The Art of the Helicopter John Watkinson - Karatunov.net

350 The Art of the Helicopter
air using only stored energy in the rotor, which would suffice until forward speed had
been attained.
Cierva autogyros achieved jump-start using a delta-one (see section 4.7) hinge in what
was known as an autodynamic head. The inclination of the dragging hinge pin caused
an interaction between the drag angle and the pitch angle. This kept the blades in low
pitch as they dragged back due to engine torque. As soon as the drive was disconnected
the blades would swing forwards and increase their pitch and the machine would jump
into the air.
Raoul Hafner built a jump-take-off gyroplane in which the rotor head had cyclic
control via a swashplate and a collective pitch control. The pilot could control the
jump and also set the RRPM in flight. The machine could land vertically because
the pilot could lower the collective lever to speed up the rotor during the descent and
then raise it again to use the stored energy to arrest the descent just before touchdown.
Hafner’s rotor head was essentially a helicopter head. Hafner went on to design the
Bristol Sycamore, a practical and successful helicopter and the first British rotary wing
aircraft to be given a civil Certificate of Airworthiness.
The upward inflow of the gyroplane suggests a rather different rotor design to that of
the helicopter. The blade twist should be reversed compared to that of the helicopter,
i.e. the root should have less pitch than the tip, although the amount of twist needed is
smaller. In practice, many gyroplanes are built with no twist at all, for economy.
The rotor of the gyroplane is not supplying propulsive thrust and so the loads on
the blades are reduced. Retreating blade stall is a much-reduced problem, especially
if a fixed wing is provided, and as a result many early gyroplanes cruised at speeds
considered high even in modern helicopters, especially those having wings.
Simple gyroplanes have two-bladed underslung teetering heads. If these have fixed
pitch, the two blades can be rigidly fixed together and gimbal mounted. Cyclic pitch
is applied by a single rod from the swashplate. This makes for a very light machine.
However, the combination of low weight with a zero-offset rotor requires more caution
on the part of the pilot, just as it does in the helicopter.
A gyroplane with a teetering head is vulnerable to a combination of low RPM
and low g. Inexperienced pilots may try to dive like a fixed-wing aircraft using forward
stick when they should descend by reducing power and applying aft stick. Forward
cyclic will reduce RRPM. The correct recovery from low RPM/low g is a very gentle
progressive application of rear cyclic. A rapid or panic application of rear cyclic at
low g means that the disc tilts back without the machine following and there may be a
blade strike. There have been some accidents because of this. In contrast a gyroplane
with a hingeless rotor could be remarkably aerobatic.
9.2 The winged helicopter
The winged helicopter is conventional except for the addition of fixed wings (Figure 9.2).
When this is done, the wing produces a proportion of the lift, and so the rotor produces
less lift. However, the wing also produces drag. There will be the usual induced and
profile drag as well as interference drag between the wing and the rotor.
This drag has to be balanced by thrust from the rotor. As a result the rotor tip path
axis has to tilt forward further than it would without the wing. Although the vertical
component of rotor lift is reduced, the horizontal component will be increased. The
span of a practical wing will be less than the rotor diameter and so the wing will be
less efficient than the rotor. Thus the winged helicopter is unlikely to be more efficient