----- Original Message -----

From: Ed Anderson

To: Rotary motors in aircraft

Sent: Saturday, April 11, 2009 7:31 AM

Subject: [FlyRotary] Gary Casey was [FlyRotary] Re: Rotary Engines

 

Good analysis and logic, Gary.

 

You’d make a good addition to the “rotary community”.  I have noticed over the 10 years I have been flying my rotary powered RV-6A that the problems have decreased considerably, the success rate and completion rate has gone up and first flights are now occurring without significant problems – even cooling is OK {:>).  I believe most of this improvement can be attributed to folks sharing their knowledge, problems and solutions with others - such as on this list. 

 

I know that fewer parts count is often touted as one of the rotary benefit – and while it is true that the part count is lower, the most significant thing (in my opinion) is not only does the lower part count help reliability (if it is not there – it can not break), but if you look a the design of the eccentric shaft (for example) you notice the absence of the jogs in a typical crankshaft and their stress points.  The thing is over 3” in diameter at some points and does not have the same inertia loads born by a piston crankshaft.  The parts that are there are of very robust design.  Finally, the rotary is (I believe) more tolerant of damage and tends to fail “gradually and gracefully”, it can take a licking and keep on ticking as the old saying goes.  Only extended time and numbers will provide the true MTBF for the rotary, but I believe it looks very promising.

 

Failure of rotary engines are extremely rare, but unfortunately, as with many alternative engine installations, auxiliary subsystems such as fuel and ignition frequently being one-off designs have been the cause of most failures – with probably fuel the prime culprit.  The good news is that for some platforms (such as the RVs) we have pretty much established what will make an installation successful.  The Canard crowd is fast approaching that status with their somewhat more challenging cooling requirements being over come.

 

  Having lost a rotor during flight due to putting in high compression rotors with worn apex seal slots worn beyond specs (found this out later – my fault for not being aware of this spec limit and checking it) which led to apex seal failure and consequence lost of most of the power from one rotor, I was still able to maintain 6500 MSL at WOT and fuel mixture knob to full rich – flowing 14.5 GPH – a lot of it undoubtedly  being blown through the disabled rotor.  Flew it back 60 miles to a suitable runway and made a non-eventful landing.   There was a small increase in vibration due to the power strokes no longer being balanced, but nothing bad and you could still read the needles on the gauges.  Other folks have had FOD damage to a rotor and also make it to a safe landing.  Two folks lost cooling (one loss of coolant fluid , one lost of water pump) and while they did cook the engines, both made it back to a safe landing.  So all things considered, I think the rotary continues to show that if the installation is designed properly, it makes a very viable and reliable aircraft power plant.

 

Failure of rotary engines in aircraft are extremely rare, but unfortunately, as with many alternative engine installations, auxiliary subsystems such as fuel and ignition frequently being one-off designs - have been the cause of most failures.  The good news is that for some platforms (such as the RVs) we have pretty much established what will make an installation successful.  The Canard crowd is fast approaching that status with their somewhat more challenging cooling requirements being over come.

 

My rotary installation cost me $6500 back in 1996, the primary cost being a rebuilt engine $2000 and the PSRU $2900.  I have since purchased a 1991 turbo block engine from Japan for $900 and rebuilt it myself for another $2200.  My radiators (GM evaporator cores) cost $5.00 from the junk yard and another $50.00 each for having the bungs welded on.  So depending on how much you buy and how much you build the price can vary considerably.  Today, I would say it would take a minimum of around $8000 and more nominally around $10000 for a complete rotary installation in an RV – some folks could do it for less, some for more.

 

But, regardless of the technical merit (or not) in someone’s mind, the crucial thing (in my opinion) is you need to address two personal factors:

 

1.  What is your risk tolerance?  It doesn’t really matter how sexy some “exotic” engine installation may seem – if you are not comfortable flying behind (or in front) of it, then it certainly does not  makes sense to go that route.  After all, this is supposed to have an element of fun and enjoyment to it.

 

2.  What is your knowledge, experience and background (and you don’t have to be an engineer) and do you feel comfortable with the level of involvement needed.

 

So hope you continue to contribute to expanding our knowledge and understanding of the rotary in its application to power plant for aircraft.

 

 

Best Regards

 

Ed

 

 

Ed Anderson

Rv-6A N494BW Rotary Powered

Matthews, NC

eanderson@carolina.rr.com

http://www.andersonee.com

http://www.dmack.net/mazda/index.html

http://www.flyrotary.com/

http://members.cox.net/rogersda/rotary/configs.htm#N494BW

http://www.rotaryaviation.com/Rotorhead%20Truth.htm

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From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Gary Casey
Sent: Saturday, April 11, 2009 8:36 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Rotary Engines

 

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 :-)

 

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