Herraripower wrote: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!
Skippydiesel wrote:HELP!!!
I have tried 4 times to post photos of my Sonex engine installation - every time my whole post disappears, to who knows where?
Kai wrote: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
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