Hi All!

I'm new to this site, and am just starting on a CH750. In evaluating my engine options, I came up with a question that I haven't seen addressed anywhere. It's not about WHICH engine, but rather why one doesn't exist.

All of the current LSA offerings with speed reduction units use spur gears or belts to make the reduction, but why no planetary gears.

Automotive automatic transmissions all use planetary gears, and some of the old large radial engines also used them. (See attached diagram of Wright R3350).  Gear Reduction Unit

It seems that the layout would be more compact, stronger, and keep the prop centered on the crank.

The current engine units look like afterthoughts (which many are) while a planetary unit would appear as an integral component of the engine (slightly longer snout).

Any ideas, or does such a creature already exist?



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Its even better if a reduction gear is not required at all. Less complication, less weight and less cost to overhaul.

Oh, I agree with that completely, and right now I'm leaning toward a UL260i. which is direct drive. I looked at most, if not all, of the usual powerplants, and those with the reduction gears remind me of a house with a poorly done room addition. They may be functional, but they just look awkward.

However, there are a lot of engines out there that need the reduction because of their power curve. And I can see where the smaller (lighter) engine gets by with the gears with a fairly small weight penalty for the unit.

I'm just curious why the planetary system hasn't been pursued. And IMHO it would look a lot better and it might just be stronger pound for pound.

Having worked on these gears before (a long time ago mind you!) I think I can answer this one!  Basically comes down to a few factors, torsional vibration, cost and weight. 


In order for planetary gears to absord torsional vibrations from the crank, they have to be made with significantly small tolerances  (that means big costs) that can help mitigate some of the vibrations, but they are not completely successful at it, so they must be fairly big and heavy as well so that means more weight.  For the small motor market, it's just not worth the R&D cost and build of these types of reduction drives which is why they are mostly seen on big radials.


A belt reduction is just far more simpler and cheaper to use.  Straight reduction gear mounts (like used on the new Viking) have the PSRU installed with rubber mounts to absorb the vibrations.  In the long run it's way cheaper to replace rubber mounts or belts every couple of years than to design/build a planetary gear system for our limited market!


Hope this made sense...



Thanks, Mark!

I guess the old $$ come into play again.  If this were to happen, it would have to be someone's "labor of love" to see it done rather than the profit motive.

All comments are right on. Also, the cost factor is even bigger than it might look on the surface. Torsional vibration is always a HUGE problem with reduction drives, especially metal to metal ones. The larger (and even small) production engines that have reduction gears have thousands of develpement hours, dozens of redesigns and tens of thousands of hours of testing before they are considered anywhere near to developed. Once you have eaten all those costs, NOW you need to tool up for very precise manufacture of a very low volume product. It all adds up fast.


I happen to have a discussion about this with a retired Pratt and Whitney engineer friend of mine just the other day. He described how much intellectual and finacial investment is required to develop geared drives like the ones in the big radials and the Rolls Royce Merlin and other such engines for me. It is mind boggling. No way that could be done economically for a small batch of engines. (and small is several thousand).


If you do a one-off you bypass all those developement issues, but you also loose the thousands of hours of prototype testing, the testing that weeds out the designs that have self-destruction issues. You and your one-off become the test vehicle, basically for the flying life of the airplane because one plane can never fly the time off to make the unit trustworty. That would make me very nervous. I understand just enough about torsional vibration to know that I do not like it.

Isn't the reduction drive on the Rotax an all metal gear unit?  I don't think I have seen any mention of a dampener in that unit?  Cutaways and drawings are not so readilly available for the Rotax as they are for any of the other engines out there.  They must be secret!

The PSRU can have all the gears it wants, but isn't the 912 driven from the crankshaft to the PSRU by a belt?  The crank is what transmits the  vibrations, if it's not connected directly to the gears it's not a problem! :) 

I've seen drawings before, they just are a bit hard to find...I'm guessing besides homebuilt aviation enthusiasts, who else would want them, so they are just not that common...I'll see if I can find one!


I was not aware that there was a belt in the Rotax PSRU.  That would explaine the source of their vibration separation.  The unit doesn't seem big enough to have a belt drive.  The type of housing that surrounds the Rotax PRSU looks more like a transmission housing.  I guess I haven't looked as closely as I should have.  It is burried in the coweling of the CTLS I fly regulary so it isn't somehting that I get to look over regularly.  Thanks

Joe B

Here's one....

The 912 is not belt driven, it is gear driven.  All of the current 912s have a slipper clutch built into the driven gear to help with vibration, especially at low power settings.

And that slipper clutch is why the Rotax makes that "death rattle" on start up and shutdown. It struggles to deal with the fairly violent power pulses (on start up) and compressions jolts (on shut down) associated with start up and shut down. It does protect the reduction gears the rest of the time, however. The Rotax reduction drive is the result of millions of dollars of development, thousands of hours of prototype testing and many tens of thousands of hours of use in the field. It just reinforces the points I made in my earlier post -- you need a LOT of developement work to field a useable and reliable reduction drive. A reduction drive is not, in my rather cowardly opinion, something you want to cobble together as a one-off homebuilt unit. There are a lot of very tough issues to deal with in developing a reduction drive.

Well, I never thought I'd get THIS much info with my question, but I appreciate all the replies. Very informative.

Thank you all!

I also have a new-found respect for that damper hanging on the nose of my auto's crankshaft.


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