Bill:

While everything in your post is correct, the key is not simply the deltaP. An engine with the proper baffles which make the air go where it's needed and NOT go where it's not needed cools much better than one with a high deltaP and the air going to the wrong places. Based on the Lancairs I've looked at the problem is not the amount of air available for cooling or the deltaP, but that the metal baffles around the cylinders are sub-optimally designed. Correcting that is counter intuitive in many cases and requires a serious R&D effort to accomplish. When that was addressed on the IV-P we worked on, there was more than enough air available and the use of plenums and an unusually high deltaP were not necessary.

There is way more air available than needed. The trick is getting it to go where it needs to go.

Walter


On Mar 18, 2006, at 7:30 AM, Bill Hannahan wrote:

 
 
Since pressure is proportional to velocity squared the pressure corresponding to 75% IAS is .75 x .75 = .562 of maximum dynamic (pitot) pressure.
 
The lower cowl pressure corresponds to .122 of max pressure, a difference of .44 of max dynamic pressure.
 
This assumes that the ASI static port was connected to a good static system. If it was open to cabin pressure, which is usually below static, that would make the upper pressure look better (higher) then it is, and the lower pressure worse, (higher) than it is.
 
44% of dynamic pressure at 200 MPH is equal to 100% of dynamic pressure at 133MPH, and since airplanes cool successfully at slower speeds it should be enough, are you sure the temp probes are calibrated?
 
To get more pressure there is more to be gained on top than bottom. Larger openings or diffusers aft of the openings would help, but I would start by eliminating leakage and making sure the baffles force all the air through the cooling fins, and that no gaps can open up under pressure, in flight.


BILL HANNAHAN
WFHANNAHAN@YAHOO.COM

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