I have my vents located in the plans position on one wing and near the wing/fuse position on the other (metal wing rear spar clearance problem). Both are about 1" below the wing and pointed forward. There doesn't seem to be a problem with either location. The forward facing of the vent should help some with pressurizing the tank, but the main value is that the venting that can occur (see below) with full tanks stops when you finally takeoff! Some additional considerations:
- Be sure to protect the vents from insects (mud dabbers are aproblem in Texas). I used a trick from another builder; two small u-shaped pieces of safety wire shoved into the vent, small enough to allow airflow, but preventing bug nests.
- As a backup (and necessary in my header tank venting system), cross connect the vents from each tank (1/4" tube across fuse behind the spar carry through tube).
- Drill small hole on top of the vent line(s) right after they exit the tank (or add a tee with a stub tube) to allow venting should either or both of the external vent tubes become clogged by ice or ???.
- Flat wings on the BD are very sensitive to siphoning on the ground when the tank(s) are full. So to minimize this, add small tubes inside the tanks from the vent bulkhead fitting to the highest location in the tank (differs for taildraggers vs trigear).
It is a very good idea to provide ram air pressure through the vent line. If your fuel caps do not seal well, a negative pressure can be developed within the tank because the fuel cap is in the low pressure area, slowing or stopping fuel flow. The ram air pressure through the vents can overcome this problem if the leaks are not major.
I also use and recommend cross venting between the tanks in case one of the vents gets plugged. In my plane, the header tank vent (1/4" line) goes over the spar to a tee fitting which is plumbed to both tanks, providing the cross vent. In each tank, the header vent line is carried to the outermost, upper point in each tank. The individual tank vents are projecting from the front of small sumps under the outer edge of each tank. these vents are then plumbed to the top inside edge of the tank to minimize fuel spillage when the tanks are full.
My plane (metal wing) has vent tubes coming out the bottom rear near the root that have been plugged. Someone along the way (I'm the sixth owner) evidently had problems with venting or just didn't like them. Instead, the fuel caps have forward-facing vent tubes sticking out of them 3/4" or so.
Roger Mellema, Noel Dunlap
I mistakenly said that the Dunlap wing is not the BD round spar. Noel did use the BD spar. He is using a 2.5 gal header tank but the normal fuel pickup locations. He has had a problem with the fuel vents: "An interesting phenomenon was the ability of the vents to pressurize the tanks sufficiently to cause transfer of fuel from the forward set of tanks to the rear set. Fuel would then flow in a large plume overboard, out the rear vent. If I placed a short piece of tubing over the rear vent and made it longer than the front vent, then fuel would transfer forward and the same thing would occur out the front vent. Needless to say this was the worst case situation since the fuel pickups were in the rear set of tanks. I tried adjusting tubing lengths which was kind of like trying to level a table by sawing small sections off the legs. Also this usually happened only if the tanks were over half full. My final solution is as follows:
I have never had a problem getting all of the fuel from the wing tanks since they are pressurized from ram/wing pressure air. I regularly ran one wing dry as indicated on the fuel gage and engine power loss. Switching tanks brought restored power in about five seconds. This delay, I believe, was due to the header tank hindering the flow to some degree. The vent for the header tank has a small 3/16" vinyl tubing off a small brass fitting on the top of the tank to a valve and brass tube that was on the floor forward of the landing gear box. The line or tubing vent ran along the side of the center pedestal to the manual valve so that I could bleed the air out of the header tank with the fuel valve turned on. In theory the vinyl tubing vent would allow me to observe fuel/bubbles when the header tank was full. then I would close the valve and have an intact tank system. however, it only seemed to work that way part time on the ground and I never noticed any difference of operation in the air except fuel would siphon out the valve if it was left open. Now I have vented the header tank to the top of the left wing tank. Whether pressure from that tank will conflict with the header tank operation remains to be seen. I may retain the manual valve to close off that vent just in case."
"My fuel vents are molded around a male mold, it forms a depression into the surface and the actual vent hole is on the rear surface of the depression facing fwd & down. The hope is to get a bit of ram pressure while avoiding any ice accumulation."
View the building pictures of another builder forming these depressions.