In that case, fuel gauges (float, or capacitive) would be nearly useless.
Just think how a standard carburetor works, the venturi is sucking the fuel and vaporizes it; then think of the top of the wing as one half of a venturi.
I had the same fuel siphoning happen to me in a BE60, the other pilot had refueled the other side and was in a hurry; I could see a blueish mist streaming over the wing; we returned to the field and closed the fuel cap that was hanging on the chain;
Peter wrote:
I am still looking for a decent explanation of the physics behind the fuel “pouring out” of a missing or leaky filler cap, out of a tank which has the fuel level say 10cm below the hole.
I don’t think you will….because it doesn’t happen… (except in the free cross-feed example I have above)
It happens in a PA31, I can guarantee you. I’m actually trying to find a picture taken by our crew at the time!
Dave_Phillips wrote:
It happens in a PA31, I can guarantee you. I’m actually trying to find a picture taken by our crew at the time!
That may be true … if there is free cross-flow from the opposite tank(s)… then the lower pressure will “suck” the fuel across….it will only start over-boarding when the level reaches the top… with the PA31 there is a cross-feed transfer pump….is there also a cross-feed valve? Maybe it was open (or leaking) even if the pump was off?
Or is there free flow between the main tank and the outboard tank?
No free flow at all, unless you select crossfeed which is only done when asymmetric. Outboards and mains are completely separate.
PS. The tanks are bladders.
Well I can attest to the fact that in a Warrior at least the fuel doesn’t just dissapear overboard, unlike the fuel cap which was not securely tightened. Despite the tank being full, there was no detectable loss in 20 mins flying. The cap was found on the runway and I won’t make that mistake again. Doh!
However, there does seem to be good evidence of serious fuel loss in similar circumstances.
(Not thread drift) In steam engines, we have a thing called ‘Priming’. This is where liquid boiler water is carried over into the cylinders and can have very nasty hydraulic consequences. It happens even when the water level is perfectly correct through a process of ‘frothing’ which causes the water to be picked up and carried by mechanical forces and is made much more likely by contaminants like oil in the water. Once it starts it can be very hard to stop. it’s not syphoning as such because there is not a continuous column of water, just a seething bubbly mass.
Could fuel behave like that?
Ok…the fact remains though that you will only overboard (significant volumes of) fuel if the level is at the top of the tank and the is being fed from another tank….that requires cross-feed…intentional or otherwise…. all the other talk about sloshing and vaporization are not significant…the concept of fuel at some level below the filler (where there is an air gap) being somehow sucked out is absolutely ridiculous…
AnthonyQ wrote:
sorry Ted…you’re wrong…
I don’t understand your comment?
I am saying that low pressure above the wing, is not the cause! A reduction of pressure by itself does not make sense.
I am suggesting based on the actual design, bladder/metal, placement of vents, tank bottom parallel to the earth, standpipe at the filler, position of the caps, tank is pressurised etc, are the causes.
Either by a difference in pressure in voids within the tank and or vortex of air created.
I understand the PA-31 tanks are pressurised by a NACA vent, but its a long time since I operated that aircraft, perhaps someone can confirm?
A critical point is the volume of fuel at the start of this process!
Ted wrote:
AnthonyQ wrote:
sorry Ted…you’re wrong…
I don’t understand your comment?I am saying that low pressure above the wing, is not the cause! A reduction of pressure by itself does not make sense.
I am suggesting based on the actual design, bladder/metal, placement of vents, tank bottom parallel to the earth, standpipe at the filler, position of the caps, tank is pressurised etc, are the causes.Either by a difference is pressure and or vortex of air created.
I understand the PA-31 tanks are pressurised by a NACA vent, but its a long time since I operated that aircraft, perhaps someone can confirm?
Pressurizing the tank will not cause (significant volumes of) fuel to flow out the filler unless the tank is full…and being fed from another tank…
