how should I vent my header tank ?

[Nico 2]

I have an Avid flyer STOL, and flew without problem for almost 2 years. Than suddenly I had a few engine fails. I investigated and found the problem, air lock in the shut-off valve from the wing tank to the header tank. I did replace the standard valve by a homebrew valve with 6 mm throughput, mounted an electric Facet pump at the header tank exit  and solved the problem, though many times there is still air in the line below the elbow of the wing tank excit. I have to get the air out manipulating the electric pump at the exit of the header tank. I did write an article about the airlock problem end the valve.

I also did make a header tank vent as described in the manual. But that was no success. When pointed forward, the pitot pressure in the header tank due to the vent did prevent the fuel flow from the wing tank. When pointed aft, it did suck lots of fuel out of the ventline. I could not find a way to vent properly and mounted a valve in the ventline. Now I vent the headertank at the ground and shut the valve before take-off. 

It works well, but I am not really happy with this set-up because in principle it is possible that the header tank collects air when the aitcraft makes right turns. I followed the advise to mount a level alarm in the header tank wich gives me quite a safe feeling.

I like to know if someone ever mounted  the vent in the wingtank by a riser tube next to the tank exit. Please comments.

[EDMO 1078]

If you want to vent the header tank to the wing tank you will have to run the vent tube to the upper part of the wing tank where there is air and not gas.   Kitfox has the sight gage fittings at about the center of the wing tank, and the upper fitting is in the best place to run the vent for the header tank.

     I would sand a small spot on the tank at that location, drill a hole just big enough for a small push-on double-ended connector fitting and epoxy the fitting.  After epoxy has cured, just push your vent tube onto the end of the fitting and go.

EDMO

Edited 11 Oct 2014 by Ed In Missouri

[1avidflyer 1983]

Avid had a separate vent that went up above the wing to vent the header tank.  Lot of guys put a valve in that line cause sometimes the vent could suck gas out of the tank.  I used the valve in the line and it worked fine.  Open the valve, air goes out of the header tank, close the valve after air is out.  Jim Chuk

[wypaul 448]

I have a vent/ fuel gauge line, that exits as per the plans on my Avid that works well now.  It took some fiddling to get it right and as a fuel gauge it only works at one RPM which I used my normal cruise RPM.  It is a 1/8" steel tube bent at 90 degrees like the fuel cap vent, about 4" above the wing pinched to a pinhole opening but is facing at about 45 degrees off the nose.  The clear vent line holds it in place so it is easy to adjust.

[Nico 2]

It seems that most people have the ventline as suggested by the construction manual and had to add a valve to prevent fuel loss through the vent line. I could not find an angle without fuel loss or pitot pressure in the header tank that gives fuel flow issues Paul. . I had to put the valve in the line as well as told by  Jim, and it works fine. The only problem I see is that, when I like to vent in air, fuel sprays out of the ventline when the air is gone over the plane, which I do not like too much.

Ed, can you give me a bit more details about the place of the upper hole in the tank where you suggest to vent?

I think that the recommendation to have a fuel warning alarm in the header tank when the level sinks, is quite important. It did alarm me e few times!!. Highly recommendable!!

In order to prevent fuel loss out of the ventline when venting, I like to know what you guys think about a riser tube next to the winktank exit elbow that goes to the top of the tank at that position. I know it is not the higest point in the tank, but the other end of the tank (e.g. the neck of the tube where the cap goes on) is very difficult to reach for venting without damaging the fabric. 

[1avidflyer 1983]

Nico,  how did you rig your fuel alarm on the header tank?  Jim Chuk

[Nico 2]

Hello Jim

I am an electronic engineer. I did make a metric 8 mm hole in the topplate and did fit a M8 bolt with a center hole. Through the center hole I did stick approx 12 cm of the top end of a fishing rod (hollow). At the bottom end, I did glue a very small NTC resistor of 470 Ohms and did seal everything with epoxy. Two thin (0.2 mm)wires pass through the top end of the fishing rod giving electrical access to the NTC resistor. NEVER put the battery voltage to one of the NTC wires. Than a short circuit may give fire or explosion danger. 

I designed a little circuit with a weatstone bridge that could measure the difference between two the same NTC resistors. One in the tank, one glued to the topplate of the tank. When the power applied to the bridge heats up the resistors (only milli watts and always current limited to prevent fire danger). The resistor in the tank will be cooled by the fuel and the resistor at the topplate by the topplate. As soon as the fuellevel sinks and the NTC resistor is above fuel level, it heats up approx 10 - 20 degrees more than the other NTC resistor and the bridge gives a voltage that can be connected to a LED flasher at the dashboard. Simple, reliable, easy to make your self and very low cost.

