Engine mounts sagging

[kevinh]

Hi ya'll,

If anyone is interested in playing around with bushing ideas I've converted Mike's drawing into a web editable 3d model. Here's the URL: https://cad.onshape.com/documents/aca1e ... 7f19bb890f

I'm going to try an experiment with printing a couple in high temp TPU and compare to the bushings from Sonex.

[mike.smith]

All:

Kevin noted some difference between my measurements and bushings that he has on hand. I used a micrometer to measure extra bushings I had on hand, so the dimensions were based on those. If not all bushings match then there may be some non uniformity between bushings or batches of bushings. And any given bushing is not truly round; there's some bulge factor as measured from the bolt hole center to the outside of the bushings. So there may be room for adjustment to the dimensions. I suppose the only way to check suitability is to test fit one in an actual engine mount.

[kevinh]

Ok - I printed a mount based on Mikes design and I have some extra info and questions:



( Left is a test print in a rigid plastic (just to test the dimensions against an actual engine mount - it fits perfect). Middle is the real part printed in 95A hardness TPU, right is the sonex part )

I tweaked Mike's measurements to exactly match the height of my Sonex bushings the only difference was that the thickest portion of the bushing was a little thicker on the ones I received from Sonex. So as Mike says there may be some variably in the molding process.

Questions:

• The 95A part I just printed feels very hard. Folks who have flown with firmer bushings: how hard were yours?
• I have a friend with a durometer. I'm going to go over next week and measure the sonex part, the part I just printed and the bushings Bryan is using to see how hard each one is ;-)
• Folks who have had drooping problems, do you think it is only really the aft bottom bushings that are softening up? I'm thinking about just replacing those and leaving the others stock.
• My plane is still a few months from flying, anyone who has a flying plane and wants to try a couple of these printed bushings send me a note and I'll send you a pair (if you promise to report back on this thread ;-)). Of course, no guarantees - I'm just some dummy on the internet.
• I can gets some 85A TPU to print with if the 95A is too hard. Or I guess I could go the print and fill a mold route but that would be a bit more work.

[mike.smith]

Ok - I printed a mount based on Mikes design and I have some extra info and questions:


[*]Folks who have had drooping problems, do you think it is only really the aft bottom bushings that are softening up? I'm thinking about just replacing those and leaving the others stock.
[/list]

For me its been both fore and aft bushings on the bottom.

[mike.smith]

As for durometer hardness, we just winged my bushings to something that's harder than the Sonex bushings. But the harder the bushings the more vibration is going to be transferred to the engine mount, welds, and the rest of the airframe, so one must always be careful.

I the end I still think the best solution is to brace the bottom arms of the engine mount to the top arms, to prevent movement in the first place.

[Rynoth]

Response below from Kerry at Sonex regarding my issue. I'm a bit surprised at the "you should add a washer" comment since this isn't in the Aerovee manual, however it is the action I've taken and I hope that it will provide adequate results.

"You should add a washer behind the bushing that supports the entire diameter of the cushion. That will keep it from “melting” around the flange of the motor mount.

Another trick that users of Jabiru-provided cushions do is to put a hose clamp around the bushings to provide them additional support. We’ve never found that necessary with our bushings if they are fully supported but now that yours have compressed it may give them additional life."

[kevinh]

Also - This thread was _super_ interesting: http://www.vansairforce.com/community/s ... p?t=129002

I'm going to go ahead and print a set in 85A, but I might try printing a mold so that 55A polyurethane could be poured in. I'll probably go ahead and assemble and do first flights using the sonex bushings and then swap if needed. How hard is it to swap them once the engine is on, should I be able to use an engine hoist slide the engine out a couple of inches and get in there?

[mike.smith]

Here's my Kitlog entry from the FIRST time I replaced my bushings:
http://www.mykitlog.com/users/display_l ... 271&row=58

[jeff0196]

Just remember if you have the turbo, you will have to remove it and the exhaust in order to slide the
engine forward off of the mounts.

[wlarson861]

Since first assembling my engine to the mount I have had washers on both sides of the mount bushings. After 300 plus hours I can not detect any deformation in the rubber mounts nor any sag in the engine. The bag from the kit that held the rubber mounts contained a stack of washers so I assumed they were for front and back and extras to adjust the thrust line if needed.