Motor offset

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Motor offset

PostPosted: Sun Jul 19, 2020 1:10 am

On all my previous builds I use no offset on the motor mount.
I'm putting the 120 ax on my slide 58 and was wondering if I should add some right thrust.
Would like to hear what you guys think.
Opinions please.....
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Re: Motor offset

PostPosted: Sun Jul 19, 2020 9:11 am

It’s not worth the electrons you’re using to post this. There are so many posts on this subject. Just to recap... You are not flying IMAC with full fuselage, with comparatively small control surfaces. However, if you want to experiment then you can easily add washers to create a couple degrees of engine right thrust angle offset, but like I said it’s a slab you are flying, it will strait itself out in flight, they fly nice just the way they are, if they build strait.
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Re: Motor offset

PostPosted: Sun Jul 19, 2020 3:07 pm

When I first got into 3d profiles, right thrust was a god-send for making it easier to hover and harrier. So I used it generously.

Later on as I got better at inverted 3D stuff, it started to become obvious that my beloved right thrust was creating more problems than it solved. So I started removing some of the formally beloved right thrust, and the inverted performance improved.

Amazingly using less and less right thrust didn't seem to make it any harder to hover or upright harrier. Apparently it was the lack of piloting skills and not the lack of right thrust that had been the problem all along. I just didn't realize it back then.

Now I tend to adjust the amount of right thrust (if any) so upright harriers and inverted harriers require equal but opposite rudder stick offsets. This usually works out very close to where the plane tends to hover with the rudder at dead center, and without any vertical fin offset or throttle to rudder mixing.
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Re: Motor offset

PostPosted: Sun Jul 19, 2020 4:13 pm

Thanks guys.
I seldom do inverted harriers but am hoping to retire in the near future and then hopefully fly more.
I think I will build this with zero offset like all the others.
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Re: Motor offset

PostPosted: Mon Jul 20, 2020 12:48 pm

rudder offset to compensate for slipstream, like full scale planes designed by engineers use. Look at my build threads. It is the solution.

unless you want to argue with W.F. authoritah?
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Re: Motor offset

PostPosted: Mon Jul 20, 2020 3:42 pm

Take some 1/8 by 3/8 hardwood, 4” long, draw a center line at the 2” mark, run it on the belt sander to create two tapered shims. Cut in half at the line and drill holes for the motor. Now you put them in and fly it, then fly again with out, chances are, you will prefer to have them in until you get to flying more inverted stuff. When the airplane is upright, its right thrust, when inverted it’s left thrust... if you set the plane at zero thrust, you will need right rudder when you pull a tight loop, when you do an outside loop, you need left rudder. When you have right thrust, you need more left rudder...
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Re: Motor offset

PostPosted: Tue Jul 21, 2020 9:31 pm

Thanks Mo.
So does rudder off set not have the same results that motor offset does?
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Re: Motor offset

PostPosted: Tue Jul 21, 2020 10:44 pm

iceman1 wrote:Thanks Mo.
So does rudder off set not have the same results that motor offset does?


Bob Godfrey was the only designer other than WF that I have ever seen use rudder offset. My thought is that you still end up with the same end result, you need more left rudder to compensate when inverted.
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Re: Motor offset

PostPosted: Wed Jul 22, 2020 3:39 pm

yep. when i was using shims, used the red hitec servo arms, clip off 2 lobes and belt sand them. but ruddes offset is the engineer's preferred solution.
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Re: Motor offset

PostPosted: Thu Jul 23, 2020 7:48 am

Generally speaking, 2 degrees of right thrust will be plenty for all envelopes of flight performance.
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Re: Motor offset

PostPosted: Thu Jul 23, 2020 10:34 am

Wild Fred wrote:yep. when i was using shims, used the red hitec servo arms, clip off 2 lobes and belt sand them. but ruddes offset is the engineer's preferred solution.

The worst part about using hitec servo arms, is you have to use hitec servos.
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Re: Motor offset

PostPosted: Fri Jul 24, 2020 4:59 am

Dr Constrictor wrote:
Wild Fred wrote:yep. when i was using shims, used the red hitec servo arms, clip off 2 lobes and belt sand them. but ruddes offset is the engineer's preferred solution.

The worst part about using hitec servo arms, is you have to use hitec servos.


naah, just bum the red arms no one else uses off of hitec flyers. cappy probably has 3-400 of them!
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Re: Motor offset

PostPosted: Mon Feb 22, 2021 2:31 pm

I never got that a profile engine sits on one side,thus the center line of motor be 1/4 off,must not be biggey.
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Re: Motor offset

PostPosted: Mon Feb 22, 2021 3:07 pm

You are correct. Yes, the 1/4" offset to the right side results in a slight yaw to the left, but it's not enough to make a difference worth worrying about.

It's far more important to have a strong full width motor mount instead of weakening the motor mount by cutting the thickness in half just to get the motor thrust line down the center of a 1/2" wide fuselage.

Also keep in mind that most (all?) electric motor mounts allow the motor to be centered along the width of the fuselage. Only glow/gas engines are offset.
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Re: Motor offset

PostPosted: Mon Feb 22, 2021 3:26 pm

fin offset. spiral slipstream. google it. wf has spoken.
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Re: Motor offset

PostPosted: Mon Feb 22, 2021 5:48 pm

engine offset. p-factor. google it. mcddd has spoken.










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Re: Motor offset

PostPosted: Mon Feb 22, 2021 7:42 pm

Why not adjust rudder a bit,or that be to easy. Or build offset mount,put engine back in middle,but does it matter,do el motors fly better for it or splitting goat hairs. Dont think plug good enought to know one from other,but do look wrong.
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Re: Motor offset

PostPosted: Mon Feb 22, 2021 7:54 pm

p factor only affects the gyro when changing the direction of the gyro. spiral slipstream is what affects the plane in a stable orientation.

just a few pics to show the real problem.

and that's why i use offset rudder, just like the kingairs i worked on in the army had.
Attachments
corkscrewing-slipstream.gif
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Slipstream_effect.jpg
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Re: Motor offset

PostPosted: Mon Feb 22, 2021 11:30 pm

Wild Fred wrote:p factor only affects the gyro when changing the direction of the gyro.

You are describing gyroscopic precession, not P-factor.

Wild Fred wrote:spiral slipstream is what affects the plane in a stable orientation.

I agree that spiral slipstream affects the plane in a stable orientation. And also in an unstable orientation. But spiral slipstream is only one of the forces that cause a propeller driven plane with "normal" rotation direction to yaw to the left.


Wild Fred wrote:just a few pics to show the real problem.

Very nice!

Wild Fred wrote:and that's why i use offset rudder, just like the kingairs i worked on in the army had.

I've never flown a Kingair. I've also never seen a Kingair hover. Or harrier. Or fly inverted. Or inverted harrier.

But the right thrust vs. fin offset vs. throttle to rudder mix debate has already been done on this site many times in the past. And just like political viewpoints, it really comes down to what works best for you, at this point in time.

Here's an old thread with more details about thrust offset. There are others if you search for them, but the debate seems to always end up the same. Some like thrust offset. Some don't. Some like fin offset. Some don't.
viewtopic.php?f=21&t=6223
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Re: Motor offset

PostPosted: Tue Feb 23, 2021 7:11 am

https://wiki.flightgear.org/Understandi ... d_P-Factor worth reading.

i can buy p-factor in harrier, not in hoover. propwash and torque effect in hover probably strong effect, though.
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