Interestingly, if I correctly interpret the extract on exit area I took from a study on aircraft liquid engine cooling, it would seem to imply that a “free floating” cowl flap would seek its own/best position with regard to minimizing cooling drag.  The flap would open more if the air pressure on the cowl side exceeds the free air pressure at its exit into the airstream and would be closed by the airstream until the pressure on both sides of the cowl flap equalized.

  At least that is what it appears to say to me.  However, minimum cooling drag may not/will not always be conducive to maximum cooling.  In some cases, like in climb, you may not mind more drag if your cooling is better by opening the flap more than optimum for minimum drag.

So I imagine, you would still want a control level/servo motor to control the extension.  There is more pressure on the flap than you might think.  I think it was Bill Eslick that tried a cowl flap and believe he eventually abandoned it – but, not certain if he did or exactly why (I think I knew at one time {:>) – Bill, speak up!

Ed

Count on Ed to supply the technical background :-)  That's another good rule of thumb on the maximum inlet area too  (.3 *  core area).   More doesn't do you any good.

Main point is, You don't have to be a rocket scientist to make this stuff work if you follow a few basic rules of thumb.  (The right ones that is, there are a lot of bogus ones floating around as well :>)  

Tracy