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RV-6 Wing Screws

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FitzRX7

Well Known Member
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.
 
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.
 
This has come up before, perhaps with the same airplane. They are absolutely structural and required.
 
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".
 
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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...).
 
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"
 
Snowflake said:
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...).
Click to expand...

No, underneath it stops just aft of the tanks.
 
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.
 
ChiefPilot said:
No, underneath it stops just aft of the tanks.
Click to expand...
Now that you mention it, I should know that because that's where I have trouble keeping the free end of the rubber filler strip in place... I remember it's right beside the fuel drain.

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...
 
az_gila said:
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.
Click to expand...

Gil,

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
 
az_gila said:
The joint (overlap) should be bolted per the previous link. http://www.vansairforce.com/community/showpost.php?p=663799&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.
Click to expand...

I am sure it would be Gil but that opening is tigheeeet. Unless you have eyes on your forehead or are really good with a mirror it would be tough at best.
 
JonJay said:
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.
Click to expand...
...
JonJay said:
I would be more concerned with what else the builder felt was "unnecessary".
Click to expand...

Exactly. This kind of omission in construction makes the entire airplane suspect. And if the builder was trying to brush off this issue with nonsensical talk about service bulletins (a red herring), that again makes the airplane and the builder even more highly suspect.
 
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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FitzRX7 said:
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!
Click to expand...

Good question. Easy, depending on who did the inspection. A good DAR who is familiar with RV's and did a thorough technical inspection would have caught this problem. If, on the other hand, the inspection was done by a FSDO guy without any real familiarity with RV's, then something like this could easily slip through the cracks. The legalities of the airworthiness inspection are mostly bureaucratic. It is up to the builder to make sure the airplane is actually airworthy, and that includes choosing the right DAR to do a thorough and meaningful inspection. Not everyone does.

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.
 
fehdxl said:
Gil,

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
Click to expand...

Yes... the dimpled joint would definitely be stronger in shear.

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.
 
screws

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
 
6 Gun said:
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
Click to expand...

Not the first time two different answers have come from Van's. The 6 was designed a long time ago.
 
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.
 
edbooth said:
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.
Click to expand...

Are they pre-punched on the RV-7?

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%...
 
screwed

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
 
6 Gun said:
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
Click to expand...
I wouldn't think you are "screwed" just yet. There are obviously at least two airplanes flying around out there without any screws in them and none have fallen out of the sky. So, you are at least 14 screws better than that. I do know those screws are structural, but I do not know to what degree they contribute to the overall strength of the wing. If I had to guess, not much, but that is only a guess.
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.
 
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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FitzRX7 said:
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?
Click to expand...

Personally I would not sign off the aircraft without them (And I have inspected more than a few RVs). We do not have an obligation to sign off anything we are not comfortable with.

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.
 
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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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FitzRX7 said:
So the consensus seems that if you can spell 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!
Click to expand...

1 - It is not your responsibility to tell everyone else, but I understand your feeling. No one said the wings would come off without the screws. I have been involved in wing tests. I have witnessed the screws carrying load. How much? Know one knows. That is why with things like this, you do not play games and randomly decide to not install them. I said it before, but I will say it again... any RV that has blatantly visible shortcuts taken during construction, how do you know what's there that is not visible?
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.
 
gasman said:
Also keep in mind that the wing was stress tested without this skin attached.
Click to expand...

Also keep in mind, the above can only be said about the RV-6 wing.

All models after that were tested with the wings installed on a forward fuselage / center section.
 
gasman said:
This is not a big deal to fix without removing the wings.
Click to expand...
Tell me how you would do it. Locating the rib flange under that skin from the bottom woukd be hit and miss. Drilling it from the top through a one inch gap up against the fuselage side I can't imagine. If you get that far dimpling might be possible with a pop rivet dimpler but I have not had good luck with them.
Perhaps I am not seeing things clearly.
 
az_gila said:
Are they pre-punched on the RV-7?

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%...
Click to expand...

The holes for the nutplates on the wings where prepunched on the 7 , had to use hole finder to locate the holes on the fuselage overlap. Then you have to pull the wings back out to dimple the screw holes. Would be difficult with the wings permanately attached.
 
JonJay said:
Tell me how you would do it. Locating the rib flange under that skin from the bottom woukd be hit and miss. Drilling it from the top through a one inch gap up against the fuselage side I can't imagine. If you get that far dimpling might be possible with a pop rivet dimpler but I have not had good luck with them.
Perhaps I am not seeing things clearly.
Click to expand...

With careful measuring, you could drill the holes from the bottom. With a 12" drill bit run through a piece of tubing, you could drill it from the top. Instead of dimpling the 0.040 skin, why not go with button head screws from the underside with nyloc nuts on the inside? A little tape to hold the nut(s) in a socket extension and attaching the nuts would be fiddly, but doable.

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
 
wing

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....
 
screws

Rob ill be waiting for numbers on 10 #8 screws so I can figure out load seven can carry I bet its seven times more than none. Im sure very few Rv's are built with no changes from plans planed or not.If one crashes its a mechanical problem (The Nut Holding the Stick) this has been a good thread hope we can all learn from it.
Bob
 
I've run a bunch of calculations on the RV-6 wing structure, which I have decided *not* to publish. As a P.Eng I do have liability to be concerned with. I will say that the limiting factor on loads on the structure is definitely not the spar or wing structure... To say that the spar on the RV-6 is overbuilt is a bit of an understatement... I'll leave it at that.

