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This has been an interesting thread. In the end, it
doesnt really matter how many "major" parts you have - even a minor failure can
bring you down. While I believe the basic rotary engine itself is more fault
tolerant than a recip, the peripherals used in the typical rotary install are a
lot more complex than a typical recip install. Since we rotary fliers dont have
the benefit of 70 years worth of experience flying behind the typical LyCon farm
implement I think overall our odds are considerably worse. Comes down to how
well an individual engineer's his installation and there is a tremendous amount
of variation here.
The dependence on electronics in the typical rotary
install is a good example. I may be a little sensitive to this issue
since I've been trying to find an intermittent glitch (2 times in 22 hours of
engine testing).
Mike Wills
RV-4 N144MW
----- Original Message -----
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
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!
(do you allow us outsiders in your events? I'll
park well away :-)
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