Dick, I think you will find any spur gear PSRU that provides offset not
worth
the extra weight. It remains to be seen if any of them last as long as a
Ford
six pinion planetary. The reason for this is a thing called gear line
contact length. Almost any planetary has much more gear line contact
length
than any spur gear PSRU of reasonable size. The stresses are highest in
the
gear line contact. The gear line contact length on a Ford six pinion is
about
5.4 inches long in total. To get this in a spur gear transmission
results in a monster gear box. Here is a picture of such a spur
gear PSRU. This is out of an S51 and the box weighs over 120 pounds.
The line contact length is still only about three or four inches.
The vast majority of successful geared aircraft engines, and that
includes huge radial piston engines, almost all turbo props and some
turbo fan engines, use planetaries for that reason. The WW II liquid
cooled V engines are exceptions because they really need the off set.
We have ten of thousands of accumulative hours in 100's of Ross's
and Tracy's PSRU's.
Why do you need the offset?
Paul Lamar
In order to keep the prop axis the same as the
plans LEZ, the engine has to be mounted so high that parts of it are
above the slope of the cowl.
The water pump, even a cut down version is still too high. The intake
runners and the back of the
engine all sit proud of the original slope of the cowl, which is an
extension of the slope from
the rear of the fuselage. I plan to move the alternator from the top
down to the cool side, but I
wouldn't even have to do that if I drop the engine a bit. I wanted to
raise my thrust line at
least an inch. The LongEZ was designed for 100-115 HP and I wanted to
swing a larger diameter prop
without running into ground clearance problems. With the 13B and a Ross
or RWS drive the prop
flange is farther back than the Lycoming installation which can mean
problems during rotation and
flare.
I took a hard look at the belt drive configuration you have on your
website. As far as I
know the only person making a similar setup is Ken Welter and I think
that will give me too much
offset. I still need to keep room underneath for the rads. As it turns
out, I will most likely go
ahead and use the RWS RD1A drive and work around what are probably
relatively minor drawbacks for
my installation. Since his ratio is 2.17:1 I probably can't swing as
large a prop as I would like
anyway, without the tips going sonic. I will get used to looking like a
warthog. Maybe that's an
idea for a name for my airplane. Hmm. I always appreciate any feedback I
get from you and the
other experienced engineers on the list. I always take experienced
advice seriously and have
changed my planned approach many times based on this information. Thanks
for all you do to help
those of us who are more technically challenged. Dick Gregory LongEZ
N613NC
I think there are some Ross 2.85's out there for sale. Advantage
is it is about five or ten pounds lighter than Tracy's. There is also a
short
version of the Ross as I recall.
Paul Lamar
Is it possible in your installation to not run the intake tubes over the
engine but to run them around the engine either fore or aft? Under might be
possible if you have cowl clearance outside of the exhaust system. Just a
thought. There is usually several ways of going around the bush,but some may
be more difficult than others.
smale@ncinternet.net
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