The Art of the Helicopter John Watkinson - Karatunov.net

354 The Art of the Helicopter
Fig. 9.5 With a propeller a gyrodyne can change its hover attitude using cyclic to balance prop thrust.
The Gazda Helicospeeder had a tail rotor mounted on a swivel. In the hover this
would be oriented conventionally, but in forward flight the pilot could turn the tail
rotor to point backwards and act as a pusher propellor. This machine still exists and
has been restored by Stanley Hiller. Sikorsky also tried a similar device, which they
called a Rotoprop, on a modified S-61.
In the Lockheed Cheyenne (Figure 1.12) a tail rotor and a rear-mounted pusher
propeller are both provided. The tail rotor is on the end of the tail plane to clear the
pusher. The propeller pitch can be controlled by a twist grip on the collective lever.
In the hover the pusher propeller is typically operated in flat pitch and the tail rotor
counteracts the torque as normal. However, the pusher prop can be used as a hover
attitude control. Figure 9.5 shows that with prop thrust balanced by fore or aft cyclic
the machine can hover nose up or nose down.
Accelerating into forward flight is not done using the propeller as it is far more
effective to tilt the rotor disc as per pure helicopter practice. However, as speed builds
up, the disc attitude is levelled and more power is fed to the prop. As the Cheyenne
has a hingeless rotor it can provide sufficient control power even when it is off-loaded
by the wing and no other controls are necessary. The Cheyenne was probably the
highest performance helicopter ever built. It could do 215 knots in level flight, turn
continuously at 60 ◦ bank angle, could pull 2.6 g and remain stable in negative g. The fact
that the Cheyenne didn’t enter production again had nothing to do with the compound
concept.
In the Piasecki Pathfinder the rear prop is mounted in a ring duct equipped with
deflectable vanes to provide anti-torque thrust for hovering. Whilst adequate for civil
operations it is unlikely that such an arrangement would meet military crosswind hover
requirements. The Pathfinder’s ring duct also has adjustable horizontal vanes. In cruise
these can be trimmed so that the hull is absolutely level for minimum drag even if the
CM is not aligned with the rotor shaft. The rotor is then providing axial thrust only
and so alternating loads on the mast are minimized.
The compound helicopter offers a tangible way of improving the performance of the
helicopter, particularly in the area of forward speed. Compound helicopters have not yet
reached the market, but this is not because of any fundamental performance problem.
If there is a rotary wing technology deserving to be revisited with modern materials