Fuel cap lost

[schu]

Ok, that video is pretty impressive. Timeframe?

I could probably have the first prototype in aluminum in a few weeks, but the real issue is that I don't know what the bores are for all of the aircraft out there. I've already mentioned that what is in Bob's plans doesn't appear to be what comes in the kit, but what I need most now is for everyone to measure the ID and height of their neck for all of their tanks and post them here or PM me.

Also, I haven't heard back from Kevin, and I want to understand what his plans are before I move forward. None of this makes a lick of sense unless I build 50, so I really need to have the design perfect and know that there is a demand for them.

schu

[schu]

Use a caliper or better.

[Utah-Jay]

schu

I am still working on my fuselage, in the covering process, but will look at the fuel ID when I start on the wings, love your design

[Nev]

I should have some dimensions for you later today Schu.

[Mark Dickens]

In measuring one of my tanks, the ID is about 1.725 according to my caliper. The height is hard to determine because of the weld, but if I measure from inside the tank to the top edge, it came out to about 7/8"

[Mark Dickens]

I'm sorry, let me be more specific. When the ID was measured in one direction, it was 1.72", when turned 90 degrees it was 1.73", thus my statement as an average. My digital caliper only shows measurements in 100ths of inches. In terms of the depth of the neck, you want it from inside the tank? I only have one tank at home, so I'll have to measure the other when I get to the airport where the other wing is currently stored

[Mark Dickens]

I just went out and measured the depth and like the others, it came out to 22mm

[schu]

Mark, can you measure all of your tanks and see if they are all pretty similar. Also, can you give me exact readings. About won't be good enough for o-ring fittings.

As for the inside edge. Take a piece of aluminum maybe 1/2 wide, bend it over to a 90 in a brake or vise or something, then put it down in the tank and hook the bottom then use the end of your caliper to measure from the top to the aluminum at the edge.

I don't have time to draw it, so here is some back of napkin for you:

IMG_2909.png

Thanks,
schu

[Bissetg]

Mine are ID 44mm Depth 22mm

[Nev]

Mine are ID 1.72 inches. Depth 22 mm. Apologies for the mix of metric/imperial.

[schu]

Okay, so far I have:

Bobby	1.7235	.8675	Measured a single tank
Mark Dickens	1.72 on one 1.73 on the other	.866 (22mm)	Measured two tanks but with caliper that only shows 100's
Bissetg	1.732 (44mm)	.866 (22mm)	Have no idea if it's both tanks or a single tank and seems to be rounded to nearest mm.
Nev	1.72	.866 (22mm)	Have no idea if it's both tanks or rounded to the nearest 100th

Also, Brooks Cone points out:

Then I replaced the O-rings with a Dash 323 size which is one step smaller. Now I feel the Factory Cap fits just right. Before it was so difficult to remove I felt it was a hazard to me from multiple fronts....lubrication leading to a cap loss, ladder fall risk, damage risk to the cap/tank by using a tool to help pry/removal, etc.

That's moving from a 1.75 OD o-ring to a 1.6875 o-ring, which is a 3.7% stretch around the fuel cap body. Applying that same stretch to the o-ring thickness (which I'm not sure if the math works this way), that would probably make the 0.1875 thick o-ring around .181 which would be 0.0139 smaller in the bore or have an OD of around 1.736.

In other words, Brooks points out that .014 is absolutely the difference between a nice fitting o-ring and something you have to pry off, and the measurements I'm getting from people vary .012. This means that either the bores are not very consistent, or the measurements I'm getting back aren't very accurate. Given the data and information I've heard from others about tight caps, I suspect both.

My design doesn't put any pressure on the o-ring to make it seal, while the factory design (and Kevin's design) crushes the o-ring. Because of this, I'm not sure my design will work very well. Sure, it will lock in the bore with the spreaders, but I'm not sure that it seal all that well without making several different sizes and/or playing musical o-rings which doesn't sound like fun.

At this point it seems fairly clear to me that the cap design needs to be machined to each specific bore size, or they need to crush the o-ring, or they need to be designed such that there is a rubber seal around the body against the top lip and some finger that grabs the bottom of the filler neck and draws the cap into the tank against the rubber seal.

schu

[Nev]

Measured one tank Schu. My caliper reads 100's.

[Bcone1381]

"Machine it, draft it, build it, brake it. Everytime something new gets built the whole world advances." M. Paytey

Experimental aviation has the element of learning ......

If a cap was inserted into into a tube, then hung it from a chain hoist with a "fish scale" maybe we could measure how much force it takes to remove it, then change things around like O-Rings, lubrication, try various torques on the cap.

So far, my take-a-way is from this thread.....torquing it down with a tool like what was posted by mbaerobat in Post #12 and no lubrication seems prudent to me. Our caps stay in place in a large part due to pressure those o-rings apply to the filler neck. Lubrication can only help them sneak out.

I hope to test mine some day. For now I'll build and keep an eye out for a tube similar to our filler neck.

[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.