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I was just wondering if anyone else has, or is experiencing this issue?
In the 10 hours I've flown now I need again to try and do something with the mounts. The prop hub has drooped down now that it is again rubbing on the cowling. The lower rear engine mount and top front squash quite a fair amount and essentially tilt the engine forward. I had a spare set of mounts and fitted these at about 2 or 3 hours and now they need attention again.
I forgot to email Sonex during the week and have just remembered now I'm at the airfield doing the 10 hour service.
I thought perhaps maybe I had some defective rubbers the first time but 2 sets sagging so quickly seems strange.

Does anyone have some thoughts on this or perhaps tried using Nolethane bushes? I'm not sure if they would be too stiff or perhaps may not handle the heat.

-Brett

I think it's perfectly normal to have it sag. Mine engine drooped fairly quickly but the hub doesn't rub on normal flights. However, I know when I've reached 2.8 G's when I smell fiberglass burning from the hub/cowl contact.

I've decided that on my next airplane, I'm going to fit the prop hub a bit high so that it sags perfectly into place in the first 60-90 days... :lol:

I made a couple slotted washers that can be installed behind the bottom rubber mounts by loosening the bolts and lifting the engine slightly. I noticed that the rubber seemed to be extruding around the mount so I sized the washers to prevent that. I originally made the cowl expecting there to be some sag, but the engine sagged more than I expected.

Yep. When I built my last plane I put the prop hub about 1/8-1/4" high of center (I forget). After about 50 hours it had settled right into the center.

How about giving the bushings a quarter turn every annual?

Thanks for the responses from all,

Well I have just fitted another set and will see how it goes. I do like the idea though of the slotted washers to bump it back up when these ones sag. I am really wishing I had the foresight to anticipate the fact that it would drop over time but oh well. Live and learn I guess.

-Brett

Brett wrote:I was just wondering if anyone else has, or is experiencing this issue?
In the 10 hours I've flown now I need again to try and do something with the mounts. The prop hub has drooped down now that it is again rubbing on the cowling. The lower rear engine mount and top front squash quite a fair amount and essentially tilt the engine forward. I had a spare set of mounts and fitted these at about 2 or 3 hours and now they need attention again.
I forgot to email Sonex during the week and have just remembered now I'm at the airfield doing the 10 hour service.
I thought perhaps maybe I had some defective rubbers the first time but 2 sets sagging so quickly seems strange.

Does anyone have some thoughts on this or perhaps tried using Nolethane bushes? I'm not sure if they would be too stiff or perhaps may not handle the heat.

-Brett

This is a recurring problem with the geometry of the engine mount and the softness of the rubber engine mounts (my opinion). If you don't fix it now this is what will likely happen:
https://www.flickr.com/gp/127253485@N06/4L0Dm1

If you look at the accessory plate photos you'll see where the lower engine bolts have eaten into the tops of the bottom two holes in the accessory plate. The problem appears to be that because the top and bottom arms of the engine mount are not connected, the top arms act as a fulcrum and allow the downward movement of the engine (gravity and g-loads) to flex the upper arms. The soft rubber mounts on the lower arms can't resist the movement and so the lower rubber mounts get elongated until the bolts make contact with the accessory plate holes.

You can try using oversized washers (larger diameter than the rubber mounts) so you can crank down the pressure on the nuts/bolts (there is no published torque value from Sonex) and catch the overflowing rubber. If you do, you will probably have to use shorter engine bolts or the nuts will bottom out on the bolt shoulders before you can get them tight enough. This is most easily done by removing the engine from the accessory plate (you can just leave it hanging there in space a few inches away), so you can work with just the accessory plate. I have done it both ways and I can tell you it is MUCH easier to do with the engine not on the accessory plate.

But I did all that and still could not keep the engine from sagging. I had to replace my accessory plate, then make new mounts out of polyurethane. Another Sonex owner made molds and then we tested different ratios of the 2-part polyurethane until we got something that was harder than the rubber mounts. So far NO SAG! You can see one of those mounts on the Flickr page link above.

Thanks for the response mike,

I was a little dubious to be the first person to try the polyurethane/ nolethane mounts but looks like I certainly won't be. I saw on the weekend at my local auto store the exact same size rubbers in poly for $22 for 4. The only prob is I expect they will be quite harder, I imagine, compared to the standard rubber. Perhaps too stiff for this application.

So far to change them I have just been dropping the complete turbo and exhaust, loosening the mounts then supporting the engine while it pivots up or down to change all the bushes. Bit of a rigmarole but do able.

I have a few other more pressing issues with the plane atm but I think I will look into the mounts a bit more when I get time. I hope during these 40 test hours I won't be continually modifying things. I'm a bit eager just to fly it :)

-Brett

Hi Brett--

Could you please provide brand and part number info for the poly engine mount bushings you purchased. Thanks Stuart

Hi all,
Looks like we are talking about both aerovee and jabiriu mounts from sonex. We did purchase the upgraded harder mounts (jabiriu) from sonex, for our waiex. So far so good, even in the tucson heat. May have to replace next annual, i do notice that all rubber products deteriorate faster here. Rotating the rubber bushings or extra washers may help. The poly bushings, are those for aerovee or a jab installation?
Oshkosh coming up fast. Safe travels for everyone. Enjoy!
WaiexN143NM
Michael