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Old Jul 12, 2004, 11:38 AM
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Blythewood, SC USA
Joined Mar 2000
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How to build a strong plywood motor mount?

I have a need for a strong plywood motor mount for a Hacker C50 XL, because the plane I am converting, a Great Planes Extra 300s ARF calls for the propeller drive washer to be 7 inches from the firewall. Even the best motor mounts out there for this motor will only give me about 5.5 inches and would require a spacer anyway, and with the motor that far out, turning a large 22x14 prop, I would be afraid of ratcheting the motor right off the firewall. So, where does one begin? I've built smaller motor mounts out of plywood before, but I am by no means an engineering master, and wouldn't know where to add strength and where I can take some material away for cooling. Has anyone done this before or can point me in the right direction? Thanks ever so much.

Oh, BTW, I have the two fiberglass spacers/templates that came with the motor, so I can rather easily make the actual mount itself. What I am looking for is a way to extend that mount out about 7 inches. Thanks again!
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Old Jul 12, 2004, 06:04 PM
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umrk's Avatar
Paducah Barkley, Kentucky, United States
Joined Sep 2003
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I'm not familiar with the plane, motor, or the mount that you need. But there is a way to strengthen ply, fibergalss cloth. Case in point, not knowing what i'd need for the mount on the scratch built fun fly plane on my bench right now, I made some 1/8" ply out of 2 pieces of 1/16" ply, grain at 90deg angles, bonded with thinned 30min epoxy, with 3/4oz cloth between the pieces, on a flat table and heavy books added till its cured. Let cure then cut to shape. It does add strength at a miminal weight penality. Also you can reinforce corners, joints, etc. with the FG cloth and epoxy (or finishing resin unthinned, but it takes much longer to cure)

HTH

mike
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Old Jul 18, 2004, 02:42 PM
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Blythewood, SC USA
Joined Mar 2000
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This is what I came up with...

It seems plenty strong in all axes, with no tendency to torque. Very easy to build, as well..

It's constructed out of 1/4" aircraft grade ply and 1/4" threaded rod, with 1/4x20 t-nuts on all mounts. Probably unclear in the picture are the regular nuts locking into the t-nuts behind the new front firewall and original firewall.

Comments before I take flight with such a homegrown contraption...
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Old Jul 18, 2004, 04:17 PM
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Georgia
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Quote:
Probably unclear in the picture are the regular nuts locking into the t-nuts behind the new front firewall and original firewall.
Yes it's a little unclear to me.
I see the T nuts on the front of the front ply plate bolted to the gear drive. I assume there are nuts on the back side of that right?
I don't see how it mounts to the firewall though. Can you give a little more detail.
I made mine from aluminum bar stock to the length I needed. These motors need to be secure, as they put out some torque.
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Old Jul 18, 2004, 05:34 PM
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Blythewood, SC USA
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Sure, here are some pictures that may make it a little clearer. The nuts are actually couplers for threaded rod. I felt that they would provide more bite at only a minimal weight gain. Not pictured in the pictures are the, yet to be installed, lock washers.
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Old Jul 18, 2004, 05:55 PM
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Georgia
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That's how I thought it was.
I don't see anything to keep the rods from moving rearward in the firewall. Looking at the front, the rods appear to go through a drilled hole in the firewall with the nuts on the back side of the firewall.
Another possible issue is the 8 screws on the front that go into the drive. With them being in just plywood I would keep a close check on them.

Bill
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Old Jul 18, 2004, 07:53 PM
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Blythewood, SC USA
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There are T-nuts in the original firewall. And you're right about the gear drive. I will need to put some washers there as well.
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Old Jul 18, 2004, 08:42 PM
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Georgia
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Quote:
There are T-nuts in the original firewall.
Problem is though that there is nothing to keep the rods from moving back. Those T nuts will push right out. Don't even try to start it up like that.
You need to take them out and put them on the front side, or use flanged nuts on the front and back side of the firewall. The latter is what I would do. The flanged nut would spead the load out some on the ply which would be better.
I'll bet you had some fun threading those rods through all the T nuts.

