NACA Ducts

[Yazza54]

I want to get some more airflow around the induction system on my GTM Libra, as you can see the filter is up at high level, at the moment there is no doubt a considerable amount of heat soak from the manifold. There's a moulding at the side that could be opened up a bit but I'd like to put some kind of NACA duct directly above the filter.

I'm just wondering if I could get away with the flush type or whether it needs to be a scoop. Scoop would ruin the lines of the car a bit. But no point in fitting a flush one of its not effective... What say you? Looks like airflow normally follows the shape of the car on a coupe, so maybe a flush one would still work?









Would like to use something like this



Rather than this

[gtmdriver]

NACA ducts are expressely designed to work when fitted below the panel rather than standing proud above it.

The dimensions are quite critical though and a lot of the after market ones available are designed for appearance rather than efficiency.

Have a look here.

http://cozyserenity.weebly.com/uploads/2/1/8/0/218...

[FlossyThePig]

If you can get hold of a copy of "Flying on Four Wheels" by Denis E Ortenburger (try your local library) there are loads of references to NACA ducts used by Frank Costing, plus other stories of aerodynamics.

[rdodger]

First thing I would do is check on GTMOC for the pics of the car showing airflow around the car. From that you can probably see where cold air can be picked up around the rear clam. From memory it was quite a surprise where the air went.

Most people seem to duct from the lower naca shamed duct near the petrol cap. I did that on mine to a sealed air filter box.

Here it is

http://www.gtmdrivers.com/forum/airflow-t3230.html

[Yazza54]

Wow nice one Rog I didn't know that existed, looks like the airflow does follow the rear clam but weirdly goes through the 3 holes at the back!

[Sway]

Flat(ish) floor under the tub, then wrote open engine bay means low pressure in bay and suction through any higher pressure area...

Mine runs to the mesh fitted to the nearside naca duct, seems to be absolutely fine. Otherwise a scoop fitted to the perspex window at the slots would provide a 'ram air' effect I believe...

[Sway]

As an aside, whilst we all call them naca ducts, naca actually produced a book of pretty much every possible aerodynamic shape, much like Pantone do for colours, with empirically determined lift/drag/pressure data.

That huge body of publicly funded research is still very much relevant today, with modern plane wings being formed of several naca profiles blended together...

[JontyR]

Ive borrowed the pictures with the tape on, as it is an excellent way of demonstating why lifting the bonnet has zero effect!

Interesting thread this

[Sway]

Are you talking about raising the railing edge of the bonnet to 'help extract hot air'? If so, indeed it does feck all.

Great place to put an air intake on a front engined car (or the heater air feed, as per the Metro and the Libra!) as it's virtually always a high pressure area, and high up out of the way of standing water.

[Yazza54]

Looks like the duct will work then, I was thinking very simplistically about the air going over the car and through it but as you've explained it should effectively suck air through the duct anyway due to the pressure differential??

[JontyR]

Sway said:

Are you talking about raising the railing edge of the bonnet to 'help extract hot air'? If so, indeed it does feck all.

Great place to put an air intake on a front engined car (or the heater air feed, as per the Metro and the Libra!) as it's virtually always a high pressure area, and high up out of the way of standing water.

Yes that is exactly the point I am talking about! It always frustrates me trying to have a coversation about that.

[Sway]

Yazza54 said:

Looks like the duct will work then, I was thinking very simplistically about the air going over the car and through it but as you've explained it should effectively suck air through the duct anyway due to the pressure differential??

Yep. Any 'laminar' airflow over the duct will create a low pressure area in the duct, sacking air through it.

Laminar being effectively 'clean air' in normal speak.

Original application was air intakes on aircraft, where drag was a huge issue. So rather than a scoop sticking out and causing loads of drag (and turbulence often effecting airflow over the wing and therefore lift as well) you could provide sufficient air for a supercharged aero piston engine with virtually zero additional drag.