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I read the recent exchange about radiator problems with 320's which I believe
are mounting Mazda engines. Be advised that correct installation of a
radiator in an aircraft can be a tricky deal, but once you understand the
principles and if you are willing to do a bit of cut and try based on heat
transfer principles, you should be able to get a good result. Some hopefully helpful comments based on what I read:
1) Keep in mind that the available air side pressure to push flow through the
radiator goes as the square of the indicated air speed. To put it in
perspective, 100 feet per second at sea level (about 70 mph) yields about 2.5
inches of water ram pressure. 200 knots indicated is a bit 3 times as fast
and produces about 10 times the available ram pressure, or about 28 inches of
water, or about one psig. If you are cruising at P-51 indicated air speeds,
you have lots of ram pressure available, and so can make a radiator with lots
of pressure drop (which translates to thick core with fine fin spacing).
Going slower, you have less pressure drop available, and you need a radiator
with lower pressure drop (thinner) and you have to use more frontal area to do
the same job. Be advised that P-51 radiators are poor heat transfer designs based on
current technology. Look at a Stewart Warner oil cooler to see what a modern,
high performance, compact heat transfer surface should look like (but don't
use oil coolers as radiators: I think they don't pass enough liquid flow for a
liquid cooled engine, but I would have to check the data sheets.)
2) Air conditioning coils make less than optimal radiators because they have a
much different problem statement they are addressing: modest amounts heat flow
in a stream that can tolerate very little pressure drop (your home furnace
blower). 3) I would think that motor cycle radiators would have low pressure drop
because they normally operate at highway speeds while generating modest power
output (far less than 100%) This suggests to me that they may be too thin for
the speeds you are flying at (pressure drop not matched to available ram
pressure) and probably too small in frontal area. As I recall Jim Rham's
Lancair IV uses about 400 square inches of frontal area for 420 HP, core about
2 inches thick, but he must climb 180 knots indicated at climb power to keep
temperature in range. (If some one has better numbers, let me know. ) This
suggests that inlet, outlet, or radiator are too small and blocking flow. (I
suspect the inlet on Jim's plane.) Maybe for slower installations the right
ratio to start with is 1.5 square inches of radiator 1.5 inches thick per
horsepower. (Talk about a SWAG...)
4) Cowl flaps are GOOD for planes with wide speed ranges. The data in Hoerner
shows that a cowl flap open about 15 to 30 degrees not only increases
discharge area, but generates about half a Q of suction at the outlet (Q being
the ram pressure). Thus you can get abut 75% of Q at the front face of the
radiator if the inlet to the radiator is halfway decent, and get another 50% Q
negative at the discharge to increase the available pressure and flow at low
speed, ground, climb, etc. I have been trained in heat transfer and fluid mechanics (be warned it was
years ago) and have researched engine cooling recently as a hobby and as part
of my Lancair IV project. If you fax me a sketch of your installation
(ideally top, front, and side views) with some key dimensions and the amount
of horsepower you want to make, I will run a short computer program I wrote to
make starting guesses on inlet and outlet areas, flow rates, etc. and send you
back the results and any suggestions via a mark up of your sketch. Give me a
few days to respond. I have been working half days lately at the office (7 to
7), but I will fax the results back to you sooner or later. I want to see all
these liquid cooled installations succeed, and the knowledge that makes them
succeed to spread through the community. Your installations, can be guide
posts for others if you can get them working satisfactorily.
You can reach me via fax at 650-968-3750.
Fred Moreno
Los Altos, California
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