Sonex-B Fuel Tank Vent

[gethomas]

Want to alert Sonex-B builders of a BIG PROBLEM I have completing my airplane (snb022).

While running the engine (AeroVee2.1) for tuning, the fuel tank deformed, collapsing inward (see photo 1). Reviewed the problem with Sonex. They think a vacuum was created through the tank vent line. The drawing for tank installation (SNB-G01) shows the vent line routed on the front left inside of the fuselage and exiting somewhere through the landing gear fairing. In my opinion the drawing is incomplete and ambiguous on where and how to install the vent exit. I brought it out through the fairing (see picture 2). Sonex stated that while I have the vent line installed as shown on the plans, the plans do not show a workable vent system. They have suggested removing the tank and trying to reform it with air pressure and heat. Also, rerouting the vent to exit somewhere toward the center of the fuselage.

This obviously will require major disassembly and rework. Besides being disappointed in the Sonex, I am concerned about the integrity and safety of the tank if I can get it reformed. Will the heat and pressure damage the tank? After reinstalling the tank, will a vent exit relocation fix the problem?

Has anyone had experience reforming XLPE? Could a strong enough vacuum be created thru the vent line to cause the damage? Seems like we’re just guessing about the cause of the problem and all the re-work may not solve the problem. I’m wondering if the problem could be a manufacturing defect with the tank or a design issue between airframe and tank.

Any opinions or suggestions?

Picture 1

Picture 2

[John Monnett]

OK,
Our rotation molded tanks are flexible but almost indestructible. When we remove and install the large tanks in our jets we use a shop vac to partially "collapse" the tank to fit in it's very tight space. The tanks return to their original shape after installation. That being said, I believe the position of the tank straps and fit up is compressing the side of the tank. The material is simply buckling because it has nowhere to go.
Before removing the tank or any other drastic measure, I would loosen the tank straps slightly and use a low pressure (3-5#) air gun on the vent opening to inflate the tank (fuel valve off and tank cap on). That should return the tanks shape. A shop vac will also work as a pressure source. Although careful heating the tank surface will not harm it, I would not recommend using a heat gun to soften the distorted area unless the the tank is completely purged!

[Rynoth]

I'm curious why the vent didn't work? Can you verify airflow between the tank and the vent exit?

[gethomas]

Wish Sonex Tech Support had recommended loosening the straps. Do you think changing vent exit location will keep this from happening again.

[gethomas]

Put air through vent line and got good flow. The issue was not that the vent line was open, but air flowing over the gear fairing created a vacuum through the open line.

[onex28]

I’ve always been instructed to have a 45 degree cut in the end of the vent line facing forward into the slipstream. Your photo shows no such cut.

Onex28

[jrs]

Wouldn't a vacuum in the tank stop the fuel flow and shut down the engine ? Did that happen ?

[Rynoth]

Put air through vent line and got good flow. The issue was not that the vent line was open, but air flowing over the gear fairing created a vacuum through the open line.

From the sound of your OP, this happened while running the engine on the ground? I have a hard time believing that airflow from the prop during a runup while stationary on the ground could create enough negative pressure in the vent to actually distort the fuel tank. Could be wrong. Regardless, cutting an angle into the pipe exit that faces the incoming airflow should keep positive pressure on the vent.

[Onex107]

I can't imagine that you could generate enough low pressure to deform a tank with a vent line pointing any direction. You didn't say how much fuel was in the tank. You are showing the bottom in the first picture and it appears that the affected area is the front corner. I would suspect a more direct force, like John says, tank straps too tight or a fuel pump sucking on a tank with a collapsed vent line.

[pappas]

Gentlemen,

Apparently, distortion of the tank is common. Mine distorted on the ground, with the engine NOT running while performing the fuel flow test. I disconnected the fuel line to the Aeroinjector, drained about 4-5 gallons with a fuel flow rate of 17 gph. The tank top and bottom distorted. I assumed it was from a vacuum being created when fuel flowed out faster than the vent could handle. Note, I did not remove or even loosen the fuel cap for the test. I figured that under flight conditions that the cap would be securely attached as well.

The distortion in the tank bottom resolves when filled with fuel and returns when low on fuel. The topside keeps its depression on the starboard side forward at all times. It won't hold quite 20 gallons in this configuration and the fuel filler leans slightly to starboard because of it.

I spoke to the first Waiex B builder who lives in Texas about a year ago. He asked me if my tank had distorted because his had. He wanted to know if it was only him. It is not only him. This seems to be an issue.

I contacted Sonex and Kerry. These are his responses:

"I’ve never heard of a fuel tank collapsing and it's hard to imagine it would do so from the fuel being consumed. It strikes me that high heat would be required and/or excessive clamping pressure from the fuel tank straps."

"John has vacuumed out tanks before and he said they will collapse, but it should take much more than the fuel being drained to do it. He thinks you may have had a partially plugged vent."

I had already verified that I did not have a partially plugged vent.

I have been flying with this partial collapse for 100 hours. The only annoyance is the slightly off-kilter fuel filler.
After reading all of this again, I think I will loosen the tank straps where they attach to each other at the bottom center of the tank and see what happens. I'll let everyone know if it has an effect or not.

Perhaps Sonex should put out a notice to not over tighten the tank straps and we can avoid any more builder angst from this condition.