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adding right thrust with washers??

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adding right thrust with washers??

Old 03-29-2007, 06:06 PM
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Default adding right thrust with washers??

I have read several times of putting washers on the left side of the engine mount to gain right thrust. My question is,
when you shim one rail of the mount and "kick it out" a degree or so, then that side of the mount is no longer at a 90 degree angle to the firewall but the other side is. This would seem to twist or distort the engine mount. In other words, the 2 engine mount rails are forced into 2 different angles. How well does this adjustment work?

Bill ..............>>>>>>geaRUP>>>>>>>>>>>>>>>>>>>>>>>> >>>>

Oh, by the way , I have always had the thrust angles built into the firewall, Im just curious as to why someone would use the washer method.
Old 03-29-2007, 06:11 PM
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Default RE: adding right thrust with washers??

My Goldberg Cub sets the thrust angle this way, at first I thought it was a little iffy , but it has worked out really well.
Old 03-29-2007, 09:03 PM
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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.
Old 03-29-2007, 10:26 PM
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Default RE: adding right thrust with washers??

To put it in a nutshell, It works.
Old 03-30-2007, 03:02 PM
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Default RE: adding right thrust with washers??

Washers work on a solid firewall. I make my shims out of paper. Glue stick 20 or more pieces together with about 1/8" stagger. Trace your mount onto the paper stack. Cut it out and soak the edges with ca.....perfect wedge.

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