Sonex/Rotax Engine Bed

[Skippydiesel]

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.

I second all of Kai's comment - I am particularly impressed by the size of the sockets for the rubber vibration isolators - no problems with vertical loads.

How will/is the undercarriage attached?

[Skippydiesel]

Thank you all for your responses.

I have written to Sonex on this topic - hope to hear from them in the next day or so.

Summarising the advice so far:

Ring Mount

This would seem to be the best solution.
I would really like to go down this route BUT just got this Sonex into the air (seem to have solved the earlier cooling issues) and need to recover from all the unforeseen/unplanned expenditure.
I dont have the knowledge or skills to build one, so am dependent on purchasing the ring from Rotax ($$$!) & having the Ring/Sonex frame fabricated by someone else($$$!).

Higher Durometer Rubber
[list=]Fitting stiffer rubbers may be the most cost effective/easier/quicker answer BUT could have unforeseen consequences elsewhere, due to transferring more of the vibration & twisting forces to the Sonex engine frame & airframe. This may be my most likely attempt at a solution.
[/list]
Live with the current situation
[list=]In the short term, I think I will continue to fly with what I have got and monitor the situation as best I can. I should hear from Sonex soon & they may have some useful comments.[/list]

[Kai]

Skippy,

I have to disagree- it does not have to cost a leg and an arm. Secondhand and/or surplus ringmounts for the 912 turn up on the web from time to time. Surplus Aerovee mounts for the Sonex likewise- the core weight of the Aerovee is close enough to the 912 for CG purposes. Then you make up the famous two 12mm steel (or resilient aluminium if you can get hold of it!) adapter plates, and you are in business. No welding att all.

My two bits.

[Skippydiesel]

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

I reckon that Sonex could easily adopt 13brv3/Rustys concept - Rotax ring mated to Sonex framework - beeeeautiful!

[13brv3]

How will/is the undercarriage attached?

The Onex is a bit unique for the tailwheel plane since there's no gear sockets on the mount. It uses flat aluminum gear attached to the fuselage. Fabricating a mount with the gear sockets or nosewheel would have been more complicated for sure.

[Skippydiesel]

How will/is the undercarriage attached?

The Onex is a bit unique for the tailwheel plane since there's no gear sockets on the mount. It uses flat aluminum gear attached to the fuselage. Fabricating a mount with the gear sockets or nosewheel would have been more complicated for sure.

Thanks for that - explains, in part, why your system looks so wonderfully clean/minimalist - I have a Sonex Legacy/Rotax 912 ULS sitting on the Sonex Mk2 engine bed adapter - nowhere near as "clean" as your system.

[Skippydiesel]

Calling Casey Cooper

Hi Casey, as a fellow Sonex/Rotax owner, I would like to make direct (person to person) contact with you, to compare experince with the Sonex /Rotax engine bed adapters, through Forum private messaging.

[CaseyCooper]

Hello all,

This has always been a long winded story between Sonex and Rotax.

The original attach bar/mount Sonex sold was an attempt to finally provide an option for those that always wanted a Rotax in a Sonex.

I bought a Sonex with the whole hearted intent on having it equipped with a Rotax. Having an Aerovee or Jabiru wasn’t ever on the table for me.

On my first several flights I completely destroyed their 1st generation rails. Discouraged but determined, I began to design my own to replace theirs. I had just dumped my entire savings into a plane that didn’t work. The 2nd generation rails they now provide was my solution to the initial problem.

(Sorry to add my story time.)

As you stated Skippy, the engine moves. A LOT. Bed mounts of any kind with a Rotax tend to move quite a bit because of motor mount flex. In my opinion the 6.5” cowl opening for the propeller spacer is too small. You place a bigger prop on there and the spacer will burn into the cowling, which happened to me when I had the Sonex cowling in the beginning. I then made my own cowling with a bigger exit (about 8”) and a spinner. I have never had contact issues again. Rotax’s have a tremendous amount of torque compared to Aerovees and Jabirus, so the exit on the Sonex cowls never had to be very big. The worst thing that ever happened was the engines would sag.
I don’t know what propeller/spacer combo you’re running but you may benefit from the propeller spacer Sonex developed for the 912 to help this issue.

Another issue is where the engine finally resides when run up. The motor mount structure will eventually “settle”. To bring back some alignment and provide some resistance to the torque over, I’ve used fender washers between the tops of the Barry mounts between the rubber and upper attach angles to essentially “shim” the engine into alignment. The Barry mounts are already the hardest ones they offer. I don’t believe it’s the rubber compression so much as it is the flexing of the chromoly structure. I now have about 350 ish hours I guess with that setup with no issues.

