• Welcome aboard HomebuiltAirplanes.com, your destination for connecting with a thriving community of more than 10,000 active members, all passionate about home-built aviation. Dive into our comprehensive repository of knowledge, exchange technical insights, arrange get-togethers, and trade aircrafts/parts with like-minded enthusiasts. Unearth a wide-ranging collection of general and kit plane aviation subjects, enriched with engaging imagery, in-depth technical manuals, and rare archives.

    For a nominal fee of $99.99/year or $12.99/month, you can immerse yourself in this dynamic community and unparalleled treasure-trove of aviation knowledge.

    Embark on your journey now!

    Click Here to Become a Premium Member and Experience Homebuilt Airplanes to the Fullest!

Benefits and drawbacks of a rotary

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

orion

R.I.P.
Joined
Mar 2, 2003
Messages
5,800
Location
Western Washington
We too have done quite a bit of work in examining alternative engines for the homebuilt industry and like many out there, have concluded that the rotary makes a lot of sense. The two primary benefits of the rotary are the high potential power-to-weight ratio and the fact that the engine does not have a catastrophic failure mode. Even if you blow and eat all the tip and edge seals, the engine can still maintain about 40% power, which should be enogh for most users to get to a safe landing patch.

The drawback to the engines though are its rather unique torsional characteristics. Most folks out there seem to be of the opinion that if the engine is smooth at idle that it does not have a vibration problem. Nothing could be further from the truth.

A Lycoming displays its torsional problems at low rpm where at idle it often develops about an eight on the Richter scale. The coupling between the compression/power impulses and the propeller's spring characteristic becomes most demostrated near idle, where the system is nears the combination's natural frequency.

The rotary on the other hand is usually quite smooth at idle. Its natural frequency (coupled with a prop) is generally at a much higher rpm (about 4,300 - but this is dependent on a nuber of variables). At this high speed, if there is any torsional feedback, it happens very quickly, potentially resulting in a very abrupt failure of some part of the drive system.

We've seen this in our own dyno testing where we managed to push the bearings right out of a sprint-car gearbox that was rated at over 750 hp.

Powersport, in their development have also seen similar failures before they hit on the right combination of internal balancing and damping.

The problem therefore is not the engine but the reduction drive system you add to it. In that arena there seem to be only two realistic solutions. The first is the work done by Tracy Crook. I have not been a continuous follower but hearing from others whose opinion I trust, his research seems to be about the most concise in the homebuilt arena.

Powersport of course has also done a lot of work, but on a more commercial scale. The best news there is that they are now selling their reduction drive at a more affordable cost (about $6,500), which is certainly a lower dollar figure than if I were to design a drive and manufacture it from scratch.

We have also started a reduction drive for the rotary, based on the Morse silent chain. However, other projects have so far kept me from finishing the work. Maybe soon.
 
Back
Top