The Art of the Helicopter John Watkinson - Karatunov.net

Other types of rotorcraft 353
increased rotor thrust would initiate the acceleration, but the g-sensor would add wing
camber by lowering the flaps so that the wing lift would increase in proportion to
the rotor thrust. Whilst the 347 was very manoeuvrable indeed, all of the necessary
components added to the weight and complexity.
If wings are successfully to be applied to helicopters, they need to be designed in
at the outset. If the wing structure can also do duty carrying fuel and/or supporting
the undercarriage, the extra weight is reduced. Most helicopters have relatively poor
hull shapes and finish and the hull drag is high. If an improved hull design is used, in
which the usual external accessories are flush mounted, and a retracting undercarriage
is employed, the drag of the hull can be reduced by an amount that makes a significant
contribution to reducing the extra thrust needed by the wing.
In some machines the undercarriage retracts into sponsons outside the hull in order
to avoid encroachment into the hull space. Such structures may also be used for fuel.
Although not intended to be wings, it is quite common to give these an airfoil section
to obtain at least some lift in return for the drag.
9.3 The compound helicopter
The compound helicopter, or gyrodyne, is one in which the rotor does not produce any
forward thrust in cruise. Instead the thrust is provided by other means. Experiments
were performed using turbojets, but as might be expected from momentum theory,
these were a very inefficient way of producing thrust at helicopter speeds. Conventional
propellers give much better results.
In gyrodynes the pitch of the thrust propeller is increased as airspeed increases so that
it continues to provide forward thrust. In cruise, the cyclic control of the main rotor is
used to trim the control axis so that there is sensibly no flapping and the tip path axis
remains at right angles to the direction of flight. This has a number of advantages. The
thrust required from the rotor is reduced as it is only carrying the weight of the machine
and not overcoming drag as well. This may be reduced further by the use of stub wings.
Rotor power is reduced and this reduces the weight of the transmission. However, the
main advantage is aerodynamic. As the rotor remains edge-on to the airflow, there is
no need to increase the blade pitch to account for inflow as speed rises. This minimizes
the effect of retreating blade stall and reduces vibration, allowing a higher airspeed to
be reached.
The objections to the use of a wing in an otherwise conventional helicopter mostly
disappear in the compound helicopter and are replaced by advantages. The thrust to
overcome the drag of the wing does not come from the rotor, but instead is supplied
by the propeller. The benefit of a higher load factor is obtained along with potential
for higher speed. The winged helicopter has difficulty if the engine fails at high forward
speed because the wing lift prevents the rotor obtaining upflow. However, in the
gyrodyne, following an engine failure at speed the propeller can be set to the windmill
brake state and used to create shaft power by slowing the machine down. This power
will maintain the rotor speed until the forward speed drops sufficiently to lose wing lift.
In the Fairey Gyrodyne shown in Figure 1.11 the anti-torque tail rotor is replaced by
a forward-facing variable-pitch propeller mounted on a short wing at one side of the
main rotor. In the hover the anti-torque propeller pulls forwards and the main rotor has
to be tilted backwards using the cyclic control to prevent forward drift. The Gyrodyne
captured the world speed record in 1948, but development was halted following a fatal
fatigue failure having nothing to do with the compound concept.