Like Chris has said, there’s probably a handful of Sonex’s flying with the current Sonex mounting structure building time and with opinions on the setup. I currently only know of one airplane that has a substantial amount of time with the configuration, and a couple that have flown at all.

A bed mount has always been the “cheap” route for manufacturers to get away from the additional ring mount cost. However it’s ended up causing some headaches which I don’t know if they were worth the expense.

This isn’t a problem with Rotax engines. But I assure you there are solutions coming that will solve these issues and bridge the Sonex/Rotax gap once and for all.

Skippy, feel free to pm me or post on here. I was replying to you on the forum so that this may be a lesson/learning opportunity for others going this route. But I would like to see pictures of everything you’re describing. Your current set up, cowl alignment, changes you may have noticed, etc. I’ll do what I can from here to give you pointers to getting things ironed out. I’ll take some pictures too and give you information on what I’ve done to help the process. I’ll take some pictures when I’m in the shop on Monday.

[Skippydiesel]

That was a great "letter" Harraripower - my thanks for the trouble you have gone to.

First - I have a Sonex Legacy/Rotax 912ULS/Airmaster CS /Whirlwind 3 blade prop. The prop is within Rotax weight limits but is heavy at over 11 kg.

I have recently "converted" from Sonex provided, Barry Controls 22001-13 mounts, to -14 mounts (the next stiffness up).

I have made/fitted Cupped Snubbing Washers to try and improve the resistance of the lower mount rubbers, to lateral movement.

Sonex recently sent me larger OD spacing washers, ACV-P54-06 & 07, to replace the set they sent me earlier - I have fitted these.

All of the above, made some improvement to the engine movement but I still had a contact between left rear exhaust header/pipe and engine frame.

Yesterday I removed the offending exhaust header - using a piece of tight fitting water pipe, on the side I did not want to adjust, I progressively squeezed the pipe with Vice Grips, to cause a dent/depression at the point of contact. After reinstallation, I followed up with a short engine run to 50C oil temperature and shut down in the normal way. The pipe did not contact the witness tab on the engine frame. I will do a proper flight test in about a week and report back then.

If you would like to contact me through the person to person channels, I will send you photos of my setup.

[Skippydiesel]

Hi Casey,
Sent you a whole heap of photos - hope you received them and they gave you further insight into my Sonex/Rotax installation (I would like to see yours).

I think my installation differs from yours, in the following ways:

Mine is a custom cowling - engine/cowl clearances were designed in from the start, so for the most part not a problem. I have had no issues with the prop clearance. Have had rubbing where the exhaust exits (fixed) and in the vicinity of the fuel pump (fixed).
My most intransigent problem has been the left rear exhaust header contacting the framework (working on it)
My current mounts are Barry Controls 22001-14

I don't have a prop extension and think that fitting one would likely exacerbate the engine movement.
As mentioned I have quite a heavy prop. In one of the photos I sent, showing the (original) rear Barry 22001-13 mounts, the lower rubber donuts, show significant lateral displacement/distortion. The front mounts were fine. I believe this displacement shows the effect of the heavy prop causing a "whipping"/"pendulum" motion on shut down/start up, with the front mounts acting as a pivot point, generating a larger movement at the rear mounts. The fitting of Cupped Snubbing Washers was in large part a response to this symptom (seems to have worked)

I note with considerable interest that you have fitted the Barry Controls Mounts 22001-15 ("The Barry mounts are already the hardest ones they offer."). I am intersted in any observations you may have regarding the different performance of the -15's compared with the -13's (Sonex original) .

We had a break in the prevailing weather yesterday and I have managed to do 1.8 hrs flying - two starts, 3 touch & goes and two landings. Again the tendency of the left exhaust header to touch the frame has been markedly reduced but there is still a faint touch mark on my "witness" material (soft aluminium)

What to do:

Can dent the offending exhaust header some more, to improve clearance. Easy, nil cost/minimal down time and as almost have sufficient clearance may be the way to go but what of future softening/sagging of rubber mounts?
Make a new exhaust header with better clearance - Without a doubt this is the ultimate solution - costly and with a significant down time.
Install Barry Controls 22001-15 ("...the hardest ones they offer.") Would this give just that little bit more of necessary resistance to lateral movement? - What may be the negative impacts?
Try to fit one or more anti frame twist rods - If access allows, 1-2 rods across frame to reduce the tendency to flex - If possible, cheap, quick/no down time. What may be the negative impacts?

You have alluded to a better solution from Sonex "...I assure you there are solutions coming that will solve these issues and bridge the Sonex/Rotax gap once and for all." I assume this is a "ring mount" of some description (many good examples in this Forum). Don't want to go to too much trouble/cost trying to make the existing mount work, if this development is iminent. Any idea when it will be available?