flyrotary@lancaironline.net Mailing List Archive

Gary,

Good post. I never had the vibration issue explained quite like you put it. I think I understand now why the roary is smoother than a 6-cyl a/c engine.

On the parts count, I would like to point out that while each rotor has many little pieces, but then each rotor does the work of three pistons. So, that works out about equal.

As I see it, there are three areas where the piston a/c engine typically fails. One is the exhaust vlaves. This type of failure is usually not catastrophic, unless the head of the valve separates from the stem. Another is the cylinder attach flange which cracks and the jug blows off the engine. (This was my experience). This could easily be a fatal (to the pilot) failure. Then there is the crankshaft. I am not familiar with what happens when you break a crankshaft in an a/c engine, but I do know what happens in a motorcycle when you break a crankshaft; the engine quits in short order. This too could be fatal. In your previous post, you barely touched on the significant differences in the "crankshafts" designs of the two engines. While I'm no engineer, it is very obvious to me by comparing the two designs why the piston engine crankshaft is prone to cracks, especially when bolted to a large spinning propeller. That may be a plus for running a psru, to isolate the prop from the crankshaft. (But that is just an assumption on my part.) But I do know that it is extremely rare for a rotary to break a crankshaft.

To be fair, the rotary has failure modes too. Such as overheating due to a broken coolant hose, or a broken fanbelt. But at least the engine will continue running for a few minutes before it siezes. Then there is the apex seal failure. This is normally associated with detonation on turbo-charged engines. Being N/A, I doubt I will have to worry much about that. And then, like you pointed out, there is the psru. That may be the weakest link in most installations. But the RWS psru has many trouble-free hours by many different rotary-powered a/c. And the risk of failure can be mitigated by a good preflight inspection, and the use of a chip-detector. The planetary gearset is from the Ford turbo-diesel automatic trans and is well proven by millions of miles of heavy truck use. Electrical failure would be another catastrophic failure. That's why I run a dual system (two alternators, two batteries). Of course, this adds complexity and weight. Who knows, maybe someday there will be magnetos available for the rotary.

Concerning the fly-in, yes, Anyone interested in rotary-powered aircraft are welcome. But please rsvp so we can plan accordingly.

Mark S.

On Sat, Apr 11, 2009 at 7:35 AM, Gary Casey <casey.gary@yahoo.com> wrote:

Just to add a few more comments and answers to the several excellent comments posted:

How many parts does it take to make a rotary rotate? Well, "parts aren't parts" in this case. Mark was right in that there are maybe 4 "major" components, but you have to define major. A piston engine certainly has far more major parts. Is a valve a "major" part? I think so. Is a rotor corner button a major part? Not sure, but probably not. Is each planet gear in the PSRU a major part? I say yes, and the PSRU is an integral part of the rotary engine. As someone correctly pointed out, it's not how many parts, but the reliability of the total system that counts. Just looking at the history of the rotary (which, from the implication of another post) it's not that good, but I don't think it has anything to do with reliability of the concept. It's more to do with the experimental nature of the builds and installations. My original point, perhaps not well expressed is that to say there are just 4 parts is an oversimplification. But let's face it, to put in an engine that has had many thousands of identical predecessors is less "experimental" than one that hasn't..

Are we ES drivers more conservative? Probably so, since the ES is probably one of the experimentals most similar to production aircraft, and not just because the Columbia (can't force myself to say Cezzna :-) was a derivative. Therefore, it tends to attract conservative builders and owners. Not surprising then that almost all ES's have traditional powerplants, with the most excellent exception of Mark. While there may be more, I know of only two off-airport landings caused by engine failures in the ES in almost 20 years of experience. One was caused by fuel starvation right after takeoff (fatal) and one was caused by a PSRU failure in an auto engine conversion. So our old-fashioned conservative nature has served us pretty well.

Yes, I was assuming that the rotary had electronic fuel injection and ignition, but that by itself doesn't change the inherent fuel efficiency of the engine. Direct injection does have a potential to improve BSFC because the fuel charge can be stratified. It will probably decrease available power, though. I think the best rotary will be 5% less efficient than the "best" piston engine(same refinements added to each). But I stated that as a simple disadvantage - as Mark pointed out, it isn't that simple. The rotary already comes configured to run on auto gas. The piston engine can also be so configured, but the compression ratio reduction would reduce its BSFC and maybe durability advantage. The total operating cost is certainly significantly less if auto gas can always be used to refuel. I assumed in my assessment that it will only be available 50% of the time. The real disadvantage, which I failed to state, is that the extra fuel required for a given mission might be 5 or 10% higher and that negated the weight advantage, if only for long-range flights.

Is the engine less expensive? I did a thorough analysis of a direct-drive recip auto engine installation and my conclusion was that if the auto engine were equivalent in reliability to the aircraft engine it would likely cost just as much. Is the same true of the rotary? I'm not sure, but you have to consider the total cost, including engineering of all the parts in the system, not just the core engine. I would love to do a rotary installation, but I don't think I could justify it by cost reduction.

It wasn't mentioned in the posts, but some have claimed the rotary is "smoother" than a recip. I at first resisted that notion. Sure, any rotary given sufficient counterbalancing, is perfectly balanced. A 4-cylinder opposed recip is not - there is a significant secondary couple. The 6-cylinder opposed engine is perfectly balanced, but only for PRIMARY and SECONDARY forces and couples - higher order forces have never really been analyzed, although they would be very small. And then consider the forces within the engine that have to be resisted by that long, heavy, but flexible crankshaft. So it isn't the mechanical balance that gives the rotary an advantage. Let's take a look at the the torsional pulsations, comparing the 3-rotor against the 6-cylinder: A 6-cylinder engine has 3 power impulses per rotation, as does the 3-rotor, so they are the same, right? Wrong. They both incorporate 4 "stroke" cycles, meaning that there separate and sequential intake, compression, power and exhaust events so that is the same for both. The power event, which is the source of the torque impulse, takes 1/2 of a crank rotation for the recip. In the rotary the power event requires 1/4 of a ROTOR rotation, but the rotor rotates at 1/3 crank rotation - the result is that the power impulse lasts 3/4 of a CRANK rotation, 50% longer than in a recip. Therefore, the torsional excitation delivered to the propeller, PSRU and to the airframe is significantly less than for a recip. And if you analyze the actual forces imparted, they go down by the square of the rpm. The torsional vibration amplitude goes down by a factor of 4 just because the rpm of the rotary turns about twice as fast. If you've skipped to the bottom of the paragraph, as you probably should have :-), yes the rotary is "smoother" - a LOT smoother.. (my apologies to rotary purists, for simplicity I used the word "crankshaft" for both engines)

But just because you can burn auto gas should you? The biggest problems with auto gas in recip aircraft have nothing to do with the engine, but with the high vapor pressure of the fuel - it is more prone to vapor lock. The fuel systems of certified aircraft are not particularly well designed with regard to vapor lock. "Fortunately", rotary engines typically have no mechanical fuel pump and are forced to rely on electric pumps. Fortunately because the pumps can be located at the very bottom of the aircraft and close to the fuel tanks, making vapor lock much less likely. I would caution any builders to consider vapor lock possibilities very seriously, much more so if you intend to run auto gas. when I was going to do this I planned to put one electric pump in the wing root of each wing, feeding the engine directly(the check valve in the non-running pump prevents back-feeding). Redundancy was by a "crossfeed" line that could connect the tanks together.

And thanks, Mark for - probably incorrectly - referring to me as a "good engineer". I'll have to put that in my resume!

Have a good day,

Gary
(do you allow us outsiders in your events? I'll park well away :-)