Fuel cap lost

[triumphantduke]

Looks awesome.

Devil's Advocate:
-are all components large enough to NOT plug a fuel port, if it were to disassemble
-if the profile of the vent is not the same, has the icing propensity changed
-like you said, preventing movement would be great to prevent debris/failure from chaffing

Angel's Advocate:
-You're a genius if you can also resolve fuel spillage from expansion, without breaking any ventilation rules (like no traps)
-Incorporating a key so you don't come back to an empty plane lol

[Mark Goldberg]

Thought I would chime in here with some comments from me and Bob. Just spoke with him about this thread.

First - he and I have no issue at all with someone using a different fuel cap. That is your choice as homebuilders.

But I would like to say again that in 1200 hours on my 4 place with me and a bunch of demo pilots flying - there was never a lost cap. And Bob with cumulative 2,000 hrs + on his planes has never lost a fuel cap. I would say that maybe 10-12 caps have been lost by customers over the 20 years. Bob says he tightens his down and then tries to pull it out by hand. If you can not pull it out by hand pulling hard - Bob feels that there is no way the breeze would loosen a cap and allow it to depart the plane. Sometimes it does take two tries to get the cap tight with the air vent facing forward.

Over the years the biggest complaint I hear some times is that the cap is hard to get out. I then ask if any lubricant was used on the o rings. Maybe half the time the answer is no. Mostly some lubricant solves the issue. I use a little bearing grease I would keep in a zip lock baggy, and Bob uses a little oil from the dip stick. What is usually NOT good to use is Fuel Lube. That is slippery stuff and can allow a gas cap to come out.

In the past I urged the builders with tight caps to turn down the OD of that area .005-.009. Makes a big difference. An easier solution is what Brooks suggested (and Bob approved) to use one size smaller o ring so it compresses the o ring more and can solve a too tight cap issue.

Once Bob looked at a drawing for a fuel cap that he thought would work fine but had one problem with it. If something in the cap broke or the threads galled - you could not get the fuel cap out without destroying the filler neck of the tank. So what I am saying is that the green grass over the hill might or might not be as green as it looks. I know Schu well and how capable a person he is. This cap he is working on might be fantastic. No problem from me or Bob with him designing one and offering it.

I really believe that most issues with the fuel caps are because Bob and I have not provided his guidance on how to use them as he wants. Hopefully soon we will have some written guidance with the caps on how best to use them. Mark

[Battson]

Totally agree with all these comments. Well said Mark.

[spinningwrench]

Has anyone measured the actual roundness of the fuel tank opening. Welding may well distort the diameter to where one O-Ring size will not work. It would seem a clamping arrangement that seals the upper lip would be a more consistent solution.

[Nev]

Grant is currently using that exact solution........

[Nev]

NZ Bearhawk Patrol builder Graham Johnson has a background as a toolmaker. After I told him of the issue, Graham came up with a couple of ideas. His initial thoughts were to try and make the original fuel cap work better. To do this, he made a test rig to check the amount of force required to remove the fuel cap in different scenarios. He also turned a mock filler neck from nylon.

The criteria we used was:
- It had to easy to install and remove with one hand (modest strength), and no tools.
- Simple to get the vent facing forward with sufficient and consistent O ring compression every time.
- To explore Grahams idea we wanted to use the existing fuel cap, and minimal machining.

Original Fuel Cap
We started with the original fuel cap, as it comes with the supplied -324 O rings. The original fuel cap required over 25 kgs of force to pry off when secured properly without lubricant. However the difficulty installing and removing it is why so many of us used a lubricant or a screwdriver to assist.

Using grease as a lubricant, the fuel cap then required such little force to remove that the weight of the test rig provided more than enough force with a reading of around 4 kgs. When we wiped much of the grease off, the friction force increased to 9 kgs. With such little force required, in combination with the hydraulic action of the sloshing fuel, it's easy to see why this is problematic.

