The Art of the Helicopter John Watkinson - Karatunov.net

Other types of rotorcraft 349
De la Cierva’s free flying models were successful but they had fixed surfaces with
no controls. As a result the problem was only revealed on the full-size prototypes,
several of which were destroyed before flapping hinges were adopted. Blades mounted
on flapping hinges cannot transfer rotor moments to the hull. With hinges fitted, the
application of the conventional elevator caused the hull correctly to pitch with respect
to the rotor. As was shown in Figure 4.22 this would cause the control axis to deviate
from the tip path axis, resulting in an application of cyclic pitch to the rotor causing it
to follow the attitude of the hull. The machine would no longer roll over when elevator
controls were applied, and it would roll when the ailerons were used.
However, even if flapping hinges are fitted, this is still not enough. In translational
flight, a flapping rotor will suffer from flapback due to asymmetry of lift, and will
also suffer inflow and coning roll. The tip path axis will be aft of and to one side of
the hub axis. The flapback is countered by propeller thrust, whereas the inflow and
coning roll must be countered by steady application of opposing aileron in order to
fly straight. Flapping hinges alone could not and did not control inflow and coning
roll. De la Cierva soon discovered that the gyroplane is capable of very slow flight
where conventional aircraft-type ailerons and elevators are ineffective. These were soon
abandoned in favour of the tilting hub method of cyclic control described in section 4.15.
In a fixed pitch gyroplane application the tilting hub allows a fairly simple mechanical
system. The rotor shaft is gimbal mounted and the blades have flapping hinges (and
thus need dragging hinges and dragging dampers). Tilting the hub is possible because
of the flapping hinges and this causes a cyclic pitch application. In stable forward flight
the control axis (which is the hub axis) will be forward of the tip path axis because of
flapback, and to one side because of inflow/coning roll. In other words the hub must
be tilted with respect to the tip path axis.
The reader is cautioned that many of the popular explanations of blade flapping
in gyroplanes are hopelessly flawed. The most common flaw is the assertion that the
flapping hinges were essential to the gyroplane and by association to the helicopter. This
is nonsense and the origin of it is easy to see. Many technical treatises on the gyroplane
and the helicopter, for example the classic work of Gessow and Myers, analyse with
respect to the control axis. To enable straight and level flight the control axis must
be tilted and of course all rotors, hinged or hingeless, flap with respect to the control
axis. The same erroneous account is seen in any number of general readership books.
It seems that someone somewhere confused the tip path axis and the control axis and
wrote an incorrect account that subsequent authors simply repeated without question.
The cyclic control requirements of the helicopter and the gyroplane are identical. As
was seen in Chapter 4, the tilting hub with flapping hinges gives exactly the same
cyclic control as a hingeless head with feathering hinges and a swashplate. Gyroplanes
have been built with no flapping hinges and helicopters have been built with
tilting hubs.
In early gyroplanes the blades were started by pulling a rope wound round a drum
and brought up to speed by taxiing. Later machines drove the rotor from the engine to
start it, using the ground contact to oppose torque reaction. The drive was disengaged
before flight. The simple gyroplane takes off in the same way as an aircraft by accelerating
along a runway, although for a much shorter distance. It can lift off as soon
as the blades are turning fast enough. Using a flare, a gyroplane could lose most of its
forward speed and need only a very short landing run. This allowed it to land in places
from which it could not take off again. The solution was the so-called jump-take-off
gyroplane. This would use the engine to drive the rotor on the ground in fine pitch
until its RPM was somewhat in excess of that required for flight. By disconnecting the
engine from the rotor and applying a higher pitch, the machine would jump into the