1) tell me about them

2) Can they be used as inlets to engines as well?

I can only give you a very general explanation of them.

NACA inlets are air intakes that are streamlined into the fuselage. They're named for the National Advisory Committee to Aeronautics (NACA), the forerunner of NASA. Some are used for engine cooling, some as ram air inlets, others for lord knows what. You see them sometimes on the side of engine nacelles, and I can think of at least one commercial aircraft (727) with them on the underside of the wing.

As for being jet intakes...since they're so small and undoubtedly don't move quite enough air at what's probably a high pressure, I doubt you'll see them used as engine inlets...at least for propulsion's sake.

The beauty of these inlets is that they scoop air with very little drag, much better than the typical scoop that protrudes into the airstream.

NACA inlets or NACA ducts (as seen under the wing), are well formed as they will not allow ice to form on their surface. They have the shape of an airplane.
Under wing, they are used as ventilating inlets/outlets for fuel tanks and they must not be blocked by ice. NACA found out that the specific aircraft like shape is ideal to slow down air at that specific point and moisture will not freeze. So the duct will always stay free of ice.

Kim

Naca inlets were in fact used as engine intakes on the North American YF-93A,initially called the F-86C.Two were built but a production order for 118 F-93A's was cancelled.One of the two built was used by NACA at Moffat field until 1956.

One of the smallest jets around, the BD-5J used NACA inlets for it's engine.

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Photo © Bruce Highland

And if these inlets were bigger, say 12 feet long, would there be more air going in? What about a potential for shockwaves near the critical mach number and beyond? Surely if there is a dip in the fuselage, the air would speed up just like the hump on a wing surface speeds up the air causing lift.

I'm not saying NACA inlets can lift just that I am sure there may be shock waves coming off of it at a critial speed, eh?

I can't comment on ice formation, but Avt007's brief explanation is best. The NACA duct (sometimes called called NASA scoop) formula can be scaled up or down and modified as necessary to facilitate intake volume at designed speed envelopes without creating surface aerodynamic drag. The drag created by what the directed air is being used for is another story.

They are also very popular on racecars (IE: NASCAR, Formula and Indy cars) as are many other aerodynamic tricks...Jack

Hi guys.

Here's some photos of Naca Inlets.

I believe the Naca Inlet in the belly / wing root fairing on the 747 (in the second shot) is a ram air intake for cooling the heat exchangers for the air conditioning packs.

I don't know what the 3 Naca Inlets are for on the engine cowling of the Mitsubishi MU-2 turboprop in the third shot. Does anbody know any details? They're obviously for the purpose of supplying air to engine components ....I think. 

The first (set) of photos are the ones I'm the most interested in.

Why do Embraer ERJ-145 Regional Jets have a Naca Inlet near the tip of their nose? (just behind the radome). What aircraft system needs an outside air supply from way up front like that? Could it be for cooling avionics?

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Photo © Andy Vanderheyden

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Photo © Pekka Lehtinen


It was mentioned that Naca Inlets are shaped like an airplane. I agree. An airplane like the Concorde!

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Photo © Philippe noret


Chris  

NACA inlets are best for low-drag air intakes where the flux through the inlet is relatively small compared to the flux over the whole surrounding surface.

Therefore, it's possible to use them as an engine intake, but it's rarely the best option. At a few hundred m/s, a more conventional air inlet might have more drag, but is less likely to suffocate the engine.

Or - as an example at lower speeds - to feed the engine a given amount of air, the extra drag of NACA intake air would be larger than the drag of ambient air on a conventionally-shaped inlet.

Off the top of my head...

Consider a graph of total drag (vertical axis, x) against some function of air intake and airspeed (y and z). Adjust the functions suitably to get a relatively flat plotted surface. The plot for a NACA duct should have a more pronounced (diagonal) trough around the "optimum" air intake for a given speed, whereas a conventional intake will have a broader, shallower (diagonal) trough; and that trough can be angled more steeply towards the y axis with a change of intake size.

Therefore, if you can be sure that the NACA duct provides the right air intake rate for your application, through a range of airspeeds, then go for it.

However, they're considerably less effective if you have to resort to forced induction of some kind (ie you have a fan behind it) and, let's face it, it's often not appropriate to scale it to the size where it would be useful for propulsion.
Look at which aircraft uses a NACA duct for an engine intake. What else is different about this aircraft compared to all others?  

All the above is little more than sleepless fumbling, sorry. I should dust off my textbooks!  

Hey bob....

It may be sleepless fumbling but it is very good sleepless fumbleing  

The ERJ's batteries are up front. If I remember right at least one of those ducts in that photo is for battery cooling/ventilation.

Hi guys.

> E1FAIL, Thank You for your reply. OK, so the batteries are up front on an ERJ 145, and it's very possible that at least one of those Naca ducts is for cooling them and keeping them ventilated.

Is it safe to say that the batteries on all airliners have a source of air for cooling & ventilation?

> Bobrayner, very interesting info. Regarding what else is different about that small jet compared to other aircraft, well, it's engine looks to be around the size of a beer keg! 


Chris