ACRE wrote:
Hi Paul, what do you mean by Chev Small Block based V12 Thunder Mustang? Is it 1
and1/2 SBC motors, like in a Lyc360/540 idea? Dave McC
Yes basically. The cylinder spacing, rod length, rod journal diameter,
main journal diameter, cylinder bore, valves sizes, etc., etc., are more or less
the same as the Chev small block. No doubt it has 50% more power but the
structurally life is probably less as the crankshaft torsional problem is
far more severe due to the extra length. It took Rolls Royce years to solve
the V12 crankshaft torsional problems and then they never got it much
above 1200 hours TBO. Of course the life of a Mustang or Spitfire
in combat in WW II was rarely more than 300 hours due to other factors.
According to Taylor the major problem with automotive cranks is fillet radius.
The automotive engine cylinder spacing is too close to allow adequate fillet
radius in the crank cheeks. This was a major factor in the downfall and bankruptcy of
the Chev big block based Orenda V8 aircraft engine. If you are going
to design a V8 or V12 aircraft piston engine you need to start with a clean
sheet of paper. V engines (and all other inline piston engines)
are trying to shove their crankshafts through the bottom of the blocks.
A horizontal opposed engine will weigh less due to the more efficient
structure. Lycoming built some horizontal opposed air cooled 8 cylinder engines
and they too had problems with the crank despite the wider cylinder spacing.
The beauty of the Wankel on the other hand is the shaft is straight through
yielding no concentrated stress problems up through four rotors and maybe more.
Paul Lamar
There are several implications related to the piston engine crankshaft problems.
Since the crank is unlikely to fatigue in less than two million cycles
and the race length at Reno is about 15 minutes. Having said that lets calculate
the number of cycles an engine goes through in a typical Reno race.
Lets say he is running the V12 at 6000 RPM flat out that is only 90,000
revolutions of the shaft at max power during the race. Given 2,000,000 revs
before the crank must be replaced means you could run about 22 Reno type races.
Running at 5000 feet means the NA
engine is not generating peak power so that too helps. If you reduced the
bore and consequently the displacement to limit the peak HP that too would
help. If you shortened the stroke to reduce the displacement that would help
even more as the rod journal to main journal overlap would be increased.
Another thing would be to never exceed some manifold pressure such as 20
inches of Hg or less reducing the continuos power greatly.
There is some manifold pressure where the engine will live indefinitely.
What that is is unknown but most auto engines are designed to run indefinitely
while generating only about 50 HP or less. That include the 400 HP Chev V8.
What you wind up with is an engine that has a rather poor power to weight
ratio or limited life for use in an aircraft.
An over the road tractor trailer diesel engine on the other hand is
designed to run at peak power almost indefinitely. Such a 300 HP engine
would weigh in the neighbor hood of 2000 pounds. I have always maintained
such an engine installed much further aft in a crop duster airframe would
make a great banner or glider tow airplane. Unfortunately it would be illegal
in the eyes of the FAA.
BTW Crossflow ("The future of aviation." :)) bragged to me at Sun & Fun they
were going to clean everybody's clock at Reno this year with one of their
multi 100 HP turbo Subaru engines installed in a White Lightening. Needless
to say they did not show! I suspect realty sank in when they could not exceed
250 MPH let alone 300 MPH. The fastest airplanes at Reno in the Sports class are
generating over 600 HP.... for awhile.
Paul Lamar
BTW I forgot to add that Nemesis was given three $50K (or more) engines by Lycoming.
Not something your average millionaire can afford.
Paul Lamar
The Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site. http://home.earthlink.net/~rotaryeng/
Copyright 1998-2004 All world wide rights reserved.