Vance Jaqua wrote:
Paul - Per my calculations, you actually understated the accelleration
loads on the piston engine crank pin. Accell is near true sinasoudal
- or r omega squared at max - perhaps a little higher as
a function of rod length to stroke (I have a fairly sophisyticated
computation of that some where in BASIC) so a 5 inch stroke at 2500
sees about 440 "G"s for about 3080 pounds with your 7 pound piston and rod.
However for a 1/2 inch offset ecccentric (you did not tell me what it
really is) at 700[0] rpm the "G" forces are about 690 "G" sinc the rotor
weighs at least twice an aluminum piston and rod) we will use 15 pounds
for over 10,000 pounds force.
I think you mean 7000 RPM. But you are wrong on the rotor forces.
The weight of the current cast iron rotor is nine pounds and not fifteen.
The centrifugal force of the nine pound rotor on its bearing is m*V^2 / r.
Assume the radius r is 0.5 inch or .04166 foot.
m is the mass in slugs.
V is the velocity around the circle in feet per second.
Circumference of the rotor's CG path is pi times the one inch diameter or
3.1416 inches or .2618 foot. Therefor at 7000 RPM or 116.6 RPS the velocity V
is 31.24 feet per second. Squared that is 973.
Weight of the rotor is nine pounds so the mass is 9/32 or .28125 slugs.
Therefore the centrifugal load alone on the rotor is .28125 times
973 divided by .04166 foot or 6568 pounds or 730 G's.
Never the less, due to combustion forces that partially cancel
the centrifugal force, the loads at 5900 RPM are way less than
this and are at a minimum according to this chart on a somewhat
smaller engine.
Given that there are less cyclic loads in this endurance limits are
usally roughly half tensile - so the rotors pin is still more highly
loaded.
I would not say it is more highly loaded as the crank pin in a Lycoming
is about 2.25 inches in diameter while that of the rotary is three
inches in diameter. The loads reverse on the crankpin while on the
eccentric shaft they do not reverse. Further more there are no great
jogs in the e-shaft such as found in crankshafts that cause high stress
concentrations. The 13B e-shaft will live at 10,000 RPM while the engine
develops up to 700 HP. Try that with a Lycoming and you would have
a basket full of shrapnel.
Both of these answers are really not all that staggering
with reasonable structural engineering, as evidenced by the relative
success of both engine types. Catastrophic failures are usually
the result of operating way out of design limits added
to a defeciency or failure in cooling and or lubrication. Special
purpose piston engines are regurally
operated well over 10,000 rpm for usable durations - even those big
"stock" V8s operate over 8000 rpm long enough to finish rather long
races.
Yes if you make the piston engine small enough it will rev to 10,000
RPM with out failure. A model aircraft piston engine revs to over
20,000 RPM. Or make it the same size and go to 12 cylinders.
The exact same scale effect applies to a rotary. If you
make a four rotor with the same displacement as a 13B two rotor
it may well rev to over 20,000 RPM. It will also further increase its
excellent power to weight ratio advantage over the piston engine.
None of this demonstrates indisputable superiority
of either design. If you rate them against success in the world market
place - the piston engine is the clear winner - conventional is conventional
because it works purty good. Vance
I obviously disagree as there are way more factors beyond the superior design
and durability of a product that determines its success in the world
market. I think the single most important factor, in the case of the wankel rotary,
are the lack of understanding about how it works and its advantages. The
next most important factor is the huge amount of manufacturing infrastructure
that would be obsolete and need to be replaced.
The use of the rotary in aircraft is expanding exponentially.
A really big event is about to happen when Tracy Crook and Dave Garber
get the twin turbo rotary 700 HP 350 MPH huff and puff finally perfected :-)
I would not be surprised if a completely successful flight did not happen
before Christmas. I will try to be there when that happens to take
pictures and report back to our readers.
Thanks Vance for your insight on engines. I greatly appreciate your effort
on this to check my calculations and improve my arguments in favor
of the rotary :-)
Paul Lamar
Paul - your arithmetic must have gotten a bit slippery since I stomped out your numbers
on my trusty calculator and got the same 690 "G" as by using r Omega squared - which
is the same as your equation using peripheral velocity - not an issue :>) tho.
I have a set of equations memorized that are very fundamental and widely
applicable like f = ma! From there I derive the necessary formulas.
It takes me longer with more work but I usually get where I am going eventually.
The interesting thing was looking at the figures. With piston engines near rated rpm
the pressure loads are not really an issue, but the wankel has such a high "piston"
area that it overides the dynamic inertial forces over a large part of the operating
range. The second interesting fact is the high cyclic loading as a result of this.
They are not showing full reversal, but the force cycles from about 500 lbs
to 8800 lbs with what looks like another little loop inside the big one. Bearing
pressure loads get pretty high also when you convert kg/cm^2 to psi as well
Just think - you need a lot of force on the short "arm" of that offset of the
eccentric to make those foot pounds you see coming out the shaft..
The Wankel is indeed an interesting engine.
Those loads are the main bearing loads and they are not really a function of
RPM. They are indeed a function of combustion pressure. Yes, as the volumetric
efficiency varies they too vary. If it were not for those combustion loads
there would be no load on the main bearings at all as the engine is in
complete and total balance.
Nobody ever has trouble with the mains unless you are running
50 psi boost or 65 inches of Hg. Of course at that point you are looking
at 700 HP from a 200 pound engine plus the weight of the turbo :-) Able Ibarra
surprised me a couple of years ago by telling me that the limitation
to the amount of HP one could get out of a 13B were the bearing loads.
He was working at the time to improve the bearings by increasing
oil pressure to the 100 psi level among other things. At that time
he was at the 700 HP at 10,000 RPM level. 800 HP came later.
Improvements can be made in the main bearings by adding another
one between the rotors. Not easy with out going to split end housings
or segmented eccentric shafts such as found in the three and four
rotor Mazda engines.
Here is a jpg of a CW 4 rotor design using a split housing approach
and a main bearing between each rotor. Sorry about the bad scan. The original
is in J. Norbye's book. Patent number 3,062,435. Somebody could
look up the patent.
Are you planning to be at Santa Paula this weekend? Timmy and I are considering
making a morning run that way for a bit of first Sunday, and a visit with Alex,
and who ever else we happen upon. - Vance
I'll be up there today to get the Cessna out of the shop. I may fly it
today if it is ready. I will try to meet you at Rich's.
I don't know about this weekend as I am still doing wife maintenance.
Paul Lamar
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