Gary,
Thanks for adding a more technical tone to this discussion. Yes, I was not accounting for all the misc pieces needed to make the rotary run, but then I wasn't considering all the little pieces needed to make a conventional piston engine run either. Of the pieces that normally fail and end up poking out through the engine case, I think you'll agree that the rotary has significantly fewer of those. In fact, I have never seen a rotary with a thrown connecting rod. ;-)
Having a liquid cooling system is a two-edged sword, but its not anything that can't be overcome with good engineering. For coolant lines on my installation I used aluminum tubing connected to the engine and radiator via "Wiggins" couplings. I monitor coolant pressure, coolant level, and coolant temperature. Of course, if I catch a Canadian goose in the radiator, it will likely loose its ability to cool the engine, but then you have the same risk with an air-cooled engine.
As for the bsfc, do your numbers reflect the modern EFI systems, or carbureted engines. Tracy Crook realized a significant improvement in bsfc when he switched from carburetors to EFI. The new "Renesis" rotary engine has a better bsfc due to the side exhaust ports. Anyway, I prefer to consider it in "dollars per air-mile". By the time you factor in the cost savings for purchasing and maintaining a rotary engine over a certified engine, and that the rotary runs happily (prefers) on 89 UL fuel (half the cost of avgas), the cost per mile tips significantly in favor of the rotary. (Reading the recent post about the $2300.00 oil pan practically brought tears to my eyes.) I guess its the German in me that caused me to seek out something better, or different.
Ahhhh... you mentioned the magic word, "turbo-charger". I built my engine with the intention of turbo-charging as it was initially turbo-charged in its former life. After much thought, I decided to follow the KISS principle and go N/A. But there's a little voice in my head that keeps whispering "turbo-charge". With the rotary's high energy exhaust gasses, turbo's are a natural solution. Yes they add weight, but not much more than my current exhaust system. Stay tuned...
One thing that I hadn't mentioned that could be considered a negative for the rotary engine is that very few A&P's know anything about rotary engines. Heck, very few auto mechanics know how to work on a rotary engine. But, if I'm there with my repairman's certificate in hand, who needs an A&P? Also, rotary parts are less plentiful if you get stuck in some hole-in-the-wall town. But there is always UPS overnight.
Gary, thanks again for your thoughtful post. I'm not trying to convert anyone to a rotary engine, I only want to see it get a fair shake.
Mark S.
P.S. I've CC'd the Fly Rotary group as they need something to talk about (the list has been rather quiet lately).
On Fri, Apr 10, 2009 at 7:13 AM, Gary Casey <casey.gary@yahoo.com> wrote:
I'll certainly have to commend Mark on the great work with the rotary engine. I agree with his comments on almost every count. But...
You probably should count ALL the parts in and around the engine to have a fair comparison. For example, the air-cooled aircraft engine cooling system has essentially no moving parts, unless you count the vernitherm. Yes, the 3-rotor engine has only 4 MAJOR moving parts, but each rotor has about 50 components. While that's not necessarily good or bad, it's not an inherently simple solution. It rejects more heat to the coolant and more of that to the oil (rotors are oil-cooled), making the cooling system larger and potentially more complex. And the exhaust is hotter and contains more aggressive pressure pulses, which have to be taken care of by some sort of muffling. The ideal muffler is probably a turbocharger, which can work very well on account of the pressure pulses, but it probably takes a special high-temperature turbo that can tolerate the up-to 2000 degree exhaust. The turbo adds weight and complexity, but perhaps not more weight than an effective muffler. The fact that the engine is inherently round and concentric with the output shaft is a good thing, but probably more attractive for a wing-mounted engine than one in front of the fuselage. The rotary engine almost requires a speed reduction unit to make the power/weight come out favorable, and I was not impressed with the design of the then-currently available units, although they seem to work okay in practice. One big thing that bothered me is that the efficiency is inherently lower than that of a good piston engine, partly because the compression ratio is limited to less than about 9 and the surface-volume ratio the combustion chamber is higher. This penalty is probably 5 to 10%. All that being said, the big attraction to me was, as Mark said, the rotary will rarely completely fail, even if the coolant is lost. The apex seals might disintegrate and parts warp, but it will most likely continue to produce power for some time, unlike a piston engine. A long time ago we were testing many rotaries and occasionally we would see a loss in power. When the engine was shut down it welded itself together even though it was still producing power. And the very things that make it less efficient contribute to the fact that it can tolerate a variety of fuels. And with boosting it can be made to produce a lot of reliable power.
I seriously looked at 3 different approaches - a standard aircraft engine, a direct-drive automotive piston engine, and a rotary. The eventual deciding factors were that the automotive engine came out heavy and the rotary engine burned more fuel. I really do like the rotary, though.
Gary
From: Mark Steitle <msteitle@gmail.com>
To: Lancair_ES@yahoogroups.com
Sent: Thursday, April 9, 2009 2:27:21 PM
Subject: Re: [Lancair_ES] Rotary Engines
Dave,
Since there were no other replies, I figured I would give my 2-cents worth.
I have been flying a 3-rotor Lancair ES for almost 2 years now with a total of 110 hrs on the Hobbs. While it hasn't been without some teething pains, all-in-all, it has been a very positive experience and I would choose a rotary again if/when the opportunity presents itself.
While I did the FWF myself, my installation and the Mistral are both closely related. As an example, I could bolt a Mistral intake and/or exhaust directly to my engine, and probably interchange many parts with the Mistral 3-rotor. The Mistral folks have taken much of the rotary engine technology, and refined and pakaged it into a (soon to be) certified product.
My reasoning is based on my belief that the rotary is inheretly a stronger engine (pistons are cast iron vs. aluminum), with only 4 moving parts. If you read the recent AOPA story about the Cessna 400 blowing an engine over Pennsylvania in the night, well, I had a similar experience in a Cessna 152, only not at night. Like the chap in the AOPA story, we too just barely made it to the nearest airport, with oil pouring out from the cowl onto the runway. Since that incident, I have been very leery of all conventional piston engines. Hence my decision to go with a rotary.
Gross weight on my ES was 2060 lbs. I typically climb out at 7000 - 7200 rpm (2400 - 2500 prop rpm), climbing at between 1000 fpm and 1300 fpm, burning 16 - 18 gph, 15 gph in regular cruise (6000 rpm) and around 10 -12 gph in economy cruise (5100 rpm). (Keep in mind that the pistons (rotors) turn at 1/3 the speed of the crankshaft, so they are only turning 1733 rpm in economy cruise.) I can run either 100LL or mogas (w/o alcohol) without worry and can lean the mixture aggressively without worry of hurting the engine (no exhaust valves to burn). I can pull the throttle to idle whenever and not risk shock cooling the engine. Being fuel-injected, it will start cold, hot, or anywhere in between. What's not to love?
I mentioned some teething pains... those consisted of an early cooling problem which was solved with an auxilary water-to-oil exchanger and a cowl flap. I have also had a series of oil leaks, all from the oil pan not being properly sealed. I finally pulled the pan, cleaned and resealed it. Problem solved. The toughest issue to resolve has been finding a muffler that could withstand the pounding of the rotary's exhaust. I'm pretty sure that issue has been resolved by switching to a DNA racing muffler, but I don't have enough hours on it yet to state for certain.
Hopes this helps answer your question(s).
Mark S.
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