Dale Smith wrote:
Paul wrote:
We should be able to demonstrate an improvement in BSFC if and when
we get an effective variable pitch prop. In my C182 the best
economy is obtained by increasing the pitch and throttle opening and
decreasing the RPM. Most people will tell you not to exceeded "square" in
an aircraft piston engine.
You bet, Paul, plus the sequence of power application, i.e. lead with a prop
RPM increase, follow with a throttle increase ... decrease power with
throttle first, follow with rpm decrease ... is vital (on a recip.,
especially bigger ones.)
What square means is 2200 RPM and 22" of manifold pressure.
Supposedly if the pressure rises above the RPM you are lugging the
engine and it is bad for it. You are risking bearing or rod failure.
This is called "overboosting" i.e. too much throttle for too low a prop RPM
setting, but minor deviations are common. Lugging is generally refered to as
only occurring at the low end of the power band when too much throttle is
applied too quickly. Pings (detonations) occur ... especially with with low
octane gasoline, that are murder on a reciprocating engine's innards (if
allowed to continue). Rotary too, I have learned from here. But I am
unclear as to what exactly causes detonation in a rotary? And I believe you
mentioned that damage occurs almost instantly? And the necessity of a
detector/sensor/automatic throttle reduction or EFI cutout?
Reciprocating "Square" power settings at @ 75-85% of maximum Prop RPM just
seem to work out the best for leaning (by torque indicator readings) to just
lean of stoichiometric maximum torque, for best efficiency at any given
normal cruise RPM. This may well be 65% throttle.
Since the rotary is more robust than any piston engine we may be able
to open the throttle wide (at low altitudes), increase the prop pitch
and decrease the RPM. The lower RPM will result in less engine friction
and the wide open throttle less pumping looses resulting in better
BSFC or lower fuel burn for the same speed.
This is why I am personally so excited by your constant speed prop project.
It provides the flexibility to harness the power profile of the rotary
engine. You are dealing with the heart of the problem when you divorce
throttle position from prop RPM and essentially just tie it to prop "bite" (a
simplification).
You have shown graphs with "sweet spots" in BSFC around 5000 RPM. I assume
this is true of the "new rotary" (not yet on the market) also, although I
believe they rate it at higher RPM with the lighter rotor? Are they not
bragging about the BFSC with the new engines' porting? A two rotor, STOCK,
rated at 289 HP?
I was talking to Jim Mederer about that yesterday. Mazda published an
SAE paper a while back on the side port but they revealed almost nothing. Jim
and I agree the SAE has gone down the tubes. Most SAE papers are now mostly
advertising. In other words Mazda is not talking. Jim however said he talked
to the chief engineer in charge of the engine and the guy was very positive.
Everything
about the engine was vastly improved according to him. The power is up
at 6000 RPM as well as peak power up around 9000 RPM.
This principle is widely used to improve the MPG of cars. My Ford
Cougar cruises down the freeway at about 70 MPH and 2000 RPM.
Nothing new. Over drives have been used for decades.
Paul Lamar
Perhaps the best efficiency criteria is miles traveled per pound of fuel
(MPG), but most people would rather "push it up a bit" and travel at faster
than L/D max airspeeds. Speaking of which, I would hope that any power
failure mode or "failsafe fallback" of the EFI programming should be capable
of at least 55% of rated power. Do you know if this is the case with Tracy's
system?
Sincerely, Dale Smith
Yes, as I recall Tracy has a limp home mode I think. Tracy? Also don't forget
you get two complete computers and the engine will run fine on either one.
Paul Lamar
The Aircraft Rotary Engine Newsletter. Powered by Linux.
http://home.earthlink.net/~rotaryeng/ http://www.linux.org