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How much should I offset the engine mount

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Old 02-28-2012, 09:24 PM
  #1
mchandrayan
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Default How much should I offset the engine mount

I have recently purchased a 90-120 size Sukhoi 26 ARF
The firewall has very distinct right thrust built in and the manual calls to offset the engine mount so that the spinner is centered.
Now the problem is the manual does not say how much the offset should be neither are the center line or offset lines marked on the firewall.
Though I have marked the center, I need help in figuring the offset.
Any help would be appreciated
Thanks
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Old 02-29-2012, 07:26 AM
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LesUyeda
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Default RE: How much should I offset the engine mount

Draw a scale top view of the firewall, and the crankshaft, and find the solution graphically.

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Old 02-29-2012, 07:28 AM
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Default RE: How much should I offset the engine mount

If you make a simple square from two pieces of wood, shaped like a letter L. make sure it is straight and the angle is 90 degrees. tape or tack glue it to your firewall ..make the long part of the square come out through the center of your cowl opening. Once it does center up .. the center of that stick at the firewall is the center line to use. the are other ways to do it, but I am guessing from your question that this route might help you.. there are many variables in the problem, but I am guessing if you look at the firewall from the front of the airplane, and mark 1/4 inch to the right of the center.. that will get you close, and will be a good starting point.

One other method I have heard of is to position your fuse pointing straight up, use padding and straps to protect it and hold it secure. put your motor on the mounts or standoffs you will be using, set it on the firewall.. the motor will be pointing straight up. Now install the cowl. Position the motor through spinner hole on the cowl until it is centered, and the motor is positioned like you want it . gently remove cowl, mark holes or outline motor mount.

Not very scientific, but it should get you there.
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Old 02-29-2012, 09:25 AM
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Default RE: How much should I offset the engine mount

Hmm! The wooden square is much simpler and less likely to damage the model or engine (if they crashed down while the setup was in progress).
Thanks will try that out.. I have already marked two lines one offset 1/4 and other 3/16 lets see where do we arrive at using the sqaure method
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Old 02-29-2012, 12:01 PM
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Default RE: How much should I offset the engine mount

I sure wish all ARFS came with plans, it would sure make finishing easier for things like this.
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Old 02-29-2012, 02:48 PM
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Default RE: How much should I offset the engine mount

Make a square piece of wood larger than the mounting lugs for the engine and mark where the bolt holes are on that square. Drill holes where you marked the bolts.

Find the center of that same square and glue up a long dowel to it with a couple of gussets to hold it squarely in place.

While the glue is drying, take a pencil and tape it to another dowel so you can use it to mark the firewall through the holes you have drilled.

Now, hold that dowel against the firewall while putting the cowl in place. Move the dowel around until you get it centered in the cowl opening and take the pencil and mark the firewall where the bolts need to go.

Drill up the firewall and off you go. Spinner will be centered.

Hope that all makes sense. It's really pretty easy and you can use that template over and over for the same engine or bolt pattern.
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Old 03-01-2012, 08:40 AM
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Default RE: How much should I offset the engine mount

Install the engine to the motor mount. Measure from the spinner back plate to the back of the engine mount. Multiply this length by the sine of the offset angle, probably two or three degrees. Offset the bolt pattern on the firewall this distance from the center line. Works for me every time.

"Do the math"
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