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Here we go again...

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Rotary power....


All you have to do is mention that phrase and you polarize the (mostly canard designed) homebuilt community.


However, I wanted to be able to discuss (by virtue of this forum) the reasons why builders are opting for the rotary (and don't say because John is doing it, although that may be reason enough... ;) ) engine in their projects.


Why open the discussion again? I think there are a few misconceptions about the advantages/disadvantages of using a seemingly superior converted auto design engine. Discussion may help review and bolster confidence (or not) and may help to lessen psychological procrastination.


So, this isn't about whether or not you are using rotary power (as in 12A 13A 13B 20B Wankel Mazda, Pratt and Whitney doesn't count), you have decided to use rotary power because.........???


So is it because of cost? Weight? Power?

Are there a few tricks up your sleeve?


Why would you go rotary?


(hint: Most Lycoming advocates assume $$$ is the motivation)



running to the side after lighting the fuse...



Back to building... #618 Cozy MK IV


My Cozy web pages, courtesy: Rick Maddy... :cool: WN9G :rolleyes:

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For me it was the first time I heard about "shock cooling" a certified aircraft engine while beginning flight training. Not long after that, I saw an ad in an aircraft magazine by a certified aircraft engine manufacturer claiming that their quality of engineering now matches that of the automobile industry.


As far as the rotary goes, I don't buy the arguments that because of it's low part count and rotation instead of reciprication makes it more reliable than other engines. For an eye opener, look at the number of used RX7's with under 100k miles for sale that have new engines installed because the old one blew. (?) Its not the part count that makes the engine reliable, its the implementation. There are plenty of Honda and Nissan sports car engines that have lived hard lives but have yet to give up the ghost.


That said, I do believe that the rotary can be MADE very reliable. Its design also has very attractive "fail-safe" charactoristics that I like that standard cylinder based engines do not have. The lower price of purchase (vs a low time Lyco or Franklin) and maintenance expense are just a bonus. :D

This ain't rocket surgery!

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Advantage of auto conversion: Once you create a reliable power plant, things like maintenance, repairs, service parts, and overhauls are trivial. Parts are readily available, at very affordable prices. Many low horsepower examples are flying.


Disadvantage of auto conversion: Not all components are highly developed and readily available. Not a great number of flying examples. High horsepower applications are less common.


Advantage of Lycoming installation: All parts are available. Large number of flying examples. Many people knowledgeable about the basic engine and installation. Weight, horsepower, and performance are well known.


Disadvantage of Lycoming installation: Typical installation is a used engine, or engine rebuilt from used parts. Service life is quite often unknown. When engine needs overhaul, it may be very expensive. Cost of ownership is unpredictable.


Things that are debatable:


Initial cost of installation: An auto conversion is not necessarily cheap. A Lycoming engine is not necessarily expensive. (Everything is relative, of course.)


Reliability: Auto conversions typically rely on a reduction drive of some type. New intake and exhaust systems are sometimes required. The reliability of these auxiliary systems becomes the weak point of the system. The Lycoming engines also have failure modes, some of which are dramatic. The comparison of number of failures is meaningless, as the number of hours flown by auto conversions is miniscule compared to Lycoming hours.

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  • 2 weeks later...

Well i started to look for some and found that the 20's r hard o find but now i wonder about the 3 roter (9 sides), it most be smoth and fast. i want that or 280 hp or so,i'v call atcins and some dude in floida i thing but no replie from ether. got a friend at work who got a friend that owns a yard, and has his eye on a new 1, a c sea..i think, has any one build a rx8 that flys ?

Steve M. Parkins

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I hate to sound so common, but it really is a combintaion of all of those. As to the price issue (and yes, it is important), one of the additional advantages, to me, is that I can buy different componants of my powerplant assembley at different times (i.e. core Renesis, PSRU, cooling etc). This helps with cashflow.


From what I can see the rotary seems to win hands down when it comes to on going care and maintenance. As has been mentioned, you could have a couple of extra rotaries waiting in the hanger for less than a lyc overhall.


I really like the way the rotary works. The three moving parts may not be convincing to all, but to me it is just "too cool" (technical term). I test drove the RX-8 and I was impressed with its extreme smoothness. It may not translate to aviation but it was nice.


I think it is only partially accurate to say it is an unprovan design. True, it is not as proven in aviation, but it is a proven design. If I am not mistaken, hundreds of thousands of rotary have been used successfully. No, not in aviation, but there is tons of data out there on the use of the design. I like that Mazda/Ford has done a huge amount of R&D for us. I think most of the potential problems comes from the supporting systems (yeah, and home engineering). Problems with subsystems is also a problem with Lycs(plus the problems with the engine pulling itself apart <g>). In fairness, when a Lyc does have a problem, such as with the jugs, we all seem to hear about it.


Also, I resent the pricing structure of the certified stuff. I do not buy the excuse that the prices are so high due to fear of litigation, R&D and small market (sure, they play a part)and as a result the buyer is getting a far superior product. But I think it is an excuse to soak the aviation public and perpetuate the process. Look at the difference between Dymon or Blue Mountain and Garman/King/Chelton.


After being commited to the rotary for about a year now, I truely think if someone gave me a new lyc, I would still choose the Renesis....of course, I would sell the Lyc to pay for my Renesis and glass cockpit <g>. All that being said, more power to you Lyc guys.


I have baught into the arument that the Rotary is the perfect aviation engine that someone mistakenly put in a car.




All the best,



Christopher Barber

Velocity SE/FG w/yoke. Zoom, zoom, zoom.



Live with Passion...

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