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Author Topic: NACA vents vs. Hood Scoop question  (Read 2032 times)
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MAYOMAN
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« Reply #15 on: September 09, 2018, 11:24:17 AM »

Submerged duct (NACA) air inlets are an efficient low-drag design that can be used effectively on Landracing vehicles. Unfortunately, the principle is not well understood by the Landracing community for various reasons, including lack of technical information. As a result sometimes the optimal benefit of the NACA design is missed. An example is when the NACA duct is located at the leading surface of the vehicle, more like an air scoop.

The proper location of the NACA submerged duct is in the thin boundary layer parallel to the air stream, on the flat sides or top of the vehicle body.

There are a large number of declassified published studies from NASA [NACA] regarding submerged ducts on aircraft to provide low drag air inlet to the fuselage for both engine combustion air and other purposes, such as cooling. An early study, and most-cited, is NACA Advance Confidential Report No. 5120, An Experimental Investigation of NACA Submerged-Duct Entrances. Frick et al. 1945.

Examples of the correct application of the NACA submerged duct can be seen in the attached examples from the Honda Hawk streamliner and Project BUB which Woody and I collaborated on several years ago.


* NACA side air cooling inlets.JPG (60.8 KB, 851x701 - viewed 80 times.)

* NACA top turbo inlet.JPG (68.72 KB, 741x966 - viewed 72 times.)
* NACA5 side.pdf (29.56 KB - downloaded 20 times.)
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MAYOMAN
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« Reply #16 on: September 09, 2018, 11:27:22 AM »

Here are some more views of the NACA submerged duct design. I hope these help.

* NACA5.pdf (104.5 KB - downloaded 20 times.)

* NACA and diffuser.JPG (34.22 KB, 1032x600 - viewed 75 times.)
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MAYOMAN
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« Reply #17 on: September 09, 2018, 11:30:19 AM »

I also thought that sharing the calculations for the Project BUB supercharged application might be helpful. Obviously this is a specific application, but illustrates the design process.

* NACA DUCT RATIONALE.pdf (11.02 KB - downloaded 34 times.)
* NACA Duct Sizing Worksheet.pdf (118.11 KB - downloaded 34 times.)
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WOODY@DDLLC
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« Reply #18 on: September 09, 2018, 12:07:52 PM »

This is a [very special] NACA duct application that took quite a few studies to see if it would even work properly - much less be optimized!  shocked angry cheers
http://www.crp-usa.net/windform-sp-naca-intake-duct-broke-record-bonneville-run/
Couple more views on my webpage, too.
Many thanks to all the NACA personnel for all their work and published data - they were on their game! cheers
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Lemming Motors
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« Reply #19 on: September 17, 2018, 05:44:52 AM »

I purchased a copy of the Allan Staniforth book and have read the section on NACA ducts.
It is written for non-aerodynamicists and made a lot of murky things clearer - thank you for the reference.
I worked through an example and did find deriving some of the data from the graphs difficult (too small a scale at the lower end for differentiation of duct size).
I went on-line (there be dragons) to derive things like engine air requirements empirically and compared to the book - I calculated 254 cu feet / min and the book shows 300cuft/min for a 2 litre at 9,000 rpm. Rounding up is probably a good thing for this purpose.

One thing that my fluff filled brain is failing to comprehend though (okay, more than one but this one for now);

Since the duct size is determined by the engines air requirements at specific revs and inlet air flow is proportional to speed - what speed do you calculate for? Or put another way; lets assume max speed is 200mph but on the way there one is pulling maximum revs in each gear - won't the lower airflow at say 100 mph through the duct sized for 200 mph essentially reduce engine efficiency?

Staniforth suggests an assumption of 50% duct efficiency to be conservative and therefore to use one duct twice the calculated size (rather than two smaller ones where the incoming air from one may disrupt the other) - does this 200% overage adequately compensate through the speed range and answer my above question?

These threads are fantastic and I sincerely appreciate the time experts take to share information and / or direct enquiring minds.

John
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QikNip
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« Reply #20 on: September 17, 2018, 02:04:52 PM »

I purchased a copy of the Allan Staniforth book and have read the section on NACA ducts.
It is written for non-aerodynamicists and made a lot of murky things clearer - thank you for the reference.
I worked through an example and did find deriving some of the data from the graphs difficult (too small a scale at the lower end for differentiation of duct size).
I went on-line (there be dragons) to derive things like engine air requirements empirically and compared to the book - I calculated 254 cu feet / min and the book shows 300cuft/min for a 2 litre at 9,000 rpm. Rounding up is probably a good thing for this purpose.

One thing that my fluff filled brain is failing to comprehend though (okay, more than one but this one for now);

Since the duct size is determined by the engines air requirements at specific revs and inlet air flow is proportional to speed - what speed do you calculate for? Or put another way; lets assume max speed is 200mph but on the way there one is pulling maximum revs in each gear - won't the lower airflow at say 100 mph through the duct sized for 200 mph essentially reduce engine efficiency?

