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RV-6 Wing Screws
Hey guys, I'm new here and I'm trying to buy a flying RV-6.
We noticed that there are some screws that seem to be called for in the plans missing from underneath the wings, where they attach to the belly skin of the aircraft. I've been told various things as to that they are important and that they are not, and I'm just trying to get to the bottom of it. Thanks! |
If the wing skin meets the belly flush then I see no reason to put them in. Mostly the screws help make the skin flush. I guess they could flex over time and crack if it is not flush but not really a huge concern.
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This is actually where the belly skin overlaps the wing. I'm never built one, but from my rookie level of reading plans it appears like 10-ish screws that go through each belly skin->wing skin->inner wing rib and attach to nutplates mounted in the wing. 20 or so screw/nutplates in total, 10 each side.
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This has come up before, perhaps with the same airplane. They are absolutely structural and required.
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But I was told that there are no airworthiness directives or service bulletins that exist for this issue. (I'm not really sure what all that means, it's just what I was told)
How long would it take to fix something like this? |
It is not a service or airframe issue, it is a construction issue. Who knows what impact it may or may not have on the strength of the wing. I posted a link to a post about a 7 with the same issue.
The fix would require wing removal, not an easy job. I would be more concerned with what else the builder felt was "unnecessary". |
Has anyone got any pictures of this? I'm having a hard time visualizing it and I own a -6... But I didn't build mine either. Doesn't the wing root fairing go all the way around? Guess I'll find out again at this year's annual (due in about a week...).
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Absolutely JonJay
I fully concur and for all the same reasons. If you call Van's and ask them, the answer is, it absolutely is a structural matter.
Steve "The Builders Coach" |
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The screws that are missing are #8 screws. There should be #8 plate nuts in the inboard edge of the lower inboard wing skin to accept the screws attaching the belly skin. You could just install the screws if the plate nuts are there. The question is, what structural loads have been imposed by the lack of these screws? If the plate nuts are not there, something important was missed by the builder. Maybe you should keep looking. BTW - good job noticing this omission and investigating it.
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I can't remember ever seeing screws under there on mine, either, but i'll definitely be checking it the next time i'm at the airport... |
The joint (overlap) should be bolted per the previous link. http://www.vansairforce.com/communit...99&postcount=7
You might be able to use dome head #8 screws and self locking nuts installed from the top if you remove the upper wing/fus. fairing strip. If this is possible, you might be able to drill the holes and insert nuts/bolts without taking the wings off. Check with Vans to see if this is acceptable - it probably is. |
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What's the shear strength of a dimpled screw joint versus just a hole with a dome head screw and nut? My gut says the dimpled is stronger, which is what is called for by Van's in the plans. Sure, anything is better than nothing, but if one wants the full 6 positive G capability... I suppose some sort of dimple extension/riveter apparatus could be fabricated to make dimples without removing the wing...sounds tough though. Fly safe, -Jim |
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Well, this wasn't the builder that mentioned the bulletin thing. The guy who's name is on the aircraft as the builder actually passed away 10ish years ago.
It seems like I've got the right idea then, that these are completely necessary for the plane to perform as advertised. Hopefully it gets fixed soon. Thanks for everyone's inputs, I really appreciate it. One last thing, how easy would it be to get the FAA examiner person to sign off an aircraft as airworthy with a problem like this? Why wasn't it caught at whatever kind of inspection gets done for that? Please excuse my ignorance on this, hopefully one day I'll build a plane! |
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So if you're still interested in buying this airplane, you should seriously consider having it inspected from head to toe by a qualified person familiar with the type. Not for legalities, since it has already been deemed legally airworthy, but for your own peace of mind, to know what if any other major deficiencies in construction may be lurking in there. With something as blatant as what you've found, there's bound to be more. |
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Thats exactly why I said to check with Vans first...:) It would be difficult to get fingers, tweezers, swivel sockets extra down in the narrow gap as JonJay says... but if you can get in there, it will be less work than wing removal on a -6, and even less if it's a -6A. |
I have to go along with everything said to this point.