However, there may be other factors. I looked closely at the wing-fuselage interface and wing and spar construction, but didn't look further into the fuselage structure or loading to see what may be going on in the fuselage. Maybe the limiting factor is the fuselage, and the load of the pilot pushing on the seat pan... It would be interesting to mount a GoPro on the underside of the wing, looking at that overlap, and go do a loop... To see if the gap opens slightly. Or maybe some things you don't want to know. :)

I'm not going to pursue any further analysis right now... For many reasons. One, my -6 is 17 years old and shows no ill effects. Two, I talked to someone who had to remove the wings from his -6 for a repair, and it soudns like it really wouldn't be that hard to retrofit as per plans: pull the bolts from one wing at a time, separate the wing far enough to gain access, and then install the hardware as specified on the plans. A little more work than putting round-head screws in without removing the wings, but I hate the idea of using round-head screws... I'd rather pull the wings off than add drag. :)

The third reason is, regardless of any analysis I could do, even if I proved beyond any shadow of a doubt that the screws aren't needed, if I ever wanted to sell the plane, *not* having the screws would likely prove detrimental to my resale value.

So the TL;dr is, it's not that hard to retrofit, and sounds like it might be a fun experience (yeah, my definition of "fun" may be suspect... But I did just spend two nights poring over plans and running calculations for "fun" too... :).
 
Very interesting result... I will make sure my screws are in place when I get to that stage. Hopefully by the end of the year.
 
For the OP: I would get in writing from Vans that the screws are required (if that is their determination) and use that letter as a negotiation point with the seller. If the rest of the aircraft passes inspection, I wouldn't let the screws turn you off completely.

On a -6 (conventional gear), it is really not that hard to unbolt the spars and pull the wing out far enough to install the screws. There is also the option of using click-bond nutplates and a screw dimpler to install the screws without removing the wing. It would be a challenge, but not impossible.

You can see the the gap between the fuselage and wing is very narrow on the -6. Not much room to work in. Probably be better to just pull the wing out a foot and support it with a sawhorse.

FP03062012A0003Y.jpg
 
rvmike said:
I don't get it. They are in the plans, Vans said they have to be there. Would you buy a car with only one lug nut on each wheel. As someone else said (what else was left out?) What are you people arguing about??

Time for another beer
Mike
Click to expand...

I think part of the confusion is that different people have received different answers from Vans. Apparently one helpline tech said they are not structual. However, if Ken says they are required, then they are 100% in my book.
 
rvmike said:
I understand the Vans conflicting answers, but they are still on the PLANS. Done with this issue
Click to expand...
Many things are on the plans that aren't structural.

The reason this generates so much discussion is that from an engineering perspective, it just doesn't jive. You've got an .032" lower wing skin, that suddenly joins to an .025" lower fuselage skin. That's a *reduction* in cross-sectional area, when heading down the wing toward the center of the airplane. Usually, you *increase* cross sections heading inboard... Look at the spar for example. So if there is load being carried inboard by that skin, all of a sudden it's getting dumped into a smaller structure, which means a stress concentration. That's an unusual choice, from an engineering perspective.

On the other hand, this overlap looks a *lot* like a wing root fairing. The location is ideal... It's the section that's beyond an arm's reach from the leading edge from the front, and hidden behind the flap from the rear. And it aligns perfectly with a continuous piece of fuselage belly skin. With the belly skin extended a few inches, you have a fairing that only needs to be installed once... Subsequent inspections can be done by removing just the top fairing, on which you can reach all of the screws without laying on your back dropping screws all over yourself. That's an excellent design choice, from an engineering perspective.

Again: Not saying they are or are not structural. But it's not 100% clear why they're needed, when people have reported here and elsewhere that the -6 wing structure was tested to its limits at the factory without them.
 
Exp aircraft

Ok I have to tell the truth I thought I was building an Exp. Aircraft its not the only thing I didnt change I called Vans and ask about the brake line running across the firewall and ask why not use a little extra plastic hose inside and put bulkhead on each side I was told because thats the way its always been done I didnt want my fluid getting hot from exhaust so i have a bulkhead on each side and I think its an improvement.I wanted to build my own F.I. booster pump I posted for advice not one reply but I couldnt not do it so I have a nice booster pump for $250 .Its not in the plans but when I built windshield fairing ran two bids of carbon fiber over roll bar.Not in the plans but that 200hp angle valve looks good up front with its 74" BA.Not in the plans but I moved my stops for aileron to outboard bracket and used #3 bolts as stops. Not even going to talk about about all the speed mods but I love my Exp. RV-6.Good thing Van didnt stop at the 3!
Bob
 
Snowflake said:
The reason this generates so much discussion is that from an engineering perspective, it just doesn't jive. You've got an .032" lower wing skin, that suddenly joins to an .025" lower fuselage skin.
Click to expand...

I am pretty sure that the belly skins on all the 2 seat RV's is .032.
 
This thread has gone off the deep end. No ONE structural component carries all the load. Loads are distributed among many components. If you want to fly your RV-6 without these screws, that's your right.
Just don't call on me to sign off your inspection.
 
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