Different O Rings
Next, we tried a variety of different sized O rings, looking for a simple solution. We tested each combination with and without a lubricant. After trying many combinations, there were two potential candidates. An imperial -323 (one size down from the supplied -324) showed promise. However the inside diameter of the -323 is lower, so after stretching it to fit the unmodified shank, we were back to the same OD as the -324 and it was difficult to fit and remove by hand,

We then tried a metric 34mm ID in 4.5mm thickness. This worked better, but required quite a lot of force to tighten and compress the O rings out to provide enough engagement. An ideal size would seem to be 34mm ID and 4.75mm thickness, but we haven't sourced one yet.

Filler Neck Groove
Graham then added a very small groove to the inside of the filler neck, to engage with the top O ring. Although not a mechanical lock as such, the groove increased the "friction" in all situations by at least 50%. This made a huge and measurable improvement, but it does require a specialist tool to add a groove. Graham has designed a tool to do this, but at this stage we're still exploring ways to do it without the groove.

Engagement Pin
One thing we noticed is that when lubricating the O rings, we lost friction in the lower fuel cap body, and often it spun as we tried to tighten the cap into position. This meant that we were not able to tighten the fuel cap properly when it has been lubricated, and probably explained why a number of them have departed inflight. Not only did the lubricant reduce the friction between the O rings and the filler neck, it also prevented the same amount of ring compression from being applied, resulting in the cap not being able to be tightened. Another issue was that even without lubricant, it sometimes required more than one attempt to compress the rings sufficiently, with the vent facing to the forward position. It could be too tight, or not tight enough.

To prevent this, Graham added an engagement pin (in the photos below). This made a significant improvement. Now we were able to fully tighten the cap whether lubricated or not, because the lower body was held firmly in place. It also allowed us to insert the cap, then twist vent fin a precise amount every time (we used 180°), knowing that the lower body was fully engaged. The result is that the vent faces forward every time, with the same amount of thread engagement and ring compression.

Adding the engagement pin is relatively easy. It requires a hole drilled for a press fit of the pin in one part of the body, and a slightly larger hole in the opposite body part.

Reduction in shank diamter
Finally, we decided to try turning the main shank slightly smaller. The reason that this was done last, was because it does require some specialist modification to the fuel cap. It could be done with a file, or done during the manufacturing process. The diameter was reduced about .005". This then allowed the -323 O ring to fit with slight friction both inside and outside. It allows an easy press fit (one hand) into the filler neck. Removal can also be done with one hand and required it to be wiggled out. We then tried this on the real tank, and it worked well. The filler necks we measured are not exactly round, but there was enough tolerance to account for this. We didn't turn the shank down further at this stage, but will first try it in flight before going further. We also tried the original -324 O rings on the narrower shank, but they were still difficult to fit and remove the cap without lubricant.

Where we're at currently, is having the fuel cap shank turned down to fit a -323 O ring, the addition of an engagement pin, no lubrication, no groove. With a 180° turn upon installation and only modest strength required, it currently gives a force of 15 kgs to cause the cap to come off, which we think is likely to be sufficient (60% more than the original -324 rings with grease as lubricant). Anecdotally, we couldn't pull the cap off by hand once it was tight. With the original fuel cap and lubricated rings, we could.

If we decide to add an inside groove to the filler neck it increases the force required to detach the fuel cap to 22-24 kgs - very similar to the force required to remove the original fuel cap when it's used without lubrication. The cap can potentially be done up tighter, but given that you're usually perched on the wing strut with one hand outstretched, we tested with what we thought was a reasonable amount of strength that most people could use.

If the correct size O ring can be found, then we're optimistic we can do without reducing the shank size.

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[Bissetg]

Thanks to you guys for putting the effort into this.

[AKKen07]

Wow! That is awesome.

[Bissetg]

6B34C205-92AB-463A-8125-F96F9E3837A1.jpg Until this matter is resolved I’m using a 33-57 mm tridon clamp to ensure the caps stay secure. Despite being agricultural and inelegant it seems to work.

