Header Tank Fuel Vent/Ram Head Presure.....Houston, we have a problem!

Attachment 2607

Attached is a drawing of a internal fuel tank using gravity fuel feed. Drwg 'A' is acording to the plans. Should work fine as long as the vent doesn't get clogged.
Dwg 'B' is a tail dragger setting at rest. Notice the fuel level is full and covers the vent. Fuel runs out.
Dwg 'C' shows the vent line routed to the rear. Fuel runs out only easier.
Dwg 'D' has the vent moved to the front. No fuel spill?

OK, I'm getting there.

Dwg B: you just stopped for fuel. The FBO filled it all the way up with cold fuel on a hot day. No problem, right....WRONG. At least I think there could be wrong. Fuel expands, fills the vent line, you start and Ram air holds the fuel in the line (like a finger over a straw). Is there enough Ram air to lift the fuel back to the tank??

Dwg C: Same basic senerio. Just easier to happen. Like when the fuel sloshes back at full throttle.

Dwg D:Seems like a better location, till the fuel expands.

My question: Is there a problem?? Will prop wash and air stream lift fuel 36" to clear a vent line? If it will, then why do we have fuel pumps on wing tanks? Why not just use Ram air to lift the fuel to the header tank. I know of two times where the engine quit on takeoff after the plane was just filled with fuel. Would it be worth adding a second Ram vent. A boost pump won't clear a mud dauber or bug. Maybe a check valve to let the vent air in. The best I've found is no Ram, vent is straight down. But, I've seen several tubes bent into the air stream.
Seems like there should be a more fail safe fuel delivery system just in case gravity fails.

Thanks,


Bob

You are forgetting that A) the carburetor is below the center of mass of the fuel and there is some suction or siphon effect from that direction, and B) the ram air into the fuel vent does not have to push all of the fuel back up to the tank, just provide some pressure towards the tank to allow fuel to flow to the engine.

That said, the full throttle, nose high condition is the worse case so a prudent designer will put the vent at the forward end of the fuel tank. And spilling fuel is expensive and bad for the environment.

Go look at the fuel systems of Piper Cubs.

I think that most airplanes that are not set up for acrobatics do not have their fuel vent lines run to the bottom of the airplane. Cessnas vent under the high wing. I would have to look at where the fuel vents for Mooneys and Pipers are.

As for bugs in the fuel vent, a simple cover keeps them out. But I do note that I do not see many other owners putting covers with Remove Before Flight streamers on their fuel vents. But I will note from observation that the bug does not generally cause the engine to quit on climb out. Since the airplane is climbing and the trapped pressure in the tank is greater than the outside air pressure, the engine runs fine until you are in cruise. Once level and the engine has a chance to burn enough fuel so that the reduction of fuel volume in the tank brings the trapped air pressure down to where it provides increased resistance to fuel flow to the engine, that's when the fun begins. And descending makes the problem worse....

Best of luck,

Wes
N78PS

Quote:

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Originally Posted by BobMo

Attachment 2607

Attached is a drawing of a internal fuel tank using gravity fuel feed. Drwg 'A' is acording to the plans. Should work fine as long as the vent doesn't get clogged.
Dwg 'B' is a tail dragger setting at rest. Notice the fuel level is full and covers the vent. Fuel runs out.
Dwg 'C' shows the vent line routed to the rear. Fuel runs out only easier.
Dwg 'D' has the vent moved to the front. No fuel spill?

OK, I'm getting there.

Dwg B: you just stopped for fuel. The FBO filled it all the way up with cold fuel on a hot day. No problem, right....WRONG. At least I think there could be wrong. Fuel expands, fills the vent line, you start and Ram air holds the fuel in the line (like a finger over a straw). Is there enough Ram air to lift the fuel back to the tank??

Dwg C: Same basic senerio. Just easier to happen. Like when the fuel sloshes back at full throttle.

Dwg D:Seems like a better location, till the fuel expands.

