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Minimum Fuel Tank Vent Line Diameter

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HomeBuilt101

Well-Known Member
Joined
Oct 20, 2015
Messages
384
Location
Aguila AZ
What is the recommended diameter for a fuel tank vent line that will supply a 350 HP engine?

Is the diameter requirement of the fuel tank vent pipe dependent on whether the end of the fuel vent pipe is a simple cut off pipe end sticking straight down from the belly of the wing, or if the pipe end is arranged with a NACA scoop, a pitot tube design (like C-172), or the usual 45 degree cut end with the beveled edge pointed into the air stream?

Lets say the airplane will have one vent line will provide the vent for two 45 gallon fuel tanks and the configuration of the tanks is such that the top surface area of the fuel in both tanks will be about 2000 square inches. I cite the surface area of the top surface of the fuel because if the fuel vent end is pointed forward like a pitot tube, I can imagine that a pitot tube sticking out in the air stream at 180 MPH can create a fair amount of positive air pressure and this pressure being ported into the fuel tanks will spread out across the 1000 square inches of fuel surface area in each tank as well as the side walls and the top inside surface of the fuel tank...however the walls and top of the tank will not move so the actual force is going to be pushing down on the surface area of the fuel... so I can imagine that the diameter of the pipe can be a concern.

Kind of like a hydraulic floor jack...a small diameter squirt of hydraulic fluid pushing against the large diameter of the lift cylinder piston and inside the actuator there is a great deal of force on the actuator cylinder walls but since they do not move the actual force is being exerted down onto the piston surface area...so in a fuel tank the air pressure forcing its way into a small diameter pitot tube would ported to the fuel tanks and push against the very large surface area of the fuel creating a lot of force and unlike the floor jack needing to get stroked to exert pressure...the air pressure from the pitot tube would be continuously forced into the fuel tank as long as the airplane is moving...no?!?!?

To think of this in extremes can be helpful...A super small pitot tube opening would be way too small to effectively move the air through the pipes and on the other hand a super large diameter pitot tube opening like 2000 square inches would only push down on the fuel surface area on a 1 to 1 ratio (so that would not work for more reasons than that)...but there has got to be a sweet spot of diameter to create an efficient air flow plus sufficient pressure...

In this design, there will be a one way check valve plumbed into the system out in a void in the wing where the check valve could still draw air from this dry vented area in the event that the pitot tube portion becomes blocked with debris or ice the fuel pumps will be sucking away and the check valve will open up against its slight spring pressure.

Any ideas?!?!?
 
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