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Why is Engine Mount Set off Center

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Why is Engine Mount Set off Center

Old 09-10-2008, 02:55 PM
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chevisn7
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Default Why is Engine Mount Set off Center

I have two Extra 300s ARF planes I am building. One for myself and one for my son. Both by a different manufacturer but identical in every other way. One has the engine mount built with a 1/4 inch off center angle to the left as you would face the plane. It is very obevous this was intended as one side of the engine mount is longer than the other. When I mount the engine the end of the prop shaft is 1/4 inch off center. The other plane has the engine mount srtaight and level from every angle. Whats the deal? Why is one set at an angle? Is this angle needed and for what reason? It looks odd when the cowling is installed like the prop is crooked and installed incorrectly. Thanks in advance for any help you can provide.
ChevisN7
Old 09-10-2008, 03:17 PM
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TedMo
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Default RE: Why is Engine Mount Set off Center

Often a design will have right thrust incorporated in it to compensate for the torque of engine. In your case one manufacturer decided it would benefit by it the other didn't. I would prefer to have it not there until I fly and determine if it needs it. I can always change by shimming the engine if needed. Should be interesting to find out which of the 2 planes flies best.
Old 09-10-2008, 04:29 PM
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JohnW
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Default RE: Why is Engine Mount Set off Center

Yeah, compensating for the right thrust angle. There is a yaw effect seen single prop planes due to p-factor and asymmetrical flow. The right thrust is designed to help offset this yaw. It is not possible to ever get the right thrust “perfect” as the yaw effect varies with speed and throttle. So what is generally done is the right thrust is set for cruise speed. Basically every plane needs some right thrust for more honest tracking, the amount varies, but it is typically a couple of degrees.

Because of the right thrust angle the engine is set at, the motor mount will need to be attached to the firewall off center to compensate such that where the crank meets the prop is back at center. This would mean the mount is shifted to the left on the firewall (looking from behind as if you were in the cockpit.)
Old 09-10-2008, 06:18 PM
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Gray Beard
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Default RE: Why is Engine Mount Set off Center

John brought it up at the end there but to remind you, when setting up A plane always think of yourself sitting in the plane, that way we all know what is right and left and speak the same language.
Old 09-10-2008, 09:58 PM
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chevisn7
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Default RE: Why is Engine Mount Set off Center

Thanks for the info. I moved the engine per your suggestion a little off center to bring the prop back in line with the center of the cowling. It still looks very odd to see the spinner sitting at such an angle to the front of the cowling. Question: If I straighten out the engine could any thrust requirement be compensated for by adding a little rudder. I am very picky about how things look and this engine sitting at such an obvious angle is driving me nutty. The 1/4 inch angle built into the engine mount is placing the prop shaft at a 5 degree angle to the front of the cowling. Ie; one side of the spinner is sitting 1/8 of an inch from the cowling and the other side is a 1/2 inch away from the cowling.
Old 09-10-2008, 11:09 PM
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JohnW
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Default RE: Why is Engine Mount Set off Center

5 degrees is quite a bit. Are you sure that is correct? It may be correct, but you might double check that. Yes, you can fudge a bit of right thrust via rudder trim and a throttle->rudder mix, but it isn't perfect and this technique is generaly only used by precision aerobatic pilots to tweak a plane that already has right thrust. If the look bothers you, try rotating the cowl such that the spinned back plate lines up OK with the cowl. This of course means the whole cowl is now at the 5 degress too. If you ever look very closely at some planes, most notably pattern ships and large (35%+) 3D ships, you will see that often the cowls were designed to "hide" the engine angle with a asymetrical cowl. It may look odd, but this is what is required to get the planes to fly straight without unwanted yaw.
Old 09-11-2008, 06:29 AM
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da Rock
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Default RE: Why is Engine Mount Set off Center


ORIGINAL: chevisn7

Thanks for the info. I moved the engine per your suggestion a little off center to bring the prop back in line with the center of the cowling. It still looks very odd to see the spinner sitting at such an angle to the front of the cowling. Question: If I straighten out the engine could any thrust requirement be compensated for by adding a little rudder. I am very picky about how things look and this engine sitting at such an obvious angle is driving me nutty. The 1/4 inch angle built into the engine mount is placing the prop shaft at a 5 degree angle to the front of the cowling. Ie; one side of the spinner is sitting 1/8 of an inch from the cowling and the other side is a 1/2 inch away from the cowling.
OK, now that is a bit out of the ordinary. Most cowls are made to match up to whatever offset the designer chose for the thrust line. Even most ARF mfg's know to do that. They make their cowls offset so that the spinner backplate has a uniform gap behind it, just like JohnW mentioned. What kits are they?

BTW, it really is better to have a model fly well than it is to have it look good sitting in the pits.

Actually, 1/8" to 1/2" is way out of the ordinary. What manufacturer?
Old 09-11-2008, 02:38 PM
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chevisn7
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Default RE: Why is Engine Mount Set off Center

Manufacturer unknown. Made in China is all the box said. Bought the models on E-Bay. I will go with what you have suggested, aligning the cowl with the backing plate on the spinner. Like you said better to have a plane that flys well than one that looks good in the pit. Now the other plane has me worried with no thrust built into the mount. I will add a few degrees just to be safe and anign the cowl as you have suggested. I reduced the thrust angle to 2.5 degrees and it looks much better. If that is not enough I will change it back. I am a beginner at RC in todays terms. Flew quiet a lot 30 years ago. What will the plane do if it does not have the correct amount of thrust built in. How do I know to add thrust angle or recuce it. Other forms may already have this info. Just point me in the correct direction.
Thanks
ChevisN7

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