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The Art of the Helicopter John Watkinson - Karatunov.net

The Art of the Helicopter John Watkinson - Karatunov.net

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356 <strong>The</strong> <strong>Art</strong> <strong>of</strong> <strong>the</strong> <strong>Helicopter</strong><br />

Fig. 9.6 <strong>The</strong> flaw in stopped rotor proposals. As rotor speed falls, <strong>the</strong> reverse flow region becomes enormous<br />

and leading and trailing edges interchange.<br />

Tip jet convertiplanes are interesting and educational, but are unacceptably noisy<br />

as production items. More significantly, developments in fixed-wing STOL technology<br />

have delivered simpler solutions in many applications. Stopping <strong>the</strong> rotor in flight is<br />

simply too difficult. Consequently most <strong>of</strong> <strong>the</strong> recent progress made in convertiplanes<br />

has been in machines that tilt <strong>the</strong> rotors in some way to become propellers creating<br />

forward thrust. This is an elegant approach because <strong>the</strong>re are no redundant propellers<br />

in <strong>the</strong> hover and no redundant rotors in cruise.<br />

A prop rotor or tilt rotor machine can take <strong>of</strong>f like a helicopter, but by turning<br />

<strong>the</strong> rotors about a horizontal axis forward flight will be achieved. When <strong>the</strong> rotor axes<br />

are horizontal <strong>the</strong>re will be no rotor lift at all and <strong>the</strong> wing must produce 100% <strong>of</strong> <strong>the</strong><br />

lift. In this mode <strong>the</strong> tilt rotor is an aeroplane with large propellers. Compared to a<br />

helicopter, drag is reduced considerably and <strong>the</strong> top speed and range are significantly<br />

improved. <strong>The</strong> load factor also becomes that <strong>of</strong> an airplane and so <strong>the</strong> tilt rotor should<br />

be highly manoeuvrable.<br />

A rotor that provides enough hover thrust will be somewhat overspecified in forward<br />

flight and will be inefficient because it will work at a low L/D ratio with a high pr<strong>of</strong>ile<br />

drag penalty. <strong>The</strong> solution currently adopted is to make <strong>the</strong> cruise more efficient by<br />

reducing <strong>the</strong> rotor size. This increases <strong>the</strong> disc loading in hover, and drives up <strong>the</strong><br />

downwash velocity and <strong>the</strong> power required, but this power is not used for very long. It<br />

may be an advantage to use different RPM in <strong>the</strong> hover and in forward flight. With <strong>the</strong><br />

rotor always in axial flow, <strong>the</strong> alternating forces experienced by <strong>the</strong> helicopter blade do<br />

not exist and <strong>the</strong> rotor does not need to be detuned against vibration. It is thus quite<br />

easy to change <strong>the</strong> rotor RPM for cruise.<br />

<strong>The</strong> ideal would be a variable diameter rotor and although <strong>the</strong> forces involved are<br />

considerable this is not considered insoluble. With high disc loading and a wing, <strong>the</strong><br />

tilt rotor is not expected to have good autorotation performance and it would appear<br />

inadvisable to rely on a single engine.<br />

<strong>The</strong> Bell-Boeing Osprey (Figure 9.7) tilt rotor is a true helicopter in <strong>the</strong> hover. It has<br />

contra-rotating rotors that have cyclic and collective pitch. <strong>The</strong>re are two engines, one<br />

in each wing tip nacelle. <strong>The</strong> entire engine and transmission tilts with <strong>the</strong> rotor. To give<br />

protection against engine failure <strong>the</strong> two rotors are connected by a cross shaft so that

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