Re: REFLECTOR: duct installation

["David M Parrish" <dmp@radbsd.mcg.edu>]

From:           	Alan Shaw <wingco@iu.net>

> 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 
area. 

> 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 
pressure increases.

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 
always better.

> 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.

---
David Parrish