|
Message
Caution: this is NOT a
heads up comparison of the latest in prop technology.
Background: I originally
flew with a 12 year old MT three blade 80 inch prop intended for the EngineAir
V-8 and lower altitude cruise. It was optimized for lower RPM, lower
altitude, and more horsepower, hence the larger diameter. I retired it due
to sensitivity to rock damage, a continuing hazard here in
Australia.
I replaced the old MT with
the latest Hartzell 3-blade scimitar prop available through Lancair. It is
78 inches in diameter, has very thin blade tips, and the blade tips are swept
back roughly 30 degrees. These changes would be expected to reduce Mach
effects at high RPM and cruise speeds. They do.
Notes on data: The MT test
was done in August, 8500 feet, DALT 9280 ISA+13, 50F LOP, wide open throttle,
22.7 to 22.9 in Hg, cowl flaps closed.
The Hartzell test was done in December, 8500 feet, DALT
10,500, 50F LOP, wide open throttle, 22.3 to 22.6 in Hg, cowl flaps
closed.
Engine data via Electronics
International MVP-50. Flight data via Chelton EFIS. TAS has been
carefully calibrated with an uncertainty of plus or minus 2 knots, probably
better than plus or minus 1 knot. Autopilot held altitude and
heading. I did not record weight with the MT test, but with the Hartzell
the new empty weight with heavier prop and interior completed is 1987
lbs. For the flight test I added 400 pounds of pork, 75 gallons of
fuel, and perhaps 10 pounds of oil bottles, tow bar, charts and rubble in the
airplane for a total of 2836 pounds departure weight.
The comparison is shown
below. Measurement resolution is one knot on the
Chelton.
The benefits of the Hartzel prop show most clearly at
higher RPM as expected. The larger diameter MT actually went slower when
operated well past its optimal speed.
For bragging purposes, I cranked the RPM all the way up
(2680, actually) and set the mixture at best power. Result was 248 knots
at the same flight conditions as above burning 17.6 GPH. The MVP 50
showed 76% power, but accuracy of that number is unknown and probably a bit
suspect. Result is 14.1 nautical miles per
gallon.
Economy cruise would be 2300 RPM, 50F LOP (61% power
according to the MVP 50) and yielded 232 knots at 12.2 gallons per
hour. Result is 19.0 nautical miles per
gallon.
For laughs, I calculated the simple flat plate area for
several of the cruise conditions, still assuming 85% prop efficiency (a guess),
15.6 HP/gallon per hour (apropriate for 10:1 pistons and LOP according to
GAMI) and I got 1.91 square feet, about a 10% improvement on the stock
Lancair IV as reported by Martin Hollman. The cost: perhaps 1000 hours of
additional work for drag reduction, primarily ahead of the firewall.
If any are interested, I used a prop spinner mounting
method that hides all the screws with the new MT prop with Lancair
spinner. Must be worth at least 0.003 extra knots! Looks nice and
slick. If you are a fanatic, let me know and I will publish some
instructions as "builders tips."
Fanatic Fred
image001.gif
|