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Old 03-29-2007, 09:03 PM
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khodges
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Default RE: adding right thrust with washers??

If you have the engine firmly screwed to the rails, and move one side, the other will follow. If it's a nylon mount, it will give enough; a metal mount may strain the screws some, though. What really changes is the prop center, if you don't also change the position of the holes in the firewall. Consider: if the mount is centered on the firewall, on the horizontal and vertical centerlines, with no thrust offset, then the prop shaft is right down the middle of the imaginary extension of this center point. If, then, you "jack up" one side of the mount to give some thrust offset (right thrust), the far end of the prop shaft will now sit to the right of center. The more right thrust you dial in, the further the tip of the shaft moves off center (or the prop flange, which is the important part). To counter this, the mount itself must be offset to the left on the firewall. The exact amount is dependent on how far from the firewall the prop flange is. Very roughly speaking, if the flange is five inches from the firewall, the left offset of the mount will be about 3/16 to 1/4 inches for about 2 to 2-1/2 degrees of right thrust.

For a trainer or a plane without a cowl, it really won't matter too much if you don't reset the prop center. But, if you have centered your prop shaft hole and mounted your cowl, and then change the thrust angle, your cowl hole won't match where the prop flange now is. You either have to offset the cowl (which can look kinda funky), or redrill the firewall and move the mount to the left.

Personally, I prefer no right thrust. That's what the rudder is for, IMO, to counter the torque reaction and P-factor. Some planes are more difficult to fly with no thrust offset, so to "tame" it , right thrust is added, even if it was designed without the offset. It won't hurt a thing to put a couple of washers under one side to add some right thrust, there is usually enough play in the screw holes to allow it.