The Art of the Helicopter John Watkinson - Karatunov.net

Fig. 4.23 A fixed-pitch tilting head needs no more than a gimbal and control rods leading to the cockpit.
Rotors in practice 143
translational flight a steady application of cyclic feathering is required to overcome lift
asymmetry.
Early de la Cierva autogyros had aircraft-style control surfaces and no rotor control at
all. The rotor was equipped only with flapping hinges. In forward flight, lift asymmetry
would result in extra lift on the advancing side, but this would precess to an aft tilt of
the disc. Figure 1.10 showed that the thrust of the airscrew overcomes the rearward
component of rotor thrust due to the aft tilt. As the attitude of the hull does not
change when the rotor tilts aft, the mast is now tilted forwards with respect to the tip
path plane. The result is that the rotor has cyclic pitch applied. In an autogyro of this
kind the rotor will flap back until the correct amount of cyclic pitch is obtained for
the airspeed. Whilst the autogyro flies in this way, it should be emphasized that the
helicopter cannot and does not.
In later de la Cierva machines, the aircraft control surfaces were dispensed with and
the pilot could tilt the rotor head for pitch and roll. This approach is used to this day for
control of lightweight fixed-pitch autogyros. Where there is no shaft drive or collective
control, the rotor head can be very simple, as shown in Figure 4.23. The rotor requires
just a teetering hinge and the head is mounted on a gimbal so the pilot can tilt it. The
tilting head can only apply cyclic pitch. Where collective control is required, a feathering
bearing will be needed and so the tilting head has no advantage. Hafner was the first
to build an autogyro with cyclic and collective control and the mechanical solutions he
devised would be used in his subsequent and successful Sycamore helicopter.
4.16Dragging dynamics
In-plane blade motion is variously called ‘lead lag’, ‘lag’ or ‘dragging’. This author
prefers the term dragging because lead lag is cumbersome and lag can be confused with
phase lag; another thing altogether. Section 4.6 illustrated the mechanisms responsible
for dragging. In many rotor designs, dragging hinges or flexible members are provided