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Have just installed the latest Sonex fix for excessive Rotax engine movement.

Have precisely one hours flight time on the latest mod - two additional washers in each rubber stack.

Removed cowling to inspect for movement /contact signs.

Movement : may be very slight improvement - early days for signs of abrasion/polishing. The exhaust tail pipe has contacted the cowling (as before washer fix). Have used "witness" paint on all previous signs - most good.

Possible bigger problem: Both rear engine mounts - bottom rubber (lowest in stack) has become displaced sideways - right side the most obvious.

Front rubbers: As installed, ie nothing unusual.

Your advice/comments will be eagerly read (Is it okay to go flying?)

Sorry to say it but until someone comes up with a ring mount engine mount or rigid connection at engine and resilient isolators on the firewall, like most 912 engine installs are done, you will continue to be R&D

Ercoupechris wrote:Sorry to say it but until someone comes up with a ring mount engine mount or rigid connection at engine and resilient isolators on the firewall, like most 912 engine installs are done, you will continue to be R&D

During our development work getting the Edge engines into the Sonex airframe, we looked at this. We quickly came to the concusion that the ring mount was the only way to go, and developed an engine mount for the Sonex A accordingly: look up Sonex turbo on YouTube. However- making a welding jig and making the mount was a LOT of work! And unless you had a certified welder that was willing to work for free, costs would be close to astronomic in a shoestring operation. There was no way we would be able to recover some of the expenditure, not to mention the glaring absence of paying customers. So the rather voluminous welding jig was dismantled when nobody showed any interest: one chapter closed!

Having said that; there are industrious people who have had the fortitude to successfully go down that road, so it can be done. I know of one guy in Sweden and one in Argentina. I am sure there are more.

Anyhow, what you see below, is what Edge came up with. If memory seves me right, the basis for this taildragger mount was a Sonex product for the Aerovee. After all these years, it is still flying.

I am seriously considering obtaining firmer rubbers - I know they are available.

What I don't know is, what the pros/cons may be in fitting firmer rubbers - what think you?

My Sonex Legacy has the thinner, than B model, bolts connecting the Sonex/Rotax adapter. There are 4 of these, in shear and they just don't "look" to be adequate. The reason I mention this, are my concerns that stiffer engine mounts may put additional pressure (vibration/twisting force) on these bolts - comments?

Skippydiesel wrote:I am seriously considering obtaining firmer rubbers - I know they are available.

What I don't know is, what the pros/cons may be in fitting firmer rubbers - what think you?

My Sonex Legacy has the thinner, than B model, bolts connecting the Sonex/Rotax adapter. There are 4 of these, in shear and they just don't "look" to be adequate. The reason I mention this, are my concerns that stiffer engine mounts may put additional pressure (vibration/twisting force) on these bolts - comments?

I would try to get a hold of Casey Cooper as I believe he is using the Rotax rails that Sonex provides. I could be wrong but I seriously doubt there’s more than a handful of those things (Rotax rails) actually flying in the fleet right now. The design that Sonex decided to go with was a compromise mainly surrounding their thoughts on the high cost of using the Rotax suspension ring. They could’ve gone with the alternative of resilient mounting the engine mount to the firewall and picking up the four mounting points on the rear of the Rotax engine rigidly. Those who don’t use the Rotax suspension ring almost always go with this alternative method where cost is a concern.
This is one area where they should’ve gone with the best solution, not the least expensive one. It’s my belief that people that are spending the kind of scratch that it takes to put a Rotax on one of these airplanes, would rather have the better solution. I did a little research and the best I can tell, the Sonex design closely mimics, but is not exactly, what the Czech sport cruiser bed mount uses.

The issue with those confounded iRMT´s, is that they very often have a point: several points, as a matter of fact. The advantages of the vertical suspension layout for the Rotax 912 is no exeption, and those who feel that the Rotax ring is on the pricey side, have new mounts for the Sonex keep popping up all over the place.

See these two mounts. The black one is from some genius Down Under, the other one flies in Sweden. And a chap in Argentina refined my own crude work by making a sculpture mating an Aerovee mount to the R912

Hi Skippy,

For what it's worth, I used to work at a motorcycle manufacturer that used rubber mounts for engines with a fairly significant imbalance...
That said, my caution for changing the durometer (stiffness of rubber) of an engine isolator will also cause a change in the energy transfer from the rotating engine likely causing some vibration level into the frame of the aircraft. Stiffer isolators increase the energy at higher frequencies. Softer ones damp the vibrations at lower frequencies.
The harmonics in the airframe noticed with one engine type and mount may not be observed in the same airframe with a different engine and the same durometer mount. 'Balance factor' is a thing. There are companies that specialize in 'tuning' isolators for specific installations if the budget will allow the science to do the development of the 'best' solution - there are always trade-offs for slightly less evil somewhere.

It really is a relatively inexpensive experiment, and maybe you won't hear any new buzzing. Structural engineers might caution you on fatigue life. The vibration energy (excitation) has to go somewhere...

Keep us posted!

I'm very glad I chose to use the ring mount. My only regrets are not adding some right thrust angle, and not using a softer damper. I used Lycoming sized conical dampers, and they're a bit too stiff.

An interesting thread indeed!

Reminds me of my own trials and tribulations when I went through this. Now I have spent the entire evening fanning around on the internet, looking for pictures and drawings for R912 bed mounts: it should be emphasized that a successfull R912 bedmount seems decidedly possible. However, what strikes me, is that these other bed mounts without exeption have their rubbers positioned far outside the circumference of the oil sump, wheras with the Sonex setup the rubbers are positioned almost on the centerline under the sump, thus robbing the design of any arms to react torque. And thanks to the psru, torque is considerable on these motors, causing deflections of a magnitude not experienced with direct drives. In addition, there is little to prevent the motor from wobbling up and down: IMHO everything on the mount is just spaced too close together.

However, that is what I feel. Others might have another opinion- comments?

13brv3 wrote:I'm very glad I chose to use the ring mount. My only regrets are not adding some right thrust angle, and not using a softer damper. I used Lycoming sized conical dampers, and they're a bit too stiff.

Rusty- beautiful! Very nice and clean design.