Actual Flying Rotax Installs

[CaseyCooper]

Skippy,
That’s interesting the previous owner didn’t want to switch. I would like to understand his reasoning.
I do want to say though, the chromoly is not failing in any way. It’s just moved about 1/16” which could easily happen just in jigging when welding up the mount and in no way is it going to cause failure. But that 1/16” over a distance from it to the propeller flange caused about a 1/4” down and a 1/4” left.
Other manufactures do a 1-2 degree thrust offset to try and manage this phenomenon the Rotax has. They’ll shim the flange up and right so that it returns to center under load. So in no way is this anything new to the Rotax community. ICP produces the Savannah, a Zenith 701 type aircraft, that’s an SLSA and they use this method just by using washers to get the desired angles. Rans uses them on the s12 bed mounts as well. After I weld in a intermediate support in the mount I’m going to monitor its flexing. If it solves it, it will very likely be standard on all the Rotax mounts. But as a temporary solution I’ve shimmed the bed mount with 1/16” fender washers (under the top bed angles between the angle and bushing) and put the flange into a slight offset and it’s been working great. The addition of washers that thin hasn’t caused any strange distortion. It still keeps the bushings at their standard compression. I’ve purposefully put the plane through g loads, high attitude and stress to try and get more movement but it’s been steady so far. It’s just steps to refining it as much as possible. As we all know, Sonex tends to over engineer and build a safe product.
That’s awesome you have a project so close to completion! If you have any pictures it would be cool to see. Everyone’s install is a little different, you might have some good ideas on the build that others haven’t thought of yet

[Zack]

Zack,
I’m going to try and offer as much information to you as I can, which hopefully other people will learn from as well. As stated previously in the forum I finished my install last year almost 200 hours ago. I destroyed the original Sonex mounts in just hours. I then designed the new bed mount they’re using (the aluminum bracket system) to adapt to the chromoly structure to solve issues at that time. The problem I experienced am experiencing now is a motor mount flex or sag if you will in the chromoly structure. Really the proper mount to have is a ring mount. It would allow for better room for components, it would be lighter, and discourage certain characteristics of a bed mount. Plus a ring mount would allow for turbo charged models, like the 915, which requires the exhaust/turbo system to remain standardized so it would not work with the current bed mount.
With all that being said, my left side parallel bar that the aluminum mount bars attach to (your left side when sitting in the plane), has slightly sagged due to a lack of support. Only about a 1/16” but that translates to a bit at the propeller flange. In talking with Mark, I’m going to do a field upgrade to my motor mount to remove the sag, increase support strength, and hopefully solve the issue. Being that my plane is the only one flying with this mount system and the steady accumulation of hours, it has kind of been turned into a test monkey fortunately/unfortunately. So I will be doing some further modification and I will let everyone know how it goes. With your current set up, your propeller flange WILL NOT, and I’m not saying this to be a Debby downer, stay centered in your cowl and you will end up with an off center spinner. (Low and left if sitting in the plane) and it will look more pronounced with your set up. Your cowl setup is gorgeous. I just don’t want to see it potentially be damaged or have problems. My original cowl for my plane was made with the intention of seeing what worked and what doesn’t, and I’m now in the process of refining the cowling.
I work on Rotax’s everyday and have many hours behind and in front of them. My friend who overhauled my engine before I put it in is a performance Rotax builder, iRmt instructor, mans the Rotax booth at Airventure and one the go to (if not the go to) guys in the U.S.
We’ve talked a lot about Rotax “do’s and don’t s.” The 912uls/ul (carbureted) lends itself well to dual exhaust set ups, or 4 into 1 set ups. As long as you have proper tuning (as you’ve stated) you’ll be fine. You just don’t want any pulses canceling or a surplus of back pressure to keep the engine from breathing properly. After all, an engine is an air pump. It only pulls in as much air as it can push out, it is an exchange for energy. You ideally want a pulse to be accompanied by the next pulse at the proper rate to help cycle air through the engine properly.
However, on the 912is you absolutely have to use their stock exhaust (muffler). Being that it’s computer controlled the exhaust plays a big roll in its operating parameters (o2). So in modification to the exhaust system, it must be “tricked” into thinking its stock but just simply tuning it to stock parameters. Which you are attempting to do so kudos. The problem with your current setup is that the way to do that is to fit a muffler under the mounting truss but now you have radiators in the way. (Which I do also, my radiators are in the exact spot yours are). So that’s a factor to account for. I know at Sonex they’re mounting their radiators to the side allowing for the side ducts on the cowling to provide cooling, then using the under space for the exhaust. Your current exhaust work is a thing of beauty. The only thing I worry about is on hard starts or killing the engine, a Rotax bucks a bit because of its high compression and usually a large propeller up front. So I worry about your exhaust slapping the support it’s directed around. I’m not saying it will, but it’s possible.
Again, I offer my experience just as that. I have no intention of belittling anyones project. Yours definitely is being built with some care. Keep it up!

