I think this is a well written little article by Greg Richter on the 13B cooling
problems. It is from his web site.
http://www.Bluemtn.com/greg/aircraft.html
Greg mentioned he is not yet entirely happy.
I don't agree with everything he says here but it covers the basics very well.
The comments on the gaps around the rad are particularly important.
I think the rad should be much thinner for one thing so it works better
at low speed, high climb rates. Some sort of cowl flaps should be used
as well instead of a thermostat to reduce the cooling drag.
Greg is a busy man so he may not get around to this until forced :-)> PL
"Cooling
Cooling is via a 4" thick custom aluminum cooler measuring 14" x 10". I tried an auxiliary fan and plenum chamber flowing 1250 CFM to keep
things cool on the ground. A trio of 4-3/4" electronic muffins fans similarly pressurize the oil cooler plenum. Found out, we didn't need either
one and we ended up replacing all that heavy exotica with scoops and small muffin fans to exhaust the hot air on the ground. Seems that cooling is
the holy grail of auto conversions, but I didn't have any problems right out of the box. Here's why:
Instead of going to the junkyard and picking whatever looked "beefy" I calculated the amount of heat rejected by the engine at various fuel burn
rates. If the engine is burning X pounds of fuel per second you can easily figure out how much shaft work is being done by the prop and the
difference in the enthalpy between induction and exhaust gases. The rest of the energy (that which didn't go out the tailpipe as warm air and didn't
turn the prop) is heating up the oil and water. How much radiator that works out ot be is an exercise in table lookups and airflow. Radiators are
not vastly efficient, and the hotter you can run the engine the easier it is to cool. Razzle Dazzle has a 180 degree F thermostat, and runs about 190
degrees F most of the time. I've set redline temp at 220 degrees F.
Ducting is a pretty big deal since the difference between calculated and actual performance is all duct and baffle. I had a 3/16" gap between the
radiator fan and it's shroud that cost nearly 30% of airflow. 30% is a big difference -- enough to say "doesn't work" when it actually works just
fine. I heartily recommend measuring differential air pressure in your plenums with a water tube manometer to see how much pressure you are
generating. Take that number and your ground test data and you can compute how fast you need to be moving to generate that much air from the
scoops, and then how much air you will need at full power and at what speed you have that much. Great to know things will work before you fly
'em. More Fun, Less Drama.
Pushers don't have a propblast filling their scoops so I submit there is little chance of making a cooling system work without fans or cowl flaps.
With a tractor type airplane you get plenty of air from the Really Big Fan out front without having to add more little ones.
The oil sump holds 11.5 quarts of oil (6 more than we need) while the cooling system holds a little over 1.5 gallons of coolant. Not much, I know,
but the amount of coolant only contributes to the thermal intertia of the system, not it's transfer function. Which in non-engineer speak means
that once the coolant mass gets hot it stays hot until something cools it, so more coolant is just more dead weight. The huge oil volume was to
prevent foaming from the rotors, and from the wet air pump I use to drive my gyro. Since I could never quite get the oil separator working
satisfactorily, I now have a Rapco air pump on the back of my radiator and will be ditching about 50 pounds of excess oil weight. Geez, the things I
learn.
The cooling system is all aluminum, TIG welded and connected with AN -24 fittings and hose. The pressure cap is set for 16 PSI and a small
coolant overflow tank catches the coolant expansion and returns it to the tank on shutdown."
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