OK, Paul O Wise One,

You were correct. I put small pieces of foam at the ends
of my pressure sensing tubes and now things make sense.
I thought with a small tube with about 1/16" ID the velocity
effect wouldn't be there, but I was wrong. These are very
small pressures we are measuring.

It was a 20C morning, full power climb 5200 RPM 90 knots all
the way to 3500', oil temp reached 230. Level off and cruise
at a lower RPM than normal to get things cooled off, 4900 RPM,
120 knots, oil temp eventually dropped to 210.

I seem to have about constant pressure drop accross the oil
cooler of 3-4 inches of H2O, in the correct direction.

Climbing at 90 knots, there is 1" of positive pressure on the
inlet side and 2" of suction from the big fan at the exit side.

Cruising at 120 knots, there is 3-4" of positive pressure on the
inlet side and no suction from the exit.

Shouldn't I be getting more pressure on the inlet side than this?

Perry

You should have the manometer on Friday morning. After you get the oil
cooler working you can use the manometer to measure some pressures
on the back side of your cowl to see what is going on.
Any negative pressure on a negative slope will have a drag
component in the axial direction.  A ducted fan could, under the 
worse conditions, cause an increase in fuselage drag by increasing
the air velocity over these surfaces, canceled of course by
the thrust. Never the less there is a large loss in efficiency.

A long gradually tapered cowl would be the ideal shape for a ducted
fan and or a small diameter prop for that matter. Charlie Airesman
proved that with his direct drive and a 2 foot  prop shaft extension. 


Paul Lamar
 
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