Hello from Colorado. Haven't seen any notes on rotaryaviation web
site lately, so wanted to contact you re: 1. do you have a
compressor map for the 13BT??, and 2. any autopsy on what may have
happened to the stock turbo??
I have never seen a map for the
stock turbo, not because I haven't tried. It is an HT-18 (Hitachi), I hear
it is very similar to the T3-60 trim that you were looking at. No official
post mortem. It is clear that the axle fatigued and broke at the base of
the turbine wheel (same as both of John Slade's failures). Blades probably
damaged by hitting the turbine housing, but it may be that a damaged blade
caused an out of balance that led to the axle fatigue.
I am still trying to figure out what may have caused what happened to
your turbo at 14,000 density altitude and 36"?? My guess is that it went
out the top of the map...either overspeed and/or surge, but without the map, I
can't do more than guess. I am amazed that the engine didn't experience
detonation and failure if the compressor map looks anything like I
imagine. Since quick response is one of the primarily requirements
of a stock turbo and they operate normally at 5,000' DA, operation at high
Pressure Ratios is not normally a prime requirement.
I was at 12500 MSL (about 14k DA)
and 32.5"MAP, 6100 RPM, EGT 900C, stock waste gate. IAS was 160 kts. TAS
190 kts.
Attached are some jpg hand/head scratching which
lead me to believe a T3 with a "60" trim should work just about right.
Please excuse the ramblings, but if you study it just a bit, I think it will
come clear. If you compare the bottom typical cross country operating
flight profile on the "vertical" page with the operating points on the RH
compressor map (T3-60 Trim) of the "horizontal" page, it looks to me as though
one could operate right in the heart of the high efficiency portion of the
map.
You are right that it does look
about right. But here are some things to consider.
1) As far as I know all
compressor maps were made at sea level, you cannot necessarily extrapolate to
high altitude. In other words, suppose you see a point on the map which
has a P/R of say 1.5 with 'x' lbs/min. Moving to the point on the map with
a p/r of 2.0 and the same mass flow does not mean the the intake air was less
dense (moving up in altitude), it reflects a state where the intake air is still
at sea level but the engine rpm is slower (same mass flow but higher
density=lower volume) but the compressor is turning faster (different sized
turbine or waste gate more closed). Changing the intake density will
change the compressor map - and no one makes those maps except for aircraft gas
turbines.
2) The curves are
interpolations of a series of data points and are rough approximations
only. They also don't take into account several factors which no
doubt will have an effect such as temperature, humidity, turbulence of the
incoming air, and condition of the blade surface. True, factors
which probably have a minimal effect but make the maps that much more
inaccurate.
3) Those are the compressor maps,
but it is not the compressor that is failing, it is the turbine. There was
no damage to the compressor side at all and no indication of surge (compressor
stall). The compressor map does not address the important issues for the
turbine such as max speed, heat tolerance, back pressure or duty
cycle.
4) You are not taking into
account the effect that turbo back pressure will have on the power output of the
engine. In my experience, and from the reports of others like Tracy Crook,
that back pressure is very detrimental to the power output you will see for a
given MAP.
Sustained operation over 40" at any altitude
seems like a bad idea to me. Also at 14,000 DA, if the compressor is
producing 36" MAP at the intake (~38"-39"@compressor output) creating
~200 HP @5,500 rpm counting 2-3" in losses through the
system) it must have a Pressure Ratio
of approx. 39"/(30"-14") = 39"/16" = 2.44. This is pretty far up the map
very close to the surge line (very bad) on most turbo of the size we would use
for 150-250 HP.
I don't see why 40" continuous is
necessarily a bad idea in itself. If you are able to keep the coolant and
oil temps in line, the rotary should easily be able to handle the duty
cycle. Similarly, with a properly sized and cooled turbo it should not
have a problem either. At altitude, you simply need a bigger turbo to make
the same 150-250hp.
As I have said before, you have already done almost exactly what I am
planning with your installation since we take off at 5,000' and fly at
10-12,000' Eastbound and 15,000 westbound here in Northern Colorado your
installation is of great interest to me. My only different plan is to
put the coolant heat exchanger in an underslung location. (See attached
jpg of installation which I created for a friend.) This is primarily to
reduce crowding in the cowling and allow the intercooler and oil cooler to be
cooled from cowl inlets. Congratulations and thanks for sharing! I
got quite a few good photos at Copperstate.
IMHO, since you are starting out
from scratch anyway, you would be much better off fitting a more universal
exhaust manifold from the start, such as a T3 or TO4 compatible. That way
it will be much easier the change the size of your turbine and/or compressor if
things don't work out.
I think you will probably be OK if
you keep the stock turbo under 30". That should give you good
normalization but won't even produce 180h.p. in my experience.
Best regards and POR,
Same to you Doug, good luck with the
installation
Dave Leonard
(POR?)
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