Staniforth suggests an assumption of 50% duct efficiency to be conservative and therefore to use one duct twice the calculated size (rather than two smaller ones where the incoming air from one may disrupt the other) - does this 200% overage adequately compensate through the speed range and answer my above question?

These threads are fantastic and I sincerely appreciate the time experts take to share information and / or direct enquiring minds.

John

John:
I'll try to get my hands on a copy of that  book! Thanks for your input. cheers
Rick
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WOODY@DDLLC
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« Reply #21 on: September 18, 2018, 09:20:45 AM »

Some NACA duct history: https://www.flyingmag.com/scoop-naca-scoop

From the above article:

Today we see them everywhere, and they are widely used in precisely the applications-engine induction and cooling air inlets, oil radiator inlets, fuel tank vents, cabin ventilators, all sorts of mysterious apertures on Firebirds and Lamborghinis for which the original researchers deemed them unsuitable. [What did they know!?!? angry]

A NACA duct application: http://melmoth2.com/texts/NACA%20inlet%20sizing.htm
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Lemming Motors
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« Reply #22 on: September 18, 2018, 11:23:16 AM »

Thanks Woody
I have read the melmoth (plane) article before - it made more sense this time around (as I research and read more widely) but to be honest  I wasn't trying to keep up with the math regarding mm Hg etc. so I may have missed some of the nuance.
John
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Certainly sir; a lick of salt, a sip of gas and a twist of Lemming. More Lemming sir?
Just a squeeze.

A Squeeze of Lemming it is sir.
QikNip
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« Reply #23 on: September 19, 2018, 08:43:12 AM »

Woody:
Ditto John's comment. Both articles were enlightening (even if some of the engineering math escaped me) and increased my NACA understanding. I suspect that in the end, I'll need your direct assistance if I opt to go that route. The good news there is that we live an hour or so from one another!
Rick
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« Reply #24 on: September 20, 2018, 12:29:35 PM »

Here's another question...what level of efficiency would be reasonable to start with in terms of sizing the system? If the total of the static area of the carburetor throats is 16.32 square inches and we assume a .7, level of efficiency, that would result in an intake system with a minimum area of 23.32 square inches throughout - meaning an average diameter (it the system was round AND my math is right) of 9.04 inches. At .6 efficiency that diameter increases to 11.03 inches. As an aside, is the foregoing syphering generally correct?
Rick
P.S. Since this thread is getting kind of long, as background the engine is naturally aspirated, 122 CI and shifts at 8,300 RPM's and by my calculations would consume 293 cubic feet of air per minute at that speed.
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« Reply #25 on: September 20, 2018, 01:16:11 PM »

It is pretty hard to believe that the throat area for a 122 CI engine is 16 square inches...that is, about 4.5" diameter.
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jacksoni
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« Reply #26 on: September 20, 2018, 01:19:30 PM »

And 23.32sqin is , rounded up, a 5.5"pipe, not 9+.

My 122" engine is fed through a relatively long 3.5" pipe quite happily
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Jack Iliff
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QikNip
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« Reply #27 on: September 20, 2018, 02:17:42 PM »

It is pretty hard to believe that the throat area for a 122 CI engine is 16 square inches...that is, about 4.5" diameter.
\

It has two dual barrel Webers. Each of the four throats measure 2.28" in diameter. That's four radia of 1.14" each. If my math isn't wacky (not saying it isn't), using the formula for the area of a circle (Radius squared times 3.14, equals 4.08 square inches per barrel. That value times four gave me a total of 16.32 square inches of throat area.
Rick
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« Reply #28 on: September 20, 2018, 02:25:48 PM »

And 23.32sqin is , rounded up, a 5.5"pipe, not 9+.

My 122" engine is fed through a relatively long 3.5" pipe quite happily

Jack:
It's encouraging to hear you 122 CU'r is happy with a 3.5" pipe. By chance is your system delivering any boost that might be aiding in air delivery? Also, can you share the math that corrected my~9" result, converting from 23.32 sq. in.?
Rick
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jacksoni
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« Reply #29 on: September 20, 2018, 04:42:53 PM »

Well hope I haven't screwed up but: Area=23.32. Circle area= Pi RSq as you have said. 23.32/Pi=7.42. Sqrt 7.42=2.72. =R X2=Diameter=5.44.

I am not convinced my system gives boost but maybe a little. It Wilmington I could not see a definite difference on mapped MP to ambient. Maybe I make up the losses through the system. Intake is at corner of the bumper which is high pressure area. 3.5" to intercooler which the mfg says flows enough for >700hp and with less than 1" pressure drop. right angle turn in the cooler to 3.5 pipe that makes 90* turn to the small plenum above the intake pipes. Under hood air is  going to be considerably >100* at Bonneville. I have 58-62* going into the engine at the end of the run. I have not run at Bonneville without the intercooler. I did make one pass at Wilmington without it and it was definitely slower.
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Jack Iliff
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 G/GC- 169.741  2009
 G/GMS-178.835 2010
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