OK lets put some numbers on this stuff. One cubic foot of water weighs
65 pounds so if it was sitting on a plate each square inch would see
65/144 or .45 psi so 12 inches of water is .45 psi. If you were
seeing 4" you would be seeing 4/12 times .45 psi or .15 psi.

Putting it in other term one inch of water is .0375 psi.

If you had a flat plate going 100 MPH perpendicular to the air stream
at sea level you would have 26 pounds per square foot or about
.18 psi.

That is called dynamic pressure.

Paul Lamar

Mike Wills wrote:

Paul,

  So Chuck measured 1 1/2 inches (or .05625 psi) of pressure within the
cowl on the exit side of the radiator when he sealed up the left inlet. And
a 1 sq in inlet on the side of the cowl intended to recover dynamic
pressure to cool a coil should see .18 psi. There is a positive difference
here of .12 psi so the air should flow into the cowl not out. But again
perhaps what is being said here is that a NACA inlet is a poor way to
recover dynamic pressure.

  On a few RV4s that Ive seen NACA inlets have been installed in the canopy
skirts for cockpit ventilation. This is a well known problem area for
getting adequate ventilation on the RV4. Some of these installations have
had VGs installed in front of the inlet or on the inlet "ramp". In
questioning the owners of these airplanes the initial install of the NACA
inlet resulted in very little inlet airflow but the addition of the VG
transformed them into very functional inlets. Anyone else ever heard of or
seen this use of a VG to improve the function of a NACA inlet?

Mike Wills

Yes vortex generators will energize the boundary layer and help the NACA duct
work. To cool your coils use a small blast tube from any high pressure area
or move them behind the firewall.

BTW We owe a lot to Chuck for doing those cowl pressure measurements. I think the
reason he did not see more of a drop in cowl pressure when he blocked off
the cheek inlets is because the air is leaking in around the nose of the
PSRU where it goes through the cowl behind the spinner. I noticed that 
Turbo Lance was well sealed in this area and Piper was trying to do 
a similar cooling scheme to ours. Also I think Chuck has a few leaks 
around his radiators.

Nobody to my knowledge has sealed this prop area so far. Air leaking here 
and around the radiators subtracts from cooling as it increases the pressure 
in the cowl and reduces the pressure on the front side of the rad.
It is a double whammy.

The local air velocity on the front of the cowl is near zero while the the
pressure is near dynamic pressure. In that case it is quite easy for
air to flow sideways behind the prop spinner backing plate.

The reason we don't think about this is most air cooled aircraft engines
don't seal this area and air flowing in here adds to their cooling.
The bottom of the engine is then sealed to the cowl BELOW the 
nose of the crankcase and the spinner.

This is very very easy to fix. IMHO this is also a major reason why
we have had so much trouble so far cooling the rotary. We should
have found this out years ago if people had been doing their
assigned home work :-)

Ten years ago I had a Continental engine rep tell me the reason my 
O-470 was blowing oil out the breather despite the piston rings being 
in good shape was because of the dynamic pressure on a bad front crankshaft 
oil seal. I replaced the seal and sure enough it stopped blowing oil. 
Strange but true.

BTW Chuck thanks again and please run your measurements at 150 MPH
as the pressures will be twice as high as at 100 MPH with better resolution.

Paul Lamar
 
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