NACA type airbox intake

Not directly RV related but thought there may be a few people here who had done this or seen it done or had an opinion on the idea.

I am looking at using this NACA style scoop as an intake to a filtered airbox (its actually for a Pitts Special - carburated O-320).



It will be placed in the usual place, about 1ft below the spinner with the narrow portion towards the top, the intake hole is 2.5". With the slope of the cowling the intake hole is essentially facing directly forward.

Just wanted opinions on if it is likely to work well or if I'm wasting my time.

Well, maybe

It might work (only sue to its location) -- but this type of intake does not generally produce any MP increase on piston engine intake systems due to 'ram effect'. You would be better off with a scoop sized to about 2x the fuel controller throat area, and try to include an expansion area that allows the air to slow a bit, but it would seem to me that this won't be possible in this particular installation...

That is a nicely done part indeed!

Carry on!
Mark

I recently installed a similar inlet up towards the front of my RV7A. Results are not great but I do think I am getting a bit of MAP increase from it. I was able to form mine in such a way that the opening is straight in, the air does not have to turn.

I plan to experimt with the installation of a "helper scoop" to see if it increases the MAP any. I have seen 30.5" down on the deck at speed.

My inlet air foes through a K&N Appollo then in the engine..

Randy C

I like the looks of this for use in place of the white plastic Van's NACA scoop...would it fit as a direct replacement in the appropriate locations on the side of the fuse in a -7? (That is, are all NACA scoops identical, I guess?)

Dragonfly said:

Not directly RV related but thought there may be a few people here who had done this or seen it done or had an opinion on the idea.

I am looking at using this NACA style scoop as an intake to a filtered airbox (its actually for a Pitts Special - carburated O-320).

It will be placed in the usual place, about 1ft below the spinner with the narrow portion towards the top, the intake hole is 2.5". With the slope of the cowling the intake hole is essentially facing directly forward.

Just wanted opinions on if it is likely to work well or if I'm wasting my time.

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If you look at the fast guys, the real fast guys, (Relentless, Nemesis) all of their engine air intakes stick out from the cowl. I assume to get away from the boundary layer generated by the spinner. My cowling is the same way. I think the only way to get a NACA duct to work properly is if it is mounted on the side of the aircraft in a way that the pointy end it facing the relative wind. Even then, it has to be done perfectly. That carbon NACA duct looks good, but it seems to have rounded edges on the "ramp". That I think can defeat the entire purpose.

For a good read on induction inlets..

http://www.n91cz.com/InductionSizing/Induction_Inlet_Sizing_rev0.pdf

I'm starting to agree and will probably change the way I attack this not. Its not a true NACA duct in the sense of the word.

I will probably go to a protruding intake as you mention above, tried and true method which works and is easy enough to do.

N54SG said:

... Even then, it has to be done perfectly. That carbon NACA duct looks good, but it seems to have rounded edges on the "ramp". That I think can defeat the entire purpose...

Click to expand...

Good point that is lost on most people. NACA ducts have to have sharp edges to maximize efficiency/air flow.

I started my career at Hawker Siddeley in Kingston, one chap who was very knowledgeable on air intakes dismissed the NACA duct as a way of getting significant amounts of air inside an aeroplane. His view was that any intake worth its name must stick into the airflow and grab some air - otherwise you just don't get any mass flow down the intake. If he was pushed further his opinion was that most 'NACA ducts' that he had seen weren't really as they did not correctly implement the required shape - (see here). My guess is that the person who moulded the duct in the picture copied something else not very well and didn't look at the original spec. For an engine intake I would get something out into the airflow!

Pete

That Chap....

....must never have seen Rutan's canards. Over half of those flying, and certainly the most efficient ones, have NACA inlets for engine cooling. They work really well when done properly.
Very true that it has to be shaped correctly to do what it is designed to do and many have butchered them up and wondered why they didn't work, including me.....

You'll get some dynamic pressure recovery with most any hole below the spinner just because of location...see a fuselage CFD pressure plot.

As for canard cooling intakes, most with NACA ducts are in the armpit area?

The keys to NACA duct efficiency is its shape and location. Actually, it is less of a "scoop" and more of an escape means for the boundary layer. I built a set for cockpit vents on my Hiperbipe (which happens to have long, flat sides). They are mounted in the doors, and deliver an incredible amount of air. However, they are dimensionally as close to ideal as possible (sharp edges, etc), feature minimal ductwork (none, actually), and are located well aft on the aircraft.

That said, reasearch indicates that NACA ducts don't work very well in a curved surface or areas where there is minimal boundary layer, such as the front of the cowl. In this case, a pitot style inlet is your best bang for the buck. The inlet should be in clean air, and as close to the TE of the prop as you can get. Further, the duct should be divergent so that the inrushing air has a chance to slow down and increase density. Correctly designed and executed, you will see MP above ambient, giving a true boost.

NACA

No Dan, most are on the belly, but I suspect you knew that! I do have a very efficient cabin air supply from a perfect little NACA in the armpit of my wing. Some Velocities have been successful with them on top and sides but, as has been noted, that isn't a good spot for the best pressure. I really don't see how it would work to cool an RV unless applied on both sides. But then if that worked well it probably would have been seen at Reno already. A local fellow has done some pretty extensive improvements to his Lancair in this department and I think he's been referenced before as it is directly applicable to your designs.
See: www.N91CZ.com

I make a direct replacement of Vans standard NACA duct out of fiberglass.

NACA airbox intake

One of the original NACA research papers (NACA ACR 5I20) on the submerged duct entrance has this to say about carb air inlets (P18 - Field of Use for Submerged Inlets)
"Submerged inlets do not appear to have desirable pressure recovery characteristics for use in supplying air to oil coolers, radiators, or carburetors of conventional reciprocating engines. The required diffusion of the air and the range of inlet-velocity ratios is too great to give desirable characteristics at all flight conditions."
"...it should be noted that submerged entrances have a definite advantage over other inlet types for certain inlet and air-flow requirements. The design of such inlets is more critical than that of other types because of the effects of boundary layer thickness and local velocity fields."

However, a later paper (NACA RM A7I30) concludes (P18 also):
"Other applications could include some ducting systems involving cooling and carburetor air. If this type of entrance could be substituted for the proturding scoop-type of inlet, the aerodynamic neatness of the aircraft would be greatly enhanced".

Not sure of the definition of "neatness" here.

NACA RMA50F13 addresses submerged inlets located on the rearward portion of a fuselage. Due to the thicker boundary layer at this location, the use of sidewall deflectors is necessary.

The submerged inlet is intended to eliminate external drag compared to external duct entrances and it does this very well, but the shape and location on the aircraft is critical. Some kind of instrumented mock up of the proposed location and flow requirements should be made to compare with an external duct to verify if it is an advantage or not.