Engine Offset? - RCCanada - Canada Radio Controlled Hobby Forum
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Old 08-29-2012, 10:52 AM   #1
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Engine Offset?

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OK Guys;

Here’s a question for the Giant Scale enthusiasts…

I know about adjusting the engine angle to compensate for thrust by adding a few washers on the engine mount etc…


The other day I was looking at a review for a 50cc AC and the fellow mentioned a ‘Engine Offset Calculator’ – which (when the math was done) told you how to move the engine over a little bit to accommodate the thrust/torque without ‘Tilting’ the engine.

So… is this ‘Required’? Are current manufacturers building in the thrust angle? or What is the situation??

The Link to the Calculator;


The Math

Engine offset

The formula for measuring the engine offset is: Engine Offset = (engine length) * sin(thrust angle)

Here is an example for the DA 100. The length of the engine from the back of the mounting plate to the back of the spinner is 6.5". If I want to have 2.5 degrees of thrust built in then: Offset = 6.5" * sin(2.5) = 0.2835". This is approximately 9/32. Which means that to have 2.5 degrees of right thrust, you will need to move the centerline of the engine 9/32 to the RIGHT (when looking at the firewall) for mounting so that the center of the prop is in line with the center of the firewall (or the manufacturer stated centerline).

Thrust angle

How do you find out how many washers you need, or how much longer to make the left spacers versus the right spacer to get the right thrust you want? In other words, if the firewall has no thrust built in, then you will need space out the left mounting holes of the engine more than the right to put in right thrust. The equation to do this is: Extra Length = (distance between mounting holes) * tan(thrust angle)
To give an example, the distance between the mounting holes on the DA 100 is 2.75". If you want a 2.5 degree right thrust angle, then: Extra Length = 2.75" * tan(2.5) = 0.120". The left side of the engine will need to be extended 0.120" farther than the right to have a thrust angle of 2.5 degrees.

The No Math Method

The simplest and easiest way to come up with offsets-no math involved.

First come up with your required angulation, lets say 2 degrees. Take a large sheet of paper draw a long straight line. On the left of the paper make a cross hatch marking your starting or measuring point. Take a protractor and line up the cross hatch with the mark on the protractor and mark your 2 degree angle. Now draw a long diverging line at a 2 degree angle from your original line starting at your cross hatch mark. Now say your spinner is 10" from your firewall. Measure 10" from your cross hatch and measure the width of the 2 lines at that point. That dimension now becomes your engine offset dimension. If your engine mount itself is say 4", measure from your crosshatch out 4" and measure the width of the 2 lines at that point and that becomes the distance you need to tilt your motor mount.

Simple enough?
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Old 08-30-2012, 02:24 AM   #2
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Re: Engine Offset?

Whatever works for you, I can see where the site could be very useful thank you for posting! Personally I've had SOH CAH TOA drilled into my head hard enough that they may be my last words.

At any rate most of the ARFs I've purchased in recent years not only had the thrust angles built in, but also offset crosshairs on the firewall already marked to center the engine on based on the recommended firewall to prop washer distance.

Last edited by Wheels_78; 08-30-2012 at 02:45 AM.
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