I am going to use a Rotax 912 ULS engine in my single place Onex experimental. This engine weighs about 170 lbs and it will be replacing the VW conversion engine normally in the Onex which weighs 190lbs. So I am doing the math below to figure out how many inches I need to move the new engine forward to compensate for its lighter weight. Using the weight and balance diagram below, the number of inches that the cg of the engine is forward of the wing is 48.6”. So the moment is 190 x 48.6 = 9,234.
If the new engine is 170 lbs, to get the same moment I would need to have the arm be:

9234 = “X” x 170
“X” = 9234/170
“X”= 54.32
So I would need to move the engine forward about 6” (54.32” – 48.6”=5.72”)

Is it really that simple or am I missing something?


On a separate note, while I am waiting for the engine to be delivered, I am working on PTT in the lav. Just because I am a single place experimental doesn’t mean I can’t run like the big dogs.

One thing you need to make sure is that you are using the center of gravity of each engine when figuring it's position. The wight distribution of the engine may be that the CG is not in the center. Also have you considered all the accessories?

Realize its the CG that matter not the Moment.....
So make its the CG of the two motors your moving, not just the prop flange.

Also does the 140lbs include radiators etc.... ?


Lastly if the new motor is really that much lighter... then you need to compensate for the fact that the empty wt will be 40lbs less as well, so you don't need as much moment.....

Do you expect the airplane to be the same empty weight as the one listed?

Is your wt similar to the Jeremy example?

Is there anything heavy (like the battery) you can move around a bit when your finished and really understand where the finished CG is?

Its going to get somewhat complicated if you move the motor forward as that diagram uses the front of the spinner as reference and you will no longer have a clean reference....

What matters is where the final CG is with respect to the wing....

Are you going to make a new cowling for it? How about use a heavier constant speed prop and leave the engine in the same location. That Onex would rip with a CS prop.

Now that is a very interesting solution! I am going to cost out some constant speed props.

I am planning on fabricating a custom engine mount and a custom cowl, so I can move the engine wherever it needs to go, and hopefully I will come up with a slightly better
looking cowl than the stock one on the Onex.

Be care with calculating locations since your 0 point, or datum will probably physically change with moving the engine forward. You'll have to measure out an equivalent point to be the datum or dramatically change the math equation if you change the datum to another known place, like firewall or MLG.
Have a knowledgable person check your figures.
Know of a couple first flights that had tragic outcomes due to W&B errors.
Long distances in the tail can be used to add a little weight as permanent ballast to fine tune changes in the nose. Lot easier to do than having to add weight up front.

Good luck and have fun with your project.

Do you have a reasonably accurate W&B estimate for the completed airplane including the VW engine as-installed by the plans? If you do, then you could subtract the weight and moment of the VW engine. Then you can add the weight and moment of the Rotax engine. If you don't like the new c.g. then try a different moment for your engine... and so on.

That's basically the same process if you remove and replace things like avionics or accessories (i.e. lightweight starter) or adding new features (shoulder belts in an old Beechcraft, for example). Pretty hard to go wrong.

By "reasonably" accurate, I mean the way you're building it compared to the plans. Homebuilt plans and advertised empty weights are usually bare bones, while most homebuilders add a bit of weight from more avionics, soundproofing, comfort, etc.