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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
----- Original Message -----
Sent: Wednesday, December 14, 2005 3:17
PM
Subject: [FlyRotary] Cowl Exit
Looking back thru my notes, I seen a posting that
Buly's plane needed 126 inch of outlet air to make the cooling system
work. Is this amount necessary for tractor installations? At
present I have an opening 4 - 1/4" x 11 - 1/4", with the exhaust to exit short
of the opening to help siphon out the cowling heat. I really don't want
to get everything buttoned up and find out it isn't going to cool. What
are you fellows using on your RV's. Thankyou in advance.
JohnD I need to get some foam for remolding the cowl, what can I
get at the lumber yard that will sand and work
well?
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