Regarding this;

{ the Andair check valves (at least the model I got) are marked 1psi required to push air out the valve.

So each wing would have about 50 square feet top and bottom or about 100 lbs. total force trying to blow the wing apart.}


There are 144 square inches in one square foot. 1 psi is equivalent to 144 pounds per square foot. The 320/360 with extended fuel tanks has about 15 square feet of wing tank, so about 2,200 pounds of force on each skin at 1 psi.

Regarding this;

{ The ES has tiny NACA scoops in the wingtips like the IV with the thought that they would slightly pressurize the fuel tanks to help the fuel feed to the engine.  Sort of makes sense to me.  I have no idea if it works at all.}

The pressure produced by a NACA scoop is not proportional to its size. A poorly designed or poorly located scoop may produce no pressure gain at all. A well designed NACA scoop located near the front of a body, that is, where the shape is still expanding, such that you can see the inlet standing directly in front of it, may well produce full dynamic pressure at zero flow, like a pitot tube.

Out of curiosity, I have looked at many fuel vents at fly-ins. There is great variation in type and location. Some are located such that I would expect them to produce near full dynamic pressure.

Dynamic pressure increases with the square of velocity. At 200 knots CAS it is 27 inches of water, about 1 psi. 300 knots equals 62 inches of water, 2.2 psi, 317 pounds per square foot trying to blow the skins off the wing.

http://www.iflyez.com/manometer.shtml

Regarding this;

{Having a check valve or float valve in the vent line is common in certified aircraft so I disagree that vent lines must be free flowing in both directions.  The twin cessnas use a float system to block the vent when fuel is present. This has led to problems with unanticipated blockage of the vent line leading to ADs like this one:}

I think the AD makes a good case for free flowing vent lines.

Imagine you have a vent with a float valve. You have an early morning departure from Phoenix and top the tanks the night before. Something comes up that delays the trip one day and your plane sits on the ramp where temperatures skyrocket during the day. The fuel expands, the float valve closes, and the wing is structurally damaged by expanding fuel. The damage may be indicated by a big fuel stain on the ramp, then again it may not be indicated until you take off.

I am somewhat suspect of multiple vents to one tank. I met a builder with a BD4. He had doubts about the wing tip vents, so he had drilled numerous holes in his inboard mounted fuel caps. I pointed out that with gasoline, the fuel vapor concentration is usually above the combustion limit, but with the fuel cap in a low pressure region, and the tip vent getting ram air, there would be a continuous flow of fresh air through his tank that could make it combustible. A static spark near the vent could set it off. He thought that was interesting.

A check valve in the cap allowing air in, not out is probably OK as long as the main vent is not in a negative pressure region, but it might allow water in while sitting on the ramp.

I’m a big fan of the Keep It Simple principle regarding fuel vents. I only top off a few times a year for long trips and regard the possibility of a little spillage taxiing out as a small price to pay.