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Re: REFLECTOR: duct installation
From: Alan Shaw <email@example.com>
> Don't we need electric fans to cool on the ground?
If the radiator exhausts into the cowl just in front of the prop, the
prop may give enough suction for ground ops. I'm leery of fans,
since they restrict airflow.
> Doesn't the coolant pressure effect the heat exchange?
Only in the fact that higher pressures equal higher coolant
temperatures, which produces greater heat transfer per radiator
> Don't thick cores work very well at high velocities?
Maybe. If you look at the oil coolers for many of the big WWII
aircraft, they have very little ducting (Little or no diffuser, so they
basically see flight velocity air.) and have very large and long
airpaths through the core. With high velocity air, you want low
restriction to keep the flow up, but you also have to have enough
surface area to transfer the heat, so the air has to be in contact
with the core for a longer time.
The other way is a high surface area density, which also gives a
more restrictive air path. (This is most common in modern
radiators.) Here you want low air velocity with a higher pressure.
The diffuser gives the tradeoff between the two ends. As the inlet to
radiator face area gets bigger, the velocity of the air drops and the
The biggest problem with high velocity cooling is going to be finding
suitable radiators at a reasonable cost.
> In theory at 200 mph we should get lots of cooling right?
> In practice most cooling (water or air) installations I have seen
> don't work properly at high speeds because the air piles up way
> out in front of the intake scoop and goes around it. In
I think a lot of the problems come from not looking at the
aerodynamics and thermodynamics of the cooling system closely
enough. My Cessna 175 has a huge cowl inlet, yet has a history of
heat problems if the plane is flown at too low a speed. Then look at
the Lancair IV. It's engine is probably producing twice the power of
mine, yet it's cowl inlets are a fifth or less the area. Bigger ain't
> What good is the "correct" size radiator if not much air is going through it?
Ah. The sixty-four thousand dollar question...
That's were the ductwork design becomes important. The diffuser
can slow the air down and increase the pressure so the air goes
through the radiator, but a poor design can waste half or more of
what you gained.