Landracing Forum
Tech Information => Aerodynamics => Topic started by: QikNip on September 05, 2018, 05:55:45 PM
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I've been reading up on hood scoops and have thus far concluded that they can create additional drag if oversized, oxygen starvation if undersized, unexplained negative aero effects even if right sized ... and that they are in general a black art! After reaching the foregoing conclusions, I began to wonder about the use of NACA vents mounted in the hood, collectively plumbed into a fairly large collector box, feeding the carburetors - what I hope would be more equalized doses of cooler air than occurs under the hood at speed.
At issue is the question of how many vents would be needed to provide the requisite air to supply a 122 cubic inch motor spinning at 8300 RPM's achieving a top speed of 155-160. Secondary consideration would be any boost that might come as a bi-product of the cooler, calmer intake air.
These specific questions aside, I'm fully open to hearing that this is a thoroughly dumb idea! I throw myself on the mercy of the aero educated here. :-D
Rick
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Rick, scoops are simple! :-o :x :-P :-)
http://www.engineprofessional.com/EPQ2-2014/files/inc/812367d726.pdf
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Woody:
I read it once. Now I'll read it some more! I think your comment in the article..."It isn't that simple" belies your message above! :-D
Rick
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NACA ducts work as long as you use the correct formula.
I've seen many advertised by online vendors in the automobile
arena and none if not all are real NACA ducts. My experience
with them was in road racing. Ask George Poteet why he got rid
of his big scoop when building the new speed demon. Just my
2c from an amateur wannabee aerodynamacist.
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Maybe you need an "Air Intake" – like this Monza that's held the C/CFAlt records at both Bonneville and El Mirage since 1992.
And, same motor, set 7 Street Roadster records at El Mirage and Bonneville, too, in a barn-door Model A that's way outclassed now.
PM me for a description.
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NACA ducts work as long as you use the correct formula....
Won't name names but someone real famous with a big world record that uses them told me he wouldn't use one on a NA vehicle, they run blown. A blown engine can suck in the air it needs if you give it a place for that to happen. Not that a NACA duct wouldn't work, but a lot of variables that would almost dictate wind tunnel testing to make sure in your situation that it would work.
This might possibly help with scoop design that wouldn't hurt drag too much....
http://purplesagetradingpost.com/sumner/bvillecar/bville-scoop%20info-1.html
Sumner
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Stainless and the guys with the Bockscar have ran a NACA duct inlet for years and have the data to show that if it is correctly designed and made and put in the proper place on the car surface they do work. As I remember they actually got theirs from some military aircraft and it works for them. I have a piece to that is to fit where my present scoop fits that will allow me to make a NACA duct inlet, but it is a next year project. Flat out to make the WOS meet next week!
Rex
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If you want to know if your NACA duct works, paint it... if you can, it doesn't... :-o
If is really difficult to paint, because the paint shoots through it when the gun passes to paint the floor, then it probably does.. there is more to it than just the shape...
Yes ours came from our favorite airplane, was part of an engine pod that was scrapped after a fire...
:cheers:
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I missed the part in any of this that says "air flows from high pressure to low pressure".
I guess it's not important.
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"air flows from high pressure to low pressure"
Generally true, but if the air has mass, which it does, and velocity, which it may, the converse is possible.
So, "Ramcharging".
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I missed the part in any of this that says "air flows from high pressure to low pressure".
I guess it's not important.
If you are using a NACA duct I think that is the principle that makes it work, as long as you have velocity and a correctly shaped duct which I think maximized the amount of air captured.... but like I said, there is more.... the correct shape on the rear lip has a lot too do with how well the duct works. It also needs go be sized for your needs. In the past we have measured positive pressure in the airbox fed by the duct.
If you think 1 is good so 2 must be better..... nope tried that.... no positive pressure.
But Panic, if you wish to educate us on the theory behind the NACA duct, feel free to do so... whatever you think is important :-o
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And the quest began :cheers: https://ntrs.nasa.gov/search.jsp?R=20090012113
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An air inlet in the center of an aerodynamically shaped nose may be better than a NACA inlet or a scoop . It gets full ram air pressure for any speed and actually reduces drag on the body slightly compared to a hood scoop by reducing the amount of air the nose needs to push aside . This only applies to a correctly streamlined nose .
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Some of this was discussed in "Engine Compartment Venting" in Aerodynamics. I may have more to share from an earlier project - but need to get an okay from Woody.
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to Woody sharing :cheers:
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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.
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Here are some more views of the NACA submerged duct design. I hope these help.
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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.
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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! :-o :x :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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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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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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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!?!? :x]
A NACA duct application: http://melmoth2.com/texts/NACA%20inlet%20sizing.htm
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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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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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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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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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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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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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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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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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Not all those throats are inhaling at the same time which is why the air requirement calculation divides the revs by 2.
My Euro version of the calculation is litres x (revs/2) = litres / min and then convert litres x 0.035315 to cu ft and I get 293 cu ft min.
The Jenvey throttle body air box only has a 3.5" intake and they were more than happy to supply that knowing a 122ci capable of pulling up to 9,000 rpm.
Rik I am assuming your are talking an S2000 motor as your spec and shift points are remarkably identical to my calculations or is that just a weird coincidence (cue doo doo doo doo music).
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Not all those throats are inhaling at the same time which is why the air requirement calculation divides the revs by 2.
