Not sure I follow you about exit larger than inlet being gospel. IF you mean subject to translation errors and the cumulative alterations of many scribes I might see your point. Everybody going fast has smaller exits than inlets.

 

As I stated that is because the inlet must be sized for WOT climb where you are moving a lot of air. At that point the outlet must be larger as well like 150% to draw the air through and allow for expansion. OR you must use an exhaust augmenter-take your pick.

 

As the speed increases and the power level drops in cruise you need to close the outlet down (unless you want a large drag penalty). Ideally you should close the inlet down as well. Since it really doesn't matter much on a Tractor (because of external diffusion and an ugly front end aerodynamically anyway) and variable inlets are more complicated it does not typically make sense to fool with closing the inlet down.

 

So ideally you should have an inlet sized to cool the thing in climb, followed by an exit that is variable from around 150%-70% or so depending on how you plane to operate the aircraft.

 

You want the minimum air flowing through the cooling system at any cruise operating point to create the least drag. Unless you are in full power climb, and then you just want to cool the thing.

 

A single size for your outlet is a point design and will create excess drag or inadequate cooling at other operating points.

 

Monty

 

 

To many replies already, all mixed up, mission acomplished! :)

 

I don't know wether the mentioned airplanes had cowl-flaps, and if yes, wether they are big enough to accomodate 150-70%, but they do use exhaust augmentation, but I don't think it figures that big - it seems it is more aerodynamic streamlining within the cooling ducts, using the proper pressure differentials/building the ducts in a way that they take advatage of the existing differential pressures around the cowling/airframe.

Basically it looks like a real thorough clean-up according to Hoerner, K&M and K&W.

In the Kitplane article it is mentioned that the Mustang II does use the exhaust to augment, but it does sound like "just to milk the last bit out of it".

If you are able to eliminate around 30% of total drag just by cleaning up the cooling act - hell, I will be doing everything possible to make it slick!

When I get to the point I will try to interview both guys in detail - if no one else gets to them first:)

 

Thomas J.

 

PS: Another study in smooth cooling is Cory Birds "Symmetry" - Scaled Composites.....