Sender: <marv@lancaironline.net> (Marvin Kaye)
To: flyrotary
Date: Thu, 01 May 2003 22:57:00 -0400
Message-ID: <redirect-2330357@logan.com>
From: "Ed Anderson" <eanderson@carolina.rr.com>
References: <list-2330202@logan.com>
Subject: Are Blowoff valves where its at?
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> Posted for "Tracy Crook" <lors01@msn.com>:
>
> Not having any turbo experience I hesitate to give an opinion on this but
> here's my take on it.
>
 > This is not meant to discourage anyone from using a turbo  but it is
> imperative that you develop a good mental model of how it works prior to
> leaving the ground.  This is not hard to do.  Just a basic understanding
and
> few good ground rules about wastegate operation is all it should take.
>
> Tracy Crook


I would like to join Tracy regarding turbo.  While I have not turbocharged
my 13B, I have built my own turbocharger installations for two automobiles
back in the mid 70s.  While I finally got the bugs worked out (after shatter
pistons and bent piston rods) and experienced the rush of beating anemic
corvettes from 0- 60, it was worth it - however, it certainly was not
without challenges.  A few which left me blowing oil out the valve cover and
over the windshield, shearing piston ring lands off pistons, etc.

Therefore, I sort of cringe when I hear people talking about controlling
their boost with manually operated boost controllers.  My experience was
that when the boost came on, it would very quickly head out of limits very
fast.  Taking your foot off the accelerator would quickly reduce the boost
if you caught it soon enough, but it builds exponential and with the free
breathing rotary I would think the problem would be even worst.  Perhaps
other folks have quicker reaction times that I do, but I would want to
ensure that:

1.  If not using a waste gate, ensure that the maximum boost achievable is
within your set limits. In other words the size A/R of the turbine and/or
combination with exhaust restrictions ensures that your max boost limit is
not exceedable.  You need to be careful with this approach for while it
would be perfect operating at one altitude - it could lead to compressor
surge at higher altitudes.

2.  If using a waste gate, ensure that you have some means of cockpit
control.  The reason is that as you increase in altitude and keep a steady
boost the boost ratio over ambient pressure continues to increase to the
point you could enter the surge zone of the compressor.  There may also be
some reason you might want to pull more power without the resulting boost.

3.  A blow off valve is certainly better than no safety net, but not as good
as a wastegate or properly selected A/R for the turbine in my opinion.  The
reason is that the operation of the blow off valve itself can push a
compressor into surge mode or more likely result in overspeed of the
compressor (less resistance to the compressor, turbine spins  compressor
faster and faster - at least until the effect of less boost takes hold)

4.  If everything is done conservatively then you may get away with some
less that sterling design decisions, I certainly am not trying to discourage
anyone from the turbocharging route - I intend to dive into that challenge
in the next year or two myself.  Read some of the available reference
material on turbochargers and while there is not much out there on
turbocharging aircraft, some of the references to turbocharging Pikes-Peak
Mountain race automobiles addresses the effects of changing altitude.

The 1971 printing of Hugh MacInnes "How to Select and Install TURBOCHARGERS"
has a chapter on High-Altitude Turbocharging that is well worth reading.
Chapter 14, page 107. One comment worth nothing "If the engine is to be
operated at several different altitudes (I wonder who fly at only one
altitude?) then an aneroid type control should be used to prevent
overboosting at lower altitudes"
and there are a number of other cautions and suggestions.  Well, worth
reading if you can find a copy.

There are undoubtedly other considerations that could be made, but once
boost starts climbing things happen awfully fast.  FWIW

Ed Anderson