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Bobby J. Hughes wrote:
Analysis only valid if cores are identical in construction (resistance
to air flow will likely be different with the different cores).
Charlie,
Good point. We do have a known relationship of how the cores behave
using the temps before and after the exit area was enlarged. My notes show oil at 220-225 and water steady at 190 before enlarging.
Mark reported oil at 215 climb and 190 cruise and water at 160 (cruise?)
after enlarging. Exit area improvements. 95-95 deg OAT if my notes are correct. Oil 225 -215= 10 climb
Oil 220-190 = 30 cruise
Water 190 - ? = ? Climb (no data)
Water 190-160= 30 cruise.
It is clear that enlarging the exit area is made improvements to both
oil and water cooling. Bobby (hope I never need to understand the other 80% of the equation)
I'm not a fluid dynamics person, but I can read & see what seems to work for others, & the stuff that really works has just as much attention paid to exit as entrance.
Tracy pointed out earlier that smoothing the airflow *out* makes a big difference.
Enlarging the exit works, but it seems to be a brute force method. Some of the newer accounts I've read indicate that the exit can actually be smaller than the inlet if everything is done right.
If there's no shaping of the exit air flow, it will be totally chaotic in the cowl until it tries to re-accelerate from ~40mph back to 180 mph to squirt through the exit area. Slow & chaotic seems to me to equate with high pressure & high drag, just what you don't want behind the heat exchangers. Letting the air leave the heat exchangers & go where it wants seems to me to cause the same problems, on a slightly smaller scale, that separation causes in the inlet duct.
The P-51 has an exit duct. The fastest guys flying conventional air cooled engines in homebuilts also have exit ducts. They attempt to confine & shape the area behind the heat exchangers (cylinders/cylinder heads) so the air doesn't have room to continue to slow down & 'recover' pressure before it needs to speed up again at the exit point. They also put big round (as in 3" radius) transitions at the bottom of the firewall, so the air doesn't separate when it tries to turn the corner from straight down to straight aft.
Does this stuff pass a sanity check with those that know more about aerodynamics? If so, am I explaining it where it makes sense?
Charlie
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