ps. Would someone precisely define
"pressure recovery" for me?
Regards,
Tommy James<><
By pressure recovery I mean
the 'recovery of the dynamic pressure due to the forward velocity of the
airplane, this is what is measured by the airspeed indicator. There is a
formula to calculate dynamic pressure at any given airspeed. Don't
remember it off the top of my head but a web search on "Dynamic air pressure"
will no doubt turn it up. I recommended the ASI instrument because it
takes all the formulas & math out of the measurement. The
pressure goes up as the square of speed (really fast).
Example: The dynamic
pressure at 120 mph is 7.6" H2O. Before doing a bit of inlet work, my
right side rad only had around 3.5" pressure at this speed. The left rad
had 8.1" (Super good, got a .5" boost from prop blast). A
slight reshaping the right side inlet boosted it from 3.5 to around
5.5 . Doesn't sound like much but remember that the
back side pressure was around 2" which meant that the differential pressure
across the rad (front to back) was only 1.5" By raising the front side
to 5.5 the differential pressure is now 3.5" (more than doubled!).
5.5 is still not great so I'll be doing more work on the inlet to get it
closer to what the left side is getting. I'm explaining all this
in inches of pressure because Paul insisted on using all those water
manometers when we did the test. Couldn't fly faster than 120 for the
test because it would blow the water out of the manometers if I did.
Gets much simpler with the ASI instrument.
Keep in mind that the test
only gives you the raw data. Interpretation is still up to you.
For example, Paul Lamar concluded that the right thing to do was to reduce the
back side pressure on the rads (using a cowl flap, louvers in cowl, etc) since
he considers the 'right' value there to be zero. I had the opposite
conclusion.
Here's the thing that makes
it confusing. If you look only at cooling results, Paul was
right. I could have gotten equal or better cooling improvement by using
a cowl flap. BUT, this improvement comes at the expense of added
drag, especially at cruise & top speed (racing). By improving the
pressure recovery instead, I got the improved cooling with almost no
increase in cooling drag. This is the ultimate goal in aircraft cooling
design.
Tracy
Crook