Wow, that's a lot of great feedback, thank you. I'm disheartened to hear the mount is sagging and needs further refinement. Could you please share details of the field modification so I can leave space available as I locate things? Is this issue serious enough to cause you to ditch the bed mount in favor of the ring mount? I have a lot of sunk cost and time working around the bed mount, and I need to decide if I should pull the plug. It's very frustrating that Sonex advertises a Rotax mount and universal cowling when issues keep cropping up.

Regarding the exhaust, we did our best to follow the 912iS Installation Manual during the design. They have a section titled "If a genuine Rotax exhaust is not used" and states: "The shape and configuration of the exhaust system is essentially determined by the free space available in the aircraft." This is where I find myself, and I haven't yet seen an un-modified stock exhaust on the Sonex bed mount. Because of the sloping firewall, you actually have quite a bit of space behind where I mounted the radiator. It's just that you need a custom routing to get there, and the price of the custom work (I don't weld) and the stock Rotax muffler to modify isn't a sound financial choice when you can just build your own. We had to compromise on the "ideal" length of the primaries from the rear cylinders to route around the trusses. We placed the EGT probes per the Installation Manual. IIRC, the ECU is a closed loop EFI with the EGT sensor a key input instead of an O2 sensor. I too worry about rocking motion of the engine in the mount, and have designed in some clearance. I hope it's enough.

[Zack]

Here is a picture of the nearly completed exhaust off of the aircraft. You can see the mufflers and EGT mount locations. We're going to merge these into one outlet on the centerline downstream of the radiator exhaust. I should have picture of it completed this week.

[CaseyCooper]

Zack,
Believe me, I know how disheartening it is. When I originally flew my plane after the engine install (with the very original attach bars) there was fiberglass burning smells, vibrations, all sorts of things that made me wonder if I made the right plane choice (all in under 10 hours). Every flight it seems like I had to tweak things. I originally talked with Sonex and found out they had never installed a Rotax, they just produced the mount, so I was on my own. That’s why I went on to design/produce the updated mounts and test them myself. They were very receptive and eager to help after presenting them with my issues and proof of concept for some changes. But I’m not going to ditch the bed mount just yet. I’m going to try the field upgrade and see what happens. If it doesn’t work, I’ll probably throw in the towel and design a ring mount. I’m trying my best to try and refine it so future people have a successful experience. Sonex is finally about to finish building their first Rotax Sonex and begin their flying and testing fortunately.
As far as the your exhaust, that is a thing of beauty. I can’t believe how good it looks! I’m sure you’ll have success with it. I’d definitely be interested in seeing how it turns out. I’m sure Sonex would too. They’re stuck (last time we spoke) on the exhaust
Please keep us updated! It’s a very nice build you have going.

[Skippydiesel]

HELP!!!

I have tried 4 times to post photos of my Sonex engine installation - every time my whole post disappears, to who knows where?

[GraemeSmith]

HELP!!!

I have tried 4 times to post photos of my Sonex engine installation - every time my whole post disappears, to who knows where?

Resize pictures to 1,000 pixels or less on long edge and give them captions in simple text with no punctuation - seems to work best for me. Otherwise the software seems to throw a lot of errors

[Skippydiesel]

Thanks GS

[Skippydiesel]

Engine to cowl clearances ??

What are measurement are people using for their Rotax to interior of cowl clearance ??

As movement is magnified the further the part is from the source - how much clearance are you giving the exhaust tail pipe to cowl ??

[Kai]

I think it really depends of what kind of engine mount you’re planning to use, how hard your islolators are, and where exactly the exhaust pipe exits the cowling.

Especially on pushers with a bed mount, I’ve seen some hefty movents during startup. Concerning the bedmount in a Sonex, Herraripower should have some answers.

In my own Sonex, I have the standard R912 ring mount in combination with the isolators Sonex provides for the Jabs, as well as the standard R914 exhaust system. The cowling clearance around the exhaust exit pipe is some 3/8’’, which is ample.

Thx

[Skippydiesel]

I think it really depends of what kind of engine mount you’re planning to use, how hard your islolators are, and where exactly the exhaust pipe exits the cowling.

Especially on pushers with a bed mount, I’ve seen some hefty movents during startup. Concerning the bedmount in a Sonex, Herraripower should have some answers.

In my own Sonex, I have the standard R912 ring mount in combination with the isolators Sonex provides for the Jabs, as well as the standard R914 exhaust system. The cowling clearance around the exhaust exit pipe is some 3/8’’, which is ample.

Thx

Engine mount is Sonex adapter for Rotax installation. Currently the two bar obsolete model, however I am working towards obtaining the latest 4 bar (probably a bit "stiffer"/less engine movement)

As the Sonex adapter utilises the bottom 4 Rotax mounting options, I expect most movement top side of engine, in the lateral plain and at engine start/shutdown.

As for the exhaust tail pipe, it exits at a shallow angle, in line with firewall. As you illustrate, I expect this part of the system, being furthest from the "pivot" point to exhibit the greatest movement. Your 3/4 inch, is that 3/8 in on each side ? (this would be so much easier in metric).

Would love to send a photo but so far have had no luck in uploading photos