Fuel Vent Location - Why?

[aparchment]

Why do the instructions have us putting the fuel vent lines out through the cockpit floor. It would seem to be a better idea to 1) not bring any more fuel into the cockpit than necessary and 2) keep the fuel vent as far away from the exhaust as possible.

Wouldn't it make more sense to vent the fuel right at the wing root?

[weezbad]

you can do the rocket style vents. i did the standard. i dont know if vans has them go as high as possible for aerobatics or what. but i felt the rocket style vent's MAY vent actual fuel in turns.

[Spike]

Dunno. I believe the bottom fuse skin that fastens to the bottom inboard wing is load bearing. Maybe they didn't want to put a hole in it?

[ptrotter]

I plan on using the rocket style venting in the wing root. I don't think it will vent much more fuel than Van's method.

If the plane is level, the tank end of the vents are above the fuel level due to the dihedral of the wings. In this case no fuel will enter the vent line unless the tanks are extremely full and the temperature rises enough to push some fuel into the vent line. This is probably not very likely.

In a bank, the high wing will have the tank end of the vent above the fuel so no fuel will enter the vent. The low wing will have the open end of the vent above the fuel, so no fuel can leak out.

Now, in a bank, the vent line of the low wing will fill with fuel up to the level of the fuel in the tank. When you level off, this will now run down toward the wing root due to the dihedral. I expect that this is what Van's is trying to prevent from leaking out by making the vent line loop above the wing as much as they do.

However this is a very small amount of fuel. With a 1/4" vent line the maximum that can be inside the vent line is about 2.4 cu. in. .125*.125*3.14*48 (assuming the tank is about 48" long). By making a fairly large loop in wing root, I think this will eliminate most, if not all, of this from leaking out the vent.

At least that is my theory.

[captain_john]

I had given the Rocket style vent some thought. Antony, I agree with you about keeping it out of the cabin. Theoretically, you won't have any fuel in the lines, but there will be lots of vapor!

The reason it goes so high (higher than the level of the fuel is to mitigate the likelihood of fuel BELCHING out of them on a hot day. It simply has more distance to travel.

Paul's rocket style ones could have as much distance, but for the fuel to find it's way back into the tank is less likely. There will be standing fuel in the bottoms of the loops. In Van's design, there is no chance for standing fuel. That standing fuel could end up on the hangar floor if you have a change in temperature, and that is likely.

I don't see the location to be a problem. I bet you could put it most anywhere. Even of it were to catch fire, there isn't enough of it to spread for lack of other present combustibles.

My two...

CJ

[flytoboat]

I don't think the vents care what angle the bank is as long as it is a coordinated turn... (Check Bob Hoover's YouTube video where he pours a glass of tea while doing a roll)

I plan on using the rocket style venting in the wing root. I don't think it will vent much more fuel than Van's method.

In a bank, the high wing will have the tank end of the vent above the fuel so no fuel will enter the vent. The low wing will have the open end of the vent above the fuel, so no fuel can leak out.

Now, in a bank, the vent line of the low wing will fill with fuel up to the level of the fuel in the tank. When you level off, this will now run down toward the wing root due to the dihedral. I expect that this is what Van's is trying to prevent from leaking out by making the vent line loop above the wing as much as they do.

At least that is my theory.

[ptrotter]

[quote="flytoboat"]I don't think the vents care what angle the bank is as long as it is a coordinated turn... (Check Bob Hoover's YouTube video where he pours a glass of tea while doing a roll)

I don't think that applies in this case. As long as the outboard end of the vent line (inside the tank) is submerged, and the vent is not blocked, fuel will flow into it up to the level of the fuel in the tank. As the plane rolls back, depending on the speed of the roll, the fuel in the line will either flow back into the tank (if the roll rate is slow), or inboard (if the rate is higher).

CJ,

I don't any fuel will end up on the hangar floor in either case. I don't think heat expansion will push much fuel into the vent line and as long as the vent line is long enough, it probably won't matter what its orientation is, heat probably won't expand it enough to push it the length of the tube. Only fuel that enters the line due to a bank is likely to spill out and that will most likely occur at altitude. Unless, of course, you are doing knife edges down the runway

[4kilo]

Paul,

I assure you that if you are in a coordinated turn, the fuel has no way of knowing if you are banked, straight and level, or even inverted (so long as you have positive G's).

The actual problem is uncoordinated flight (slips and skids) and parking on an uneven ramp. Since it is unlikely anyone would be uncoordinated long enough to vent any major amount of fuel with the "coil in the wingroot" vents, I can't imagine any problems with this system in flight.

