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Oct 27, 2017, 10:48 AM
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Brynmr's Avatar
Discussion

Gluing motor mount


Opinions on gluing aluminum motor mount to balsa? I'm thinking Amazing Goo/Shoe Goo or JB Weld but not sure which glue would work best. Also wondering if I should coat the balsa with finishing epoxy first.

Thanks
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Oct 27, 2017, 08:36 PM
B for Bruce
BMatthews's Avatar
Just use a regular epoxy. The filling in JB weld gets in the way of getting a really good joint. I'd also rough up the aluminium where it'll be glued. Like really coarse roughing. Even to the point of scoring it with a knife or scratch awl. And a sealing coat isn't needed or wanted. Just do it directly.

One other thing I'd also suggest is that you epoxy the mount in so that the front of the edge is flush with the face of the nose. Then after that epoxy cures cut and fit a 1/32 or 1/16 plywood former to the nose that has a center hole just a hair smaller than the aluminium mount. The plywood will bind and lock the balsa all together around the nose and at the same time for a lip that the mount can't push through without dragging the model with it.

The plywood nose former is especially wise because I'm seeing a lot of end grain from the sides and triangular shaping stock. And balsa isn't all that strong in that direction. The plywood nose former will lock it all together far more effectively.
Oct 28, 2017, 02:56 AM
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Brynmr's Avatar
Thanks. I'll try the epoxy and scoring but if I added the plywood, it would reduce the 5 or 6mm of motor shaft I have now for the spinner collett to grip, to scary short. I'm afraid it wouldn't hold.

Here's another view. The motor mount does fit snuggly into the nose. My other option is to get Pete Houve to machine another motor mount with deeper dimensions. That would at least give me added contact. This mount was a spare I happen to have.
Last edited by Brynmr; Oct 28, 2017 at 03:24 AM.
Oct 28, 2017, 06:27 AM
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portablevcb's Avatar
I gather from this that you have a front mount motor. Can you change it to a rear mount and put in a firewall for it? How big is this and how much power does the motor have?

I never liked front mount motors unless I had space for structure around it, and in this case there isn't much.

If you stay with a front mount then I'd have a mount made with some 'tabs' that could be screwed into the sides of the cowling. Reinforce those parts of the cowl with thin ply 'beams'.

charlie

PS it looks like you might have space in your current configuration to drill and tap the Al mount to accept screws that go in radially through the cowl.
Oct 28, 2017, 08:24 AM
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Brynmr's Avatar
2M LiL Bird about 24 oz. AXi 400 2212/34 Sailplane Outrunner Brushless Motor. Kv (rpm/V) 710 Max Power 200W. I managed to make enough space for the motor so that's ok. But I'm thinking I may have to have another mount made. Ordinarily this wouldn't be a problem but low funds at the moment.
Oct 28, 2017, 02:21 PM
B for Bruce
BMatthews's Avatar
I'm still a touch worried about the durability of just the balsa. If you're that tight on the shaft length then I'd still put the plywood ring on the nose but trim out the center to allow the mount to extend out as you need instead of touching against a lip. Not as ideal but better than nothing.

I do see the plywood nose former you have but it's been so cut out that there's a lot of spots that could simply split under some good torque. The thin facing plywood would aid in avoiding that. The last picture also shows a set of sprung fingers intended to center up the mount and perhaps lock it within the model. But it seems like you're using it the "wrong way around" ? It almost looks like it's intended to insert from the front and hold the motor stator bell up against the wood and "pinch" the motor into place. Yes? No? Or if you do glue it in then the fingers will form a lovely "tooth" for the glue to hold onto. Just blast it with some degreaser and dry it well. And in this case since there's gaps to fill I would use something like JB Weld so the filler powder aids in avoiding the glue running all over the place.
Oct 28, 2017, 08:53 PM
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Brynmr's Avatar
Quote:
Originally Posted by BMatthews
I'm still a touch worried about the durability of just the balsa. If you're that tight on the shaft length then I'd still put the plywood ring on the nose but trim out the center to allow the mount to extend out as you need instead of touching against a lip. Not as ideal but better than nothing.