[Mark Goldberg]

I forwarded Bob the posting about the testing that you did in NZ. He feels that unless you do the testing with just engine oil on the big o rings - that the results you saw are not really of much use. If you did the testing with Fuel Lube (Easy Turn) - then even less useful results came out of the testing. Bob would like you to try using just engine oil as a lubricant and see how the caps behave. Mark

[Nev]

Hi Mark, We didn't test aircraft engine oil specifically. I'll get back to you on that. We did use two different oils, and one type of grease. Definitely we noticed a difference in lubricity between them.
EZ-turn wasn't tested, we wouldn't have been able to get a reading, it's reduces the friction too much. (sometimes we were able to remove the cap by hand, even when tightened).
We used the standard supplied fuel cap as a bench mark. We felt that one factor in using any type of lubricant was that fuel is able to thin or dissolve grease and oil, thereby increasing the O ring friction. (Anecdotally, this sometimes made it difficult to remove the caps even when I had used engine oil to help install them on my Bearhawk). With EZ-turn, it isn't dissolved and the "slipperiness" remains.

[Nev]

Mark, we tested today using Bob's method of engine out - specifically W100. It held good up to 12 kgs of force. Although that's a significant reduction from the un-lubricated cap, it's still not too bad. The major discovery is that as soon as we added lubricant, the cap could not be fully tightened every time, because the lower body spins. We then reduced the modified cap shank to 34mm and installed -323 rings. The friction decreased significantly, but was still at 12-13 kgs. It's much easier to install and remove. We think it will hold well, however Graham is keen to try a small taper to see if we can improve on it.

[Bcone1381]

Awesome Write Up, Nev! Wow!!! You guys are very creative and I learned a lot from your testing. Our community will attract and thrive with material like this.

I'm thinking outloud.

-The lower cap body control with the Engagement Pin for positive control the O-ring crush is awesome. I had been accommodating it, but never saw it as a risk like you did. You two codified, exposed and fixed it. Great work.

-You say you removed .005" from the shank surface where the O-rings ride. Would you declare a shank dimension for us? I suspect that slight variations exist (change in venders?)

[Nev]

Hey Brooks, Thanks. Mostly Grahams work of course.
Yes I'll get a shank dimension for you. (Left my calipers at the hanger!)

[Nev]

We discovered today that the shanks are not identical in diameter. We have 3 that measure approximately 34.35mm. The one we altered was slightly wider initially, and Graham has now turned it down to 34mm. If we turn it down any further it won't have enough holding friction. We are going to try a couple of other ideas, then will report back.

[Nev]

Can anyone confirm what material the original O rings are ?

[Mark Goldberg]

They are called VITON o rings here.

[Utah-Jay]

OK, I finally took the fuel caps out of the bubble wrap. I assume the large O-Ring goes on the position I put it, it looks like the white nylon washer goes between the two pieces. Where does the smaller O-Ring go?

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It seems there is a gap between the O-Ring and the two different threaded pieces.

[Nev]

See if this helps.....

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[Utah-Jay]

My wings are still in the crates (coming out in a few months), but this thread got me curious about the fuel caps. After Nev posted the photo the design makes perfect sense. Of course I am clueless having not used one yet, but through some trial and error I would think that one you got the proper index it is a fool proof design. The only think I am surprised a bit about is that there is not a type of “Y” hanging down from a chain in the tank to retain the cap during refueling. IF someone was so inclined it would be an easy addition. But finding the correct cap index and then possibly tweaking the O-Rings the system should be damn good

[Mark Goldberg]

There is a little hole at the bottom for a cotter pin that keeps someone from unscrewing it enough to drop the bottom into the tank. The fuel caps were not originally designed with the hole in the bottom. We added 1/4" of length so there was room for that hole for the cotter pin. Bob agreed to let us add the 1/4" of length even though it adds weight.. Mark

[Utah-Jay]

Mark Goldberg I saw that hole and the cotter pin makes a lot of sense, thanks!

[whee]

I've been thinking about this and wondering why I've never lost a cap. There has to be something uncommon between those that have lost caps and those that haven't.

