Bill Freeman wrote:
BCGARDNER@aol.com wrote:
Paul,
The latest (May) Sport Aviation has an article on page 38 that chronicles a
guy's Velocity with a GM 4.3 liter V-6 in it. Aside from the fact that he
chose the wrong engine ;^) I was wondering if you or other list members had
comments on his engineering of the adaptation, including his P-51 inspired
cooling system.
Barry Gardner
Wheaton, IL
***SNIP *****
Barry and Paul,
I would worry about the cast iron crank with the prop
belt side load, which the crank was never designed to take.
At least he did take the power off of the flywheel end, which
is a BIG improvement over the other end, but perhaps not good
enough. It appears to have no outboard bearing, as do most of
these type of PSRUs.
With 220 hp @ 4500 rpm and assuming a 6" diameter small toothed
pulley on the crankshaft, the torque works out to be :
Torque =( 5252 * HP)/RPM = (5252*220)/4500 = 256 ft-lb
256 ft-lb * 12 in/ft = 3081 in-lb torque at max power (climb).
For a 6" diameter pulley, the tension in the belt is
3081 in-lb/3"radius = 1027 lb SIDE LOAD ON CRANK
Even at 75% cruise power, this is 770 lbs side load on the
crank. If he uses a 4" diameter pulley, the direct side load on the
crank and (mostly) first main bearing increases to 1540 lbs max
and 1155 lbs in cruise!!!
This is not something that the crankshaft designer
ever had in mind, and I fear that the crankshaft will crack from
fatigue relatively quickly. Without knowing the dimensions of
the crank and the design allowable stress for the particular alloy, it
is difficult to accurately assess the risk of failure, but it is a good rule
to avoid adding LARGE loads in systems that you have not done a
thorough, accurate engineering design review. This is especially
important with more modern designs which are much more carefully
analyzed than older designs ever were, and therefore have less
"overbuild" in the structure which used to be necessary to cover ignorance.
Since weight is a MAJOR big deal in gas mileage, all auto mfgers are
doing a lot of work to lighten their cars, and this means less capacity
for overloading parts before they fail.
This sort of PSRU needs an outboard bearing to reduce the bending
loads in the crank. The difficulty is accurately aligning this outboard
bearing so that it will actually carry the load without causing all sorts of
weird loads in the crank and bearing due to misalignment.
Cantilevered PSRUs (spur gear or belt type) put very large loads in the
crank that are not included in the original design calculations. This is
the main reason that epicyclic (planetary) gear reduction is so attractive
for auto conversion. Epicyclic systems are also very compact and offer
alternative ways to flow the power, depending on your needs.
Bill
You know Bill I totally agree with the above. In fact I have
a drawing of a belt PSRU on the ACRE NL web site that calculates
the side load.
I thought it had an outboard bearing. Did you see it
at Oskosh or Sun N Fun 98? There are no pictures in the magazine
of the lower part of the PSRU.
The cast iron crank or something else will break sooner or later
anyway with or without an outboard bearing. Those engines are
just not designed to run at 165 HP (75%) continuously. Now
if he runs it at only 100 HP it should last three or four hundread hours
maybe 500. Trouble is you don't know exactly when it will break.
It is not like several hundread thousands of these engines are
out there flying for thousands of hours with or without cast
iron cranks. Car service at 40 HP average does not count.
It is kind of stupid to build an auto engine and not put
in aircraft quality parts when they are available. If they
are not available is is idiotic to consider such an engine.
Even car racers don't go racing with cast iron cranks.
At least the winners don't. Of course aircraft quality parts
are only half of the issue. The detail design is the other half.
Now before somebody jumps on here and says the rotary is a car
engine the situation is entirely different. The eccentric shaft
is over designed and made from forged 4340. There is little else
that can break unlike a piston engine with a bunch of other
parts jerking around. (Jerk is the rate of change of accelleration.
....really!)
A Velocity is not a good airplane to crash in landing at
75 knots. Twice the energy of an airplane landing at 50 knots.
(E = M*V^2)
Paul Lamar
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