I would not consider buying this aircraft without a complete condition inspection by someone very familiar with the RV-6. I have found things that 99.9% of FSDO inspectors would have missed simply because they are not familiar with a particular model. The most important thing that I've found on an RV-6 is 2 missing steel splice plates where the 2 wings mate. This is very critical and would never be noticed by someone not familiar with the RV-6. The missing screws you mention are necessary for safe operation of the aircraft. Good job on noticing this discrepancy. There are probably others. Make sure they are found. |
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I had a talk with Vans about theses screws and they are not there for strength there to hold the two together like the faring on top which is not there for strenght .Thats what the spar is for!
Bob |
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Here is another great advantage for the prepunched kits. Back in the "6" days you actually had to study the prints to measure and decide where to drill all the holes. Now with the prepunched, you know something must go there (99.9%) of the time. With those older kits and inexperienced builders, there was always the possiblity that something was overlooked...pays to have another set of eyes while building.
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I thought they were #40 holes and you leave out every third rivet and drill out for a #8 screw later. I bet a lot of the should be empty rivet holes got filled...:) I think this particular row of holes may be in the pre-punched 0.1%... |
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Jon your right about different answers I called about 6 months ago when I was installing my wings and talked to tech and ask if all were nessary and was told no called back today and spoke with a young fellow and he said yes so what to do now i have seven on each side and eleven is the correct number so im off 4 on each side sure dont want to pull wings off flying or now.
Bob |
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If you want a formal written answer from Van's, you have to email it, follow up, and if your patient they will get an "engineered" answer back to you. PM'ing you shortly with some additional advice. |
How about drilling dimples? You could add stainless steel flush washers to the screw head side. I think with a flat washer on the nut side you'd have a very strong assembly.
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A few comments....
Screws at this location weren't specifically designed into the wing attach system, but static testing has proven numerous times that it does transfer some level of wing load into the fuselage/center section.
I personally would not build/own an RV without them. I have done prebuys on a few RV's without the screws. I always list it as a deficiency with the airplane. Most importantly in my opinion (and already mentioned) is that these screws missing is for me an indicator of a lack of planning and attention to detail. As already mentioned, an airplane with these screws missing would make me look very carefully at the entire airplane (just one of the reasons a person familiar with a specific model needs to be involved in prepurchase inspections) |
So the consensus seems that if you want the wings to meet design strength, the screws are required. A couple questions then.
If the seller was unwilling to pay to install the screws, and unwilling to lower the price enough so that I could pay to have them installed, how can I: 1 - Insure that everyone who might try to buy this plane is aware of this problem so they don't buy it and have the wings fall off? 2 - Recover my good-faith deposit that the seller has refused to return to me, citing that the plane has flown fine for a short time, and that 2 random people say the screws aren't needed. I understand that specifics of the aircraft shouldn't be cited here. Also, does anyone know if the DAR or FSDO that originally signed off the plane would pull the airworthiness certificate knowing this, or because it's an experimental is pretty much anything acceptable? Thanks again for all the information, you guys are amazingly knowledgable! |
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On the other hand, from a "legal" standpoint, it would be acceptable. A FSDO inspector not familiar with the RV-6 might not even notice. Remember, issuance of the airworthiness certificate only requires that the aircraft meet the requirements for amateur-built certification. The builder is the one who states that the aircraft is in a condition for safe operation. |
Also keep in mind that the wing was stress tested without this skin attached. If this is the ONLY thing keeping you from purchasing the aircraft, then purchase it and correct it later. This is not a big deal to fix without removing the wings.
