Paul et al,

I have what is probably a basic question.

I have often read that a propeller induces adverse vibrations into a
 drive train and that a belt driven PSRU is the way to isolate those
 vibrations from the rest of the components and engine in general
while allowing the propeller and engine to each operate in their most
effective rpm range. Yet I also see a lot of planetary (and other?)
gear drive units, especially in the information shared here, that
would seem likely to transmit those propeller vibrations. So in the
applications being discussed here how is a direct drive permissible?
Is it because the aircraft (e.g. the time to climb) are limited
purpose and limited life and thus not likely to be adversely affected
in the time they are operational?

Just curious and hoping to understand better. Thanks for any
responses!

Brad Chubb

The planetary was universally used in aircraft engines for years.
Only in WW II was a spur gear used in the water cooled V12's. All the
radials had planetary gear box's including the largest aircraft
engine ever made in quantity. That was the P&W R 4360 Wasp Major. It
 had 18 planet gears. The latests in large P&W turbo fan engines for
 airliners is a planetary gear box that is 98% efficient.

Direct drive requires a lot of displacement for a limited amount of
HP. The 2 rotor rotary engine is small displacement as aircraft
engine go at 160 cubic inches. However the time to climb engine is a
 record amount of HP (650)  for it's weight of under 250 pounds with
 gear box.

BTW The rotary is also a record amount of HP per cubic inch of
displacement at 4 HP per cubic inch. The former record holder was the
Offenhouser race engine at 3 HP per cubic inch.

The TTC record holder Grumman Bearcat had 2 pounds per HP to lift
while our TTC aircraft (Vans RV4 basically)  only has 1.5 pounds per
 HP to lift. Weight per HP is the name of the game in high climb
performance. Ask any truck driver :-)

We are about done with the installation and we are waiting until Dan
 Gray gets back from vacation to test run the engine on the
aircraft.

Paul Lamar


Paul, I had one other question about the P&W planetary. Are there two
outer gears so they can put one on from each side? That was the only
way I could think of that I could think of to put them all together.
there are splines on the sun gear to fit the drive shaft. There are
also on the outer ring gear but they are split so it looks like they
both come in from the outside. Bill Jepson


I don't know. The device is on display at the P&W museum in
Connecticut.

Paul Lamar


I can not remember from the last photos but if the planetary is built
with two carriers with opposing helix, then for removal the planet
carriers could be removed from each end. The two carriers could be
through bolted.

Dale Davies

Dale, I agree that the carrier's could be bolted together. But if you
look at the P&W planetary the Herringbone, ( double "V"), gears
internally all appear to be single piece. I'm not certain how the
final assembly is done.

Bill Jepson


It appears that the ring gear is split.  That would allow you to
assemble the sun and planet gears into an assembly then slip the two
 ring gears on from each side to complete the assembly.

Mark LaPierre



Yes Mark,
That is exactly what I implied. The fixed gears would need to be very
precisely timed so they wouldn't load the planets and sun gear.

Bill Jepson



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