[EDMO 1078]

Nico,

     FYI, Kitfox - and I think spruce - sells a low-fuel alarm that goes into the header tank.

I am attaching a photo from my Kitfox 4 builders manual showing the upper and lower sight gage fittings and a third fitting for the header tank vent tube.   My tanks only have 2 fittings for the sight tube, so I will put a T fitting in the upper one for my vent tube.

You can put your fitting anywhere along the line where I have put Xs - the highest point on the end of the tanks should be best.

Hope this helps.

EdMO

[Nico 2]

Thanks a lot EdMO

If I put the vent tube at that place, I can attache a long tube inside the tank to reach to the far end of the tank wich is higher than the root end. Do you think that helps or is this altitude sufficient to ensure a good venting?

I saw the level meters at Spruce, but the price was a little high. When you make it your self, it is only a few dollar. I designed and made electronics for aircraft and spacecraft, so no problem at all. It is super simple. When there are more people liking electronics, I can post the schematic diagram.

Kind regards

Nico

[akflyer 1427]

I don't have a vent on my header tank, it vents from to which ever tank has the least fuel in it. I have not had an issue in over 500 hours this way. Do you have good vents in the gas caps?

[Nico 2]

I have only 1 tank, that may be the difference. I had the Spruce fuel valve in the line between the header tank and the wing tank. After 2 years of flying I had an engine stop. At ground, I investigated free flow and could not find an issue. Tested the engine, cleaned the carbs, nothing wrong. The next week, all ok. The week after that, engine stop in air. Investigated angain and I noticed NO free flow at all. I saw air below the shut-off valve. I knocked the air bulb until it passed the shut-off valve and the fuel started to flow again. I took the valve apart and saw that internally is is only 3 mm throughput!!! Very bad if the connections hav a 5 mm hole, than you fo not expect a throughput of only 3 mm.I informed spruce about it.. I replaced the valve with one that has a 6 mm throughput and the problem was gone. 

I did read that there are approx 6 accidents due to airlock in our kind of planes, so I took the issue serious. The rest you can read above.

[EDMO 1078]

Nico,

     You don't need the tube inside the tank - whether there is gas above the fitting would not matter, since the head pressure at the bottom of the tank will always be more that at the top.   Leni is correct about having 2 tanks, and you only have 1, so you need this.

I would be interested in the schematic for the warning device.   I know something about electronics, but think the computer logic is beyond my old age!   I don't like the idea of mixing electricity and gasoline - I would never put the capacitor type gauge in my wing tanks - I have let the smoke out of too many resistors and capacitors. 

I had lunch with my Grandson today.  He is an EE graduate, but I don't think he could wire my plane correctly according to FAA regs!  Yes Leni, I know its an experimental, but there are limits to experimenting! 

Ed

Edited 12 Oct 2014 by Ed In Missouri

[Bandit 286]

I only have one tank and never have vented the header tank. My wing tank is vented though.

[EDMO 1078]

I can guarantee that an airlock will stop the flow in a system - We proved it after adding wing tanks to the J3 - an air bubble between the wing tank and the header will absolutely not let fuel flow until the fuel in the header gets below the inlet tube, and we have pressure tubes on the wing tank caps.  The header tank is vented at the cap.  We could add a vent tube at the inlet, and it would work, but it would be illegal. 

 It is a little worrisome when the gage on the cap starts to bottom out, and you wonder if the fuel will flow from the wing since you cant see thru the fuel tubes.  If you have altitude, and put the nose down far enough, the airlock bleeds out and the gage starts heading back in the right direction. 

I realize that different systems act differently - the STC for the J3 puts the inlet on the bottom of the header tank, where the Kitfox inlet is in the top of the tank and should not have the problems of the J3.  I will still vent my header to the wing tank.

EdMO

Edited 13 Oct 2014 by Ed In Missouri

[Nico 2]

Hello bandit Please take care. I had the same for 2 years and suddenly had two engine stops. The headertank CAN collect air, and an air bubble in the line from the wing tank absolutely can block your fuelflow. I saw it happening. I drained my header tank completely without nowing due to an airlock and had an engine stop during takeoff (I still could brake....) and one in air. I did cut the engine inmedeately when the RPM went down, the engine kept idling. I looked for a place to land and saw the fuel pressure rising again. I was very close to the airfield, so opened the engine slowly to 3500 RPM and could reach the runway... See also the contribution of Ed. I consider he FAA advice to have a levelsensor at the header tank in our type of planes a very good suggestion. I feel a lot safer and it warned me a coulple of times. I will draw the diagram to a readable version and will post it Ed. Thanks a lot for the info. I absolutely will go this way. I sprayed enough fuel over my plane...

