# Can a single-bladed rotor with cyclically varying torque emulate a swashplate?

Instead of a swash-plate cyclically varying the pitch of the rotor blade(s), could a similar effect be generated by cyclically varying the thrust of the motor on a fixed-pitch single-blade rotor? The thrust would be at a maximum at one angle of the rotor, and at a minimum at 180 degrees from the first angle. The blade would still have some inertia at minimum thrust, but ideally the thrust from the motor would be a larger factor.

## 1 Answer

Probably not as you describe. You would need to vary the speed of the rotor throughout the revolution. You might need to double or halve the speed for part of a rev -- that much acceleration would take tremendous torque and does not seem practical.

However, there is some research, papers & another, and patents out of U Penn about a 'Pulsed Cyclic' idea.

The rotor blade has a 45-degree hinge such that pitch is coupled to lag. When the rotor is pulsed (bump in torque), the blade lags, and the blade pitch increases. Pulsing the blade as it rotates gives them cyclic control.

All of their work that I've seen works with two-bladed rotors. However, I suspect you could also apply it to a one-bladed rotor if you wanted.

• Thanks for the response Rob. That UAV from UPenn was actually what got me thinking about other ways of controlling thrust cyclically. With a small enough rotor (say a 40mm prop), would doubling/halving the speed really be too much torque? The UPenn rotor seems to change speed from 180 to 220 rad/s within one cycle on a relatively large prop. Commented Feb 26 at 21:26
• Lets assume you can get away with a 1/rev speed change -- max speed at some angle and minimum speed 180 degrees later. The longer you need to hold those max speeds, the greater the acceleration needed. For now, lets assume you can get away with sinusoidal velocity. $\omega=3+sin(\theta)$ has this shape and an RMS of just over 3 (3.0822 rad/sec). Its derivative is $\dot{\omega}=cos(\theta)$. Which has peaks at +-1 rad/sec^2. If you know the blade inertia, you could calculate the torque required ignoring aerodynamics. Commented Feb 26 at 22:05
• In practice, I think you would want a more-square wave shape to get good control authority. Commented Feb 26 at 22:05