Engine Gear Drive:


It's been a long time since the AirSig forum.  Back then, there were few choices for a PSRU and the people that made them were quite proud of them.  Everyone wanted between $3000 and $4000 for what amounted to unproven, undocumented, non-"engineered" piles of gears or belts and aluminum.  Torsion vibration was on everyone's discussion menu and no one had any real numbers when asked about strain testing or torsion measurements.

A guy was pretty much left to throw his decision to the wind and go where it landed.  Here is how I came to my decision to use the Marcotte PSRU.

The Ross unit was having noise problems.  They worked great, but, it was feared that the noise would lead to problems later in life.  This was later proven.  Ross had several units in operation, again with "some" hours on them.  The downfalls of this unit included, IMO, the reliance on engine oil pumped from the engine for lubrication.  Also, the fact that it caused the prop to rotate in the reverse direction and the drive housing was really long.marcotte_w_ross_2.jpg (40879 bytes) This created a problem when I did a rough CG calculation.  I passed on the Ross unit.

Eggenfellner didn't have any real hours on his units yet.  He had a neat idea.  K.I.S.S.  He converted a 4x4 transfer case with a little machining and came out with a nice unit that solved a couple problems.  The prop shaft turned the correct direction, the unit allowed installation of the stock starter in a way that allowed it to be tucked in closer to the PSRU, it kept the prop hub on the crankshaft centerline and it didn't rely on the engine oil  for lubrication.  He hadn't done any torsion or strain testing either.

Marcotte had started building gear drives for use in V8 powered aircraft for some sort of charity work.  He had several units with "some" hours on them, but, still no real torsion or strain measurements.  This drive had the most good things going for it.  It had the prop turning the correct direction.  It didn't rely on engine oil, it contained it's own oil.  The offset to the crankshaft put the prop 1.3" above the crankshaft centerline, which, in my installation the added room for the intake manifold was nice.  The hours he had on units and the fact that the design came from a much higher horsepower application were main selling factors on this unit.  Everything else was a bonus.

The only changes I had to make to the Marcotte unit were the addition of an oil overflow container, oflow.jpg (45760 bytes) similar to a radiator overflow, and two brackets at the bottom two mounting bolts as a safety measure to hold the unit to the block in the event of the bell housing lower mounting thread failure.  The upper bolts could be installed completely through the bell housing with a nut installed eliminating the chance of threads pulling out of the block.



bracket.jpg (30082 bytes)

The brackets idea was actually designed by Marcotte and implemented into a bed mount for the Subaru that he is currently selling.  The basis for the brackets is that the bolts go into aluminum threads in the block.  The lower threaded holes are not as deep as the upper holes. The bracket ties these lower bolts to another pair of holes just under the block.  My brackets look different than his, but they perform the same task.

The internals of the Marcotte drive are very robust.  Much larger than those in the Ross or the Eggenfellnerguymgear.jpg (20641 bytes)  This may be overkill, and I admit, it does cost some weight.  The overall weight of the Marcotte drive is 43lbs.  I've heard that the Eggenfellner is around 36lbs.  I haven't verified this though.