Thorp Air Command - T18.net

I've been doing internet research on sizing for NACA inlets. Trying to figure the size I'll need for the IO-320. I have a figure of 250 Cu feet of air required for the engine displacement. Anybody else out there using a NACA inlet instead of a scoop on the bottom of the cowl. Most photos in the Gallery show scoops. The others I know of take air once it's in the pressure cowl area. It's time to cut and I'd rather do it right the first time....

Thanks, Terry

Terry,
I installed NACA inlets on my Twin Comanche that has IO 320 B1A's. I bought the kit from Knots 2 You about 15 years ago as a STC kit, and they have worked great. There unit housing includes a area for a bracket air filter element and a very easy disconnect / connection method of mounting. You may want to look at there web page for ideas. This mod has been approved by the FAA as a STC and has gone through all of the hoops associated with that approval, that may make cutting into the cowling a little less stressful.
Phil
118BC

I found a Naca inlet with a 2 1/2 outlet that will feed air to the inlet for the fuel servo. It’ll get delivered next week and I’ll let you know how stuff progresses

After my nearly 30 hours of Phase 1, It's become apparent that my NACA inlet for the fuel servo isn't giving me the results I thought it would. I have a little more engine baffle sealing to make the engine happy, but I think its time to change the inlet to a scoop. With the NACA I've been limited to about 24.7 inches of MP- and in making small modifications, I'm being pointed to the problem is the NACA inlet- time to swap to a scoop that feeds air right to the fuel injector servo.

I've also used a NACA scoop for the oil cooler, i'll tape a scoop like piece to see what the results are. It may get changed to a scoop as well.

Terry, I’ve played with NACA ducts quite a bit. I would argue that the difference in MP is most likely not the duct. I would triple check that you’re getting full throw on the throttle. That’s the most likely culprit. Also with regards to the NACA duct I would do some research on the design, most guys do not implement it correctly, on the aft lip that lies flush with the outer surface there should be an “airfoil” shape to help drag the air in. 99% of the implementations I’ve seen exclude this. Good luck!!

_________________
Lee Walton
Houston, TX
N51863,N118LW
KEFD

I think the NACA inlet placement on the cowl is as critical as the shape. On the Beech Sierra, initially the oil cooler inlet was a NACA inlet on the left side of the cowl. It didn't work due to the prop wash spiraling around the cowling. They wound up putting a scoop on the left cowl cheek to grab the air. I also noted that the NACA inlets on the 767 A/C packs worked better on the left than the right side of the airplanes. The left inlet was located more to the front of the fairing, not the bottom as on the right side. Back to the Beech Sierra, the induction inlet is under the prop spinner and raked back at an angle. The airplane would recover 1 inch of manifold air pressure (MAP) due to ram rise. Max MAP at sea level is 29.7" per the POH. I added a "Frog Lip" to the sides and bottom of the inlet and recovered another inch of MAP at cruise airspeeds. This was with a Bracket air filter and what I believe is too small of filter area for an IO-360. Still, I would have station pressure MAP at altitude with wide open throttle. I'm not sure a bigger induction filter would have done much better, but one of my engineering club members is convinced it would.