An uneven ramp could be slightly more problematic. If the airplane is parked such that the vent inlet in the tank is below the level of the fuel in the tank, then any expansion in the tank (warming on a sunny ramp or changes in barometric pressure for example) could push enough fuel through the vent to start a siphon. Van's high routing of the vent is an attempt to break any siphon which might start in a situation like this.

Since I have not heard about any siphoning problems with the rocket style vents, I would not expect this to be a major concern. I would still, however, make an effort to not park the airplane with one wing considerably lower than the other.

One last point; if the pick-up of the fuel vent is below the level of the fuel, expansion of the vapor trapped in the tank above the fuel is actually a much bigger player than expansion of the fuel itself (unless the tank is absolutely full).

Pat

[captain_john]

Pat, yes. I was assuming a mostly full tank. That is where I plan to leave it.

CJ

[ptrotter]

Pat,

I have to disagree with you on this one. When the plane banks, as soon as the tank end of the vent line is submerged, the line becomes part of the tank and the fluids will equalize in level so fuel will enter the vent line. When the plane rolls back to level, the fuel in the line must move one way or the other depending on what forces are in play, since the line is not level. In a gentle roll, unless you are in a skid, the fluid will move downhill and therefore toward the wing root. In a skid, you might get some of the fluid to re-enter the tank, assuming the tank end of the vent is above the fuel. In most case, however, the fuel will come to rest in the lowest spot in the line. If you bank again, unless the forces are exactly right and centripetal forces keep the fuel pinned against the side of the line, the fuel in the line will move again, most likely downhill. Van's vent line has a high enough loop that the top of the loop will probably remain high enough except in a very high bank angle that the fuel will not leak out. The Rocket vent will probably allow some to leak as it would be a downhill run. One solution is to put a check valve in the vent line with a bleed hole that will allow the free flow of air into the tank but block fuel from exiting the tank. The bleed port would compensate for slow increases in pressure due to heat expansion or atmosphiric pressure changes and prevent the tank from over pressurizing. The fuel leaking via the bleed port would be minimal.

In the case of Bob Hoover's roll or loop, centripetal force keeps the water pressed against the bottom of the glass. I doubt if very many people can hold a loop or roll that perfectly for very long.

Paul

[4kilo]

Paul,

I guess we will have to agree to disagree then. A basic law of physics is the equivalence principle - that is; it is impossible to tell the difference between gravity and forces provided by inertial forces. If you are doing a turn and the inclinometer ball is falling to the inside of the turn, then the fuel in your tanks will do the same thing. If the ball is remaining in the center of the inclinometer, then the fuel is remaining in the same orientation in your tank as it had when the wings were level.

Pat

[flytoboat]

Paul,
For the same reason the tea stays in the glass for Bob Hoover, the fuel in your tank will remain level with the centrifugal forces and therefore will not move through the vent lines. Again, the fuel doesn't care what your bank angle it is as long as you are in a coordinated turn (i.e. the ball is centered.)

[weezbad]

what about the ram air in the fwd facing scarf cut? this is gonna prevent fuel from being in either vent line except when filled up and no fwd motion is taking place. this vent is as much as to prevent a vaccum being drawn on the tank as positive pressure. there is more going in than is coming out...this is one way fuel gets water in it.warm air replaces the fuel during flight cools and the moisture condenses out of it. i guess what im trying to say is that in flight it would be unlikely to get fuel in the cockpit from the vent lines. im really only typing this so my boss will think im at work

[TomC]

Paul,

Don is correct. Another example would be the fuel pickup. During a turn, if the fuel ran to the outboard end of the low wing's tank, the fuel pickup would be uncovered, with bad consequences. This does not happen because centrifigal force and gravity are balanced in a coordinated turn. In a poorly coordinated turn, the fuel would move in the direction of the ball. This is also why you can fly a left traffic pattern with the right wing tank empty.

[ptrotter]

Another basic law of physics says that liquids in connected compartments seek the same level and when the vent becomes submerged, the line becomes part of the tank (since the other end is open) and the level will equilize to whatever the tank levels are. Since whatever forces, whether gravity or inertial, act on the tank as a whole, they will not differentiate between the part of the tank that is the "real" tank and the part that is the vent line. In fact, by keeping the net force the same by doing a coodinated turn, it "requires" the levels to equilize since the fuel does not know that the forces are not due simply to gravity and the law of fluid level equilization must still apply. That means that there will be fuel in the vent line up to the level of the tank fuel and when the orientation of the tank changes, the fuel that is in the line must go somewhere. If you roll back slow enough, the fuel will flow back into the tank, but if you roll back faster than the size of the line will allow the fuel to flow back, it will flow the other way as soon as the vent line slopes inboard. That fuel will settle into the lowest part of the vent line

To take the extreme, let's say you are doing straight knife edged flight. You have simply changed the orientation of the plane with respect to the earth's gravitational field. The only force is that of gravity and down is now toward the low wing and fluid will flow that direction and the fluid level in the tank and vent line will equalize.

