Venting flush fuel caps?

[LukeS]

So, my newly acquired bearhawk project came with flush mount fuel caps, one already installed. I had planned to build Bob's fuel cap design, but do feel somewhat honor bound to not undo any of the original builder's work if at all possible (the caps also look really sweet). My question is how to vent such an installation, as Bob's caps had vents built in. Unfortunately I purchased the project second hand, so I have no way of knowing what the original builder's plans were. The man I bought the project from had no intention of ever building it. He was using the project to bookmark hangar space while he shopped for a plane. Can I just add a vent on the highest point in tank?

[mswain]

You can add vents to the high side, but you've got to be mindful of when you are on a slope that fuel doesn't run out. Some people use check valves or you can vent the right tank(s) o the left wing and vice versa (or never park on a slope).

When I built my tanks I put the vent bung on the inboard side of the tank and ran some 1/4 fuel line inside the tank to the outboard forward corner to help with the slope issue. I am also venting my tanks to the opposite wing.

[LukeS]

Have any pics? I have another complication. The original builder decided to alter the fuel tank design by using the top wing skin as the top of the fuel tank.

[Battson]

Bad idea in my opinion.

How do you be certain you can keep the water out? Look at the issues Cessna & Piper have had in the past.
My biggest fear in backcountry flying and generally parking outside, is water ingress and engine stoppage in-flight.

Bob's design is even more thoughtful that it originally appears, in a number of areas.

[LukeS]

I can only guess what the original builder's plans were, but that guess would include continued use of Proseal or the like, and maybe the use of a sloshing sealer to boot (all of which add unnecessary weight). The caps themselves are triple O-ring sealed. I'm more of a "keep it simple and light" guy myself, but some folks can't help trying to fix what isn't broke. The more I think about it, I'm starting to convince myself that scraping the original builder's tanks in favor of welded tanks with Bob's vented cap design would be the way to go.

[mswain]

Have any pics? I have another complication. The original builder decided to alter the fuel tank design by using the top wing skin as the top of the fuel tank.

I guess he was going for some hybrid version of a wet wing? I personally like having the separate tanks vs. a wet wing setup.

[LukeS]

I think so. I would prefer Bob's tanks, and I may just ending up going that way. These are also not welded, instead they're riveted with some sort of sealer, which I'm leery of. I'm guessing it stems from a fear of welding aluminum, which I'm not personally afraid of. I have an AC tig setup at home, so I may just scrap these tanks, sell the flush caps ($170 from spruce), and build Bob's vented caps. This is what I had planned to do originally before I bought this project. I just hate to discard someone's work.

[LukeS]

Pessimists are only pleasantly surprised.

[bestbearhawk1231]

Prosealed and riveted tanks work well when done properly, it is actually my back up plan if the aluminum welding of the tanks doesn't work out. Proseal is a royal PITA, reeks similar to rotten eggs, and magically attaches itself to all articles of clothing, skin, and hair. The trick to proseal is surface preparation, correctly mixing it, and having your ducks in a row before you start. If you decide to go that route, cover your bucking bars and rivet sets with masking tape for quick clean up, wear the worst clothes you own, and wear rubber or nitrile gloves. Once you are done riveting, go back and cover each tail with a dab of sealant. Make sure you scuff the mating surfaces (and any surfaces that will be in contact with with the sealant that you want it to stick to) with a red scotchbrite and wipe everything down with isopropel alcohol or MEK then don't touch it with bare hands until the sealant sets up. Use MEK or acetone for cleaning up your tools before the gooey junk sets up or you will never get it off. One trick we used sealing up corporate jet pressure bulkheads was to keep your hands wet with Formula 409 cleaner, proseal will not stick to things that are wet with 409.

Just a heads up if you decide to scrap the existing tanks, you better hope he didn't prep the surfaces correctly or you will have one he** of a time removing everything. Good luck with your endeavors and keep us posted.

[bestbearhawk1231]

One more thing, if you put a bit of proseal on the rivet or in the dimple before you buck the rivet and cover the tails after they are bucked you will have a quite a bit higher success rate when pressure testing. Make sure to wipe the outside surface (visible/painted) with acetone or MEK before it sets up or it looks like hell and paint won't stick. The rag doesn't have to be dripping wet, just damp. You are looking to remove the sealant so it is flush with the mating surface. If you want to make it look pretty (sealing antennas, gas caps, etc.) get some of that blue plastic painters tape to create a "zone" for the proseal to sit. After the proseal is smeared into place but before it sets up simply remove the tape and you are left with a nice clean line of sealant. Works slick, works well with RTV also.

[LukeS]

So, only one of the tanks is close to completion, and it's so close that I think I will finish it just to see if it works. The tank uses a small piece of upper wing skin as the top of the tank, which is already drilled and dimpled to match the tank. If I scrap the tank, I'll also be scraping that skin. So before I throw my arms up and surrender, I will close up the tank with the pro-seal method, pressure test it and if it passes, I'll fill it full of fuel for a few months. If it doesn't work, it's still good riveting practice. Thanks for all the advice guys.

[LukeS]

So I spoke to Bob today about my issue with the original builder's fuel tank changes because I was concerned that the changes he made to the upper skin were a bad idea, and not approved by the designer. Bob confirmed this, so I'll be getting some practice drilling rivets to replace an upper skin. The plus side is that now I don't have the anxiety over a potentially leaky riveted and sealed tank. I'm posting some pictures of the changes as examples of "NOT APPROVED" changes. I had originally felt I would like to honor the original builder by trying to undo as little as possible of his hard work, but with something like this that's going to screw up "my airplane", it needs to get changed back to plans spec. Again I'm very glad to have Bob as an ally to help with these challenges. It may seem like replacing the skin is a set back, but I think that when compared with the headaches of trying to make these design changes work, it will all even out and I'll end up with an airplane that I'm happier with.

[bestbearhawk1231]

On the plus side, it looks like he did good work...but good call on the reskin decision.

[LukeS]

Yeah, I was resistant to undoing his work, because it was obvious he put a lot of effort into it, and a lot of money when I start to add up all of the nut plates that I now need to drill the rivets out of. So if anybody needs nut plates, let me know.

[Battson]

What was the prime concern with his modified design? I can think of a few things including lessened structural integrity and leakage due to cyclic flight stress / strain - I am interested to know what you and Bob were most concerned about.

[LukeS]

Bob didn't get into specifics, just noting that the upper skin was designed to be continuous from the bottom of the front spar to the top of the rear spar. I'm sure it contributes a lot more strength and rigidity than the way the original builder of my project split it into two pieces. Also this builder intended to secure the upper skin above the tank with lots of screws and nutplates, which I now need to drill out. It was designed to be riveted. So because the screws were larger than the original rivets spec'd out in the plans, Bob recommended that I go with the soft rivets since they will be larger to prevent from distorting the new skin. I'm going to have lots of spare nutplates if anyone needs some for something. All in all, Bob's design just leaves less opportunities for things to go wrong. The plus side is that the original builder hadn't started to assemble the other wing yet, so I don't have to fix anything on that one.

[Enewton57]

Also notably missing is the Hat sections that run from main spar to the rear spar to reinforce the area where ribs are missing because of the fuel tank.


Eric Newton - Long Beach, MS
Bearhawk Tailwheels LLC
Bearhawk Manuals

http://mybearhawk.com

[LukeS]

Yeah, I'll have to make some hat section too. I can't decide If I want to sell the Aero locking flush caps I have, and build Bob's vented caps. The vented caps would simplify the install, but don't as look as cool to me.