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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