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Tracy,
Probably not accurate for most airplanes. There are
so many variables involved. I would think in an optimized system the exit temps
will be higher for an air cooled engine. Most of the air cooled engines we're
familiar with have very low energy density compared to the WW2 radials. I mean
look at one of those corncob engines that pumps out multiple thousands of HP
sometime. I always wondered how in the HELL they managed to cool those things.
One way was they put a lot of fuel and also water through them at max power. But
even at cruise power there just isn't that much in the way of airflow area and
fins there. The main barrel of the cylinder gets some cooling from the intake
charge and the oil, so it's not that hot.The main hot spot is going to be
around the exhaust valve and there you are definitely limited by the amount of
fins you can put on the thing. I'd bet the sodium cooled valves and lots of oil
directed at that area is the only way it works. That's where liquid cooling
has a decided advantage. My guess is in the typical light aircraft there
is too much air passing over the cylinders so the exit air is not that hot once
the airstreams mix again. Plus you still have to cool the oil. Most
installations are far from optimal. Practicality dictates always. If you
really optimized it and restricted the airflow where it wasn't doing that much
good I would bet you could get closer to the 2X number(at least for the engine
minus oil). Look at the cooling inlets on some of the formula one aircraft.
Airplanes where people are really paying attention to the details have very
small inlets. It really used to bother me why all the rotary installations had
to have such great big inlets to cool properly compared to the fast guys
at Reno pumping out gobs more power. One day the DeltaT thing
smacked me upside the head.....DUH!!! Then there is also the
ADI fluid. ;-)
The great thing about liquid cooling is it lets you
smooth out all the hot spots and move them somewhere else....but nothings
free.
A well executed liquid cooling system will beat a
poorly executed air cooled setup...no doubt. But nobody's going to build a waste
heat 200 mph ram jet, no matter how hard they try.
Congrats on getting the new ride in the air BTHW
;-) I've been following with great delight, just no time to comment. I should be
cleaning up the shop right now.....but I'm goofing off a little
today.
Monty
P.S. I'm going to have to go back and read up on
Ed's Delta trick.....He's making the rest of us look bad
again....;-)
----- Original Message -----
Sent: Thursday, December 24, 2009 3:23
PM
Subject: [FlyRotary] Re: Meredith Effect
- Spitfire
I'm wondering if that figure for airflow is true
(2x airflow for water cooled vs air cooled). All the measurements
I have seen (not many) indicate that the exit air temperature on a Lyc
installation is not significantly different than on our water cooled engines.
The total heat per HP is not that different so my assumption is that
the CFM requirement is not much different.
The only advantage
the air cooled engine's higher Dt gives you is that it requires far fewer
square inches of surface area to transfer a given number of BTU with a given
number of CFM. Our advantage is that we can add surface area a
LOT more easily than an air cooled can. You can only put so many fins on
a cylinder head.
But I may be missing something. Other
thoughts?
Tracy
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