Installing right thrust and whats the measurement - RC Groups
Feb 16, 2012, 11:05 PM
I'm a pilot, 100 yrs too late
Discussion

# 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.
 Feb 16, 2012, 11:18 PM KE your cub. 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
 Feb 16, 2012, 11:42 PM I'm a pilot, 100 yrs too late 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.
 Feb 16, 2012, 11:47 PM KE your cub. 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.
 Feb 17, 2012, 02:39 AM Registered User 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.
Feb 17, 2012, 08:07 AM
ARFs Are Me
Quote:
 Originally Posted by DGrant 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.
 Feb 17, 2012, 10:30 AM Visitor from Reality 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
Feb 17, 2012, 12:17 PM
Registered User
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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Feb 17, 2012, 01:58 PM
An itch?. Scratch build.
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.

### Images

Last edited by eflightray; Feb 17, 2012 at 02:01 PM. Reason: Added picture.
Feb 17, 2012, 08:48 PM
Bellanca Kruesair

# 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?