This video shows the operation of a very simple mechanism with which to achieve collective control.
The design in question consists of a modification of a flexplate/deltaplate used in model autogyros. The flexplate rotor consists of a flexible fiberglass strap with an extension on each side, near to where each of the blades attach to. These extensions are connected to ball and socket linkages which connect to the main gear of the rotor. As these linkages are angled relative to the rotor shaft, and the rotor itself is allowed to rotate relative to the rotor shaft, torque applied to the gear induces a change in collective pitch.
As a consequence of this mechanism, rotor collective becomes a function of motor torque. This means that as more throttle is applied, collective pitch increases.
There are two main theoretical advantages to this system over an equally simple fixed pitch rotor:
1) Height changes can be done faster as collective increases instantaneously with throttle pulses. The rotor does not have to accelerate to a higher speed for lift to increase. Rather, rotors with large inertia will only accentuate collective changes due to the greater torque required to accelerate them.
2) The rotor can be allowed to auto-rotate. Blade pitch can be adjusted such that when no motor torque is present, the rotor enters a collective setting which allows efficient auto-rotation. A curious benefit of this is that when power is lost, auto-rotation is entered instantly without delay.
