<< Lynn,
 >
 >    What is your take on the apparent cavitation mentioned by Franois in
 >
 >  the previous post?
 >
 >    Kelly Troyer

  The problem seems to exist only on their test stand not in the aircraft. So
as an engine problem it does not exist. The pump ring and rotor are made of
powdered metal and will tolerate cavitation only for a few minutes. They have
much more time on the engine than that and even more flight time, so it is a
test stand thing they ran into. The pump, or should I say the pickup tube does
run into a bit of a vena contracta that limits the upper end of pump
performance before the top output can be achieved. They have made the change to the
pickup (adding a bell shape to the end of the tube, but that was apparently no the
cause.

It is a kind of thing you would only see at the end of a long straight-away
on a really hot day if you had used a multigrade nonracing oil. The engine will
foam up the oil after a long run above 9,500 RPM. Multigrade oil is the worst
at doing it, and nonracing oils have very little anti foaming agent included.
So, the oil foams up, the hot oil is transformed into a higher viscosity from
the heat, entrained air displaces the oil and the vena contracta is maxed out
and pump output sags. So you start the lap with 100 PSI and end the lap with
80 PSI.  No big deal at all, as once below 9,000 in a few places, the windage
tray starts to burp the air out of the oil and things are good again. This is
not true cavitation because there is no damage to the pump even if you do this
for years.

With racing oil there is no problem. With synthetic oil there is no problem.
With a low viscosity dino oil there is no problem. With Max revs below 8,000
RPM continuous there is no problem.

If I had to put the pump back inside the engine, I would use the racing beat
pump with heat treated rotors and the "O" ring groove at the rear of the pump
housing. There will always be some suction leakage and pressure leakage from
the pump body iron junction, so that would help a bit. You can adjust the
relief to give well over 100 PSI and what more would you need from a pump? Anything
over 65 pounds of oil pressure is just fine below 9,000 RPM.

So, they did something on their test rig while measuring pump output that
gave them a start when it cavitated. And now they are reviewing their setup to
see why. It is not a problem in the engine. It is on a test stand.

On the bigger picture, I feel that it is important that these people are
successful in getting this certified. There have only been experimental advances
in aviation for ages in this country and around the world because of lawyers.
The government has to step in and limit liability claims of these nut cases who
sue because of red hair.
Or because Dad flew his Piper into cumulogranite.

We should have spent the last 40 years developing newer better aircraft
engines not dying behind the 1920s designs our great grandfathers trained on. Part
of every penny you spend on aviation and related businesses is a tax that goes
to the trial lawyers of this country. You didn't get to vote on it, but taxed
you are anyway.
Every product has a hidden charge for liability insurance. We must reform
tort law in this country or the whole world will pass us by. The first step would
be to never vote for a lawyer for anything, not even dog catcher. But I run
on again.

Lynn E. Hanover

 
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