"Morgans" wrote in message
...

"Bill Daniels" wrote

Why would the fuel boil? Glycol/water coolant doesn't boil if the
engine
temps are normal.

How are the temps going to stay normal, once the fuel has gotten up to
engine operating temperature? Remember the premise that the skins will
not
get rid of the heat fast enough? Someone has proven it here before. The
fuel will then get hotter and hotter, until it is boiling. The change of
state may then keep the engine from melting down, at least until all of
the
fuel is gone.

You know, I just don't buy the "skin radiators won't work" theory. The
pre-war Schneider Cup Seaplane racers did use skin radiators to cool some
really big engines. There are LOTS of reasons skin radiators weren't used
on WWII fighters - bullet holes being one. Since then, piston aero engines
have been air-cooled.

About 10 years ago I did a crude experiment. The fuel tanks on a PA-28 are
wet leading edge cells with only the wing skin between the fuel and the
airstream. I filled the tanks on my Archer II from a fuel truck that had
been sitting in the summer sun all day, measured the fuel temperature in the
right tank and went flying in the cool evening air using the fuel in the
left tank. 15 minutes later, after landing, I measured the right tank fuel
temperature again. It was a LOT cooler than when I started. This is an
experiment that anybody can do.

Using the tank wetted area, the before and after fuel temperature, the OAT
and the specific heat of AVGAS, I calculated the heat rejection of the tank
as if it were used as a radiator. There was huge heat flow from the fuel in
the tank to the airstream. It looked as if it would be larger than the
heat rejection of the O-360 in the Archer's nose if the fuel temperature
were as high as coolant would be.

Now maybe if the fuel were at 200 F, the heating of the boundary layer would
trip it to turbulent flow and create a lot of drag but I doubt that an
Archer has much laminar flow anyway. If the fuel tank/radiator were in the
propeller slipstream where it belongs there wouldn't be any laminar flow to
trip.

Bill Daniels