My procedure for refueling is basically:

1. Twist cap 180 deg to loosen. Always 180 deg so I know where to position the cap so it faces forward when tightened.
2. Gently rock cam back and forth while lifting up to remove the cap.
3. Add fuel to the bottom of the filler neck. I don't like the cap sitting in fuel, it happens sometimes but I prefer it not.
4. If the cap was difficult to remove then I take a couple of drops of oil from the engine breather tube and apply it to the o rings.
5. Install cap with vent facing to the rear and the top lip against the filler neck.
6. Tighten cap. Should be the same 180 deg turn to make it tight and clocked correctly facing forward.

Perhaps some fill the tanks fuller and the hydraulic pressure from fuel expansion pushes the caps out. The seems pretty unlikely to me as the fuel would just flow out the vent holes unless it expands quickly or the vent is constricted.

The only issue I've had with the caps is the threads on one galled. I was able to get the cap off, chase the threads and then applied anti-seize to the threads on both caps. No issues since.

[Bissetg]

Hi Whee, my experience was pretty similar, except like Nev and others I was having to pry the caps off with a screwdriver. I think Marks advice of turning the shaft of the cap down a sniff would have helped this but both Nev and I were advised to lube the caps with EZ lube which we did. It made the caps much better to put on and off but reduced the locking friction in the filler neck. In hindsight I probably overfilled the tank (I usually leave a little headroom in the tank) and with a bit of turbulence I suspect the slosh hydraulics the cap out. It’s interesting to see that others have been working on solutions to this problem in the past, I’m sure we will soon get at least one and maybe more ways to address this. Until it happened to me I thought Nev hadn’t put his cap on properly.

[Nev]

This is interesting Whee. I never had any issue with my caps staying on initially. The problem was I couldn't get them off. I was having to use a screw driver. It was also difficult to install. I could only ever get a 90° twist out of the threads (not 180°). Sometimes it would turn further, but that turned out to be the lower body turning with the threads and wasn't actually tightening it further.I suspect that the small variation in shaft size might be affecting how consistent they are to do up.

It was because of the difficulty installing and removing it that I added a lubricant to the O rings. But as a result, the lower body spins even more. Grahams idea of adding a pin completely stops this and a 180° twist always provides exactly the same amount of compression every time. Currently we have a cap that is much easier to install and remove, and can be tightened to remain secure. It does need to be tightened properly, but the force to remove it now is very similar to the original fuel caps with engine oil on the rings.

One thing I discovered yesterday is that the addition of fuel on the O rings after installation and tightening, significantly reduces the locking friction. It appears that the lower ring is keeping the upper ring relatively dry.

Graham has ordered some more tooling for his lathe and next week we are going to try a couple of small changes.

So far what I've learn't Is that every change we make causes a couple of other things to change. There seem to be pros and cons to each possible solution. One example is I made up some locking tabs so that a 30° twist of the cap itself locks the tabs into place, before tightening the threads. It stops the cap coming off mechanically. However when fixing the tabs to the cap, you need to have them in the correct spot so the threads can then be tightened the right amount. If the O rings subsequently compress over time, the vent will have to turn past the forward position to fully compress the rings.

[Nev]

I have had a few caps disappear in flight.
The method that seems to be working for me is the application of some fuel to the O rings when placing the caps in the neck and then tightening the cap tightly using a tool that I made.
Finger tight doesn’t seem to do the job.
The tool is made from UHMW plastic but my original was made from hardwood.
Haven’t lost a cap since. Still carry a spare cap just in case….

I'm going to try the tool idea. We've currently got a cap that installs and removes very easily, if a 180° turn is used. But we're not sure if that's tight enough. The tool might be an easy way to solve that.

I'm thinking especially if the tool had a bottle opener on the other end. Every self respecting Bearhawker would want one in their survival kit

[robcaldwell]

I used the tool method. A flathead screwdriver to get the cap started so I can rock it back and forth for removal. Then I use an aluminum adjustable wrench (fuel fitting tool) to tighten the cap back on. 1/2 turn doesn't get it for me, so I go 3/4 turn for a snug fit.

[Nev]

Actually I've got 3 fuel caps sitting in front of me and all 3 are different. The smaller vent hole size has an inlet area of 1.75 ²/mm. The larger vent hole size has an inlet area of 9.0 ²/mm.

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