Sonex-B Fuel Tank Vent
Posted: Thu May 28, 2020 9:08 am
by 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
Re: Sonex-B Fuel Tank Vent
Posted: Thu May 28, 2020 9:47 am
by 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!
Re: Sonex-B Fuel Tank Vent
Posted: Thu May 28, 2020 1:14 pm
by Rynoth
I’m curious why the vent didn’t work? Can you verify airflow between the tank and the vent exit?
Re: Sonex-B Fuel Tank Vent
Posted: Thu May 28, 2020 1:34 pm
by gethomas
Wish Sonex Tech Support had recommended loosening the straps. Do you think changing vent exit location will keep this from happening again.
Re: Sonex-B Fuel Tank Vent
Posted: Thu May 28, 2020 1:38 pm
by 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.
Re: Sonex-B Fuel Tank Vent
Posted: Thu May 28, 2020 2:04 pm
by 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
Re: Sonex-B Fuel Tank Vent
Posted: Thu May 28, 2020 2:49 pm
by jrs
Wouldn’t a vacuum in the tank stop the fuel flow and shut down the engine ? Did that happen ?
Re: Sonex-B Fuel Tank Vent
Posted: Thu May 28, 2020 3:01 pm
by Rynoth
gethomas wrote: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.
Re: Sonex-B Fuel Tank Vent
Posted: Fri May 29, 2020 10:42 am
by 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.
Re: Sonex-B Fuel Tank Vent
Posted: Sat May 30, 2020 9:29 am
by 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.
Re: Sonex-B Fuel Tank Vent
Posted: Sat May 30, 2020 10:44 am
by Area 51%
pappas wrote:Gentlemen,
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.
Page G01 on my plans (Legacy) view C-C states “Only Snug Bolts enough to secure Fuel Tank in place. (overtightening will deform Tank and Longerons)”
Re: Sonex-B Fuel Tank Vent
Posted: Sat May 30, 2020 11:29 am
by dirkverdonck
Hi Guys,
I was unhappy with the tank straps for my Waiex-B, tried to adjust them but replaced them in the end with 4 half-straps that bolt together in the middle. Much easier to install the tank and the fit around the edges is a lot better.
The vent line will exit under the cowling as in my Onex and will have a 45°cut to the front to slightly pressurise the tank in flight.
About the plans: one should consider the fact that Sonex kits are not ‘plug and play’ kits but some common sense and fiddling is sometines needed to achieve the required results. I applied this in the build of my Onex and it still works as a dream after two years of puttering around!
PS: I hope my Waiex-B tank doesn’t collapse in a year or two during the first flight.
Re: Sonex-B Fuel Tank Vent
Posted: Sat May 30, 2020 8:50 pm
by pappas
Well,
Color me embarrassed… again, for not seeing the reference in the legacy plans you found. That same warning may be in my “B” plans. I’m not even going to check! I will assume that it’s in there.
I defer to the position on these subjects that I usually do, which is: I probably screwed it up! 
Re: Sonex-B Fuel Tank Vent
Posted: Sat Jun 06, 2020 1:48 pm
by gethomas
While doing ground runs on the runway the engine stopped. The fuel tank had deformed apparently disrupting the fuel flow. The fuel level was down to about 2gal, the day was hot and I suspected that heat was causing the problem as John Monnett suggested. Maybe the vent is not causing a problem.
So, I did the following test on the tank:
Released straps and removed tank front bracket attached to glare shield. See Photo 1. This was to eliminate the possibility of tight straps as John described.
Used bicycle pump to pump in air back through vent line to reform tank. Photo 2.
Photo 3 shows reformed tank.
Placed airplane in the sun with sun on right side. Ambient temperature 81deg; temperature on right side of fuselage 104deg
After 30 minutes the tank deformed. Photo 4.
This shows that there is a problem with the tank heating up causing it to change in shape. Is this a compatibility issue between airframe and tank? A tank mounting problem? A manufacturing/quality problem with tank?
Any suggestions on how to resolve this problem? I read on the forum that there are Weix-B builders flying with deformed tanks.
Re: Sonex-B Fuel Tank Vent
Posted: Sat Jun 06, 2020 4:37 pm
by GordonTurner
Hmmmmmm. They have been making these tanks for a very long time. It doesn’t seem like people have complained about this all these years, I never saw it anyway. So did they change the chemistry of the tanks? Compared to the pictures, my old tank circa 2012 looks more pale-ish and translucent-ish than what I am seeing in the pictures above.
Gordon
Re: Sonex-B Fuel Tank Vent
Posted: Mon Jun 29, 2020 6:54 pm
by dtibbo
My tank collapsed too, never flown, open vent line. I’ll try loosening the straps and see what happens
Re: Sonex-B Fuel Tank Vent
Posted: Sun Jul 12, 2020 10:38 am
by fastj22
I will report that my B-model tank has not deformed. I placed my vent near the fill neck and routed it through the firewall and down the side edge with a 90 degree turn facing into the airflow. It was made with a 3/8th inch steel automotive brake line.
Re: Sonex-B Fuel Tank Vent
Posted: Sat Nov 07, 2020 9:57 am
by gethomas
ditibbo: Were you able to get your tank reformed?