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Date: Sun, 26 Jun 2005 00:07:16 -0700
From: Tim Andres <tim2542@sbcglobal.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Coolant Leak
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David Leonard wrote:

> Ok, here is my guess:
>
> On 6/25/05, *John Slade* <sladerj@bellsouth.net 
> <mailto:sladerj@bellsouth.net>> wrote:
>
>     OK, Dave. If that makes sense, explain this.... :)
>         After start-up - 29 psi (24 psi cap)
>
>  
> coolant is expanding rapidly.  The pressure rating of most caps is a 
> rough estimate at best.  It is determined by the metal spring.  That 
> metal spring will also change the force it applies based on it's 
> temperature (though I am not sure which direction and how much).
>
>         Run-up  25 PSI
>
>  
> Coolant is still expanding though possibly at a slower rate and/or the 
> spring is now also warm and exerting less force.
>
>         Two trips around the pattern - zero PSI the entire time
>
>  
> Now you are getting good air flow after climb out and the temp drops a 
> little from its peak value, so the volume of coolant contracts and 
> sucks a little back in from the overflow - zero psi.
>
>         third downwind - 8 PSI.
>
>  
> Now you have reached steady state temperature, or perhaps a second 
> slight rise in temp as the overall under cowl temp rises (all heat 
> sinks under the cowl become saturated with heat).   Also, there is 
> continuous vaporization of the coolant, which rapidly re condenses 
> after leaving the hottest areas. The amount of this vaporization is a 
> function of the pressure and temp.  So it will seek some equilibrium 
> at a pressure that is greater than zero.  ie. more vaporization 
> increases the pressure, which in turn limits amount of vaporization.
>
>         taxi back to hangar - 10 PSI.
>
>  
> Perhaps slightly warmer now (especially in those backward flying 
> airplanes) and the vaporization:pressure equilibrium has shifted a little.
>
>         After cool down - coolant still full to the brim.
>
>  
> Yea, no reason for me to think you have a leak.   (I can convince 
> myself of anything  :-)
>
> -- 
> Dave Leonard
> Turbo Rotary RV-6 N4VY
> http://members.aol.com/_ht_a/rotaryroster/index.html
> http://members.aol.com/vp4skydoc/index.html

This may be related to Al Wicks reason for running with a cup of air 
trapped in his system, a buffer if you will.....Tim Andres