I put cowl flaps on my Lancair  IV along with a top cooling air plenum and I also went to fairly extreme measures to eliminate cooling leakage around the engine.  Inlet area remained unchanged (6 inch diameter) although the inlets were raised 1.5 inches for a straight shot in and moved outboard 1.0 inch so that I could insert a boundary layer dam between spinner and inlets.  This removes the boundary layer coming off the spinner as well as the slower-moving separated wake flow at the shank of the propeller where it enters the spinner.  I drastically cut the exit area with cowl flaps closed compared to the stock Lancair outlets which are meant for turbo engines at 25,000 feet.  (I believe turbo engines would gain no benefit from closable cowl flaps – they need all that air flow at altitude.)

 

Net result was that overall flat plate drag area  went down (compared to stock) perhaps 7-10% with the cowl flaps closed, and it runs cold at 65% lean of peak, CHTs below 300F, lower than 250F at low altitudes.  With cowl flaps open (and lots of exit area) I can climb unrestricted at Vy (135 knots, best power) starting at near sea level on a 90F day.  Difference between cowl flaps closed and faired and wide open and deflected outward about three inches is about ten knots.  That does NOT mean a ten knot speed improvement over stock.  It means that when the cowl flaps are open, they are draggy. 

 

I have measured pressure drop across the engine and even with cowl flaps closed, it is too high (too much cooling air, confirming temperature data).  So last week I made modifications to reduce the closed cowl flap exit area another 25%.  No testing yet.

 

Would I do it again?  Maybe.  It was a lot of work.  Estimate of the overall speed benefit: perhaps 3.0-3.5%.  It sure is nice to climb unrestricted on a hot day. 

 

Fred

 

 

Thanks, Gary.