I do see the plywood nose former you have but it's been so cut out that there's a lot of spots that could simply split under some good torque. The thin facing plywood would aid in avoiding that.
I could do that - cementing it to the sides of the rim. Ordering a new mount that extended further into the nose would help too.

Quote:
The last picture also shows a set of sprung fingers intended to center up the mount and perhaps lock it within the model. But it seems like you're using it the "wrong way around" ? It almost looks like it's intended to insert from the front and hold the motor stator bell up against the wood and "pinch" the motor into place. Yes? No? Or if you do glue it in then the fingers will form a lovely "tooth" for the glue to hold onto. Just blast it with some degreaser and dry it well. And in this case since there's gaps to fill I would use something like JB Weld so the filler powder aids in avoiding the glue running all over the place.
Here's a few pics of these aluminum mounts. I have them in all my Art Hobby planes. This is the first time for a woody. I had a ply mount installed as per building instructions but it didn't accommodate the motor I finally ended up with. So you're thinking JB Weld now instead of epoxy?
Oct 29, 2017, 02:27 PM
B for Bruce
BMatthews's Avatar
Quote:
...So you're thinking JB Weld now instead of epoxy?
Well, JBWeld IS epoxy. It just has some sort of metal powder mixed in as a filler. The neat thing is that due to the filler it doesn't run much before it sets. So I often use it where I need to build up an area or fill a bigger gap. But you could get the same effect by using regular epoxy and adding cabosil, micro balloons or just wood dust and mix it in. Whatever is easy.....

Those mounts are sure sexy looking.....
Oct 30, 2017, 08:30 AM
Registered User
Brynmr's Avatar
Quote:
Originally Posted by BMatthews
Well, JBWeld IS epoxy. It just has some sort of metal powder mixed in as a filler. The neat thing is that due to the filler it doesn't run much before it sets. So I often use it where I need to build up an area or fill a bigger gap. But you could get the same effect by using regular epoxy and adding cabosil, micro balloons or just wood dust and mix it in. Whatever is easy.....

Those mounts are sure sexy looking.....
Ok, I didn't know that. I've got lots of epoxy and micro balloon powder so I'll go that route. Pete Hoeve sells the mounts for approx $40 shipped. Love them.
Oct 30, 2017, 01:02 PM
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DGrant's Avatar
It looks like that mount might do well with "pinning" it, as well as epoxying it.

From the pics I'm seeing, it looks like there's sufficient clearance(as long as you stay away from the "X-beam and those screws)... to install some dowels into the sides of those machined hole on the side of that mount... right through the side of the fuse/firewall.

Again of course you'd have to map it so you'd miss the x-beam screws, but either a piece of wood dowel, or a piece of carbon-fiber rod, glued into the side of the fuselage/firewall at four points might not only strengthen that mount in there, it would definitely help with torsional strength.. as I'm concerned that perfectly round mount would spin/torque at any given point when you hammer the throttle.

I've pinned many firewall/mount set-ups over the years, and not one has failed... but it's mostly wood-to-wood stuff... but that little round mount you have there does have an area for pinning machined right into it, at least from my view point. Good luck with it.
Nov 01, 2017, 12:21 PM
Registered User
Brynmr's Avatar
Quote:
Originally Posted by DGrant
It looks like that mount might do well with "pinning" it, as well as epoxying it.

From the pics I'm seeing, it looks like there's sufficient clearance(as long as you stay away from the "X-beam and those screws)... to install some dowels into the sides of those machined hole on the side of that mount... right through the side of the fuse/firewall.

Again of course you'd have to map it so you'd miss the x-beam screws, but either a piece of wood dowel, or a piece of carbon-fiber rod, glued into the side of the fuselage/firewall at four points might not only strengthen that mount in there, it would definitely help with torsional strength.. as I'm concerned that perfectly round mount would spin/torque at any given point when you hammer the throttle.

I've pinned many firewall/mount set-ups over the years, and not one has failed... but it's mostly wood-to-wood stuff... but that little round mount you have there does have an area for pinning machined right into it, at least from my view point. Good luck with it.
You have me thinking on this, DG.


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