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<< Lancair Builders' Mail List >>
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Posted for Epijk@aol.com:
In a message dated 12/30/2001 8:38:41 AM Pacific Standard Time,
glcasey@gte.net writes:
<<<...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." ...>>>
With a 77 inch prop at 2800 RPM and a TAS of 210 knots, your tip speed at
5000 ft (std day) will be 0.917 mach, and at 10000 ft (std day) it will be
0.935. My experience says: be very cautious with predicted performance curves
when they suggest better than 86% efficiency anywhere, and beware of any
numbers at tip speeds above 0.85. Calculate your "J" ("advance ratio") and
power loading numbers for the anticipated performance envelope, and manage
your expectations with real data. 'nuff said.
<<<...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)....>>>
There are several good reasons for that.
<<<...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....>>>
AND
<<<...I hope to run a full-power manifold pressure of 40 inches, but in real
life I expect 45....>>>
The mechanical compression ratio alone isn't much good at predicting peak
cylinder pressures. Other significant factors include: inlet air density
(function of MAP and inlet air temperature), camshaft lift, duration,
lobe-spacing; intake flow path characteristics, and several more.
<<<...At cruise power the engine mechanical and thermal loading will be about
the same as your average truck engine at 60mph ...>>>
I'd really like to see those numbers. "AVERAGE TRUCK ENGINE" is awfully
vague. Numbers are far less so.
<<<...so I expect no reliability issues, even without high-tech
components....>>>
With a 9.0 mechanical compression ratio and 45" MAP and no "..high-tech
components...", you could be in for a bit of a surprise, especially
regarding:
<<<...To start I expect 500 hours between valve-jobs/head gasket replacement
and 1,000 hours TBO....>>>
BTW, you never stated a target displacement, but "no hi-tech components"
suggests a GM crate engine of 350 CID. Close?? If so, 325 HP at 2800 RPM from
350 CID is 263 BMEP. On a GM crate motor??
Not trying to rain on your parade, but If you're interested, I'd be happy to
provide you with more design data, including stuff like the hydrodynamic
wedge operation of plain bearings and some (real-life) experiences with
bearing loadings v. surface velocity; steady-state heat rejection experience;
and more.
Jack Kane
LML website: http://members.olsusa.com/mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please remember that purchases from the Builders' Bookstore
assist with the management of the LML.
Please send your photos and drawings to marvkaye@olsusa.com.
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