# I Rubber band plane physics

1. Jun 7, 2017

### zerow333

Hi all, I am working on a rubber band plane (something like this: http://www.instructables.com/id/Rubber-Band-Powered-Aeroplane/) and I got a question regarding to the physics on these plane. As there is only one propeller and one rubber band, when the rubber band untwist, why doesn’t it rotate the aircraft in the opposite direction of the propeller??? Is it because of the moment of inertia of the plane is much larger than the propeller so the resistant is much greater, hence only a minimal rotation that is not really visible to our eyes? But then why real plane need to counteract this torque but these rubber band plane don’t?

Many thanks in advance

2. Jun 7, 2017

### scottdave

If the plane is perfectly symmetrical, you should see the body of the plane rotate in the opposite direction of the propeller rotation. As you suspected, it will be less, because of the greater moment of inertia. The wings could be adjusted so that one side deflects the air a little more than the other side, which would create a torque, pushing against ts tendency to rotate. For a single engine aircraft, that is my guess as to the way it is done. With an even number of engines, half of the engines could run clockwise, and the other half run counterclockwise.

3. Jun 7, 2017

### rcgldr

The moment of inertia of the wings helps somewhat, but there's still some roll response. The roll response is taken care of by dihedral in the wings, that produce a counter torque when the plane's wings are not horizontal (or whenever there's a crosswind component). The combination could result in a mild turn, but weather vane effect from the vertical stabilizer would reduce the turn. The spiraling component of propwash on the wings and vertical stabilizer also reduce roll response (there may be a yaw response to high power at low speed).

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