From: "Marvin Kaye" <marv@lancaironline.net>
Subject: Re: [FlyRotary] Re: turbo wastegate requirements
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Date: Thu, 11 Sep 2003 10:54:25 -0400
Message-ID: <web-2575962@logan.com>
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"Russell Duffy" <13brv3@bellsouth.net> wrote:

>>>I'd love to know how you're doing this, since it's exactly what I need.
I've never seen any sort of valve that's referenced to MAP.  The only thing
I can imagine is an electric valve that's electronically controlled.  <<<

Since I'm nowhere near completion, this is one of those details that I've yet 
to work out... all I know is this is the right way to do this job and how I 
will attempt to setup my system.  It seems to me that if the popoff valve is 
spring controlled, then it should be a simple thing to size and rate the 
spring to an absolute pressure.  Let's go through this little bit of theory 
and you let me know where the holes are... 

Your recent post explained the relationship between MAP in inches of mercury 
and PSI of boost, that being 1psi = about 2" MAP.  That says to me that at 36" 
MAP the deck pressure available is 18psi.  Imagine that the popoff valve face 
has 2 square inches of surface area.  Opposing that 18psi would require 9 
pounds of spring pressure against the 2sqin popoff valve flapper or poppet or 
whatever mechanical device is used to control the MAP inside the intake 
manifold.  Once the MAP reaches 36" (18psi) the pressure on both sides of the 
valve flapper are the same... if the MAP goes higher it overpowers the spring 
and forces the flapper door open, venting the excess pressure beyond 18PSI. 
 If you remember that the "A" in MAP stands for absolute, we are simply 
defining the amount of pressure available inside the manifold.  Since the 
spring rate is an absolute value that is unaffected by ambient pressure and 
the area of the valve face is fixed, I can see no mechanical reason why this 
shouldn't work. (This is essentially how the alternate intake air doors work 
on most of the Lancair IV fleet... if the air filter clogs (ices up) and 
causes a bit of suction inside the air box, the little spring is overpowered, 
the alt air door opens and allows the intake to breathe using the air 
available inside the cowling.)   It seems a simple problem in mechanics and 
not one of plumbing, at least to me.  What am I missing?

        <Marv>