N200LZ: Chapter 23 - Cowling & Cooling

[TMann]

I see a lot about the NACA type scoop vs. the P-51 style scoop.

What is the bottom line when it comes to performance and drag trade-offs. I know that Waiter swapped out his NACA style on his rebuild but that was based on a Lycoming power plant.

 

My plane will be using a mazda rotary with the radiator in the area below the engine so the armpit/downdraft solution is out of scope here.

[rsrguy3]

This is a question that I want to know as well for my upcomming prodject.

I have heard that the NACA is very sensative and must be totaly dialed in to work properly. Is it all that much more aerodynamic than a scoop?

 

-Guy

[--D--]

A proper NACA *should* give you less drag, all other issues aside. However, the "other issues" (like shoving air through a radiator) can't be tossed aside. There's a good reason the P-51 doesn't have a NACA on the belly...

 

Keith Spreuer recently replaced his NACA with a "P-51" male scoop due to inadequate dP across his radiator (Subaru). He posted this to the Cozy list...

 

"My conclusion is that the P-51 style scoop with 3:1 expansion and

less than 11 deg expansion rate is key. The P-51 scoop is much more

compatible with a radiator installation. This is because the fine fin

arrangement of a radiator requires a higher delta P than the wide fin

spacing of an air cooled engine. The P-51 scoop does a much better

job of delivering that pressure."

 

Pics and graphs and such are here (look under "P-51 Scoop" and get a cup of coffee because it takes an eternity to download anything):

 

http://briefcase.yahoo.com/kspreuer

 

The graph shows twice the dP, 73%+ dT, and he reported no noticeable loss in performance (increase in drag). Plus, his plane looks more like a P-51 now.

 

D