aft. As the air-speed increases, a conventional wing gradually takes over the job of providing lift. Intermediate nozzle positions can be used for low speed flight, and for short take-off and landing (STOL). A broadly similar approach is used on the (STOVL) version of the F35, but an engine-driven fan is used at the front.
An alternative, which was used on the YAK-36, is to employ auxiliary lift engines to supplement the vectored lift from the main propulsion engines.
Using engine thrust to produce lift directly in this way is extremely inefficient, as it requires in the order of fifteen to twenty times more thrust than is necessary with conventional wing-generated lift. Another problem is, that in the vertical motion, hover and transition stages, the aircraft cannot be stabilised or controlled by normal aerodynamic means. In the case of the Harrier, auxiliary ‘puffer’ jets are mounted on the nose, tail and wing tips to provide stability and control (Fig. 10.21). The aircraft is, therefore, very vulnerable to failures in the propulsion and stability system during vertical flight. In practice, this has not been a major problem. Landing a conventional interceptor aircraft is if anything more hazardous. The disadvantages of direct lift are largely outweighed by the operational advantages of vertical take-off and landing, as was well demonstrated by the Harriers during the Falklands conflict.
