In my opinion, one
of the things that we tend to not pay sufficient attention to is the extent
of the demand we are putting on turbos. Without getting into the
specifics of boost ratios vs ambient pressure, suffice it to say that a
turbo compressor has to work considerably harder at altitude to maintain the
same pressure ratio in the manifold.
The ambient
air pressure is of course less at 8000 MSL therefore density less as well
and the turbo simply has to turn faster to create the same amount of boost
pressure out of the less dense air. The only way the turbo can do that
is to turn faster at altitude. So you have that hot rotary exhaust
pushing those small turbine blades, faster and faster.
As we have
discovered, the stock RX-7 turbo succumbs quickly to those stresses (simply
not designed for them). Even with the TO4 compressor mod, which
clearly helps by reducing the turbine rpm needed to produce X boost, the
stock turbine and bearing just do not appear up to the stress for long
duration – although 130 hours is better than I thought the stock bearings
would do.
A precaution I
would take before flying back with the turbo that condition is to disconnect
and plug the oil line to the turbo. Should the bearing have taken a
worst beating 0r continues to deteriorate, you would not want to be dumping
that precious engine oil into the exhaust – although there should be plenty
of smoke to tell you when that condition occurred – if there is anybody
around in West Texas to let you know {:>). So I would disconnect
and plug the oil line and coolant line to the turbo – just to make certain
no leaks occurred.
Just my 0.02.
Glad, nothing more happened to the engine or you, Dave.
Ed.
From:
Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of George Lendich
Sent: Saturday, June 21, 2008 6:24 PM
To:
Rotary motors in aircraft
Subject: [FlyRotary] Re: Another Turbo
Bites the dust
It's
amazing how fast 130 hrs goes when you are having fun.:-)
I
took off the exhaust today and too a look. One of the blades on the
turbine wheel is gone, but the axle seems otherwise intact, though it may
be off kilter and that is what is causing the resistance when I try to
turn it. So I am to sure if the problem was primarily the bearing,
or overheat, or overspeed. According to my calculations, turbine
speed should have been well within the green area. (way down around 60k
RPM - near the bottom of the chart.)
One difference with this
fligh... I usually run well lean of peak, but this particular leg I
was running a little late so I decided to push it up a bit by running a
best power setting. I figured I was OK because I was high enough
that I was only showing 21" MAP. But the failure happened in descent
when I kept best power as I came lower and the engine RPM came up to about
6300 (more exhaust available).
Combination of higher than normal
EGT's and 'you number is up, turbo'???
Dave
Leonard
I follow your threads
carefully as you have put so much into the Turbo installation, I don't
know anyone who has done more but I'm sure there are, it's just we/I am
not aware of them. From what your saying, I think your right on the
money - heat is the killer (in this case).
I personally would like you to
try something remote as per suggestions by others. I think Paul's idea is
sound, but may be still to close to the heat for my
liking.
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