Sunday, March 18, 2012

Vertical/short take-off and landing - INTRODUCTION


Fig. 18-1 Michel Wibault's ground attack gyropter (concept) 1956
1. Vertical take-off and landing (VTOL) or short take-off and landing (STOL) are desirable character- istics for any type of aircraft, provided that the normal flight  performance  characteristics,  including payload/range, are not unreasonably impaired. Until the introduction of the gas turbine engine, with its high power/weight ratio, the only powered lift system capable of VTOL was the low disc loading rotor, ason the helicopter.
2. Early in 1941, the late Dr A. A. Griffiths, the then Chief Scientist at Rolls-Royce, envisaged the use of the jet engine as a powered lift system. However, it  was not until 1947 that a light weight jet engine, designed  by  Rolls-Royce  for  missile  propulsion, existed and had a high enough thrust/weight ratio for the first pure lift-jet engine to be developed from it.

3. In 1956 the Bristol  Aero-Engine Company was approached  by  Monsieur  Michel  Wibault  with  a proposal to use a turbo-shaft engine and a reduction gearbox to drive four centrifugal compressors which would be situated two on each side of the aircraft. The casing of these compressors could be rotated to change direction of the thrust (fig. 18-1). The concept incorporated two original ideas i.e. the ability to deflect the thrust over the complete range of angles from the position for normal flight to that for vertical lift and a system where the resultant thrust always acted near to the centre of gravity of the aircraft.

Fig. 18-2 Lift/Propulsion engine.
4. The  principle  proposed  by  M.  Wibault  was developed by using a pure jet engine with a free power  turbine  to  drive  an  axial  flow  fan  which exhausted into a pair of swivelling nozzles, one on each side of the aircraft. A further development was to use the fan to supercharge the engine, exhausting the by-pass air through one pair of swivelling nozzles and adding a second pair of swivelling nozzles to the exhaust system from the engine turbine. In this way the  first  ducted  fan  lift/propulsion  engine  (the Pegasus) evolved (fig. 18-2).

5. Subsequent experience with the Pegasus engine in the Harrier V/STOL fighter aircraft (fig. 18-3), lead to the development of the short take-off and vertical landing (STOVL) operational technique. In this way the additional lift generated by the aircraft wing, even after a short take-off run, provided a large increase in the payload/range capability of the aircraft compared to  a  pure  vertical  take-off.  Vertical  landing  had several operational advantages compared to a short  landing and so was maintained.

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