Fuel is all but still in the tanks, even in still air. Sloshing of fuel against a leaking gasket, in combination with a flow of air through the tank vents to the cap will sip out fuel. The locally turbulent flow around the cap (think PA28 fuel tank cap) will contribute regardless of vents. In case the cap is gone, air turbulence in the opening will provide the transport function, more intensely so.
This is all due to the horizontal nature of fuel tanks in airplanes (low distance from the fuel free surface to the opening) and lack of anti-sloshing features or fuel/air separator.
Arne wrote:
Fuel is all but still in the tanks, even in still air. Sloshing of fuel against a leaking gasket, in combination with a flow of air through the tank vents to the cap will sip out fuel. The locally turbulent flow around the cap (think PA28 fuel tank cap) will contribute regardless of vents. In case the cap is gone, air turbulence in the opening will provide the transport function, more intensely so.
This is all due to the horizontal nature of fuel tanks in airplanes (low distance from the fuel free surface to the opening) and lack of anti-sloshing features or fuel/air separator.
+1
Especially dangerous when turning out of the wind…
I don’t know why his thread is so long. There is no doubt the low pressure area above the wing is strong enough to suck the fuel out of the wing. It is strong enough to lift the plane into the air, for heavens sake.
You loose about 3/4 of the fuel if you don’t realise it and turn around, right until there is almost nothing left.
You are making this much too complicated.
Timothy wrote:
If you are happy to have fuel streaming out within a metre of 1600°F exhaust pipes, then you are very, er, brave.
I believe the OP was for a wing tanked SEP ?
Like an in-flight door ajar issue, in most types, it is a non-emergency , but on a Lancair for example, it is a real emergency since they have been known to come off and take out the empennage.
So sure, no doubt that a fuel leak from a nacelle tank on a twin is more serious than a wing tank fuel cap leak on a SEP.
EuroFlyer wrote:
ou loose about 3/4 of the fuel if you don’t realise it and turn around, right until there is almost nothing left.
.
This is totally dependent on the aircraft type and the tank configuration .
Your anecdotal experience cannot be carried over to different types
There is no doubt the low pressure area above the wing is strong enough to suck the fuel out of the wing. It is strong enough to lift the plane into the air, for heavens sake.
Hmmm… no that view sounds good but doesn’t compute in engineering terms 
It would work if the fuel was gaseous, not liquid. It would work with a liquid if the earth’s gravity was zero (but then you would not need wings). It would also work if the fuel vapour pressure was lower than ambient (then you would need a pressurised fuel tank).
Peter wrote:
but doesn’t compute in engineering terms
Well, maybe this is too simple for complicated engineers then, and other important factors are left out of the equation :)
As in: we had it in a PA28 once and didn’t realize it until well after takeoff. There was a constant stream of liquid coming out of the nozzle, and nothing could be done. When we landed at the next airport, the tank was nearly half gone.
The fuel in the tank isn’t just calmly sitting there like the clever engineers seem to assume. It’s sloshing back and forth and right and left like a swimming pool during an earthquake. The wider the surface, the more waves are being created and reciprocating. That’s why a water bucket typically is deep, not wide. Ever tried to transport water in a baking tray ? It won’t work. You WILL loose water.
The fuel slops out of the tank even on the ground when you rock the plane. In the air that’s even more the case. It sloshes and enough liquid gets close to the hole and gets sucked out. I don’t need to calculate that, I can see it.
EuroFlyer wrote:
As in: we had it in a PA28 once and didn’t realize it until well after takeoff. There was a constant stream of liquid coming out of the nozzle, and nothing could be done. When we landed at the next airport, the tank was nearly half gone.
A “constant stream”? Which model of PA28? Which tank was selected?
Now why should that matter ?
