John,
It really depends on how much power/heat you are
going to be producing. However, if you have --say 24 sq inch for each of
three openings (two for GM cores and one for oil cooler) then you have a total
of 72 sq inches for cooling air to come into the cowl. Now you are
going to heat the air which will naturally cause it to expand in volume, and
based on your opening size 4.25*11.25 = 47.8 sq inches for this expanded air
to get out. Your exit area is way too small in my
opinion.
A "rule of thumb" for exit
area that I have seen is no less than 1.2 ratio of exit to intake. I
have seen that ratio up to 1.7. so that would indicate you would need 1.2*72 =
86 to 1.7*72 = 122 sq inches. Personally, I would lean toward the 122
myself.
Here is a rule of thumb I just got
off the internet
You will hear rules of thumb that
the exit openings, because the air is heated and thus presents more volume
than the original intake air, should be 2.5 times the area of the intake
opening. With good extraction of this heated air, allowed by the fact that it
is exhausted into a low pressure area, the exit cross section can be as small
as the intake in some cases. In others larger, but I have never seen this exit
area have to exceed about 1.5 to twice the area of intake.
From a different
source Exit area totals
66.7 square inches which comes to about a 1 to 1.5 ratio inlet to exit
area.
Notice no indication that anything
less than intake area would do the job.
I used the solid pink foam from a
lumberyard (had to glue several planks together). I taped it with duct
tape to prevent the foam from dissolving when I put on the epoxy - work fine
for me.
Ed A
Ed,
The oft sited rules of thumb are fine. But you have to keep
in mind it all depends on your flight conditions. If you size a radiator,
inlet and exit for hot day climb, then it will be WAY too big for cruising at
75% power at 12K feet. In this case you use a cowl flap to bring the exit size
down. This restricts the flow of air through the radiator thereby reducing
momentum drag. It also causes the pressure in the cowl to go up. The air
stacks up and you get external diffusion. The high pressure heated and
expanded air now squirts out through this restriction and you gain some of
your drag back. In this case, it is possible that the exit may be smaller than
the inlet (the inlet is sized for WOT climb, not cruise). Remember the engine
is putting out way less than rated power because you are (or should be)
running LOP at 75% minus the power loss with altitude. So the heat load
is way less than what you need to remove at WOT climb on a hot
day. You should have some way to vary the exit if you want to
get the best out of the airplane in cruise, of course it is more
complex. It's all a trade off.
I must admit I like the current post following the previous.
One can only rebel for just so long ;-).
Monty