NACA ducts

[stickandrudderman]

I'm struggling to reconcile something:
We know that a NACA duct is designed to reduce air flow without inducing drag, but what is the benefit of employing a NACA duct at the (piston, non-turbo) engine air intake? I understood that a scoop with internal stabilisation of the air flow is better (from automotive engineering studies done 30 years ago!).
If it can be shown that a NACA duct is indeed beneficial in this application how does one go about calculating the optimum size duct?
I've spent quite some time googling this but can't seem to find an answer.
Any comments appreciated....

[Mike-Bracknell]

You're not talking about internal stabilisation of the airflow, you're talking about inducing swirl in order to fill and empty the combustion chambers more efficiently. It's also not about reducing airflow, it's about drawing it off. i.e. it's one of the most efficient designs of intake, allowing air to be drawn off whilst reducing the amount of drag of the remaining air.

To be honest, you're probably best working from the automotive background for optimal size openings per engine type and capacity, and then working out what size NACA duct to use to get that flow through it. That way you're likely to be able to get the breadth of data you need.

[Brian Abraham]

May find your questions answered here

http://naca.central.cranfield.ac.uk/reports/1945/naca-acr-5i20.pdf

[Agaricus bisporus]

Stick, I'm not sure you're description of a NACA duct is correct. I,m not sure it has anything to do with reducing airflow, has it? It is a drag-free airscoop for high M and super M speeds that crucially does not achieve the ram effects of other types of inlet, ie a scoop, so would be less efficient as an engine intake, be it for a carburettor or for a turbine compressor. It's use on low speed applications (cars, most piston aircraft) is, I think, very often cosmetic. If you want an extra couple of pounds of manifold pressure on the cheap then a scoop may be the way to go.

MB, I doubt the type of intake has any effect on combustion chamber swirl. That's developed miles downstream of the intake right inside the engine itself (valve and cylinderhead). There's half a yard of ducting before that where smooth flow is critical for free breathing.

[ShyTorque]

I was always taught that NACA ducts are used rather than a scoop where low drag is the over-riding design consideration.

[stickandrudderman]

Low drag is indeed the over-riding consideration.
Consider a fresh air vent; if you have a scoop ducted to a fresh air vent you'd be blasted by air at a/c airspeed, but through a NACA duct the flow will be reduced and therefore more useful without inducing drag.
Swirl is a factor in PE induction but this is long after the air intake.
Since the air intake is by definition low pressure, I am wondering what effects the use of a NACA duct has on the induction system other than reducing drag, and indeed if it is beneficial for anything other than that.
I'm also looking for a definitive formula that will, if a NACA duct is employed, determine the size of that NACA duct for a given engine capacity/rpm.

[Brian Abraham]

You seem to want to apply the duct to motoring so this may help

Staniforth. Race and Rally Car Sourcebook. ISBN 1859608469. (Practical guidance on designing and building NACA ducts for motor-racing applications)

[18-Wheeler]

You seem to want to apply the duct to motoring so this may help

And if you're going to want to vent air from inside the car, please don't use a NACA duct facing backwards like so many people do, as they don't work when facing the wrong way.

[stickandrudderman]

Sorry Guys, not for motoring, for aviating!
I'm trying to obtain the best info for the design of an induction system for PE a/c and am getting conflicting info for scoop versus NACA.......:ok:

[Brian Abraham]

Please tell us exactly what you are trying to do/achieve and advice might be more fulsome.

[stickandrudderman]

Please tell us exactly what you are trying to do/achieve and advice might be more fulsome.

Sorry but I thought I had?

I'm trying to establish a means of determining the best orifice to employ as an induction point for a non-turbo charged piston engine mounted in an aircraft.
Scoop= some pressurisation of the inlet tract which may or may not be beneficial and increased drag.
Naca duct= negative pressure differential and no drag.

This is the sum of my knowledge and I'm looking for contributions from people whose knowledge is greater than mine.
Thanks in advance.

[mike-wsm]

Can't claim any great amount of knowledge on this but here are the relevant words from the link cited above by Brian Abraham.

From NACA ARC No. 5i20:

http://mjr.org.uk/pprune-023.gif

[Brian Abraham]

how does one go about calculating the optimum size duct?
How one guy calculated the size with a bibliography of NACA reports which should be available on their technical server (not checked, I'll let you do that leg work)

NACA inlet cross-section (http://www.melmoth2.com/texts/NACA%20inlet%20sizing.htm)

Edited to add you might also look at this

The Scoop on the NACA Scoop | Flying Magazine | The World?s Most Widely Read Aviation Magazine (http://www.flyingmag.com/scoop-naca-scoop)

[Volume]

As quoted by mike, the NACA duct has been optimized as inlet for jet engines, where maximum airflow volume is the driving design parameter. If maximum ram air pressure (maximum pressure recovery) is most important, the NACA duct is not best. However, it is best looking ;) (after all it´s the customer who decides, and does not know anything about aerodynamics...) :ouch:

However, as you can see in this ASW-22 TM (http://www.alexander-schleicher.de/tm/22/220_TM10_E.pdf) glider manufacturers successfully replaced pitot style air inlets by NACA ducts for boundary layer control, which requires maximum pressure recovery with realtively low volume flow. Obviously they do work fine, even at low Reynolds number and within the boundary layer. But maybe it´s for the looks again...

[stickandrudderman]

FANTASTIC, THANK YOU BRIAN!
That's excactly the kind of material I'm looking for.