In a message dated 11/29/2002 9:13:30 PM Central Standard Time,
lors01@msn.com writes:

>
>  I'm not 100% sure what is going on but I think you would be surprised how
>  much the 50-50 coolant expands with heat.  I flight tested the new bottle
>  yesterday and verified that on cooldown the system was completely topped
off
>  with coolant (no air at top of fill port.) This was never the case before.
>  
>  This also accounts for another anomaly I've noticed over the years.   It
>  seemed like the water cooling was slightly better on the first flight after
>  topping off the system.  I was never sure because the next flight was
>  usually on a different day, different temperature, humidity, etc.  Now that
>  I've been paying close attention I have verified that only a few ounces
>  worth of air in the system makes a noticible degredation in cooling
>  performance  (about 5 degrees worth).   Not sure why this is happening
>  either but I'm happy with the results.  With more experimentation the
answer
>  will probably come out.
>  
>  Tracy Crook
>  tcrook@rotaryaviation.com
>  www.rotaryaviation.com
>  
-------------------------------------
This is just a theory, but I think that a possible explanation for the lower
performance when there is just "a little" air in the system is that the water
pump essentially 'emulsifies' the air into the water, creating many 'very'
small bubbles. These don't separate from the liquid phase very fast, and so
they circulate with the water.

If that is the case, then the coolant that is circulating is no longer 100%
liquid, so its heat capacity is no longer that of 100% liquid, and the
temperature rise across the engine must be a little higher to carry away the
same amount of heat.

Lets say the total capacity is ~1 gallon (engine + two cores + 5/8" hose),
this is 128 ounces.  Tracy refers to a 'few' ounces having an effect, if we
use 5 ounces, then that is 5/128 = ~4% air.  This would lower the heat
capacity of the liquid by 4%, and cause a corresonding rise in the required
delta-T across the engine to remove the heat.  Also, on the radiator side, I
would expect a decrease in effectiveness, so the inlet temperature to the
engine would be a little higher also (can't calculate the magnitude, but no
more than 4%).

Bottom line -- keep all air out of the system.

Bill Schertz
KIS Cruiser #4045