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It doesn't mean what random people have said. There is no argument that the airplane wasn't built per plans. Period. 2 - The seller is being totally unreasonable. This is exactly the type of issue that warrants a refund. It is not like you decided you didn't like the color. A deficiency was found that you can't come to an agreement on to rectify. He should be reasonable and let you walk. If he doesn't, my gut feel is that it is probably not an airplane worth being serious about anyway. Also, it is possible to find people willing to say anything we want them to say. It happens in the court of law all the time. I guess you had nothing in writing requiring your deposit be returned if any deficiency were found? Did someone experienced with RV-6's do a thorough inspection for you? If not, then as many of us have already said, there could be other surprises hiding in this airplane. Your last ?... It is highly unlikely that anyone would pull the C of A on this airplane. |
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All models after that were tested with the wings installed on a forward fuselage / center section. |
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Perhaps I am not seeing things clearly. |
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I'd guess someone efficient, working with a helper, could do the whole thing in a couple of hours. I'm a little slower than that. |
To be screwed on not to be screwed.
I was talking to Ken Kruger several years ago about doing pre-buy inspections and asked if there was anything he felt was important that was sometimes missed. He specifically said the screws that attach the lower skin of the wing to the belly skin were sometimes omitted, and that they were structural.
As for an DAR missing them, I believe that most of the DAR's inspect lot's of different types of aircraft and can't be expected to know where all of the bolts and screws go or need to be on each aircraft type. They inspect what it there and with there knowledge and experience look for things that would commonly be incorrect as per 43-13b. There are DAR's that limit themself's to, or do a lot of RV's and have built at least one for themself's, I would expect them to have a better base knowledge of Van's aircraft. It's hard to inspect something that is not there. RT |
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I would quess that the RV 3-4 and 6 were tested without a center section in place when loaded to failure.
Bob |
So as it turns out, mine doesn't have them either! I never knew... The builder did put some thought into that joint, however, as he did finish the edge with the edge rolling tool to give it a slight bend. As a result, the skin sits very nicely against the bottom of the wing. I don't think it's a case of laziness by the builder, as attention to detail on the metalwork on the plane is excellent everywhere... The builder planned from the start to to polish, and it wouldn't do to have bad metalwork.
I suspect he looked at the plans and saw nutplates on top holding a filler fairing, and saw nutplates on the bottom performing what looked like the same purpose. Knowing he could get a nice fit with an edge-rolled sheet, that's the choice he made. There is no evidence of any flexing in this area, and no sign of the belly skin moving relative to the wing skin... No little scratches that would indicated any motion. And that's after over 600 hours of trouble-free flight, at least some of which has included aerobatics. It appears from the comments in this thread and others, that the past questions about the belly screws were specifically in relation to the 7/7A, which has a different wing mounting configuration, and the 7/7A wing configuration was tested with that skin screwed in place. It further appears that the 6/6A wing was tested *without* that skin screwed in place. The other thing that comes to mind here is that relying on this skin to take a structural load is kind of counter to how an airplane is traditionally designed... Typically you rely on the skins to transfer the load to the spars, and the spars to transfer the load to the fuselage. Anything else is a bonus, but not something you should be counting on. I will have to think about this further... I'll go grab the plans from the hangar and run some numbers and see what kind of effect those screws would have. But when you compare the load-bearing capability of 10 #8 screws to the load bearing capability of the spar itself, I'm not sure the effect is going to be significant... |
I know it is beating a dead horse... but regardless of how much thought appears to have gone into deleting these screws (or anything else), it is still a deviation from the original design.
In my mind, this opens the door for the potential of other dangerous deviations that may be far less obvious. I in no way mean to imply that an RV not built exactly per the plans is a death trap... but it might be. Also, making comparisons of different models with just a very small amount of publicly known info regarding differences of how they were tested, is not a good way to decide on whether a modification is sound or not. It has been pointed out that at one time the head of engineering at Van's stated the screws in question were considered structural (and I agree with him). This is a person that oversaw the design of wing spars and other major structure. Most everyone excepts the viability of of those without question, but then they second guess the need for some screws which the same engineer said he felt were important? Venting mode now on... I am endlessly amazed at how many people will add weight to their RV, beefing up this and that "because it just didn't look strong enough", but then casually dismiss something else because to them it appeared rather insignificant.... sigh... vent mode off now.... |
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