Question on the location of a NACA duct

About 2 hours ago, we were just about to cut into our fuselage to install a NACA duct just below the cowl cheek like we?ve seen on dozens of other RV-4?s, when our mechanic came up with a new idea.

He suggested mounting it on the avionics access panel between the upper engine cowl and canopy. It seems to us the advantages far outweigh the disadvantages, but because we?ve never seen one there, we thought we might be overlooking some obvious problem that someone here might be able to point out.

Advantages;
1. The biggest advantage I see is no interference with our feet and the scat ducting.

2. Once in the fuselage, it?s there for good, but if it?s on the panel, we could remove or relocate it, as a new panel would be relatively easy to fabricate.

3. Possibly more air flow due to an almost straight run from the NACA duct to the eyeball vent on the lower right side of the instrument panel as opposed to having 2 90 degrees turns if it?s mounted on the fuselage.

Disadvantages;
1. Water is more likely to enter the duct on the rare occasions when it?s raining and outside on a ramp, but if we mount in on the lower rear side, it will only be off vertical by approx 20-30 degrees and our canopy cover would cover the opening.

2. Having to remove the tubing from the NACA duct every time we pull the panel.

3. What are we overlooking?

I think the most important consideration is whether it'll get air flow. It all depends on whether that area is high or low pressure, and from what I've read this can be a little hard to know without testing. Lots of folks have put vents in various places only to find that air actually flows OUT through them. For what it's worth, I put an RV-10 style vent on either side, right below the cheek. I'm using no ducting and am confident that those two little doors are going to grab a ton of air and keep my legs and feet cool. When closed, they're just about invisible. I might also install a Vetterman vent on the side skirt at each seat to cool the face and head.

Low n Slow said:

He suggested mounting it on the avionics access panel between the upper engine cowl and canopy.

Click to expand...

I think you will find that this is a low pressure area and will get little if any air flow.

Air Vents

If you put them under the cheek, watch out for the location of the rudder cables on the inside. Don't ask me how I know.

On the side

My buddy has his under the right side cheek cowl and it provides a ton of air. If you mount the intake a half inch below the cheek cowl, the ducting will be more than an inch above the rudder cable. The ducting goes up in front of the bulkhead into an outlet that is attached to a piece of aluminum angle that goes across from the left side to the right, behind the instrument panel.



Regards,

I like the picture of that installation Pierre. I want my air vent system install such that IF the plane was sitting outside in rainstorm (not that it ever rains here in Oregon...) the water would have a tendency to "self drain" from the vent system...which it appears this would. The install on my RV looks almost exactly like this.

NACA Duct location

Hi All
I located 1 NACA duct on the bottom of the left wing to provide air to the back seat and I pull air from the back of the engine baffle for the front seat
Your mileage will vary

Fresh Air...

Tom,
I tried every location for NACA ducts in the Bandit back in the day. CanopySkirt X2, low airflow. Fuselage left front side, OK but warm air comes in at lower altitudes. What I finally installed and LOVED were Larry Vetterman's twist out vents. Open into slipstream, close flush, lots of air. Installed 3, 1 just above throttle on left side, same place in back seat, right side front cockpit. Aircraft Spruce sells them for a high price. Larry Vetterman sells them to RVer's alot cheaper. Here is what they look like...
http://www.aircraftspruce.com/catalog/appages/ultimatevent.php
http://www.vettermanexhaust.com/
Worth a try and they don't leak when closed...
Rob Ray
1500 RV4 experimentation hours...
HR2
Vetterman Vent and Naca Open on left side...
[img=http://img384.imageshack.us/img384/4560/img2595fn4.th.jpg]

Tom, I looked at the NACA vent and gave up on it for my -4 for all the reasons you have been given. I went with this. This one is for p2.

As SZ said they should certainly catch the air.

I have two, one on the right down low, you can just see it here. Bottom left of picture. Double click to enlarge.

Smokey's route is nice, but a bit spendy.

Cheers, Steve.

Well, the first question is, why are you using a curved, divergent submerged (NACA) duct in the first place? Have you studied the NACA report on it and its two brothers, the straight, divergent submerged duct and the parallel-wall submerged duct and know what each will do? Look at a car; where does it get its vent air? At the base of the windshield, right? That's why I installed a 1" diameter hole there which goes into an expander duct, then into a 110 cfm blower, then divides out into two outlets on the bottom of the instrument panel; the expander duct is supplied with a drain for those rainy days. I get flow when on the ground when I have at least 1100 rpm. The blower is really nice on those hot days. I had Kevin Eldredge put a hole in that place on Relentless, 'cause his NACA ducts weren't doing a thing. He put in a 1/2" hole there and gets blasted-out at his high speeds. Be careful in putting a duct on the side of the fuselage above the wing; that is a low pressure area which can keep you from getting any airflow at all. If you have that problem, you can solve it by putting two small, 1' long X 1/4" high, delta-shaped VGs at the start of the duct with their LEs canted inward. That's what my friend had to do on his Lancair to get air, and what I had Kevin do originally. He took them off 'cause he didn't like their look, then put in the hole.

NACA and Vetterman

I've got a pair of 1.5" NACA ducts up at about panel-bottom-edge level in the fuselage, 10" back from the firewall. They're piped to the bottom of the panel with adjustable openings. I also put a Vetterman 1.5" on each side, 4" above the wing and about calf-height. Medium results on both.