My question: Is there a problem?? Will prop wash and air stream lift fuel 36" to clear a vent line? If it will, then why do we have fuel pumps on wing tanks? Why not just use Ram air to lift the fuel to the header tank. I know of two times where the engine quit on takeoff after the plane was just filled with fuel. Would it be worth adding a second Ram vent. A boost pump won't clear a mud dauber or bug. Maybe a check valve to let the vent air in. The best I've found is no Ram, vent is straight down. But, I've seen several tubes bent into the air stream.
Seems like there should be a more fail safe fuel delivery system just in case gravity fails.

Thanks,


Bob

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Bob;

A vent line can't "pressurize" the fuel tank. It's within a couple inches of water. It's a vent pure and simple. As fuel is removed the air replaces it thru the vent. Air only replaces the used fuel.
A: everything is ok...fluid on layer of14.7 psig air on top!
B: Same as A but the fuel will pee out until the fluid is no longer able to escape.
C: If rush of fluid overcomes the use of fuel (use of fuel is what draws air into the fuel tank) then you will have a pizzing of fuel out the vent(s). Motor fuel pressure not affected.
D: This is the best design only because the taildragger configuration would allow you to put more fuel/less air in the tank(s).

Your questions in order:
No.
No and it doesn't need too. Picture both air and fuel as a fluid. Whatever is in the vent line will respond to the suction of the decreasing amount of fuel in the tank as the motor removes it to operate.
Fuel pumps on tanks is a determinition by the designer that at some time the fuel tanks "might" be put in a position BELOW the wing tanks (ever see the video of the Aviat Husky at a 50* nose high climbout?) and over come the abilty of the manual fuel pump to pull fuel uphill.
No such thing as "ram air" at a fuel vent. 1/4" vent,1/4" line and 1/4" orfice in the tank. Absolutly no way to see any significant pressure increase inside the tank.
If two planes crashed after fueling look elsewhere unless the vents were blocked (and again, no, fuel won't do it...).
A boost pump might prolong the motor running as most of them can run with a lower head pressure (suction) than your average diaphragm (engine driven) pump but unless the restriction is overcome allowing the pressure to equalize the motor will eventually quit.
Different airplanes have different vent systems. Some are angled forward while some are straight down with a 45* slice in them.

"...Seems like there should be a more fail safe fuel delivery system just in case gravity fails."
Gravity has yet to fail...


JMPO and YMMV

Chris

I will offer the info that fuel vents are NEVER pointed aft where they will create negative pressure. There are AD's on some older airplanes where the manufacturer did this originally and the problems this caused required that the vents to be turned around to face forward. The orientation of the fuel vent will force a pressure change in the fuel tank. Forward facing is best. Neutral is OK.

My Pitts has a fuselage mounted main fuel tank with the fuel vent line routed from next to the filler cap down to the bottom of the left landing gear leg. If I fill the tank to the top it immediately siphons out a quart so that the fuel level in the tank is below the vent. So it self corrects.

If you are not going to be flying aerobatics, you can use a vented fuel cap. And what Cessna did for many of the single engine airplanes was design them with the fuel vent at the top of one of the long skinny wing tanks, routed down to face forward. When that was proved to not work as well as desired, they issued an AD that had owners install vented fuel caps. So that is the answer if you really feel the need for a backup vent.

I will note that most fuel system configurations have already been tried by the major manufacturers and looking at the history of AD's can be very informative as to what works and what might not.

Best of luck,

Wes

Wes, I learned along time ago that you never say never. I still do and it gets me in trouble every time. ALL Wacos built with fuel tanks in the upper wings have the fuel vents facing AFT. It is specified on the factory drawings that they face aft. No AD's have been issued to change them. That said, I agree with everything else that you have said in your post, as I tend to agree with most of your postings on this forum.
Have a nice day.

Dave

Thanks for the info. Learn something every day. I will hazard a guess that because the fuel tanks are in the upper wing that the "head" pressure is much greater than the effect of the aft facing fuel vent.

As you note, every configuration has some different characteristics.

Regards,

Wes
N78PS