Good question Monty but I don't
know. Only looked at it down low on hot days. I'l try the
following experiment next time I fly:
Set fuel burn at a constant value
(for roughly the same BTU rejection in both cases). Measure delta T at
1000 ft then at 8000 ft. Airspeed should also be similar. Do you
think IAS or TAS should be the target?
Tracy
TAS is the number you need since that is the real
velocity through the medium that the inlet is seeing. I made the mistake
of using indicated speeds to calculate my inlets at first, they were
gargantuan! It would also be nice to know OAT so I can pin down the
density. Cp is fairly insensitive to altitude and temp. Delta t makes a huge
difference in the size of the inlet.
70-80 deg delta T at low altitude on a hot
day is really great! That means you must have a pretty well optimized set up.
I think the lower delta T numbers indicate way too much flow and not enough
diffusion to get the job done. With rads this thick and as much dynamic
pressure as we have there should be a higher delta T. My guess is the outlets
are so big that there is way too much air moving through these coolers.
Closing the outlet down should reduce the flow and up the deltaT. This of
course assumes the core volume and diffuser is up to the task. My low
hanging fruit meter is going off.
I don't think that a liquid cooled engine is ever
going to compete with an air cooled engine on minimum cooling drag, but
to get close we are going to have to stop ingesting so much air. Take a look
at a competitive F1 racer's cooling inlets (TINY). They might make 130-150 hp.
Why are we flying around with garbage can size openings for not too much more
power? Of course if you are up against the drag of the airframe, this is less
of an issue.
The numbers I am getting using your delta T of
around 70-80 F and cruise from 10Kft-18Kft give inlet
area somewhere around 35-40 in^2. For climb it is as high as 100 in^2 and
even that isn't enough in a full power slow climb on a really hot day. Since I
am not operating a WWII bomber out of North Africa, I will not worry
about that too much. ;-). My initial inlet size is 3.5inX10.5in variable to
7X10.5. with a spray bar to take care of any oops I may have made in
calculating this. I'll probably compromise and make it a little bigger, plus
it just seems scary small. But then again if you look at a P51 and
realize how much HP it was making and then look at the inlet, maybe 35
in^2 is not so small for less than 200 hp.
I saw in one of your posts where you said your
mission was 18Kft at 100 hp (LOP I assume). That is one flight condition I
have looked at as well and it gives some really enticing numbers! 50 gal of
fuel goes a long way at that flight level. My main cruise design point is
in the 200 mph range at 10-12Kft.
Monty