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.
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
http://www.andersonee.com
http://members.cox.net/rogersda/rotary/configs.htm#N494BW
http://www.dmack.net/mazda/index.html
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
Dave,
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.
George (down
under)