One part of my A/C System that has generated more conversations than the cold air that it produces is the Condenser Scoop mounted under the belly of the aircraft.
Some people love the look of the scoop (aka P-51 reference ),and others are amazed that it does not slow the aircraft down much.
The three other RV-10's that I fly with all have the same engine prop combos (stock IO-540 and Hatzell 2 blade )and once in cruise have similar speeds and fuel burn and my aircraft is the only one with A/C. Bill has often stated that there is a 0 to 1-2 knt penalty with the scoop and in my personal experience at LOP cruise with other RV-10's,it appears to be zero.Now I am sure that in a flat out max speed drag race there must be a difference but this is not how I fly.
I asked Bill at Airflow about this and received this reply that he was happy for me to share :
“Design considerations I used choosing to go with scoop;
My experience with certified aircraft air conditioning showed that many ADs had been issued because of Carbon Monoxide contamination for AC systems where condenser was positioned inside fuselage.
Mounting condenser outside of fuselage not a new idea, it has CG and maintenance benefits (access to tail cone).
What was new was Airflow Systems design efforts, with the help of wind tunnel, to create the absolute lowest drag, lowest weight, lowest electrical draw condenser installation
Starting with a scoop that was certified for a Cessna AC system we were able to reduce drag by more than 50% through multiple design iterations where maintaining attached flow seen in the photos was the goal.
AC systems without scoops also have drag because no matter what system you have, you have to force air through the condenser and exit that air overboard. *that requires inlet holes and outlet holes which are never drag free.
*Note how the tufts are attached all the way back on the scoop indicating very little separation.* We found that we did not need as many louvres as shown in the photo and that reduced drag as well
I attended the San Diego State University where the wind tunnel is located in the early 70s (finished at another university) and never knew the wind tunnel was there until years later when I read a book by one of the professors on staff there. It was built in the early 60s using a cut down P-51 prop and a very big electric motor. *i wonder if my career would have been different had I known it was there, I really enjoyed the testing process and being able to develop new parts based on repeatable, reliable data.”