[marksires 484]

Where is the header tank in your planes?  Mine is a small vertical cylinder behind the seat - no separate vent.  I only have one tank in the right wing.

Mark

Edited 13 Oct 2014 by mark

[Nico 2]

Hello Ed

Attached the diagram of the sensor.

The NTC resistor in the fuel should be small and connected with very thin wires (e.g. 0.15 mm magnet wire) in order to have a low heat mass. I did glue the NTC at the end of a holow top end of a fishing rod using a small amount of epoxy. The wires go up through the fishing rod wich is glued in a hole in the center of an Metric 8 mm bolt (I guess you will use an other kind of thread...)

In the diagram you see two measuring points MP1 and 2. Measure the voltage between the points and adjust the potmeter to approx 0.2 Volts when the NTC is in air and the other one is cooled by the topplate of the header tank. MP1 positive. 

Cool the NTC in a drop of water at the top of the tank. The voltage should reverse. Adjust the potmeter until the voltage in air and cooled are approx the same but reversed.

When you use 220 Ohm NTC resistors, the temperature of the NTC resistor in Air goes up (more dissipation) and the voltage difference will be bigger, but I found that not necessary. But when you can not get very small NTC resistors but e.g. the 3 mm drop type, than the heat mass is larger and 220 Ohms may be better.

The reaction time of the sensor is approx 3 - 5 seconds. when the heat mass is low (Thin fishing rod, small amount of epoxy).

The opamp can be about anything. I use a  LTC271  The LED current can be set by the resistor in the drain of the Mosfet. I use 100 Ohms and 4 LED's, 2 in series and two sets in parallel. Really quite bright. 

I did glue the complete circuit at the top of the tank with Epoxy.

About safetety

The current to the NTC is limitted by the other NTC and by the 78L08 and can never reach dangerous levels. Do NOT use an 7808 stabiliser, than the current can become too large when the upper NTC shorts. 

Kind regards

Nico

 

[Nico 2]

Hi Ed

I did forget to draw a component. Between the +12VDC and ground (at the input of the 78L08) there should be a capacitor of approx 10 uF, otherwise the stabilzer can oscillate.

[Nico 2]

Hi Mark

I have the same, only one tank in the right wing and a vertical cylinder behind the co-pilot seat as header tank

[EDMO 1078]

I personally don't care for the small Avid header tanks - They look too small to me - The magnum book show two of them.   Kitfox used to sell a larger plastic cylinder, but has now changed to a flatter rectangular tank which mounts behind the seat and holds about one gallon of fuel - it also has a port in the side for a low-fuel warning device which is sold separately.  I have the rectangular tank.

My gripe is the cost - Kitfox is really proud of these plastic jugs, and the last price I saw was $160 - But none of their prices seems reasonable to me.

EDMO

Nico - If I was still in Alaska, I might try putting the vent tube on the J3 - However, in Missouri, I cant get a field approval on anything - that door is closed and locked !    I love "experimental" and the freedom it gives.

Ed

Edited 13 Oct 2014 by Ed In Missouri

[marksires 484]

Ed,

With only one tank, the header tank doesn't serve any real purpose in my plane except to act as a reserve if I let the main fuel tank run dry.  Since that should take over 4 hours, AND I don't ever take off with less than full tanks, it doesn't need to be very large.  That exceeds my bladder endurance by a good 2 hours!  )

Mark

[EDMO 1078]

Mark,

     2 Hours!??  I faintly remember those days!

I think you will find that as age goes up, the bladder time goes down proportionally.  

Piss-tubes extend your range a lot....The military uses them.

EdMO 

[Nico 2]

Ed, my headertank is an aluminimum cylinder with, I think, approx 1 gallon,

[Bandit 286]

I have the small header tank behind the seat with the gas line running from the top of the header tank to the right main tank in the wing. In over a 10000 hrs I have never had any problems with it. To me the header tank is just part of the fuel line going to the fuel pump. I would take that shutoff out between the wing tank and header tank. Not sure what good that does.

Edited 14 Oct 2014 by Bandit

[Bandit 286]

I don't fly high enough to have to slip in from 5 miles out. I get scared that high up.