Ack! My ACK!

Russell Kent wrote:

An inch or two of mercury from ram air at 225 kts? I think you're dreaming.
Ideally speaking, dynamic pressure at 200 MPH is 104 pounds per square foot or
about .722 psi. Each inch of mercury is 2.04 psi, so I'd expect just slightly less
than one inch of MP *if* you had a perfect intake diffuser. Since a perfect
intake diffuser is somewhat unlikely, I'd limit my expectations to about 0.5 inch
MP.

Richard Tasker replied:

On the planet where I live it is more like each inch of mercury is 0.5 psi (.4912 to be specific). That means your stated .722 psi is in fact approximately 1.47 inches of mercury - certainly within his estimated inch or 2.

Hmmm. Maybe I'm wrong. I figured that a standard day was 29.92 " Hg = 14.7 psi. Therefore 1" Hg = 2.04 psi. And there's where I went wrong. It's the other way around: 2.04 " Hg = 1 psi. So Dick's 1.47 " Hg is correct.
Again, that's predicated on a perfect diffuser, which is doubtful. As a SWAG, 1" Hg (70% of ideal) is probably a reasonable goal (at 200 mph); at 225kts I'd use a linear extrapolation of 1.3" Hg. 2" Hg at 225 kts is still a
dream, however. :-)

Russell "always willing to learn" Kent
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FWIW....
My old style Van's ram air delivers appx .8" MP at 180 mph.


Barnyard BOb --