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Sorry to hear about blowing another turbo,
John.
I suspect that the period will vary for
different people (depending on how much and often boost is used) but
- if you use any appreciable amount of boost continuously, it sure
looks to me that you will end up overspeeding and overheating the unit which
would eventually lead to the turbine/compressor wheels
failing.
The size of the exhaust housing is probably
more the problem than the compressor - although that could stand to be a bit
larger as well. The Mazda engineers probably only designed the turbo for
the 0-60 time to give the RX-7 that "kick in the pants" feeling. So the
average driver would probably not stay in high boost more than a few seconds at
a time. Another consideration is the turbo in the car application would
get a chance to cool down a bit once the car reached top cruise speed and boost
decayed - that opportunity is much less when under constant boost (even small
amounts).
Given how much mass flow is produced by the engine
at 5000 rpm and higher, even the stock waste gate has trouble venting enough
exhaust gas. Even with the Rx7, some drivers refer to the problem of
Boost Creep whereby the boost exceeds the capability of the waste gate to hold
it down and therefore more energy spins the turbine which increases the boost
more, which ..... .
You have not mentioned any problem holding the
boost under control, so the problem could be more due to a continuos heat load,
really sort of hard to tell.
Another failure mechanism is a compressor wheel
operating in the surge zone. What is a safe operating zone at sea level
may not be at altitude at the same boost level. The pressure ratio
continues to increase (for the same boost level) as you increase your altitude
and decrease the ambient air pressure. This pushes the operating point
more or less vertically and to the left up the compressor map chart from the sea
level operating point (mass flow vs pressure ratio). If you will notice on
almost all compressor maps going that direction far enough will put you
operating in the surge zone.
I've attached a compressor map (don't have any idea
how close a match to the Mazda one - just an example). Assuming that ambient
pressure was around 7 psi at 11,000 and assuming you were flying with approx 4.5
psi boost (9 inches Hg). At sea level that and 5000 rpm would give you a
mass flow of approx 20 lbm/min but at 11000 feet it would be closer to 15
lbm/min. Also at sea level a boost of 4.5 psi would give you a pressure
ratio of approx 1.3. The same boost level at 11000 ft on the other hand
would give you a pressure ratio of 1.6. Note on the chart, both the lesser
mass flow (from 20 lbm -> 15 lbm) and higher pressure ratio (1.3 ->1.6 at
the same 4.5 boost) move you closer to the surge zone of the compressor
map. Now with this compressor map you are still OK in fact right in the
middle of your efficiency zone. But running more boost at less rpm moves
you further up and to the left on the chart closer to the zone.
However, if you had hit the surge zone I would
expect you would see pressure fluctuations, possible hear or feel the vibrations
of the turbo surging and you did not report any. So not too likely that
was the problem. Just several things which could cause it and without
instrumentation, I don't know how you can tell.
A different turbine housing with an larger
A/r and external waste gate (and now we are starting to talk additional
weight) OR the modification offered by the Australian firm opening up the
turbine housing as well as the internal exhaust gate port would be something I
would strongly consider.
I never hear it Richard? from Blue Mountain
Instruments? ever had any trouble with this turbo unit. I forget
which one it was, but it was not the stock Mazda.
Ed
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
Sent: Tuesday, July 06, 2004 1:26
PM
Subject: [FlyRotary] Second stock turbo
bites the dust
Steve, Dave and anyone else running a stock single stage turbo - you
might want to pay attention....
This
morning I managed to break Rusty's turbo. I had been cruising at 11,000
ft with MAP 38, rpm 5100 and 175 kts TAS (about 155 IAS) for almost 2 hours
when rpm suddenly dropped to 2500. A 1/4 turn of the mixture toward rich
brought it up to 3500. Fuel pressure was 40psi, but I turned on the other pump
anyway. Coolant temp dropped from 185 to 150 and EGT dropped from 1400 to
1000. Oil pressure was constant at 90PSI.
I
was 38 miles from home over a desolate little strip by Lake Okeechobee. I
figured I could glide to either field from halfway, so I headed home. The
engine ran consistently at reduced power. I maintained 11,000 to the halfway
point, then began a gradual decent to arrive over the field at 6000. Once
assured of the field I tried killing alternate sets of coils - both gave a
decrease in rpm - and alternate sets of injectors - both killed the engine,
(or reduced rpm enough that I didn't want to know).