[ptrotter]

I suppose that it is possible that in perfectly coordinated flight that the fuel will stay in exactly the same place in the tank and the vent line will not become submerged. In that case no fuel can ener the line. But as soon as the vent is submerged it becomes part of the tank and the other laws apply as well.

With respect to the fuel pickups, I agree, however we also mitigate the possibility of fuel moving away from the pickup by installing flop tubes in aerobatic planes and installing the gate on the tank rib to restrict the fuel flowing toward the outboard end of the low wing.

In any case, I'm talking real world here and I know that not all my turns are perfectly coordinated so I don't expect forces to be balanced perfectly.

[rv8or]

I can't take it anymore!!

Can we all at least agree that it's Centrifugal force?


http://en.wikipedia.org/wiki/Centrifugal_force





Joe

[flytoboat]

Another basic law of physics says that liquids in connected compartments seek the same level and when the vent becomes submerged, the line becomes part of the tank (since the other end is open) and the level will equilize to whatever the tank levels are. Since whatever forces, whether gravity or inertial, act on the tank as a whole, they will not differentiate between the part of the tank that is the "real" tank and the part that is the vent line. In fact, by keeping the net force the same by doing a coodinated turn, it "requires" the levels to equilize since the fuel does not know that the forces are not due simply to gravity and the law of fluid level equilization must still apply. That means that there will be fuel in the vent line up to the level of the tank fuel and when the orientation of the tank changes, the fuel that is in the line must go somewhere. If you roll back slow enough, the fuel will flow back into the tank, but if you roll back faster than the size of the line will allow the fuel to flow back, it will flow the other way as soon as the vent line slopes inboard. That fuel will settle into the lowest part of the vent line

To take the extreme, let's say you are doing straight knife edged flight. You have simply changed the orientation of the plane with respect to the earth's gravitational field. The only force is that of gravity and down is now toward the low wing and fluid will flow that direction and the fluid level in the tank and vent line will equalize.

Paul,
The point is, the vent WILL NOT become submerged in a coordinated turn. You are correct that in a straight knife edge maneuver (i.e. an UNcordinated manuever) the fuel will follow gravity and flow to the low wing. The only reason for flop tubes is for sustained inverted flight. You can do positive G inverted flight and never lose suction because the fuel is still in the bottom or the tank even though you are upside down.

[ptrotter]

Joe,

Sorry, I don't agree.

Centripetal force is the force that pushes the object outward. That is the force that keeps the water in the glass. Centrifugal force is the force that the outward moving object pulls on the axis.

If you swing a weight on a string, centripetal force pushes the weight out. You feel the centrifugal force pulling on your hand.

The forces are opposite and equal.

I can't take it anymore!!

Can we all at least agree that it's Centrifugal force?


http://en.wikipedia.org/wiki/Centrifugal_force





Joe

[ptrotter]

Another basic law of physics says
that liquids in connected compartments seek the same level and when the vent becomes submerged, the line becomes part of the tank (since the other end is open) and the level will equilize to whatever the tank levels are. Since whatever forces, whether gravity or inertial, act on the tank as a whole, they will not differentiate between the part of the tank that is the "real" tank and the part that is the vent line. In fact, by keeping the net force the same by doing a coodinated turn, it "requires" the levels to equilize since the fuel does not know that the forces are not due simply to gravity and the law of fluid level equilization must still apply. That means that there will be fuel in the vent line up to the level of the tank fuel and when the orientation of the tank changes, the fuel that is in the line must go somewhere. If you roll back slow enough, the fuel will flow back into the tank, but if you roll back faster than the size of the line will allow the fuel to flow back, it will flow the other way as soon as the vent line slopes inboard. That fuel will settle into the lowest part of the vent line

To take the extreme, let's say you are doing straight knife edged flight. You have simply changed the orientation of the plane with respect to the earth's gravitational field. The only force is that of gravity and down is now toward the low wing and fluid will flow that direction and the fluid level in the tank and vent line will equalize.

Paul,
The point is, the vent WILL NOT become submerged in a coordinated turn. You are correct that in a straight knife edge maneuver (i.e. an UNcordinated manuever) the fuel will follow gravity and flow to the low wing. The only reason for flop tubes is for sustained inverted flight. You can do positive G inverted flight and never lose suction because the fuel is still in the bottom or the tank even though you are upside down.

Don,

That is a good point and I agree, however it is not a realistic real world situation and we need to design our aircraft for the real world. I have to worry about fuel leaking out the vent as I know I cannot always do 1 g. coordinated maneuvers.

Anyway, it has been an interesting discussion.

Paul