Hi John,

   Sorry, John. Thanks for the vote of confidence.  Was working on understanding K&W, duct and heat exchangers and  seeing so many reponses to your inqurey that I figured you probably didn't need another input. 

 I tend to agree with the initial advise you received.  If your radiator cap is at the highest point of your cooling system then I believe that all you need is an overflow bottle/jug/container.  Well -- I also recommend a 21-24 psi radiator cap.   That is all that I use. The overflow catches any -- well, overflow.  Needless to say your overflow bottom must be vented to the atmosphere.   The system on my aircraft is so effective in removing the air from the cooling system after a couple of flights, no air remains- in fact, that causes a condition that first caused me to think I had a engine problem. So I might as well mention it in case anyone else runs into it.

What apparently happens is as long as there is a small amount of air trapped in your system  then compression of that air can/will occur.  However, once all the air is expelled from the system, you get to a condition (verified that Tracy Crook also encounter it - don't know of anyone else reporting it) I call cold fluid Hyraulic lock in that there apparently is apparently no more air available for compression. So when the engine and cooling system components cool down, that contraction squeezes the coolant (and discharges some into the overflow).  So what remains in the engine has no room for any further expansion.  Note: An expansion chamber would probably preclude this phenomona, but at additonal weight and complexity.

I first noticed that after about 3 flights (during which that process apparently expells any significant remains air after a refill of the coolant system) that immediately  (within 3 seconds) upon starting the engine that my coolant pressure would go to 18-20 psi - long before the coolant heating could possibly have caused it.  I ,at first thought, I had a combustion leak between the rotor chamber and cooling system and was having the coolant system pressurized by combustion gas (that is one of the symptoms of a cooling "O" ring failure).  However, there was no loss of fluid, no indications of combustible in coolant (frequently will  see what looks like thin oily sluge on top of the coolant)  and once airborne (or the engine heated up)  the pressure would drop back to a nominal 4-9 psi (depending on power setting and OAT - note: I do no use a thermostat). 

Tracy confirmed the same condition (and we have different cooling system, his in parallel, mine in series, his with 5/8" lines me with 1" lines, etc.). I believe that what we are seeing was the cooling pressure sensor diaphram being pressurized by the kinetic energy in the cold coolant flow perturbed by the water pump and coolant flow.  Since without any air in the system and the fuild occuping all the space, any small pressure pertubation is no longer absorbed by the "shock absorber" of a bit of air in the system  and it shows up immediately on the pressure sensor. Once the engine heats up then apparently there is enough expansion in components through out the system that the pressure drops back down. Since we are basically talking about a liquid, as you know very, very little compressibility occurs.

Don't really know if that is the reason - perhaps some hydraulic fluid experts could offer further comment.

my $ 0.02 worth

Ed

Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com

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