One
[more] high & fast precautionary landing later and the plane was back
in the hangar. I'd thought the new Radio Shack resistors on my secondary
injectors might have given out, but no. A quick look up the exhaust pipe told
the story. The compressor wheel is sitting at an odd angle blocking the
outlet, just like last time. Apparently I was trailing a vortex of black smoke
as I descended into the pattern. This probably helped keep the spam cans
and whirly birds at bay while I took the active. :)
One
failure might be a bad turbo. A second one is enough to prove to me that the
stock turbo just can't handle the punishment of continual boost. (just like Ed
said it wouldn't :)
I
thought you guys running or planning to run the same turbo might like to hear
the story as soon as possible.
Regards,
John
(boost for all you're worth) Slade
Now
where's the phone number of that Ozzie guy????
No they use solid
state. I think it is 2GB Compact Flash. It says somewhere on their web site.
You can get 2 GB CF cards and IDE adapters for
them.
Alex
Madsen
-----Original
Message-----
From: Rotary
motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Finn Lassen
Sent: Tuesday, July 06, 2004 7:27
AM
To: Rotary motors in
aircraft
Subject:
[FlyRotary] Re: Into the blue again :-)
Hmmm.... Doesn't the Blue
Mountain and other glass panels use harddrives?
I thought that the
inside of a harddrive was sealed.
Picking a random drive on Seagate's
website:
Environmental
Operating Temperature (°C) 0 to
60
Nonoperating Temperature (°C) 70 to -40
Operating Shock (Gs) @ 2
msec 63
Non Operating Shock (Gs) @ 2 msec 350
Acoustics,Idle (Bels-typ
sound power) 2.2
No mention of ambient
pressure.
Finn
Ed Anderson wrote:
Boy, now here is an example of what kind of information we have access to on this list. Now that Ernest mentions it, yeah, I recall that the heads of the hard disk float on a cushion of air - but, I would never have thought to associate altitude with hard drive crashes! Thanks Ernest. Ed Ed Anderson RV-6A N494BW Rotary Powered Matthews, NC ----- Original Message ----- From: "Ernest Christley" <echristley@nc.rr.com> To: "Rotary motors in aircraft" <flyrotary@lancaironline.net> Sent: Sunday, July 04, 2004 5:48 PM Subject: [FlyRotary] Re: Into the blue again :-)
Haywire wrote:
Message Today we flew for 6.3 hrs and everything was great again. We calibrated a few items including the electronic governor for the IVO
prop.
What a difference that makes. Also calibrated the PSS AOA and the Dynon
AOA
and they each are phenomenal tools. The engine is running great with no major issues at all. I do have a little tweaking to do on the low MAP
table,
but nothing urgent. Then only problem that I had today was that my Sony
Vaio
laptop doesn't seem to like high altitude. I have a small Vaio and have built a place for it to mount easily and use it to display Jeppenson's FlightMap in-flight GPS program. It works great until 10,300' where it
would
then display the blue screen and then reboot. After the 4th time it
refused
to reboot again so now I'm forced to use the system recovery disc and
wipe
the disc clean. I hate to think about all the files that I said I would back-up soon... :-(. My old laptop still works fine(using it now) so
maybe
I'll try it tomorrow.
All hard drives have a spinning platter with a read/write head riding a cushion of air just above it. Go to 10,300' and there isn't much of a cushion left. The head will fall into the platter turning at 7500 or 10000 rpm. I think you'll be lucky if the drive ever works again. -- http://www.ernest.isa-geek.org/ "Ignorance is mankinds normal state, alleviated by information and experience." Veeduber
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