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<< Lancair Builders' Mail List >>
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....Assume you're using a
real (as opposed to mythological) prop with sufficient diameter to absorb
325
HP (for the sake of conversation and the benefit of the doubt, say 84"). On
a
standard day, at sea level, climbing at 100 KTAS, 2700 Prop RPM gives a tip
speed of 0.90 mach (well beyond the high efficiency range of contemporary
"real" props), with the tip-mach climbing right along with the aircraft. At
225 KTAS, the prop (at 2700 RPM on a standard day) has a tip speed of 0.95
mach at sea level and 0.99 mach at 10000 ft. (and worse on a colder than
standard day). If you've got a prop that makes 85% efficiency at 0.9 tip
mach, I WANT ONE TOO.
Now, lets assume you really do have some magic prop which will work at 0.95
tip-mach, making 2700 RPM really usable. At 2700 RPM, you'll need 632 lb-ft
of torque to make your claimed 325 HP (At 2400 RPM, a more realistic upper
limit, you'll need 711 lb-ft of torque for 325 HP.)....
Jack Kane
EPI, Inc.<<
I just left part of the comments above so one can get Jack's drift - I agree
with the comments. The difference is that I expect to use a 74 to 77-inch
prop, reducing the tip speeds about 10% from the above numbers, putting them
right in the range of "conventional wisdom." I would like to run the engine
at 2800 rpm, the highest rated speed published by MT (they don't seem to be
willing to budge from the 2800 rpm limit). The result would be poorer
low-speed efficiency compared to the larger, lower rpm combination. Runway
requirement will be longer. As to BMEP, yes, the engine will be
turbocharged, and no, I don't expect to get the same engine life as a
naturally-aspirated aircraft engine - but maybe equivalent or better than
the 350 hp TSIO-550. To start I expect 500 hours between valve-jobs/head
gasket replacement and 1,000 hours TBO. That is probably half what could be
achieved with more development. With a compression ratio of 8.5 to 9 I
expect that the peak cylinder pressures will be only 10% higher than in an
automotive application. In my "dreamland" I hope to run a full-power
manifold pressure of 40 inches, but in real life I expect 45. At cruise
power the engine mechanical and thermal loading will be about the same as
your average truck engine at 60mph so I expect no reliability issues, even
without high-tech components. Overall, the reliability running at 31
inches/2700 rpm compared to 25 inches/ 3500 rpm should be much better.
There are lots of design comparisons to be made between typical aircraft and
typical automotive engines - I think my approach makes them roughly
equivalent.
Gary Casey
ES project
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