NACA scoop for fuel vent

Has anybody considered/modified the fuel vent to a NACA Scoop? There are other airframes that use it. I’m currently seeing some variation in pressures when the tanks are full (and possible fuel in the vent line) and wondering if this would add a little more head pressure to the pump (pump longevity?) Once I’ve burned off 20 min worth of fuel, fuel pressure stabilizes. Currently have the JD air vents.

I know we should never fly into icing conditions, but it would also help for the inadvertent encounters…

Thoughts?

I researched this pretty thoroughly and did make some "auxiliary" NACA vents in addition to the standard Vans 45* cut fitting vent. This was before the JD Air vents came out and I ended up replacing the Vans vents with the JD Air vents. I'm keeping the NACA vents which I've mounted just forward of the spar, so quite a ways back from the JD Air vents and I'll likely plumb them to a tee after first checking the head pressure of each. From my research it's unlikely you'll get a higher head pressure from the NACA vents than the JD Air vents, but if you're concerned about inadvertent icing it may be worth adding. I wouldn't replace the JD Air vents with the NACA vents without a lot of testing. Mine's not flying yet.

Yes

Yes, I made my own modified fuel vents. Based somewhat on airbus design. I made the "rocket" coil in the wing root and connect the end to the flush vent mounted to the lower side of the wing root fairing. They work well. As of yesterday, 387 hours.

Never liked the tube and it is prone to icing. Also with the submerged vent I could drill many very small holes in the back end to keep the insects out.

I bought a wind tunnel and validated the design against the std plans bent 1/4 tube, same manometer reading. Of course, only when the tube was directly in the wind stream. At an angle, the submerged retained the same value, the tube did not. I made a plug for a mold, and have it in the hangar somewhere.

Photo of my fuel vent installed on lower wing root fairing and one out of the mold, before trimming and installing plenum.

why do we need to pressurize the tank? The fuel pump can pull a very strong vacuum. We have seen several cases where a plugged fuel vent caused the fuel tank to deform inward enough to look like a crushed soda can.

lr172 said:

why do we need to pressurize the tank? The fuel pump can pull a very strong vacuum. We have seen several cases where a plugged fuel vent caused the fuel tank to deform inward enough to look like a crushed soda can.

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A slight positive pressure in a tank can be beneficial in some fuel systems to assure the pump immediately pulls fuel when a different tank is selected. That is the reason RV fuel vent ports are specified to be cut at a 45 degree angle, with the open side facing the relative wind.

I like the idea and innovation of the flush vents for an IFR airplane because of the protection it might have from getting iced over. The underside leading edge of a wing (where the vents are located) is vulnerable for ice formation at higher angles of attack such as during a climb or because of angle off attack increase because of lost lift from heavy ice accumulation so without testing it is an unknown. Probably better than the protruding tube though, but once again, unknown without testing.

NACA vents inherently have a low probability of accumulating ice. They're used on some airliners as fuel vents, unheated. I'd worry about the tube icing closed well before a NACA vent.

That said, don't go into ice. If you get there anyway, get out ASAP.

David Z said:

NACA vents inherently have a low probability of accumulating ice. They're used on some airliners as fuel vents, unheated. I'd worry about the tube icing closed well before a NACA vent.

That said, don't go into ice. If you get there anyway, get out ASAP.

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I'm well aware which is why I said I liked the idea.
I was just saying don't be fooled into thinking that just because a NACA was used for a fuel vent port, that it wouldn't ever get iced over.
Is also sensitive to the installed location. The standard location of the fuel vent on an RV-10 and 14 could develop ice on the bottom wing surface (and also NACA vent) in certain circumstances

No, it wouldn't help,

NACA vents are designed to work best at much higher mach numbers than we fly at and when when some fliw is required through the vent. My view is they would not be better than the standard or JD vents.

penguin said:

No, it wouldn't help,

NACA vents are designed to work best at much higher mach numbers than we fly at and when when some fliw is required through the vent. My view is they would not be better than the standard or JD vents.

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I did a LOT of research into fuel vents (see my Kitplanes article July 2011). Although "NACA vents" were originally designed as submerged inlets for jet engine intakes, they proved to be not as effective as the ram type engine intakes we see today, for example on fighters. However, NACA did extensive testing on this shape of vent for fuel vents (NACA Research Memorandum RM E8A27b March 29 1948, NACA Research Memorandum RM E8B05a March 20 1948 and NACA Research Memorandum RME8C05 April 23 A948). They were tested at various angles of attack and also in icing conditions. They are highly recommended as a low drag icing resistant fuel vent. Although they are used as fuel vents on airliners (M ~ 0.7 to 0.8), they are also used extensively on general aviation aircraft including experimental amateur build.
The principal of operation of using the ogive plan view shape to create a vortex and draw in free stream air of course works with a flow in the throat of the vent. But the shape is surprisingly effective with almost zero flow when acting as a fuel vent. In this application, because it is submerged, it is also very effective against icing when properly placed on the airframe.

rvbuilder2002 said:

A slight positive pressure in a tank can be beneficial in some fuel systems to assure the pump immediately pulls fuel when a different tank is selected. That is the reason RV fuel vent ports are specified to be cut at a 45 degree angle, with the open side facing the relative wind.

I like the idea and innovation of the flush vents for an IFR airplane because of the protection it might have from getting iced over. The underside leading edge of a wing (where the vents are located) is vulnerable for ice formation at higher angles of attack such as during a climb or because of angle off attack increase because of lost lift from heavy ice accumulation so without testing it is an unknown. Probably better than the protruding tube though, but once again, unknown without testing.

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I used a homemade flush style vent on my 10. I just didn't want the drag from a tube. Not sure if this would help for ice. I put a tee and one way check valve in the wing root in order to provide a vent source in case the port ices over. I assume this flush port is creating a slightly negative pressure on the tank, just as many static ports can do. It is unlikely creating positive pressure. Have not experienced any fuel delivery issues in 350 hours.