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Old Feb 16, 2012, 10:05 PM
I'm a pilot... 100 yrs to late
Thermalin's Avatar
USA, FL, Palm Harbor
Joined Jan 2005
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Installing right thrust and whats the measurement

What's the best way to create 2.5 degress right thrust on a firewall.. By sanding the angle into the back of the motor mount or using a shim on the fireweall. Firewall is already in place per instructions / plan.

Also how would I know what a 2.5 degree offset would be in terms of the distance offcenter of the shaft?

I know the formula has to do with the radius of the circle. For example a 2 inch circle will be quite different from a 2 foot circle where the 2.5 degrees would be a much larger linear measurement. My radius would be the distance from firewall to propeller.
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Old Feb 16, 2012, 10:18 PM
KE your cub.
Curare's Avatar
in the gutter, again....
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With a pre- positioned firewall, I would be inclined to shim. Also, you say 2.5degrees, this is only a starting measurement, you may find you need more, or less, so if you sand the mount you wont get back what you took off.

as for the offset, ask a high-school kid to work it out.

It's simple trig
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Old Feb 16, 2012, 10:42 PM
I'm a pilot... 100 yrs to late
Thermalin's Avatar
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Good point on the starting point! Shim it is.
Yeah, I've foud some formulas but it's a pain in the arse. I just realized I have several plans that show right and down thrust and degress so I'll find one and measure that.
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Old Feb 16, 2012, 10:47 PM
KE your cub.
Curare's Avatar
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if you give me the distance from the mount to the thrust washer I can knock out a mud map for you.

If you give me the width of the mount I can give you the width of the shim too.
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Old Feb 17, 2012, 01:39 AM
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Just put a few washers behind your mount.... with a few good washers.. you can easily change all the angles for right-thrust or down-thrust... and if you want to adjust it.. guess what?.. just add or subtract a few washers.. once you find your angles you might want something permanent..but washers work fine. .. I've actually found it to be that simple.
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Old Feb 17, 2012, 07:07 AM
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Quote:
Originally Posted by DGrant View Post
Just put a few washers behind your mount.... with a few good washers.. you can easily change all the angles for right-thrust or down-thrust... and if you want to adjust it.. guess what?.. just add or subtract a few washers.. once you find your angles you might want something permanent..but washers work fine. .. I've actually found it to be that simple.
Not that I use right/down thrust that often, but when I do, washers provide the most simple and easy solution.
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Old Feb 17, 2012, 09:30 AM
Visitor from Reality
United States, VA, Arlington
Joined Dec 1996
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My custom design/build service puts in estimated sidethrust by working out the angle needed for the firewall on the initial plan, then figuring the motor mount offset position to put the prop rear face centre on the model centreline.

But any adjustments are done by me with washers, which then puts the prop centre off the model centre a little.

Which doesn't ever seem to bother the model all that much.

I'd vote for a small drawer with a plentiful supply of washers. Thick and thin per diameter are good if you're real fussy

D
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Old Feb 17, 2012, 11:17 AM
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Forget equations, Use a simple drawing with your known values to figure out the offset. Draw a T exactly 90deg. set up a protractor at 2.5deg offset of the 90deg. using your known distance from firewall move the protractor left until the arm crosses your measurement on the line. there is your offset. simple and without math. I tried to show on pics but I stink with "MS Paint" Thought that counts, right?
And definitely use shims, that way there is no issues later when you need to readjust. It seems most mounts these days have slotted holes to move the mount around a little bit to adjust this very thing.
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Old Feb 17, 2012, 12:58 PM
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eflightray's Avatar
South Wales U.K.
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I'll throw another consideration in.

Does it have a cowl and spinner ?, if so, be careful or you could end up with the cowl angled as well.

If you angle the motor from the planes center line, placing the rear of the motor on the center and then angling it, the spinner can end up away from the center line enough to look a little odd.
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Last edited by eflightray; Feb 17, 2012 at 01:01 PM. Reason: Added picture.
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Old Feb 17, 2012, 07:48 PM
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United States, MD, Elkton
Joined Oct 2011
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toe-in

off-set thrust is like toe-in in your car ....you can drive it with it messed up,but it drives better when it's correct.
Hangar flying brings this up occasionally,and the correct right thrust is acheived by trial and error....down thrust also..
in my opinion,some is better than none,so I start out with measuring from prop tip to rudder hinge line.Locate the prop horizontally,measure from each tip to the rudder hinge line ,and be sure there's less distance on the right side...On 18" props,I start out with about 3/8" total difference,and adjust from there.
Cruise in level flight ,full power,pitch straight up,shouldn't need rudder to go on a line vertical as far as it will...
'Down' thrust depends on where the wing is located.High wings like down thrust,mid-wing may not need any,some low-wing high drag aircraft may need some 'up-thrust' to change speeds without trim changes.
See why it needs to be easily adjustable?
Go play with your airplane.
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Old Feb 17, 2012, 08:05 PM
I'm a pilot... 100 yrs to late
Thermalin's Avatar
USA, FL, Palm Harbor
Joined Jan 2005
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Thanks for all the replies... thin washers is a great idea until I get it right. I came up with 2.5 degrress based in a previous post in this thread where the builder of the same model suggested 2 to 3 degrees right thrust. My concern is I didn't want any surprises on the first takeoff and was wanting to be sure I start initially close to that 2.5 degree mark.

I always offset the motor on the firewall so that the hub is centered in the cowl. I don't like how it looks off center though it wont matter much.

I'll do the protractor idea for the angle and the washers for the initial setup. Curare.. I appreciate the offer.

No1IsNormal....
This may sound stupid, but with the angle defined, I'm trying to understand how to use it. The left angled line is where the "rear" center of the motor would sit with the prophub on at the intersecion farther out?

Mike.
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Last edited by Thermalin; Feb 17, 2012 at 08:28 PM.
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