pvc backmounts for largish electrics

[eajohnson]

I've seen a few people have gone with PVC backmount arrangements. This looks really appealing to me as opposed to building up a ply box and/or using standoffs. I'm just about to do a mount for a 1/4 scale, 2000W-3000W setup (backmounted outrunner), and I've bought all my materials for a ply and standoff arrangement but the PVC install looks SO CLEAN, I'm tempted to go that way. Can those of you that have tried it post your observations and maybe some pics of how you did it?

The areas where I'm particularly interested are:
- thermal expansion - probably a couple tenths of a millimeter, has it been a problem
- noise and vibration - how does the combination of PVC and threaded rod do in terms of vibration damping - any resonance issues?
- rigidity - how rigid is this, ability to counteract motor torque
- compression at the point where the backmount meets the PVC - there is a ton of pressure on a small area of PVC here, does the mount distort over time in this area?
- thrust angle adjustment - how do you handle it in your installation, what's involved to make adjustments?
- weight - how much does it weigh, did you put holes in it to lighten, how much material did you leave in place and how did it affect the strength of the mount?

[rodsmith123]

You can review this thread and see if it helps answer some of your questions.

http://www.rcgroups.com/forums/showthread.php?t=842389

[jrb]

One I've done:

http://www.rcgroups.com/forums/showthread.php?t=752795

http://www.rcgroups.com/forums/showthread.php?t=752795

[eajohnson]

You can review this thread and see if it helps answer some of your questions.

http://www.rcgroups.com/forums/showthread.php?t=842389

Thats one of the threads that sparked my interest and prompted the question - nice work! Could you elaborate on its characteristics? How does it compare with other mounting methods?

[eajohnson]

One I've done:

http://www.rcgroups.com/forums/showthread.php?t=752795



Very nice, JRB. Did you leave the mount with all the holes or did you switch to a solid tube? It wasn't clear from the thread. My motor is set up for 2-3x the power that yours was set up for (1/4 scale plane but will be used for aerobatics and aerotow, well that's my excuse anyway). It's got tons of thrust and I'm sure torque to match so I need a good solid mount for sure.
I'd be a little nervous about it simply being friction between the motor 'X' backmount and the PVC to hold the motor in position, is there something else in there that isn't visible to counteract the twisting forces particularly as the motor and prop are accelerated?

[GrrrAmps]

I used PVC on a couple of planes with an AXI 2826. I was a little worried about the torque too so I cut notches in the PVC for the x-mount. I didn't think it was necessary, but you could also glue some blocks to the firewall to key the other end of the PVC also. IIRC, I used 1 1/4" drain pipe and the OD was perfect for the screws to just fit over the OD, so that would prevent twisting also. For a bigger motor the schedule 40 should work well. I have this on a couple planes now and it's still working fine, but I haven't used it on anything bigger than this.

[jrb]

Did you leave the mount with all the holes or did you switch to a solid tube?



Needed a bit more weight up front for balance, so si did one w/o a lot of holes.



I'd be a little nervous about it simply being friction between the motor 'X' backmount and the PVC to hold the motor in position, is there something else in there that isn't visible to counteract the twisting forces particularly as the motor and prop are accelerated?



Unless the bolts go loose there's no way any of these parts will move relative to each other.

[rodsmith123]

In one of these two photos you can see that the motor mount interlocks with the PVC. In the other photo you can see angle brackets to the firewall. I also added buttonhead screws through the PVC into the firewall at an angle. If I did this again I would eliminate the angle brackets to the firewall. I used a coupling and a plug so that I could easily set the length of the mount. I also put buttonhead screws through the joint of the plug and coupling, just in case the PVC glue joint were to fail.

[staggerwing]

Here's an approach I used on a couple of large electrics--the one shown being an 85" H9 Taylor craft.
It's cut from 3 1/2 " sq cedar (quite simply on my table saw) & is extremely light--I think 3-4 oz.
This design removes the torsional dangers (in my estimation) using spindily long spacers as the only stress these fasteners & mount see is tension & compression & adding a cut for motor offset is easy.
I cut one pc about 2' long & just cut off a len as needed.