Bill
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Old Jul 19, 2004, 10:46 AM
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Blythewood, SC USA
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Yup Bill, you're absolutely right. Duh! What was I thinking. They started to pull back last night as I was adjusting the thrust line. I'm going to do just as you suggest, and while I'm at it, also fiberglass the front motor mount.

And boy howdy do my fingers hurt from all of that threading. The nice thing about this system is its adjustability, but crap is it a pain in the rear end to get those 8" rods from the front ALL OF THE WAY TO THE BACK. A slight misalignment of the plates and they are *tight*. Fortunately, I only have that problem on one corner. Me thinks version 2 is around the corner.
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Old Jul 19, 2004, 11:11 AM
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Georgia
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One other thing that you have probably checked, but I should mention it just in case...
Be sure the cowl will clear the front plate at the corners.
Quote:
And boy howdy do my fingers hurt from all of that threading.
I don't doubt that.

Regards,
Bill
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Old Jul 19, 2004, 12:16 PM
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Claremont, SD, USA
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First let me say I don't know much about these things, I'm just a farmer, and maybe it's stronger than it looks, but there will be a lot of torque and gyroscopic forces acting on the mount during aerobatics so will it be able to resist them? I would think it would be a lot stronger if you tied the back ply mount to the fire with a ply box, or at least some diagonal braces.

I need to build a couple mounts for some bigger planes I'm building, though not this big, so if your mounts work out good I may borrow the design if it's ok with you. Of course if your plane crashes because the motor comes off??? I may have to keep an eye on this thread!

James
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Old Jul 19, 2004, 01:11 PM
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Blythewood, SC USA
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Yes, the cowl clears just fine.... I had the foresight to build the front mount based on the cowl. At least, the second time I did.. <grin>

I've since added flanged nuts to the firewall as well as a top and bottom plywood plate which has stiffened it even more. I've had a helper hold the front of the fuselage while I've tried to move the mount and I've been unsuccessful. I can't imagine needing anymore than this. I'll snap some more pics once I get home this evening.
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Old Jul 19, 2004, 05:39 PM
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Georgia
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Quote:
First let me say I don't know much about these things, I'm just a farmer, and maybe it's stronger than it looks, but there will be a lot of torque and gyroscopic forces acting on the mount during aerobatics so will it be able to resist them? I would think it would be a lot stronger if you tied the back ply mount to the fire with a ply box, or at least some diagonal braces.
Your right, James there is a huge amount of gyroscopic force applied to the mount, as well as the torque trying to twist the whole thing off the firewall. It would be best to have some means of torsional bracing especially when mounting a motor this far out from the firewall. The only thing keeping this type mount from twisting is the bending strength of the rod and the flat areas where the nuts tighten down on the wood.
As for the gyroscopic forces, the plywood will probably fail before the threaded rod(s) could buckle under compression. I worry more about the small dia aluminum attachment point behind the gear drive breaking on the C50.
Quote:
Yes, the cowl clears just fine.... I had the foresight to build the front mount based on the cowl. At least, the second time I did..
Good
Make sure you have plenty of air flowing to the motor, ESC, and batteries.

Bill
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Old Jul 19, 2004, 06:37 PM
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Blythewood, SC USA
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Quote:
Originally Posted by Bill Mixon
I worry more about the small dia aluminum attachment point behind the gear drive breaking on the C50.
Do explain. Do you mean on the motor itself or something in the mount???
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Old Jul 19, 2004, 06:50 PM
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Georgia
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Yes the motor.
look just behind the gear drive, you should see a small gap between the motor front plate and where the drive screws back to. Now look at how small the base is.
Think about where all the gyroscopic torque will end up applying load with the rear of the motor held secure.. All the load not taken up by the front motor mount plate is transfered to this point. it's something to think about IMO.
I wouldn't worry too much though because there are lots of 3D applications out there with the motor mounted this way without any reported failures.

Bill
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