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Naca Vent: Which Side

lr172

Well Known Member
I am planning only one naca vent for my overhead console. I am installing a second 2" flange fitting for easy install later, if necessary.

I am curious which side of the fuselage would pick up more prop wash on the ground and therefore, the preferred side for one Naca scoop.

Thanks,

Larry
 
Standard diagrams would imply the left side, but why not put them in on both sides? It's not that much extra work "while you are in there" :)

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We Only put one on the Port (left) side, just behind the baggage door and a little higher, works a treat plenty of air to feed the four overhead vents, even on the ground. Others have said that it doesn’t matter which side you put it on, both work, I used Gills theory and certainly didn’t need two.
 
I prefer them on both sides.

In the air, it doesn't really matter. But when on the ground on a hot summer day, the more air the better.

It doesn't really add to the cost and there's no real negative impact.

bob
 
Prop wash may be better on one side, but it would also be worth considering the possibility of exhaust on the same side. In flight I don?t think it would matter, but following the diagram posted, I would expect the chance of exhaust fumes getting into the vent on the left side more than the right side.
 
I prefer them on both sides.

In the air, it doesn't really matter. But when on the ground on a hot summer day, the more air the better.

It doesn't really add to the cost and there's no real negative impact.

bob

I am not concerned with the cost or effort, but was concerned about drag. It is hard to believe that each Naca is drag free. Does anyone know what the drag penalty is for these?

Larry
 
Right side

I had the same concerns as Jesse mentioned about the exhaust getting in on the ground, so I put mine on the right side. Plenty of air volume with just one and no exhaust fumes, although those who put theirs on the left don't seem to have any problems either. I still use a butterfly valve to throttle the air with the one NACA vent in flight, so there's certainly plenty of volume to work with.
 
I am not concerned with the cost or effort, but was concerned about drag. It is hard to believe that each Naca is drag free. Does anyone know what the drag penalty is for these?

Larry

To obtain the design benefit of a NACA scoop you should put it in an area where the fuselage cross-section is increasing. Also pay attention to the "bar" across the aft end of the opening. Proper design requires a substantial radius (compared to most installations seen).

FWIW
 
NACA

Naca vents are definitely NOT drag free. There have been studies that show them to have 75% - 85% efficiency, though. As previously posted, the location and the surrounding structural environment affects the ducts efficiency...
 
I am not concerned with the cost or effort, but was concerned about drag. It is hard to believe that each Naca is drag free. Does anyone know what the drag penalty is for these?

Larry

Some light reading from the NACA report that NACA scoops are based on: http://naca.central.cranfield.ac.uk/reports/1945/naca-acr-5i20.pdf

Note that the report states:
The data obtained indicate that submerged entrances are most suitable for use with internal-flow systems which diffuse the air only a small amount: for example, those used with jet motors which have axial-flow compressors.
Where complete diffusion of the air is required, fuselage-nose or wing-leading-edge inlets may prove to be superior.

So to paraphrase, they are great for air-breathing engines but don't use them for vents.

This should get some robust discussion going :)
 
NACA Inlet

So to paraphrase, they are great for air-breathing engines but don't use them for vents.

The original goal of this NACA AR 5I20 investigation and report was to find the suitability of low drag inlets for jet engines, so it is no surprise that the summary specifically mentions this. For example, see figure 38, second to last page where the inlets are shown on the fuselage sides just ahead of the wings. In this position they do not recover much if any ram pressure and you never see them employed in this manner. However, numerous smaller NACA vents are used on the nacelles of pod mounted jet engines for various purposes. They are also used as general purpose low drag inlets.

An earlier post mentioned they should be placed in an area where the fuselage cross section is increasing. A more general statement would be they should be placed in a flow field with a positive pressure gradient. If placed in a flow field with a negative pressure gradient they will act as an exit vent (and not very efficiently). There are lots of examples of poor placement like this on race cars and some aircraft.

To the OP's question, the NACA inlet is not drag free, but I don't think you could measure the difference in speed of your aircraft between one NACA inlet and two.
 
To obtain the design benefit of a NACA scoop you should put it in an area where the fuselage cross-section is increasing. Also pay attention to the "bar" across the aft end of the opening. Proper design requires a substantial radius (compared to most installations seen).

FWIW

Is this "radius" in the 90* corners or a radius'ed edge to the vertical oriented cut in the skin?

Larry
 
Put the scoop under the wing if possible.

While I only have 2 firm data points, my RV-4 and my F1 Rocket had 11 degree F HOTTER air coming in the fuselage sides than from under the wing. The thermometer doesn't lie.

Believe me, on a 90+ degree OAT day, the fuselage scoops were downright HOT!

YMMV!
 
For the over head console. I am not sure it matters but I put mine on the left side behind the baggage door. I also have a valve on a servo to help to control airflow. I have tons of air. I sealed up the Vans rear vents during construction. The back seat passengers only use the overhead console. They have never complained about benign to hot.
 
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