Fuel system design questions

[Zzz]

1. Gascolator under floor, or hung on firewall? If hung on the firewall, this would require a belly drain I guess between the selector and the gascolator since it's uphill?

2. Is everyone welding the tabs onto the longeron ahead of the forward door post/former? Or just letting the tee be supported by its incoming fuel lines?

3. Looking for examples of stubbed off fuel line at wing root area on fuselage. I read that hose is discouraged, but it seems like a good choice for a joint/connection to the wing fuel lines.

Any other tips? I see a lot of builders relocating their fuel valve/selectors to the left side of the flap lever.

Thanks.

[jaredyates]

I'll toss in my experience, but of course don't consider it authoritative. I put the gascolator aft of the fuel selector, under the floor, just in front of the left main gear shock strut. I didn't want to need a second drain. If you were to go with the firewall, I don't think you would want to put the second drain between the firewall and the fuel selector, unless you ran a line aft from the fuel selector and installed the drain there. Otherwise, your fuel selector would be the lowest point with all of the wheels on the ground.

I think the tabs on the longeron are a great idea and an elegant solution. I used the old method of mounting the tee on the door former and wish I had heard of the tab idea back when I was routing those lines. I wouldn't recommend letting the lines carry that fitting. It could be quite problematic if they were to fatigue and crack anywhere in that area, in part because you won't have any way to stop the flow until you run one tank dry, or if you are really unlucky and don't realize what's going on, both tanks. If you follow the 16" support requirement of 43.13, you'll likely not be able to make it work without having a structural support at that junction. If welding is a concern, you might be able to make something with unpadded clamps and right angle aluminum extrusion.

As for the wing root sections, I installed buklhead fittings in the root rib and ran 5052 lines directly to those from both sides. It was really not an issue, other than that I found it impossible to do well without the wings on. This means that you'll want to have your tanks in and plumbed to that center rib before you put the wings on, since you won't be able to get to that area to drill the center rib once the fuselage structure is in the way. Plan ahead with locating the fittings so that you don't cause interference with the flap bellcrank in the back and watch out for where you'll end up with nutplates for wing root fairings. You could go ahead and drill out the rivets in the holes that you'll be using as screw holes, and that will help. I used the SPRL fuel selector, which fits nicely under the floor if you keep it aft of the tunnel. I mounted mine just forward of the aft edge of the tunnel, and had to make a little bump for extra clearance in the stainless, but that wasn't an especially hard thing to do. As for other general tips, I would suggest that you order at least twice as much fuel line as you think you'll need. I didn't have to remake all of the lines, but the ones that required 5 tries made up for the rest.

[Mark Goldberg]

Like Jared, I like the gascolator where Bob wants it in the low point in the fuel system. Just aft of the selector valve.

My BH does not have that little supports on the lower longeron and has done fine for over 900 hours. The line is supported aft of the T where the lines come together - in a snap bushing through the door frame surround metal.. But the fuel line support is a good idea, but many Bearhawks are flying without it. I would certainly bend one up and weld/install it if you can weld.

My BH and Patrol have rubber lines attached to barbed fittings coming out of the tanks that carry into the upper fuselage where the rubber lines transition to 3/8" aluminum lines. I will say that the rubber lines have been more of a maintenance item than any other part of the fuel system.

The fuel system is a place where "experimentation" and deviation from the designers intent can have very serious negative results. Things can be done differently but I would clear modifications with Bob. Mark

[Zzz]

I'm having trouble visualizing the accessibility of the gascolator in the belly. This stuff ends up right above the SS tunnel right? Should an inspection hole be made there? A guy has to be able to sump the gascolator.

Also, this is just thinking out loud: In the event of a leaking fitting or valve in the belly, the only way to stop the flow is to...well there isn't; it all leaks out or you drain into a can from the wing sumps or disconnect at the wing root connection.

Has anyone thought about putting mini inline ball valves at their wing root connection? Safety them open, remove the levers, but it seems like a good way to deal with a failing fuel selector or gascolator in the event that it happened.

[S Lathrop]

Mark:

What kind of problems did have you with the rubber line? Would braided flex lines or using different hose and fitting types solve the problems?

[Mark Goldberg]

Zane, a little hole on the belly skin to drain the gascolator is what I have with a larger access door (nutplates) so I can take apart the gascolator at annual and check the screen.