My Euro version of the calculation is litres x (revs/2) = litres / min and then convert litres x 0.035315 to cu ft and I get 293 cu ft min.
The Jenvey throttle body air box only has a 3.5" intake and they were more than happy to supply that knowing a 122ci capable of pulling up to 9,000 rpm.
Rik I am assuming your are talking an S2000 motor as your spec and shift points are remarkably identical to my calculations or is that just a weird coincidence (cue doo doo doo doo music).
Your air requirement observation is brilliant. :cheers: As I was laying in bed last night this same thought occurred to me (only half of the cylinders on the intake cycle per revolution). As for the motor, it's a 1979 Porsche two liter single overhead cam 924 block and head.
Rick
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NACA ducts work as long as you use the correct formula....
Won't name names but someone real famous with a big world record that uses them told me he wouldn't use one on a NA vehicle, they run blown. A blown engine can suck in the air it needs if you give it a place for that to happen. Not that a NACA duct wouldn't work, but a lot of variables that would almost dictate wind tunnel testing to make sure in your situation that it would work.
This might possibly help with scoop design that wouldn't hurt drag too much....
http://purplesagetradingpost.com/sumner/bvillecar/bville-scoop%20info-1.html
Sumner:
Somehow I overlooked the link you included and only noticed them this afternoon. They were quite useful-particularly the John Burk / Tom Burkland write ups. Thanks!
Rick
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Here is a big "IF" but if we are not chasing ignition gremlins next year and are able to get multiple runs in I have a part that I have made that will fit onto the present position of our inlet scoop and will allow me to use a NACA duct for the inlet. I will try to not compromise in the adaption of the inlet from our scoop to the NACA configuration and maybe we will be able to see if there is enough difference to warrant using a NACA duct.
It is a year away and many things can change and Duke and I have a pretty long list of things we want to change but I would really love to try this.
Rex
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Here is a big "IF" but if we are not chasing ignition gremlins next year and are able to get multiple runs in I have a part that I have made that will fit onto the present position of our inlet scoop and will allow me to use a NACA duct for the inlet. I will try to not compromise in the adaption of the inlet from our scoop to the NACA configuration and maybe we will be able to see if there is enough difference to warrant using a NACA duct.
It is a year away and many things can change and Duke and I have a pretty long list of things we want to change but I would really love to try this.
Rex
Rex:
Since we have the same displacement, I'm curious as to where you're getting that NACA duct and it's size.
Rick
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"Getting the NACA duct"!!! The ones that you can buy are junk, I have lots of 3003, tin snips, hammers, TIG welder and the chart of dimensional relationships required to build a NACA duct with the right dimensions, screw buying a piece of junk when I can build my own!!!!! I will size the opening based upon 220 mph, my motor at 12,000 rpm and a fudge factor that will be determined when I start the project.
Rex
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Rex, if you want the size and pics of ours just let me know what I can tell you.
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"Getting the NACA duct"!!! The ones that you can buy are junk, I have lots of 3003, tin snips, hammers, TIG welder and the chart of dimensional relationships required to build a NACA duct with the right dimensions, screw buying a piece of junk when I can build my own!!!!! I will size the opening based upon 220 mph, my motor at 12,000 rpm and a fudge factor that will be determined when I start the project.
Rex
:cheers: :cheers: :cheers: :cheers: :cheers:
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I have just created a NACA duct calculator spread sheet from the Staniforth text, graphs and charts, and some basic formula from the interweb. Email me if you want me to send it - you're welcome to QC the math and compare it to other sources - might be useful, might be a rabbit hole, might amuse. jmellelieu@yahoo.com
John
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That basic formula works. I went NACA crazy and built moulds from 2" in length to about 12".
No shortage of accurate genuine aviation ducts here.
Thanks for sharing. You'll help a lot of guys with that. :cheers: :cheers: :cheers: :cheers:
Mike.
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Mike. You have had way too much fun with knackered ducts if you've made molds over a range of sizes :-D
What surprised me, using a 4:1 opening and a 7 or 8 degree ramp angle (and I haven't even heard the term cotan since high school in the '70's) is how long the duct is compared to its width i.e. the body / surface opening. Many of the after market ones seem much 'chunkier' - they appear to be about 2 times length to width whereas my calculations were more like 3 times.
Maybe they have much steeper ramp angles or maybe not. Perhaps they are performance bolt on's like fluffy dice. Damn - I will have to make a much taller screen to see past the dice - that's something else I didn't include in the Lakester design!
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I have just created a NACA duct calculator spread sheet from the Staniforth text, graphs and charts, and some basic formula from the interweb. Email me if you want me to send it - you're welcome to QC the math and compare it to other sources - might be useful, might be a rabbit hole, might amuse. jmellelieu@yahoo.com
John
… email on the way :)
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Mike. You have had way too much fun with knackered ducts if you've made molds over a range of sizes :-D
What surprised me, using a 4:1 opening and a 7 or 8 degree ramp angle (and I haven't even heard the term cotan since high school in the '70's) is how long the duct is compared to its width i.e. the body / surface opening. Many of the after market ones seem much 'chunkier' - they appear to be about 2 times length to width whereas my calculations were more like 3 times.
Maybe they have much steeper ramp angles or maybe not. Perhaps they are performance bolt on's like fluffy dice. Dodge - I will have to make a much taller screen to see past the dice - that's something else I didn't include in the Lakester design!
I'm still knackered. :-D