Paul, this would have no use in a tractor but in a pusher it might do some good..
Put a can on the prop hub coming back a bit into the cowl, put impeller fins on it. Some ducting around the impeller takes care of the flow.
Why not resort to an active air system and create a bigger pressure differential?
...Chrissi

BMW missed a huge bet by not truncating the exhaust pipes right at
the cowl opening for an augmentor effect.

The "cowl flaps" themselves used a very clever device. The entire ring around
the back of the cowl moved fore and aft to open or close off the 
cooling air exhaust duct. This idea could be used on a pusher with a moveable 
coaxial cone surrounding the gear box or prop extension.
 
Paul Lamar
 
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