I have a fan in my window for use when air conditioning does not seem necessary. It is in a square case. When I first started using it, I noticed that it slowly moved to the left. I assumed that the window sill was not level and vibration aided by gravity caused the movement. I put a book on the sill left of the fan to keep the fan centered. Today, I turned the fan around causing it to blow air out instead of in. Now the fan moved slowy to the right, requiring me to move the book to the right side of the case. What is the mechanism causing the above effect? I suppose the rotating fan causes a slight opposite torque on the case. This would cause uneven downward force on the sill at opposite ends of the case. The mechanism would seem to be due to this uneven force, but my intuition fails me. If I did not see it happen, I would not have figured it out from some principles of mechanics & gravity.
You are correct in your assumptions and you can find more about it by looking into "Vibratory Locomotion" http://jnaudin.free.fr/html/abssldrv.htm
Cool... before looking at Mac's link, I'd guess that there must be some slight imbalance in the motor/fan. As the heavy side is coming down, the fan is effectively lighter, and more prone to moving. So any sideways force would have more effect on moving the fan at that time. The sideways force must be that of the heavy side moving laterally, I think... but there is lateral movement both at the top of the cycle and the bottom. Assuming a slight delay (some second order effect?) in the response of the fan to the vertical movement of the heavy side of the fan, the lateral movement of the heavy side at the bottom of the cycle will have a greater effect. Anyway, I'm guessing that the fan rotates anticlockwise as its blowing air toward you.
When a vibratory imbalance is vectored downward it increases the normal force, hence friction causing cesation or reduction of sliding motion. When the imbalance is upward it reduces the load and hence friction. The movement of the fan in this case is likely promoted by the torque between the rotating fan load and the case restraining it such that the fan doesn't merely oscillate back and forth but takes on a direction opposite the side restraining the torque, which will have a slightly greater normal force than the general force of gravity. That appears to be physically correct but I'll note that it is opposite most commercial fan operations. It will be interesting to see if he can tell us the direction of rotation. There may be someother affect not being accounted for.
my guess would be a combination of two things, an imbalanced fade blade and an out of square fan box. the imbalanced blade would cause the box to move up and down while the wind would turn the box, the box could also be twisted which would cause the same effect, as the imbalanced blade moves it alternatly raises and lowers the box the twist in the box provides the turning force
It wasn't merely turning but moving laterally (sideways). An imbalanced washing machine will also walk out of the laundry during the spin cycle.
When I face the fan, it is spinning counterclockwise & blowing out. In this case the case moves to the right. When I reverse the direction (clockwise spin as I face it fan blowing into the room), the case moves to the left. The effect is not strong. When the fan is blowing in, there is a slight force pushing the case against the window framing. The leftward force must overcome friction between the case and the window frame. This almost prevents motion to the left, which gets faster if I move the fan away grom the window frame. The rotation of the fan causes a torque on the case in the direction opposite to the fan rotation. This torque results in a force counter to gravity on one side of the case and in the same direction as gravity on the other side. The case moves toward the side with the increased gravitational force. I would have guessed that it would move in the other direction.
Correct. As I was afraid the rotation is conventional and from your description the normal explanation for its motion is inadequate, somethingelse is involved. I would suspect that it is some sort of resonate frequency such that there is a cyclic harmonic vibration timed to provide more thrust in one direction from the imbalance. To eliminate some form of rachet affect on the underside of the case to the window sill you might try setting the fan on a doubled piece of plastic such as a trash bag. If it stops the motion then it is being caused mechanically by a differential traction of the frame to the sill.
