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.
Regards, Bill Hannahan
|