Actual Flying Rotax Installs

[CaseyCooper]

Mark recommended that I add more washers to try and provide more pressure on the outer bushings to reduce movement. I used these washers to encapsulate the bushings and really hold them in there. Unfortunately no luck. Like I’ve mentioned before, rear bushings showed no obvious wear. The front right, slightly. The front left showed most fatigue.

Mark emailed and assured me that they’re actively working on a solution/improvement.

[Zack]

Sorry guys, I had to render some photos so I could actually post them.

Thanks! Those AN5 bolts look real beefy compared to my AN4s.

[SonexN76ET]

After looking at the engine mount bushings extensively, I do not think there is a quick or easy fix for this. The reason is that there are two separate causes that I see:

1- Most serious is the unsupported AN4 or AN5 bolt holding up the engine through the two bushings and attach bar. This unsupported bolt experiences a tremendous amount of lashing and flexing and wear at the intersection of where it passes through the engine mount. The AeroVee mount has a better design where the bolt is inside of the pin it’s entire length.

2. Bushings sagging as discussed earlier.

Jake

[Kai]

All,

An intrigueing situation and I would be very interested to see what the manufacurer turns out.

Getting the rubber isolators loaded axially/compression is most likely the main issue here. This could probably be accomplished by discarding the existing flat bar brackets and substituting them with a double set of structural 1/2’’ aluminium angular brackets of suitable dimensions to maintain propshaft cl and accomodate isolator dimensions, one set of brackets bolted to the bedmount and the other set to the engine sump, sandwiching one set of isolators between them.

However, as a national DAR, I would be reluctant to approve such an ‘eyeball’ setup, unless the airframe designer has issued some form of acceptance. Loads and displacements would have to be calculatd, especially the rocking motion prop traction would impose on the setup. The displacement caused by the moment arm between propshaft and isolators would likely be considerable unless the isolators were very hard.

Thanks
Kai

[CaseyCooper]

Kai,
I thought the exact same thing and I actually created some for an “if all else fails” situation if there isn’t a solution sometime soon so I can fly my airplane. I used suitable mounts used on other 912 applications, such as Rans, which offer plenty of support and isolation.
I recognize you as a DAR but in this case a letter or sign off from a DAR is not needed. This is not a major modification and being in the experimental category, just requires the signature of an A&P willing to put their name on it. Technically you wouldn’t need any input from the company that builds the aircraft components because they are not technically the manufacturer. On your airworthiness certificate in the experimental amateur built category, the manufacturer is the builder. You’re just building a plane from a kit or parts from a provider. But I 100% agree there is testing and at the least, research to be done of strengths of materials involved and loads placed upon them

[CaseyCooper]

So, as something I may try if there isn’t a solution in the near future, are some motor mounts I machined based off of the blueprints for the existing attach bars. These began as 4 pieces of 9.75” long 3x3, 0.375” 6061. The measurements are exact and stepped according to Sonex’s plans. I used 6 pairs of Barry 22001-13 Green isolator bushings. I machined them to be a bed mount. As for the 6061 material there is a surplus of strength, each bushing set uses 3/8” bolts which offer plenty of tensile strength and shear. Much more material, but the bushings are loaded properly and have plenty of suspension. Cowling clears and everything fits great. Considering the original bars are 0.625” material and 4 sets of bushings less internal pins, these may be a bit overkill, but potentially more stable and durable.

[CaseyCooper]

I haven’t tested them yet. Just test fitted in case I need to switch to them later.

[Kai]

Wow!

That’s more like it: impressive! You are a much better machinist than me. However, you realize that the idea is neither ours nor new: airframe designers have in the past employed the principle extensively.

If four isolators are good, six should be better. This should add stability to the logitudinal rocking. I have no idea if it is at all possible, but since the antics of the R912 are scary during engine startup/stop, I would also have checked out the possibility to get them further (wider) apart- just a thought!

You have not asked for comments to your very good and illustrating photo’s, and I realize this is none of my business, but 30 years as an A&P tech with an inspector rating- DAR? :-) I have come to hate the hose clamp in its simplest form. They tend to live their own life and in suction line applications you never know if they really seal off air tight. If not so, this could rapidly starve your engine of oil. If no other alternative is available, make sure that they are just big enough (for even clamping, too big and they tend to go out of round), get the ones with bent up edges to avoid the clamp cutting into the hose. And secure them with a safety wire. This also goes for the big oil outlet banjo bolt under the engine (!)

Great work!

Thanks
Kai

[aripilot]

Rotax 912 installation is designed with Rotax ring mount. You also need engine mount attached to ring mount and firewall. This combo will quarantee smooth and trouble free operation of the engine.

Ari

[CaseyCooper]

Kai,
Thank you very much. I appreciate that. Any suggestions are good suggestions! So I’ll take any criticism I can get!
The Rotax definitely has its own personality. The gear reduction and compression make for an interesting start up and shut down compared to something like a lycoming or continental. That’s definitely something to take into consideration.
Maybe I will try and make some new mounts and move them further apart, but unfortunately, I don’t have a ton of room to work with but every bit helps. It’s only in the “just in case” realm right now so fortunately I have time to prototype.
There is definitely concerns when installing your oil system. However the banjo bolt/sump is not under suction, it’s under pressure, from the oil pulled into the pump at the front of the engine and circulated through. If there was a leak, it wouldn’t starve it, unless the it was so substantial the oil can ran out! Which is a scary thought in itself. Always watch those oil pressures! But I agree on the clamp. This one is, like you say, barely big enough to where I have a small tail coming out. There is definitely an art when using a standard hose clamp knowing when enough is enough. Too little, leaks, too much, curled rubber coming out of the clamp.
Thank you very much for your insight, it’s always good to have someone to bounce around ideas with!