# Is it possible to control an RC plane using reaction wheels?

I recently had an idea involving reaction wheels and a model airplane. Would it be possible to control an RC plane using reaction wheels rather that servos and control surfaces? Would a reaction wheel controlled plane be able to function similarly to a "normal" model airplane and would this be practical at all?

Yes, it would!

A YouTuber, Thomas Stanton, has actually built a helicopter controlled with reaction wheels in this video!

However, for a fixed wing aircraft, you would also have to consider the location of the reaction wheels relative to the centre of mass, and you would most likely want to house the reaction wheels internally to avoid adding extra drag.

You would also have to have a reaction wheel for each axis.

However, this should be relatively simple, so the answer to your question is yes, it is possible.

In answer to your other question of practicality, as far as I am aware there have not been enough studies in the RC sector to say for certain, but my personal opinion is no, it would not be practical due to the added mass and the problem of weight distribution, plus the limited space inside an RC aircraft and the advantageous torque traditional control surfaces give.

• I think that you wouldn't actually need to consider the location with reference to center of mass because reaction wheels generate a torque, not a force. May 14 '20 at 13:59
• @LucaScheuer that’s true, however just like your battery placement, you would want to make sure that the placement of the reaction wheels doesn’t shift the centre of mass. For example, if you were to mount it in the nose cone, it would still create a moment about the centre of mass due to the weight acting on it, so may make the aircraft too nose heavy May 14 '20 at 14:04
• @LucaScheuer, if you're flying a rigid body, that's true, but real-world aircraft tend to flex a bit. Putting the reaction wheels close to the center of mass will minimize that flex.
– Mark
Jan 7 at 1:45

## Maybe.

But it would be very challenging in practice. The kinematics of a fixed-wing airplane require it to bank in order to maintain coordinated flight. This is going to cause gyroscopic precession, which will result in time-varying flight characteristics. In particular, those flight characteristics will depend on the maneuvers which you did earlier.

### Explanation

If there were a large reaction wheel, sufficient to alter the plane's attitude, then there would be an associated large moment of inertia for the reaction wheel. This makes intuitive sense, since if the reaction wheel is going to twist hard on the plane, the plane is going to twist hard on the reaction wheel. The driving equation is $$T=I \alpha_{wheel}$$.

The actual rotational inertia would be a function of rotational velocity, $$J=I \omega_{wheel}$$. So as we accelerate the reaction wheel, we build inertia.

Now, we have an object spinning about the plane's z-axis, but we want to bank about it's x-axis. That's going to cause some gyroscopic precession torques, with the magnitude depending on the reaction wheel's current speed, $$`T_{precession} = J \omega_{roll}$$, and with the direction depending on the sign of the bank and reaction wheel spin direction. These torques appear in the pitch axis.

So in other words, the way a plane pitches as a response to a bank input will vary in function of how fast the wheel is spinning, as well as which direction the wheel is spinning. If the plane just completed a long 360 degree turn to the left and then wants to bank right, we can be certain that there will be a very different response from if the plane wants to bank right after a 5 degree right turn.

One possible solution to this problem is to use a reaction wheel to reduce the size of the control surfaces. The reaction wheel could give short, fast control responses with the control surfaces giving slow, long control responses.

I am not aware of anyone who has tried this. It might be best on a flying wing of some kind because of the large body which could house a large wheel. Conveniently, flying wings have particular issues with yaw performance so it might be workable in practice.