The problems I have with the rubber lines from the fuel tank outlets to the wing root are two. The first time I used some expensive auto racing fuel line from the speed shop, and after 5-6 years it started leaking. Only aircraft rubber lines after that. And the only other problems have been the little hose clamps seem to need tightening once every several years. Right now I have a hose clamp that needs tightening inside the wing root (Patrol) that is very hard to get to. But I am still going to do the same thing on my LSA. Mark

[Zzz]

Another question:

I noticed in one of the photos in the assembly manual that a rubber grommet is being used to pass tubing through the front door post former. I've read that this is not recommended because rubber can crack with age, putting the hard line against the edge of the drilled hole where it could be worn. Is a bulkhead fitting a better idea? I'm planning on using a 90 degree elbow with bulkhead flange for the rear former to make the hard turn from vertical to horizontal. Thoughts?

[Battson]

I used the bulkhead flange - it was a tight space to torque all the nuts correctly - but otherwise it was a nice solution. The Bearhawk book spells all this out quite clearly I think.
- and do torque the bulkhead nuts correctly (and AN fittings), they DO come loose otherwise. My flexible hoses for the brake lines worked the AN4 bulkhead fittings loose.

[Mark Goldberg]

Another option that is cheap and perhaps better than rubber is a plastic "snap bushing". Spruce sells them with the 3/8" I.D. for the BH fuel lines. Mark

[Zzz]

Jon, if you're talking about page 64 of Bob's Bearhhawk book, I have to disagree that it is clear. He describes the path of the lines but the only fitting described is the AN804-6D at the forward door former. In his sketch one must assume the rear pass-through the door former doesn't use a bulkhead fitting. Which is it? A grommet? Up to the builder I guess, hence my question.

From the AviPro PDF assembly manual:

Also reference the plans, sheet 16 and 17. There you’ll find a schematic of the fuel system with all the parts numbers called out on it.

I referenced 16,16A,16B, and 17 and don't see any fuel plumbing information. Maybe I'm going blind. I'm also the lowest common denominator of builder, so allow me to ask the dumb questions.

Thanks for the tip on torquing the bulkhead nuts.

[Zzz]

Another option that is cheap and perhaps better than rubber is a plastic "snap bushing". Spruce sells them with the 3/8" I.D. for the BH fuel lines. Mark

Thanks Mark, that looks like a handy part. Is there a down side to using bulkhead fittings at both door formers?

[Zzz]

Speaking of torquing AN fittings, are you guys using Permatex with PTFE or Gasoila or similar dope on threads? Doesn't seem necessary on flared fittings, and will invalidate torque specs. Opinions?

[bestbearhawk1231]

When doing pitot static lines on corporate jets we never used a sealant on them, the AN fittings are designed to be self sealing. If you have problems with them not sealing correctly you usually have a defective, a worn fitting, or a bad flare. They sell AN gaskets for fittings that will not seal correctly. Aircraft spruce sells them as DEL seals pn 04-0500*...the last digit number (the *) is the tube size. They run about $1.75 each which is considerably cheaper than new fittings. I would consider them to be a one time use deal. Torque values as I understand are normally dry values unless otherwise specified.

[Glenn Patterson]

I would recommend not using fittings like 90 degree elbow where you can put a nice bend in the tube. 90 degree elbows and fittings choke flow. The tube can easily be bent in a nice radius at the rear door frame. Something else to consider is using a "Y" fitting instead of tees if possible. Summit Racing sell AN fittings that are a "Y". The Y eliminates the hard intersection of a tee and is more flow friendly.

One possibility: The door frame hole could be generously over sized to pass the tube through with lots of clearance. Make an angle bracket formed to support the fuel line in the center of the hole. Split a piece of rubber hose to cushion the fuel line, slide over the fuel line and secure all with a clamp to the bracket. The door formers are not structural.

A footnote: Eric Newton & Russ Erb's manuals detail a lot of valuable information that would benefit a kit builder.

Glenn
BH727

[Zzz]

Thank you, Glenn. I did purchase Eric's finish manual just 2 nights ago and have begun reading through it. Good stuff.

[Battson]

Jon, if you're talking about page 64 of Bob's Bearhhawk book, I have to disagree that it is clear. He describes the path of the lines but the only fitting described is the AN804-6D at the forward door former. In his sketch one must assume the rear pass-through the door former doesn't use a bulkhead fitting. Which is it? A grommet? Up to the builder I guess, hence my question.

From the AviPro PDF assembly manual:



I referenced 16,16A,16B, and 17 and don't see any fuel plumbing information. Maybe I'm going blind. I'm also the lowest common denominator of builder, so allow me to ask the dumb questions.

Thanks for the tip on torquing the bulkhead nuts.

I used a grommet and a free-floating AN fitting together, and installed inspection access so I can inspect it on both sides. Grommets are easy to change, but the AN fitting is plenty thick. Be aware the fuselage flexes quite a lot when loaded up with gear, I am not sure you'd want a line stretched between two hard-points with no room to "float", but that's speculation. I guess the thermal expansion coefficients is another consideration there, if exceedingly minor. But fuel leaks are annoying too!