rph142

rph142

Well Known Member
Would a NACA vent work if it was mounted in the cowl cheek? It wouldnt take much to cut and mount them to my fiberglass cowl cheeks.

Thanks,
Rob
 
I would think that such a convex surface would be an area of low pressure. You might end up pulling air out of the cockpit.
 
Is this for cabin air, engine or oil cooling? Don't use a NACA duct (curved divergent submerged duct) unless you have the design data and can size it appropriately for your application. There is also a parallel-wall sumerged duct that is part of the same family that is much better when you want very high duct airflow. It is better than the CDSD when the velocity of the air through the duct is 0.5 or greater than the free-stream velocity. It uses a square aperture which interfaces to a round duct easier than the 4:1 to 6:1 CDSD aperture and also uses the 7 degree ramp for best pressure recovery. It should be 8.1 times as long as the aperture height-width to give the 7 degrees. A steeper ramp than this invites separation.
 
elippse said:
There is also a parallel-wall sumerged duct that is part of the same family that is much better when you want very high duct airflow. It is better than the CDSD when the velocity of the air through the duct is 0.5 or greater than the free-stream velocity. It uses a square aperture which interfaces to a round duct easier than the 4:1 to 6:1 CDSD aperture and also uses the 7 degree ramp for best pressure recovery. It should be 8.1 times as long as the aperture height-width to give the 7 degrees. A steeper ramp than this invites separation.
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:confused: Um, does that come in English?
 
Is this what you are talking about? I don't know how well it works, I just took the picture.

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I thought of your recent pictures when I saw the question Larry - thanks for posting it! I do wonder how effective it is on that curved surface though - true NACA ducts are harder to implement than people think. In my case, I have other plans for the cheek cowls, so I am looking elsewhere for cooling air....

Paul
 
Yeah I thought they would look interesting there, then I happened across pictures of johns plane "hack job". I just talked to him about their effectiveness and it's sounds like they don't work very well at low pattern speeds. It looks like I'll be pulling fresh air from somewhere else. I might end up stuffing the inverted oil tank in the cheeks.
 
elippse said:
Well, Katie, that must be some of the Latin I got in Catholic schools showing through!
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WOW, you must have been blessed with some super-smart nuns! :D

I'm glad this thread came up, because I was wondering the same thing. I'd have probably copied John after seeing his beautiful RV-3 at Oshkosh this year. http://rv3-hackjob.com/ Is there a DROOL smily on here?? :p

How about putting a duct on the side of the cowl, underneath the cheek?
 
KatieB said:
How about putting a duct on the side of the cowl, underneath the cheek?
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I looked at that Katie - the rudder pedals are there, and there's no room for the duct. I picked up a couple of the beautifully-machined Vetterman air vents that Van's sells, and am going to put them in the canopy skirt - for starters. if that isn't enough, we'll look elsewhere for cooling air....

Paul
 
rph142 said:
Would a NACA vent work if it was mounted in the cowl cheek? It wouldnt take much to cut and mount them to my fiberglass cowl cheeks.

Thanks,
Rob
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I've read that the NACA ducts work best in an area of thick boundary layer. This is usually found well aft on a flat area of the fuselage. Based on this, I have them on the doors of my Hiperbipe (which is about as flat a fuselage as you can get), and they work EXTREMELY well, even with prop blast alone. Also, there is some definite science to making these things work, and the Vans kit supplied parts are far from optimal even in the best location. Just cutting a hole in the skin and running SCAT tube to an eyeball vent is not going to work very well (not as good as it should, anyway)

The inlet lip should be slightly radiused (not just a cutout in the skin), while the ramp edges should be very sharp. The plenum cant transition to a round tube too soon, or be at sharp angles to the free airstream (both sins committed by the Vans supplied part). Lastly, the ductwork should be as short as possible. In my example, I simply have sliding door on the backside of the plenum, so no duct losses.

Based upon my experience with the -8 and my Hiperbipe, I'd say that the best place for the vent is in the side of the RV fuselage, adjacent to the occupant - no duct.

Edit: Attempts to find pictures of the "optimal" design have also led to reading that a thick boundary layer is a poor choice of location, which completely contradicts my prior study on this. In any case, as an aerodynamic device, these things need to be of the right size, shape, orientation and location to function well... Just "sticking them anywhere" is not likely to work.


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NACA came up with a family of ducts, Katie, that are referred to as submerged ducts, meaning they are mounted below the surface. There is the parallel wall, the straight divergent, and the curved divergent, which is popularly known as a "NACA duct". The straight divergent looks like the curved divergent except that its sidewalls are straight rather than curved, looking like a triangle.
The parallel wall would actually be a better choice in a lot of cases where you want a high flow volume, plus it is much easier to make and connect-to with an output duct. If you're going along at 200 mph and you have a parallel wall duct with a 1"W by 1"H by 8"L inlet, and the duct is flowing at 0.5 freestream velocity, you will get 61 CFM flow.
The usual NACA duct you see is not made correctly. It should not have a sharp edge outer lip at the inlet; it is supposed to be shaped like an inverted leading edge of an airfoil, and to keep from having separated flow, its downward ramp should be at no more than a 7 degree downward slope, so it should be at least 8.1 times as long as its opening is high. Too often also, the duct is placed on the fuselage right above the wing's leading edge where the pressure is low and sometimes, instead of air going in, it goes out! When this happens, you can make the air flow in by putting two VGs just at the start of the ramp with their leading edges pointed toward each other. The vortices shed by them will cause air to flow into the duct.
 
I would think high velocity = thin boundary layer, which is probably why these are so effective on flat surfaces. Like Paul said, there's just no good place to put one of these. I suppose some sort of scoop type vent is in order.
 
rph142 said:
I would think high velocity = thin boundary layer, which is probably why these are so effective on flat surfaces. Like Paul said, there's just no good place to put one of these. I suppose some sort of scoop type vent is in order.
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I wouldn't say there is "no" good place for NACA ducts... But there are plenty of "wrong" places. And to compound all the "wrong" places for them, people often take significant liberties with the shape and details of the duct and plenum itself. The exact specifications for the shape and details is easily found in the NACA documents - where to put them is not quite as clear.

When I built mine, I followed the NACA loft specifications exactly, so at least I had that variable nailed, as to the placement, I might have simply been lucky. In any case, mine work so well on the Hiperbipe that I am considering blocking off the "stock" duct and moving it back to the cockpit sidewall on the -8.
 
NACA Vent

Any vent, and NACA vents in particular, will only work if there is a higher pressure on the outside of the vent than there is where the air is going to. If higher pressure inside than outside, then air will flow out of the vent, not into the vent. I seem to remember the NACA papers saying these submerged vents should be located in an area of positive pressure gradient. Which means you don't want to put them on the aft end of the fuselage.

So how to find out if the pressure outside is higher than the pressure inside? Well computational fluid dynamics is an easy way to see the pressure gradients over the whole airplane, but most people don't have such programs on their computers. I would suggest using a manometer. You can tape a few small lines outside the airplane in places where you think you might want to put a vent, lead the lines into the cockpit and connect the other end of the manometer to the exit point in question, whether that is the cockpit for a fresh air vent or the cowling for an oil cooler vent for example. Yes, it's an experiment, that's why they call them experimental aircraft. One experiment is worth 1,000 opinions.
 
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