Actual Flying Rotax Installs

[CaseyCooper]

So I figured I’d take a hiatus from the keyboard and spend more time flying, less time typing.
I went ahead and tested out my mount. First thing I noticed when I ran it up on the ground was the difference in start up and isolation of vibration of the engine from the airframe. It was very smooth.
When flying it, it felt great. I did a series of high speed turns, full power climbs at high angles of attack, stall work, slips, and just regular old flying to feel the characteristics of the plane now, compared to before. I tried to impose a good amount of torque load, gyroscopic forces from axis change, and load on the engine in a hard climb. As a test if felt appropriate to impose as many loads as possible to make sure it was safe. There was no issues of wear, fatigue, or stressed components.
However, there is one issue. But let’s start here;
Aerovee’s, Jabiru’s, and Rotax’s are all flat engines. (Horizontally opposed, boxers, pankcakes, whatever you want to call them). Both the Aerovee and Jabiru have a prop flange on the crankshaft end which puts all loads at the center of rotational mass of these engines. But these engines are limited by their pitch, diameter and number of blades.
Rotax however is smaller dimensionally, and in weight, than both the Aerovee and Jabiru but it’s gear driven which places the prop shaft about 6” above the crankshaft end. Which in turn gives it an arm with an incredible amount of leverage because of its power output and ability to spin much larger and much heavier propellers. So as you can imagine, the propeller flange moves quite a bit where on the Aerovee and Jabiru it stays relatively centered. With these two you may see some rubbing on the back of your propeller or slightly on your propeller spacer but that’s fairly common and it’s never a big issue.
With all that being said, my engine still torqued over and touched the edge of my prop flange hole.
One thing about Sonex as opposed to other manufactures that primarily use a Rotax, is that the exit hole is much smaller. On any Rotax powered aircraft with a cowl, you’ll notice the exit hole for the propeller shaft gives at least about 1.5” around the entire flange/spacer to allow for this torquing over. Other manufacturers such as Aerotrek, Rans, Kitfox, Vans, use about an 8” exit hole and sometimes a center-stepped spacer to allow for this to happen. That’s why you see spinners usually ranging from 8” and up on anything using a Rotax.
On Sonex’s, my opinion, is the hole is too small for a Rotax. I am running a larger spacer, not a center stepped spacer so I am running the tightest tolerance available. With both mounts (mine and Sonex’s), the spacer rubbed on the same spot on my cowl. If you’re sitting in the airplane, it’s the 7 o’clock area of the exit hole where it rubbed. So instead of fighting it, I began sculpting out a relief in my cowl. It’s not noticeable unless you’re really looking for it when looking at my propeller directly from the side. My 8” spinner pretty much hides it. I would fly and check this wear, and sand away as needed. Eventually I got to the point I just drew a dark area with a sharpie on the wear point on my spacer, and when it was no longer being rubbed away, I knew I had shaved off enough of my cowl and the two were no longer touching. Then just went back and wiped off the mark with some carb cleaner.
So to no fault of Sonex, this is just a characteristic of a Rotax.
I went ahead and flew another Sonex today with a Jabiru 3300 just to compare the engine to airframe isolation, and my plane felt just as good if not better.
But everything is working great.

[13brv3]

Thanks for the update, and for the heads-up on the prop hole in the cowl. I'll be starting my mount soon, but using a ring rather than bed mount. I'll make a mental note about the cowl hole size if I ever get to that point.
Rusty

[Kai]

image1.png

This is what Thomas from Edge Performance in Norway is using. It's his personal mount and not a product that he offers. I like how it uses the Rotax ring mount design versus a bed mount.

Zack,

Well- yes, but not quite.

Thomas of EP made up a mount for the combo Sonex/R912 intended for his personal Sonex Legacy #0036 (importet as an EXP-class from the US years ago, and then long since resold to a different owner). I was involved in the design of this mount, and let me tell you for a fact that as a one-off the work was massive. We were contemplating setting up a small series, but try as we might there was no way we could make any profit out of it. Way later someone decided to copy his mount for Sonex Legacy #0772 (also an EXP import from the US!), and this is what the photo you selected shows. This conversion is still ongoing.

You’re right about one thing, though: for a stable installation the R912 ring mount is essential. It positions the isoators so that engine movement, particularily during start up, is limited to acceptable values.

What is important here, is not the R912 ring mount as such, but the position of the isolators. How the isolators get to this location, is another matter. I especially like the idea from the chap in australia, who kept his Aerovee mount and made up a truss ring from square steel tubing with attachments for the isolators to go with. A while back he posted pictures on this group, and his flight reports are good. My own design with a Jab 33 mount, attachment bars, and the Rotax ring, go down the same road, but moves the CG a little further forward. I previously posted some photos of this configuration further up in this thread.

Thanks
Kai

[13brv3]

I especially like the idea from the chap in australia, who kept his Aerovee mount and made up a truss ring from square steel tubing with attachments for the isolators to go with. A while back he posted pictures on this group, and his flight reports are good.

This is probably the mount adapter you're referring too. I thought really hard about doing something like this, but from past experience I know that all those extra bits of tubing are going to get in the way in a tight cowling.

http://www.sonexbuilders.net/viewtopic. ... x&start=10

Rusty

[Kai]

I especially like the idea from the chap in australia, who kept his Aerovee mount and made up a truss ring from square steel tubing with attachments for the isolators to go with. A while back he posted pictures on this group, and his flight reports are good.

