NACA intakes for carb induction - pros and cons?

[ZacYates]

Rather than risk derailing the Tsunami rebuild thread I thought I'd start a new one.

When watching the excellent home movies about Tsunami's construction I noticed she began with a NACA intake on the top cowling which later became a more standard trunk intake a la Allison Mustangs. I asked about the reasoning for that and I think it was answered.

As far as I'm aware - bearing in mind my relatively small knowledge bank compared to many members here - only Stiletto and Miss Ashley II featured NACA intakes for induction (I'm not terribly interested in MAII's NACA radiator intake in this thread).

Is losing the Mustang Smile in favour of a NACA intake a practical decision? If so, is it for drag reasons or does it help the carb "breathe" more?

It certainly makes for a sleeker nose which must be cleaner for racing and I'm curious to learn if this is true and, if so, why more inline racers haven't adopted it.

[wingman]

Re: NACA intakes for carb induction - pros and cons?

There was a third...

Vendetta used a downdaft induction system for the same reasons as Tsunami and MAII --wing location. As far as I know this intake location was not done for efficiency. I'm actually guessing that in the case of Tsunami...

I never heard anything about how the Vendetta version worked -- John had bigger issues in 1988.

Neal

[knot4u]

Re: NACA intakes for carb induction - pros and cons?

MA2 and Critical Mass are certainly among the the most "raciest" (my iPad almost crashed trying to figure out how to spell that word). The rule has always been that form follows function and beauty is in the eye of the beholder. It would be awesome if reality was a video game. I remember walking past the MA2 pit in the very early morning and being gobsmacked about how perfect the airplane looked and how tidy and professional the whole thing looked (I was jealous, my little toolcart had to sit outside all night). The Dwelles are a great group and their airplane looked like it might go super-sonic if someone ever put the spurs to it. But the truth is the Bearcat was better looking and Strega and the clones that follow it are very sexy, Tsunami was on a strong trajectory to upset the standards but unfortunately it's progress ended abruptly.

[CubersWrist]

Re: NACA intakes for carb induction - pros and cons?

This report has some CFD pictures to help with visualizing. But the conclusion is about the same as in the Tsunami thread. The benefit of a NACA duct is less drag than a protruding scoop. The downside is you lose mass flow. And a racer needs a lot of air mass flow. And lastly there is no real drag benefit of a NACA duct on this portion of the aircraft. ie a NACA duct isn't saving any frontal area or even wetted area.

Notice no one in biplane or IF1 uses a NACA duct to feed the carburetor. But they will use a NACA duct to feed their oil coolers. The applications and requirements for the airflow are different.

https://ntrs.nasa.gov/api/citations/19930090318/downloads/19930090318.pdf

https://www.scielo.br/j/jatm/a/TnXSNpGLSwKjw4dYR58WfKq/?format=pdf&lang=en

https://ntrs.nasa.gov/api/citations/19930085733/downloads/19930085733.pdf

[ZacYates]

Re: NACA intakes for carb induction - pros and cons?

There was a third...

How could I forget Vendetta?! Thanks Neal.

The benefit of a NACA duct is less drag than a protruding scoop. The downside is you lose mass flow. And a racer needs a lot of air mass flow. And lastly there is no real drag benefit of a NACA duct on this portion of the aircraft. ie a NACA duct isn't saving any frontal area or even wetted area.

Thanks for your post Thomas, I really appreciate the links.

So is there a drag benefit to losing the standard Mustang intake under the prop? Or does it just make the aircraft look fast?

[CubersWrist]

Re: NACA intakes for carb induction - pros and cons?

Hypothetically if you could eliminate the scoop you would reduce drag. But in practicality the engine needs to breath. So in all likelihood whatever system replaces the smile will be less efficient and therefore heavier.

ie you could load up a bunch of compressed air tanks into the plane. So it?d be a ?blowoff? rather than boil off. More aerodynamic but your adding lots of weight and complexity for the same if not worse performance

[Lockheed Bob]

Re: NACA intakes for carb induction - pros and cons?

I think from the 1949 photo I have that Anson Johnson had the radiator in the wings after the scoop removal. I don't remember if that was good or bad?

[Leo]

Re: NACA intakes for carb induction - pros and cons?

Did Johnson ever really get to prove his configuration?
He vented the wing radiators through louvers in the top of the wing and removed the "chin" intake, using instead a small scoop on the bottom of the cowl below the carb.
It would have been interesting to see, if the races had not ended, how the racer would have developed, and how others may have pursued mods as time went by.
It (bottom scoop removal) worked for Stiletto and GG.

[Reever]

Re: NACA intakes for carb induction - pros and cons?

I think from the 1949 photo I have that Anson Johnson had the radiator in the wings after the scoop removal. I don't remember if that was good or bad?

Went back and looked at some photos of Anson Johnson's Mustang. Scoop removed from under the spinner and moved back under the cowling to basically under the carb location? Benefit to removing the trunking to the carb?

[CubersWrist]

Re: NACA intakes for carb induction - pros and cons?

It all depends.

The important thing is the NACA duct is not magic and it is not ideal for every application.
For equal inlet size the NACA duct has less ram air pressure and less air mass flow than a standard scoop.
But it does have less drag behind the inlet itself.

So for the induction system of an airplane like a P-51 or Race #30, where the air is already being pressurized by a turbo or supercharger, a NACA duct does not make any sense. You want more pressure so you can make more power. The nail in the coffin is these scoops are in a location that does not any drag given the shape of the cowling.
Now why would it possibly be better to have a scoop farther back? You would have to consider tradeoffs in the boundary layer between the two options (inside the trunk vs outside the cowling in the prop wash)

But for cooling you don?t want ram air. You want slow air. Think of it as the longer the air is passing by the cooling fins, the more heat the air molecules can take with them. This is why Rare Bear has such a narrow inlet around the spinner. This is also what make boiloff awesome. Because the water does not vent overboard until it has absorbed the maximum heat possible before it changes phase into steam.

Now we were talking about air cooling. You can see some large NACA ducts on the Rolls Royce electric project. Those were designed for their cooling system by Paulo Iscold. The other thing I notice with most commercial uses of a NACA duct is the angle relative to the air. So the hood of a Ferrari or the bottom of the electric NXT cowling all have these angles that help pull in more air. And then you have ducting and plenums inside to further manipulate air pressure.

[ZacYates]

Re: NACA intakes for carb induction - pros and cons?

More interesting and helpful info, thanks Thomas.

Went back and looked at some photos of Anson Johnson's Mustang. Scoop removed from under the spinner and moved back under the cowling to basically under the carb location? Benefit to removing the trunking to the carb?

That was my thinking too - I was surprised to see the scoop when I recently started collecting photos of N13Y for a future model build as it struck me as more drag inducing than the Mustang Smile. But I stress it was purely an aesthetic thought as I've no clue about the finer things as explained by Thomas.