Earlier compound helicopters, such as the Bell UH-1, the Lockheed XH-51 and the Sikorsky XH-59A ABC, used jet engines for horizontal thrust. Unfortunately, the jet engine is extremely inefficient for the proposed speeds, of up to 300 knots. This eliminated the jet engine.
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The proposal in Vertiflite consists of converting a Sikorsky Black Hawk to a compound helicopter by adding a large propeller and a wing. No change was to be made to its rotor geometry.
At 200 knots, the wing was to support 70% of the weight of the aircraft. One can assume that at the proposed maximum speed of 320 knots, the wing would be supporting 100% of the aircraft. This means that the craft is now an airplane. As mentioned by CRAN, this 'airplane' must have enough power to overcome the parasitic drag and the parasitic weight of the main rotor and the tail rotor. This will probably eliminate the compound helicopter as a future contender.
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A newer idea is starting to arise, and it will eliminate the 'compound' problem. The new concept consists of slowing down the rotor (or 2 counterrotating main rotors), while at the same time increasing the solidity ratio of the disk(s). This eliminates the high compressibility effects on the advancing side and the stall on the retreating side. The rotor(s), which must be a lot stronger, then act as the wings at high forward speeds.
In addition, the stronger rotors provide other advantages such as improved control response etc.
Last edited by Dave_Jackson; 28th January 2004 at 07:50.