This is probably the mount adapter you're referring too. I thought really hard about doing something like this, but from past experience I know that all those extra bits of tubing are going to get in the way in a tight cowling.

viewtopic.php?f=60&t=579&hilit=rotax&start=10

Rusty

Rusty,

Not a bad decision, albeit perhaps not quite for the correct reasons.

Deciding for either the Rotax ring- or something similar, leaves more than enough space for anything under the cowing. However, I will admit that placing radiator, oil tank, and muffler, is a bit of a challenge: it gets crowded in there. However, it can be done as many others have already shown.

The main issue as I see it, is the Sonex cowling which is too shallow up front for the Rotax oil pump housing, the exhaust pipes, and the reduction gear housing- at least if you want to keep the prop flange exactly where Sonex intended it to be. The cowling has to be modified, and I hate composite work, so I made my new one out of sheet: no english wheel so no compound curves. It is a lot of work.

Thanks
Kai

[CaseyCooper]

Overall the Rotax mount Sonex produces is a good design. The only problem, which is obvious, it the attach bar system. With their mount I never had any issues of being able to fit my components. The oil pump clears just fine with plenty of room, exhaust also. On my aircraft I put my coolers where it seemed necessary. I fly in southern Arizona, so it’s not uncommon to see 100 degrees OAT by 9 o’clock in the summer time. So I made a scoop in place of the exhaust exit holes that are standard for the Jabirus and Aerovees. It just made sense to utilize that space, plus it created less work for myself. It allowed me to get my coolers partially in the open and let my engine run nice and cool. I haven’t flown it in the summer time yet but I may do some design changes later. My exhaust, I just wanted a sweet sounding, aggressive exhaust so I tigged up my own. However, I will say that the exhaust routing was a bit difficult but made much easier by going to duals instead of trying to join them in one muffler. The engine positioned slightly back like Sonex designed, with the use of a spacer, is exactly where it should be. Under vibration or change the components never touch. I’ll give Sonex credit, for designing it in CAD with dimensions provided by Rotax and their own files, they did a pretty great job straight off. I know that wouldn’t be something most home builders could do. However if you try to produce your own mount, that could present a lot of issues later. Unless you did an absolute stellar job it would be hard to get an A&P willing so sign off on it, resale of the aircraft, as well as support from Sonex.
Also, producing a mount, with proper jigging, proper materials, getting a great welder or if you are a great welder (I say great welder, because I don’t want a good welder making a mount for my lifeline), ensuring all proper measurements and everything is square, painting it afterward, then installing and testing it, you’re far in the hole. Far deeper than just paying Sonex for their hard work. It’s a lot cheaper, less stressful, and looks better to all parties involved when you use factory parts. They will be available for replacement parts, and technical support.
Some aircraft you have to create your own, and for those people, my hat is off to them. That is a great technical endeavor and takes a lot of patience.
I have some photos I’ll try and upload of my install as well as some fellows from France that Mark from Sonex sent me to help me get a few ideas for the project.

Kai,
If you put the prop flange where Sonex suggests for a Rotax, you do not have to modify the cowl. They want it positioned back with a propeller spacer. I used a 2” to save any issues. As I understand it, you had some input in the design of a ring mount Rotax in a Sonex? That would be helpful if you had any pictures of your input or process of helping move that along. That might be an option for some builders wanting to try and go that route. Are you intending on installing a Rotax in your Sonex?

[13brv3]

All great points. I would buy a 912 mount from Sonex in a heartbeat if they made one for the Onex. Maybe if I keep putting mine off they'll make one :-)

One thing I don't understand is why they're moving a lighter engine aft? It looks like the goal might have been to keep the prop in the same place for cowl purposes, but I'd think for CG purposes you'd want to move it forward. It must work out OK for CG purposes on the Sonex, or they wouldn't do it of course.

Rusty

[Zack]

Kai, thanks for the details! I spoke to Kerry today and they said they were aware of one customer with issues, but he didn't have any updates. He asked me to email him and he would keep me posted.

[Kai]

Kai,
If you put the prop flange where Sonex suggests for a Rotax, you do not have to modify the cowl. They want it positioned back with a propeller spacer. I used a 2” to save any issues. As I understand it, you had some input in the design of a ring mount Rotax in a Sonex? That would be helpful if you had any pictures of your input or process of helping move that along. That might be an option for some builders wanting to try and go that route. Are you intending on installing a Rotax in your Sonex?

My experience with prop shaft spacers is strained, to put it mildly- that also goes for the R912: so no spacer here.

Intending to install a Rotax in my Sonex?! No, not any more, as it has been sitting in the airframe since last summer.

If you look at the first page of this thread and my posting of August, you’ll find two pics of my installation.

Thanks
Kai

[Brett]

One thing I don't understand is why they're moving a lighter engine aft? It looks like the goal might have been to keep the prop in the same place for cowl purposes, but I'd think for CG purposes you'd want to move it forward. It must work out OK for CG purposes on the Sonex, or they wouldn't do it of course.

Rusty

As mentioned early in this thread. I moved the prop hub on mine forward 1 " of the Aeovee position using a bed mount I made. I flew also with the Aerovee turbo that felt balanced perfectly but obviously changed to the Rotax. I can't imagine how they plane feels to fly after moving the Rotax back as far as the factory mount places it. My plane is within W+B specs but feels a bit tail heavy . I'd probably not enjoy flying a Jab 4cyl one with 2 x ppl, min fuel and an overnight bag.

Have about 100 hours now on the Rotax and flew about 105 hours with the Aerovee turbo. One thing I do miss with the Aerovve turbo is the Stall turns I used to do all the time... Been a bit dubious to even attempt one with the standard Rotax carbs.