Well I did not say it but thought it many times
Nuisance said:
Paul, I have seen you make this statement before, on the Lancair list. Are there any places (internet) we can go to learn and study and really understand their characteristics?
Thanks,
I am not sure what Paul meant but I can tell you people make NACA scoops by eye ball (even commercially purchased ones) with out any of the proper critical details. If you read old NACA submerged inlets (scoop) reports there are critical details like, edges, lips, contours and aspect ratios that need to be sized properly (length, width, depth, edge radius, contour shapes). Most of time these are ignored in practice.
Many times the length to depth is ratio, is often wrong or compromised because there is not enough room, so they make it "look good" and size it to fit the space to mount it, not the needed air flow. For a given inlet size NACA scoop get very long. Most of the time the dimensions are compromised to make it fit.
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You see
NACA scoops in the hoods of some cars? The speeds are so low the drag reduction if any is questionable, especially with the shape and length. The
stock Viper scoop is just a straight sided scoop and probably works just fine.
Paul may have a totally different outlook on it, but that is what I have seen.
For example Van stock NACA cabin vent scoop is a joke if you look at the specs and characteristics NACA originally specified. Yes it works but it is neither efficient or necessarily low drag for the amount of air you can get out of it. The lip over the inlet should be three dimensional with a radius and contour for example (not flat sheet metal).
A cool example of a NACA inlets I've seen are on jets for the "pack" inlets. Packs are the air-conditioning and pressurization "air-cycle machines" used for cabin air on jets. (Piston planes use the engine turbocharger for cabin pressurization.) The NACA scoop in some of these "pack" inlet applications also have variable geometry, ramps and doors which move based on aircraft speed and configuration. Its all automatic.
Our RV's have a big bump sticking out for the cowl exit air scoop. The RV cowls are contoured around the lower vertical induction and cowl exit air (reverse) scoop, so adding a forward facing intake scoop (like we have) makes sense. Can could have recessed the belly and fire wall but did not. No doubt a tunnel cut into the firewall and belly for exit air would have been more aerodynamic, but it would make the firewall and belly structure more complicated (heavy, costly and compromise 12 other things).
The Lopresti uses a NACA scoop for his Grumman's (Tiger and Cheetah) cowls. The Cheetah puts the NACA scoop on the curve portion of the fwd lower cowl. It really looks like a big square opening when you look at it at first. I don't see the "ramp" portion doing much. Not sure if NACA did research on curved surfaces. It works to some degree no doubt. I'm sure Lopresti did his homework. The Cheetah fwd NACA scoop looks pretty before the square HOLE. The pretty shape before the inlet does not look like its doing much, but it looks good. For the size of that inlet the NACA scoop should be much longer. There is just no room for longer. That probably would be the case with RV's.
A NACA inlet for a RV, if sized properly would be long and put the outlet inside the cowl way aft, so extra hoses and ducts would be needed (with losses) to route the air to the vertical induction.
Now some Lycs (O360A4K on the Grumman Tiger) have rear fed induction sumps, so the NACA scoop might work better, but that engine would not fit in RV's. I have seen NACA scoops on the side of cowls using the O360A4K, one on a Mustang II and the other on Lopresti's Grumman Tiger. Not sure how well they work with the corkscrew airflow around the plane. The side scoop on the Tiger makes sense because internal ducting needed to get air to the rear facing carb is not too long.
Just because a scoop is flush does not mean air is not spilling out and causing drag. This side scoop looks massive. The Lopresti cowls are no doubt better than the stock Grumman cowls they replace but also more complicated. Remember the keep it simple motto. In aviation and engineering the KISS method is often the best way to go. Please don't take this as a slight against the late Roy Lopresti, who knew more about aircraft design than most people. He's on my long list of top designers, which has people people like Kelly Johnson and Jack Northrop on it.
My pet peeve against the NACA scoop is its not the best scoop for all jobs and is rarely executed or designed properly. Research on using NACA scoops for jet fighter engine intake was tried a handful of decades ago. What kind of scoop do you see modern fighters we have today? No NACA scoops are found for these applications. (I heard the B2 stealth bomber or F117 stealth fighter have some kind of submerged engine inlet or exit? Don't know? I don't have top secret clearance.
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When I see a NACA scoop they rarely look engineered. They look like it was copied from another application or made to look like a NACA scoop, fitted in available space (probably too small) for the purpose and done more for style points. When they are used they have to be designed and optimized for the application and purpose intended. Does not mean they can't be done right, just its not easy. The NASA reports on flush or submerged scoops are available on-line. There 100's of great reports on all kinds of inlets and ducts. The real problem is getting the research and practical to match. Often (almost always) we don't have the room to make it perfect so good enough and compramise is needed. The trick or tallent is making good compramises.
Ref:
NACA Report No. 713, Internal Flow System for Aircraft; Rogallo. 1940.
NACA Advance Confidential Report No. 5120, An Experimental Investigation of NACA Submerged-Duct Entrances. Frick et al. 1945.
NACA Research Memorandum No. A8B16. An Experimental Investigation of NACA Submerged Inlets at High Subsonic Speeds. I -- Inlets Forward of the Wing Leading Edge. Hall and Barclay. 1948.
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