----- Original Message -----
Sent: Saturday, July 10, 2004 7:36
AM
Subject: [FlyRotary] Three candidates for
Turbo Failure
I agree Dave.
This is of course all speculation about what
actually happened to John's turbo. But, at the moment, probably about
the best we can do. Having two failures that soon and with the same
results - loose compressor wheels - certainly indicates its probably just
not a random factor, but has some underlying common cause. I agree if
the compressor maps we are looking at are even close to the Mazda compressor
parameters, it is unlikely that John was in the surge zone. If he had
been according to what I have read, there would have been indications like
rapidly fluctuating pressure surges and sound and vibration associated.
Although with the engine at the back the sound may not have been heard and the
vibration could have been dampened - which a composite construct is good
at. So, apparently no evident to support the surge
hypothesis.
It does appear that whatever is causing it, the
stress appears to be concentrated on the compressor wheel.
Surges stress the entire rotating assembly but does particularly stress
the compressor wheel. I need to sit down again and calculate the
Pressure ratio and flow on the compressor chart after adjusting for the
pressure ratio creep caused by altitude. But, it would seem to take a
considerable increase in pressure ratio to move him into the surge zone.
The worst case situation for surge appears to be in boost with low mass flow,
in other words operating toward the left on the mass flow axis of the chart.
The further over you are the more like to enter it. But again - no
indication that is happening.
Overheating could indeed weaken the metal,
however, its my opinion that with the water cooling - overheating should not
be a problem. I mean almost all aircraft turbos I am aware of do not
have water cooling and survive for much longer, hundreds perhaps close to a
thousand hours. So you would think that with water cooling the Mazda
turbo would do even better - if overheating were the cause. Can't rule it out,
but I would put it further down the list.
My prime candidate of the three would
be overspeeding of the rotating assembly.
The turbine wheel is generally friction welded to
the shaft whereas the compressor wheel is held on with a nut. I believe
John indicated in one case the wheel was just laying in the compressor housing
and the in last case I believe he indicated it was wedged in tightly. I
don't recall how the nut is secured (other than torque). I'll have to go
down to the shop later and take a look.
I don't recall John mentioning how damaged the
compressor wheel was. I would assume that if it separated from the shaft
while spinning at 80,000 + rpm it would really tear up the wheel and
housing. On the turbine side I have seen photos where the wheel actually
tore through the cast iron housing as hard as that might be to
believe.
John do you have any photos of the wheels and
housing after the incidents??
I am still looking at the BOV and any possible
role it may play. I still appears to me that suddenly releasing the back
pressure across the compressor wheel when the BOV opens could open the
possibility that the compressor could be stressed. If the back
pressure across the wheel suddenly drops and lessens the load while
the turbine end is still getting the boost mass flow that there is the
possibly of overspeeding the rotating assembly.
I, of course, could be wrong and the
BOV effects not only be benign but actually helps prevent damage
in an aircraft operating environment as you and others have
suggested. If aircraft turbo systems had blow off valves
then we would have another data point, but of the several aircraft
systems I looked on the internet had either sized the A/r of the turbine
housing or use a waste gate type of bleed for the exhaust.
Like you say it could be a combination of
factors, but I do agree with the three you mention as to being the most likely
culprits.
Ed
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
Sent: Saturday, July 10, 2004 12:42
AM
Subject: [FlyRotary] Re: BOV more/less
air flow
Ed, Great write up! In general I very much agree with you on
all points but I want to free flow some thoughts that may modify people's
feeling about a BOV. I am even less of a turbo expert than you, but
here are some thoughts to consider.
First, I think we have to figure out exactly what is causing John's
turbo's to fry. IMHO we have 3 general candidates 1) overheating,
2) turbo overspeading, or 3) compressors into the surge region. While
it is possible that there is a combined cause, and probable that more than
one factor plays a partial role, most likely ONE factor is
primarily responsible for JTD (John's Turbos'
Demise).
IMHO that one factor is overheating. We know that heat is a
factor while according you the map you posted we are probably
not in surge or overspeed region most of the time. Also, turbo
overspeading and compressor surge are likely to present with very different
symptoms like surging pressures or a disintegrated turbo. In contrast,
overheating weakens the metals involved and would behave much like what John
Describes. In this case it seems to be turbine axle and possibly the
blades that are not up to the task.
There are 2 ways to improve the situation. Reduce the heat, of
course, and reduce the forces acting on the metals.
The BOV can be very instrumental in reducing the force acting on that
axle, and possibly also slightly reduce the heat transfer. My BOV
(which I assume is fairly standard) is not an on/off valve, but a spring
loaded valve that will open more or less depending on the pressure in the
plenum, the ambient pressure, and the pressure differential across the
throttle body. There is no way it will instantly drop the pressure
significantly. In normal operation it would never go from full closed
to full open suddenly, but rather just start leaking more and more air as
the combination of those three pressures changes. The only time it
would suddenly open is when the throttle is suddenly
closed.
While on one hand it does seem very inefficient to compress air only
to have it bleed out before entering the engine, I believe it is far more
inefficient and potentially harmful to have the compressor constantly
fighting against a large pressure differential. That pressure
differential is the largest determinant of the force on the
axle.
Further, if the turbine wheel is free to turn a little easier
(faster) it places less back pressure on the exhaust and less heat will
be transfeffed. It is true that the faster RPM increases the radial
stresses on the blades, but radial stresses are tolerated much better than
the axial stresses caused by high back pressure. (Since I have decided
that heating is the problem and not overspeeding.)
There is no doubt that we both agree on the BEST solution: a larger
turbo with an even larger waste gate placed perhaps a little further from
the engine. But until I change my set-up to fit such a turbo and
waste gate I think the BOV is a helpful addition.
Further, the BOV does provide some measure of protection against over
boost like what happened to Todd.
Dave Leonard