Pete,
The idea for the larger opening size is to accommodate the increased air
volume due to engine heating. However, the exit methodology is particularly
critical to the overall efficiency for getting the cooling air overboard and
back into the airstream. Simply cutting a big hole in the bottom of the
otherwise streamlined lower cowl will not get the job done. The positive
incidence angle of most lower cowls will have air going into the lower cowl
instead of exiting. You need to create a negative pressure area at the
exit. Some designs put a down-turned lip at the forward edge to attempt
creating the negative pressure. This works, but is terribly inefficient. A
better scheme is to make a rear facing ramp to get the air flowing smoothly
toward the exit with the exit cut off square with the slip stream. That
way, maximum negative pressure is created at the exit, and the flow rate is
maximized. If the ramp is tuned right for the cooling flow, a net increase
in thrust can be obtained. Wind tunnel tests on the P-51 confirmed 8
percent increase in thrust just cooling the water radiator.
Here is a picture of my current lower cowl during construction.
https://s3.amazonaws.com/expercraft/sidwood/144869089252d5df7076508.jpg
The 4 engine exhaust pipes exit the same ramps as for the cooling air exit.
So, there is some flow augmentation due to the engine exhaust blast.
Sid Wood
Tri-gear KR-2 N6242
Mechanicsville, MD, USA
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I working on the lower half of my cowling and need to know the multiplier to
size the exit area. I vaguely remember that the exit area should be approx.
1.5 times the total size of the inlet area. Need to qualify that memory. I
have Tony Bingelis' "Firewal Forward" book, but can't find that multiplier
listed anywhere.
Any help or info greatly appreciated.
Thanks,
Pete Klapp, building N729PK,
Canton, Ohio
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