I get what SEEMS to be slightly warmer air than ambient from the NACA's, maybe a few degrees but I haven't measured it.

I get a fair amount of un-directed air from the Vettermans, but they generate a LOT of noise and I have to re-set my intercom squelch when they're open.

I still feel like the cabin could use more air, but I'm not liking to chop any more holes in my little sweetheart. Maybe trade the Vettermans for the barn-door Van's types. Draggier but they should really pump in the air I would think. Lots of work to put them into a done airframe, dang.

G.

When I was having some speed tests done on a plane with my prop design, I found that the person's OAT sensor, mounted in his NACA duct, was giving basically the same temperature regardless of altitude. It did this on both a -6 and a -9A. It seems that some air goes in the cooling inlets, gets heated by the engine, then flows out and down the sides of the fuselage and into the cabin air inlets. I have found that the forecast temperatures for altitude, at least here on the West Coast, are pretty good and an excellent comparison to what your OAT is telling you. A surprise to me is that there is little change in temperature at altitude from day to night unless a big system moves in.

Under Wing NACA inlets

I plan on using 2 NACA style submerged inlets, 1 under each wing. I saw an RV-8 ot OSH that had them, and the owner/builder said he got copious amounts of fresh air, plus very low drag, plus no water entry issues. Not so easy to do on a finished airplane, but mine is not.

elippse said:

...I found that the person's OAT sensor, mounted in his NACA duct, was giving basically the same temperature regardless of altitude. It did this on both a -6 and a -9A. It seems that some air goes in the cooling inlets, gets heated by the engine, then flows out and down the sides of the fuselage and into the cabin air inlets...

Click to expand...

Were the ducts you describe mounted on the side of the fuselage just forward of the wings leading edge like most RV4?s?

I also want to mount an OAT probe and thought the NACA duct would be the best place. But if they pick up too much heat from the engine in the standard RV-4 location, where do most people mount their OAT probe on RV-4's?

Where does the heat come from??

This is insane.

My ducts are producing warmer air than ambient even in cruise. They are directly behind the cowl seam, and my probe is under the wing.

I used to have air pickups at the back of the engine plenum. These were toasty when taxiing, but they got to about 5 degrees warmer than ambient in cruise.

So I don't know if I buy the idea that air spills back out of the intakes and flows down the sides into the ducts. If that were true, it would say that the air inside the plenum is warm before it gets to the cylinders - - hard to believe.

BUT - - how the heck does the air get warmed up before it enters the outside ducts??? There's 165 knots of air flowing by, and most of it is plain ambient air. How does it get heated up?

Bewildered.

born2fly said:

This is insane.

My ducts are producing warmer air than ambient even in cruise. They are directly behind the cowl seam, and my probe is under the wing.

I used to have air pickups at the back of the engine plenum. These were toasty when taxiing, but they got to about 5 degrees warmer than ambient in cruise.

So I don't know if I buy the idea that air spills back out of the intakes and flows down the sides into the ducts. If that were true, it would say that the air inside the plenum is warm before it gets to the cylinders - - hard to believe.

BUT - - how the heck does the air get warmed up before it enters the outside ducts??? There's 165 knots of air flowing by, and most of it is plain ambient air. How does it get heated up?

Bewildered.

Click to expand...

From what I've seen, the cooling air inlets on the RVs are much larger than necessary.All of that air has to go somewhere. From what I've read, I guess the air strikes the front cylinders somewhat broadside, comes back forward more or less around the outside of a central core airflow, and by Coanda effect, follows the curved inlet edges toward the outside and down the sides. My prop design has much more flow in the root region, and planes that have tried my prop tend to run cooler. The tests I ran were on an RV-6 and an RV-9A with my two-blade prop, and both showed the non-decreasing temp with altitude! I guess their cabin inlets were located as called for on the plans. My 125 HP O-235 has 6 sq. in. inlets on each side for about 10HP/ sq. in. BTW, my cabin air inlet is a 1" circular hole at the base of the windshield and flows air on the ground when the rpm is about 1000-1100 and gives more than enough in flight.

smaller cowl openings?

Did you custom-fab your cowl, or just reduce the stock inlets?

I've thought for some time about how to shrink the stock inlets in a "clean" fashion, but nobody seems to have tried this - or at least posted anything about it.

I've also heard about using VG's near the NACA ducts to make them more effective - - I wonder if it might also draw some air from above a thin boundary layer and make it more cool?

Love to hear anything about this, I'm desperate for cool air.

BTW - how do you keep the water out of your windshield intake?

G.

I would think that an air intake just in front of the canopy would be a fairly good, high pressure location. I don't know about the RV-4 but on other planes I've seen tested the airflow tends to stagnate right at the front of the windscreen, building up some pressure as it decides which way to go to get around the obstacle. This may change at high angles of attack, such as when in the pattern, since the flow may detach and make that area a low pressure zone.
That said, I just installed two NACA vents 3/4" below the cheeks (one on each side) and as far forward as the rudder cables will allow. I figure at that point the boundry layer won't be too thick or be detached, which can cause submerged vents to not pick up any airflow (VGs are used in some cases to reattach the boundry layer). This position also protects them from rain ingestion but at the risk of some flow being blocked by the forward part of the cheeks. I considered every possible location and chose to put the vents where I did because I wanted minimal drag (speed!), minimal noise, maximum airflow and minimum complexity. I may do like Smokey and change them somewhere down the road but for now I think they should do the job.

Alan
N7028K
RV-4 flying soon