I'd love to see a good dyno run to nail down the BSFC better for these
motors. I think that the number is more like 0.6 lb/Hp-hr, but I have no
really SOLID basis. I think Paul's 0.5 number is possible, but probably
too low. Most spark ignition engines get about 0.42-0.45 lb/HP-hr and
the rotary is known to be "thirsty". If it can get 0.5, then it's great, but
in the absence of hard dyno data, I'll bet on 0.6, and we can see who's
right. I'd be willing to bet up to $1, but no more ;-)
The difference is significant. If you are burning 8 gal/hr, then you are
burning 48 lb/hr, and at 0.5 lb/HP-hr you are producing 96 HP. If the
BSFC is 0.6, then the HP is 80. With a pretty efficient spark ignition
engine you will have about 0.42 BSFC (brake specific fuel consumption)
so 8 gal/hr works out to about 114 hp. This about 75% of 150 hp, and when
you fly a 150 HP C172 around at 75% power, the fuel burn is 8-8.5 g/hr.
Higher compression ratio increases efficiency, so diesels get better BSFC,
like maybe 0.375 lb/HP-hr. Diesel fuel is heavier, about 7 lb/gallon. One
aircraft engine, the liquid cooled Continental Voyager series can get a
BSFC at the 0.375 level. This is phenomenal and is due to a very high
compression ratio and a special combustion chamber design. This engine
powered the Voyager "around the world on one tank of gas". Unfortunately,
it has not been commercially successful.
Bill Freeman
Long EZ builder & pilot
BSME, MSME
EAA Technical Counselor
Tracy claims at Rotomotor he is down to about the same fuel burn as a LEAN
Lycoming.
He did this by more spark advance at high altitudes. I think 0.6 is way
too high unless you are running full power, full rich. Same goes
for an air-cooled aircraft engine. The Piper Turbo Lance or some
such runs 0.65 on climb and takeoff.
I think we are going to see great things when we get a variable pitch
prop and a turbo charger. We will be able to get the same HP at lower
RPM (for less engine friction) and run higher pitch for even less fuel
burn and better MPG which is the bottom line. High boost can be the
same as high compression ratio. A lower compression ratio rotor in a turbo
motor has a better combustion chamber shape.
Paul Lamar
Sounds great, I hope you are right. If Tracy can run side-by-side with another
"identical" RV at the same speed and same fuel burn, then it's fantastic. BUT
without the side-by-side I'm still skeptical. The Navy put a huge amount of
pressure on the Grumman engineers to increase the cruise speed of the
Hellcat to match the cruise of the Corsair, both running P&W R2800s. When
Grumman borrowed a Corsair and ran them side by side, they were within
a couple MPH of each other and it varied with altitude and temperature
which one was 1-2 MPH faster. What did Grumman do? Relocate the
static port from the side of the long pitot tube (accurate) to the side of the
fuselage (inaccurate), but the same as the inaccurate Corsair. Of course,
all the books today say the Corsair was much faster, which is BS. The
airspeed indicator said the early ones were faster, but even that was fixed
later.
My point - just because one guy says "I burn 7.2 gph at 195 TAS" and another
guy says "I burn the same fuel at that same speed" doesn't mean much until
they run side-by-side. Jeff Acklund's Glassair TD 150 hp was always faster
than the Long EZs around here, at least in the hangar tales. When we ran
side-by-side, guess who was slightly faster? Not the Glassair. Even
5-7 kts faster cruise will have a very large effect on the HP required. Cube
function. If one guy is running 195 mph and one is running 207 mph, the
difference in HP is 11%. It is VERY easy to have that kind of error or even
more if not flown side-by-side. 0.45 x 1.11 = 0.5
So, I hope Tracy has done this test side-by-side, and he may have, on a
long cross country with an "identical" RV. If so, WOW! If not, well, the
test needs to be done. Identical airframes are a pretty good "dyno".
Bill Freeman
Long EZ builder & pilot
BSME, MSME
EAA Technical Counselor
Keep reading Bill. Tracy flew up 550 miles in 3.5 hours as I recall and
I saw them put 23 gallons in the fuel tank. This is a test that is very
good if there is little or no wind.
What were the actual time and mileage numbers Tracy? What did you burn going
home?
Paul Lamar
The AirCraft Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site.
http://home.earthlink.net/~rotaryeng/ http://www.linux.org