Well, I am pleased to report that my turbo, stock housing with TO4 wheel and
60-1 compressor with a fixed open waste gate has lasted over 230 hrs without
a single problem.  The solution to good enough?  Make it better.  I just
finished my annual during which I upgraded my turbo to a similar TO4 big
shaft wheel, but now it has a TO4E 50-trim compressor and the wastegate
flapper and actuator are back in place.  Why the change?  Either I am a
glutton for punishment or I am starting to pin down this turbo issue.  For
those of you not familiar with my turbo history, here is a quick recap:

First turbo - the stock second generation with waste gate flapper
installed, ridden hard and put away wet...  it was run at full throttle at
high altitudes, often at peak EGT.  It lasted 70 hrs before the turbine
wheel fell apart and broke off.  The thought at the time was that the stock
turbo was just too small for this heavy duty abuse and was overspeeding at
altitude.

Second turbo - small shaft TO4 wheel with 60-1 compressor and removed waste
gate flapper.  It was also run mostly at peak EGT but the power produced was
somewhat lower because of the bypassed exhaust.  It lasted about 140 hrs
until it suffered the same fate as the first turbo while I was in a long
full power decent (read extra high RPM and EGT's).  My thinking at this
point is that the high EGT's are primarily the problem.

Third Turbo - The one I just removed after 230 hrs.  Same configuration as
the second but I kept the EGT's under 1600, mostly as lean as I could while
keeping the engine running not too rough.  It was also run at pretty low
pressure ratios.  It could only make about 38" MAP on take off and about 40"
at full bore down lot.  At 16k altitude it was down to about 22".  My
curiosity now is weather I can perhaps increase the boost and continue to
have the turbo last, as long as I keep the EGT's under 1600.  I know,
glutton for punishment...

Other issues..

Lean of Peak operation.  During the turbo change I discovered that my stock
cast iron exhaust manifold had developed a number of serious cracks and some
advanced erosion.  Much of the erosion/oxidation was near the mating surface
with the turbo and it had started a small leak.  My concern is that all
those hours of LOP operation allows too much oxygen into the exhaust to
promote to oxidation and accelerates the problem.  The question is, should I
start running rich of peak to prevent rusting away the exhaust system
components?  ROP operations also allow me to keep even lower EGT's (for the
turbo) and smoother engine running (for the wife).  Turbos and exhaust
components that I get from cars usually have the appearance of being run
rich, rather than the dry orange color of the inside of my components.  The
only obvious downside is the obvious much increased fuel usage.  I think
that for the next couple hundred hours I have going to have to bite the
bullet and fly ROP (while at higher power settings).  Thoughts?

S.A.G. is really baffling.  Spark plugs only seem to be lasting 20-30 hrs
before SAG starts kicking in.  Noted by a distinct and marked loss of power
as I cross through about 1500' AGL after take off.  The problem is usually
intermittent and mildish at first becoming more consistent and remarkable as
the hours accumulate on the spark plugs.  The problem is always solved by
changing out the spark plugs.  I am switching to colder platinum plugs in
order to try to get them to last longer.  Perhaps running ROP will help them
last long too?

Time for a new Prop?  On a test flight yesterday, the engine was
running great.  No SAG.  At 7000' and 33"MAP I was at 6500 RPM,
about 1/3 throttle and doing 175 KTAS [201 MPH].  It is a 64
length x 77 pitch 3-blade Cato, but that is more like an
effective pitch of closer to 71".  I seem to remember being able
to get closer to 185 KTAS [213 MPH] at 6500 RPM in the past on
the same prop but I did have a little taken off because of the
low output of the previous turbo.  After some more experience I
may discover that it is time for another new prop.  Might soon
contact Paul Lipps, I liked his article in the last issue of
Contact!  mag.  If that is my worst problem, I am going to be
pretty happy.

Doing some formation flying with the guys next week that may make its way
into a movie...

Keep building!
--
David Leonard

Turbo Rotary RV-6 N4VY
http://N4VY.RotaryRoster.net
http://RotaryRoster.net

I really appreciate Dave's persistence on exploring
turbo charged rotaries in aircraft use.

The turbo life issue is not only the EGT it is the turbo RPM as well.
If the RPM is high enough the blades will disintegrate regardless
of the turbine inlet temp. The load on the blades is M times V squared
divided by R. V is the the tangential velocity of the blade
CG. M is the mass of the blade. R is the radius of the blade CG.
All rotating device suffer from this problem. As the temp goes up the
strength of the blade material is reduced. At 1600 F strength of
exhaust valve steel is down to 5000 psi which is next to nothing.
See the attached chart. Epoxy resin is 3000 psi :)

Here is a chart from Caterpillar on the situation. The turbine
MUST be kept within the limit lines of both temperature AND rpm.

I have repeatably tried to get Dave to install a Garret turbine
RPM kit. In fact I will buy it and send it to him. No luck
so far. If you send me the compressor Dave I will install
the RPM sensor for you and send it back as well. Perhaps Zack can
convince him :) We need to know the turbine RPM!!!!!

Somebody needs to try a Capacitor Discharge ignition system
to further investigate the spark plug "sag" problem.

Inconel is probably the only answer to high temp corrosion
of ferrous materials. I have seen many cracked cast iron
turbo exhaust manifolds.

Paul Lamar

Dave,


Good news on the turbine purchase.  It sounds like the same
one John Slade is flying and the Cozy Girls purchased.
They both recommended Turbonetics though the turbine is
based on Garrett's model.  What is the turbine side A/R
and trim?  P trim?    I hope you consider Inconel or
Stainless 316 for a simple exhaust manifold.     I look
forward to hearing about the MAP  at 18K'.

Doug in Japan