>
> Subject: Cooling system plumbing design
>
> There should never be any air anywhere in the cooling system up to the
> pressure cap.
> ----------
> Mercedes, Volvo and others use a single expansion reservoirs with
pressure
> cap that are filled about half way when cold. As the engine heats up the
> air is compressed - expands again as things cool down. With no
compressed
> air, the cooling system pressure goes to zero (or very slightly
negative)
> any time you reduce the power from a higher level, even at high rpm.
>
> I run a 23# cap on the filer neck (on left in photo, connected to the
pump
> inlet side, no air under it), and a 15# cap on the overflow bottle (on
right
> with cap off) - which only has a couple inches of coolant in it when
cold.
> That way there is always some positive pressure on the system, which
could
> be important at altitude. The two small lines going off to the sides are
> air bleed lines from the 2 rads.
>
> Also, I have found that the hot-cold volume change in my 20B system is
less
> than 1 quart, so a gallon overflow bottle is unnecessarily large,
especially
> for a 2-rotor.
>
> FWIW,
>
> Al
>
> ------------------------------------------------------------------------
>
> I was referring to the part of the cooling system that was actually
in the
engine.
> I am sure the water pump does not like to pump air bubbles.
>
> Your system looks very similar to what Dave had.
>
> "With no compressed air, the cooling system pressure goes to zero
> (or very slightly negative) any time you reduce the power from a higher
> level, even at high rpm."
>
> Are we talking absolute pressure here or zero pressure relative to
atmospheric?
> What value pressure cap does Mercedes and Volvo use? I suspect if the
engine
> get hot enough it could blow up the tank or the rad if no pressure
relief
were used.
>
> Ken has a pressure gage on his cooling system that indicates pressure
relative to
> atmospheric. As the engine warms up the pressure goes up and stays up
> regardless of power or RPM I think. Hard to tell for sure when you are
flying
> as the RPM never gets below 5000 during cruise. It is only a function of
coolant
> temp I think. As the engine cools off the gage reads negative
relative to
atmospheric
> and coolant is sucked back into the system from the catch can. Is this
correct Ken?
> We probably need a plot of the pressure verses the coolant temp. Then
another
> plot when the coolant is cool of pressure verses engine RPM.
>
> Continental liquid cooled engines used a system similar to what you
describe
> above but they used a 50 psi pressure cap. Consequently the 3 gallon 1/4
inch thick
> wall tank had about one or two gallons in it when the engine was cold.
Unfortunately
> the tank and the extra coolant at around 8 pounds per gallon adds a
lot of
weight.
> I assume if the pressure exceeds 50 psi coolant is lost.
>
> Here is an excerpt from their SAE cooling paper on the subject.
>
> "The pump uses a cast-aluminum, closed-impeller design,
> operating at speeds up to 8100 rpm. A pressure rise of 207 kPa at a
> flow rate of 114 L/min. [aprox 28 GPM] occurs at rated engine speed.
>
> An existing commercial, ground-vehicle ethylene glycol
> formulation was initially selected as the coolant. A 60/40%
> solution of ethylene glycol/distilled water was specified to ensure
> adequate freeze protection. Typical inhibitor packages for these
> coolants include phosphates, borates, hydroxides, nitrates, silicates,
> and triazole. Initial coolant change intervals at 250 h of
> operation or one year were selected, based on coolant supplier
> recommendations.
>
> To operate at the elevated coolant temperatures, higher
> system pressures are employed for suppression of boiling and
> cavitation. During the engine dynamometer development phase, a
> cylinder head temperature profile was obtained to evaluate
> temperature gradients by embedding thermocouples throughout the
> cylinder head. The maximum metal temperature measured at the
> combustion chamber surface is 210 C between the valves. The typical
> coolant temperature rise across each cylinder is 8 to 11 C."
>
>
> Paul Lamar ...No rotor no motor.


Hi Paul,

For some of us non-engineers who forgot that boiling temperature decreases
with pressure (and then occasionally wake up when we see discussions like
this,) remind us of what liquid cooled aircraft engines of the WWII vintage
did to solve the problem way back then. They often flew at much higher
altitudes than most of us ever plan to so they must have had it worked out
pretty well.

Robin

I have another book in my extensive collection called American Warplanes of
World War II with cutaway drawings of liquid cooled fighter aircraft.
Sorry no scanner up at the hangar.
The expansion tank of a P39 looks like a record 10 to 20 gallons.
The P51 had a yoke press. can around the PSRU of at least ten gallons.
Next to a sphere a torus would be a good light way to go structurally
speaking for a large high pressure (50 psi) expansion tank.
Here is a suggestion.

Burns Stainless has some 4 inch dia 6061 .065 wall tubing U bend on a
six inch
center line radius. Two of these welded back to back might work.
Volume would be 12 X pi X area. Area = 12.56 square inches. Volume = 473
cubic
inches or just about 2 gallons. This is nearly identical to the P51 system.
A one way valve in the vent line might be the hot (or cool) tip.

A completely sealed cooling system is probably what is required for a
aerobatic
airplane.

Paul Lamar ...No rotor no motor.

Cool Layout Paul,
With a cylindrical cutout you could even fit the starter. One of the low
positions, (other than 6:00 o'clock), would work the best.
Bill Jepson

Thanks Bill. BTW the P38 also used this system.

Paul Lamar


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