SonexBuilders.net

 studying the mount options for a 912UL on a Onex, and I'll be making a custom mount pretty soon. I was planning to move the CG of the 912 4" forward of the Aerovee CG, though I know that makes the cowl a nightmare


Wow Rusty 4" ???
I moved my prop hub 1 " forward of the origional Aerovee turbo position and found the CofG ok... could maybe have gone furthur but it flies fine and weight and balance was ok...
With two heavy guys, low fuel and a little bit in the back is on the aft limit and feels a bit funky but still is ok.. I think 4" forward could be an overkill... I have no idea about the W+B on a onex but just food for thought.

Brett wrote: studying the mount options for a 912UL on a Onex, and I'll be making a custom mount pretty soon. I was planning to move the CG of the 912 4" forward of the Aerovee CG, though I know that makes the cowl a nightmare


Wow Rusty 4" ???
I moved my prop hub 1 " forward of the origional Aerovee turbo position and found the CofG ok... could maybe have gone furthur but it flies fine and weight and balance was ok...
With two heavy guys, low fuel and a little bit in the back is on the aft limit and feels a bit funky but still is ok.. I think 4" forward could be an overkill... I have no idea about the W+B on a onex but just food for thought.

I'll calculate this a few more times before cutting metal, but I've seen others mention 6-7 inches forward, which seems like a lot. Basic engine weights are 160 VW and 122 Rotax. Rotax probably has a few extra pounds of accessories, but the oil and reservoir will typically be on the firewall, and not out at the center of the engine. I'll make the best estimate I can, and lean toward slightly nose heavy if there's any doubt, since that's easier to correct.

Rusty

Jake (originally said Brett, sorry)

It will be really interesting to see how the W&B compares with the previous Aerovee installation. Sonex was nice enough to send me some measurements between their Aerovee and 912 mount, and it appears that they moved the CG of the 912 aft about 3" to keep the prop in the same location. A lighter engine, mounted farther aft will have to move the CG aft, but maybe not enough to be a problem on the Sonex, or I'm sure they wouldn't have done it that way. It doesn't look like I can take that approach on a Onex.

Cheers,
Rusty

I haven't seen any Sonex myself personally with the Rotax factory mount. I'd have probably given it a go but with the Austalian dollar as it is and freight it was going to be well over $4000 to obtain one.
Id imagine with the factory mount my cowling would have come out nicer. So much to shoe horn in there with the Rotax. My firewall clearnace though was great.. Heaps of room for everything except the radiator. I didnt have room to sing it underneath as hoped. I don't know of anyone either trying to fit one to a Onex but like you say will be interesting to see the final CofG. Maybe with the motor a bit furthur back the radiator and oil cooler would have room to be placed furthur forward so negating the weight issue. If so would definitely ease the installation I think.

Sorry Brett. My last post should have been directed toward Jake, since he said he was using the factory mount. Do you have a link to any pictures of your installation? I fully expect a challenge packaging all the necessary bits into a Onex size cowl, but it should be interesting to try.

Rusty

I have quite a few pictures on my blog but I basically lost interest in it so it’s not fully up to date. Will paint a reasonable picture though of what I did for the install. Hope this can be of help...

sonexboomer.blogspot.com

Thanks Brett. Those pics bring back good and bad memories of a previous custom install project. I can't tell you how many hours I've spend holding various objects (radiators, oil coolers, etc) around in different spots to try to find where they will fit.

Thanks,
Rusty

I just did a weight and balance for my completed Rotax Onex. I moved the engine 7 inches forward, which turns out to be the max you would want to move it as the most forward weight and CG is just inside the forward limit by a quarter inch. That is with no baggage, full fuel and a 195 lb pilot. The CG will move aft as fuel burns off. So moving the Rotax say 5" forward on a Onex might be better. I included some pictures to show how I did the FWF.

Thanks for the excellent info George, and congrats on being this close to finished. I do have a few question though.

When you say you moved the engine 7 inches forward, where are you measuring? Are you saying the 912 prop flange is 7 inches forward of the Aerovee flange?

Did you also raise the thrust line, or is it the same as the Aerovee? I have to admit that I'm tempted to raise the thrust line an inch to help with prop clearance. BTW, what diameter prop is that?

I'd love to see other pictures of the FWF installation, either on the forum, or via email if you don't mind.

Rusty
13brv3@gmail.com

13brv3 wrote:Thanks for the excellent info George, and congrats on being this close to finished. I do have a few question though.

When you say you moved the engine 7 inches forward, where are you measuring? Are you saying the 912 prop flange is 7 inches forward of the Aerovee flange?

Did you also raise the thrust line, or is it the same as the Aerovee? I have to admit that I'm tempted to raise the thrust line an inch to help with prop clearance. BTW, what diameter prop is that?

I'd love to see other pictures of the FWF installation, either on the forum, or via email if you don't mind.

Rusty
13brv3@gmail.com

Rusty,

When I moved the engine, I moved the CG of the Rotax engine 7 inches forward of where the CG of the aerovee engine was. I tried to keep the thrust line the same. To allow a larger diameter prop, which is necessary with the Rotax, I lengthened the nose strut by three inches. I have a 64" diameter Warp Drive prop on it now. I am toying with the idea of putting on a four blade Warp Drive just for fun. I also am probably going to remove the 1" prop extension on there now as I actually don't need it for cowl clearance. Attached are a couple more FWF photos so you can see what I did. I used a Rotax ring mount and designed a custom mount going from the ring mount to the firewall.

George