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To DAR Community - I may have fouled up BIG TIME - Help!

Sam Staton

Well Known Member
A bit of background. I decided to use the area forward of the support bulkhead (which is forward of the instrument panel) as an avionics bay. All well and good so far. In deciding how to provide access to the equipment there, I decided to make the entire panel removable instead of cutting access holes in the panel. What I did was to replace every rivet in the longerons and support bulkhead with nutplates and 6-32 screws.:eek: Some of my friends at the airpark have shared with me that I may have compromised the integrity of the longerons with the additional holes for the rivets holding the nutplates. The images are attached below. I will, of course, address this with the mothership, but until I can contact them, I would like to hear from the DAR community what their opinions are. Have I rendered my aircraft an EXTREMELY expensive paperweight? If it is recoverable, any and all recommendations will be gratefully received.
 

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This is definitely a case where the only safe opinion can come from the Mother Ship. There are dozens of Monday morning engineers on this forum who will pontificate and beat their chest before giving you an answer which is worth just what you paid. Talk to Vans.
 
Will be interesting to see what the mothership says - especially since this is exactly how Rockets are done.
 
I'm very curious to know what Van's would say about this. I'm just a pilot, not an engineer by any stretch.

BUT, for what it's worth, this looks very similar to how the RV-4 does it.... Screws might be spaced out a bit further though (less holes drilled).

Edit: Looks like Michael was typing the same thing as I was thinking of it.
 
I've seen many RV's done this way, including my own RV-6. I wouldn't have a problem with it as long as the edge distance is ok on the holes. It looks OK from the pictures.

Vic
 
As others have already said, the only way to know for sure is to check with Van’s.

In the meantime (and to allow you to sleep between now and Monday), I’ll just offer that my RV-3 has a similar configuration as did my Midget Mustang.
 
Most load shear/strain is carried in the edges of a bar or plate and less in the middle. It’s why lightening holes have low impact. Think of it here as lots of little lightening holes. I bet u will be fine. But please post Van’s reply.
 
Thanks for the good words and advice!

I am very encouraged to hear about the ways other aircraft are done. The biggest concern is the ‘zipper’ effect. I’ll post what I hear from Van’s. I have a possible solution in mind, if it works out. Some of the rivet holes are very close to each other.
 
Mothership

I called Vans today, and the tech said this is beyond regular tech support. He is referring it to the engineers. He said they’re pretty busy, so he didn’t know how long it would take until I hear something, but that either he or the engineer will get back to me.
 
Edge clearance hole-to-hole would be the only concern. The stress concentration around holes actually goes down when you have a whole row of holes, and the Longeron would have been sized for the net area around the largest holes.

Since the #6 screws are bigger in diameter than any rivet that would have been used, that MIGHT be an issue, but there is probably some place along that longeron where there is a AN-3 bolt through it? If the remaining material around a AN-3 is sufficient, then it obviously is for the #6 screws.
 
Worst case is they make you put some reinforcement in but I bet you a dozen donuts (brave man) they won’t. You don’t have a paperweight. It is actually a very small reduction in cross section going from a number 40 hole to .156 or whatever #6 is and even less if it was a #30 hole. The fact that there are a lot of them vs a few makes no difference. It is the minimum cross sectional area. Also keep in mind that a designer will dtermine the load for such a member, then calculate the required cross sectional area, then since there is never a standard size that matches they will use the next larger size. So there is likely some margin. But don’t use that as an excuse to get out the hole saw!

And I am an aero engineer so I am only partly guessing.

Keep calm and rivet on man! ;)
 
When I was at that stage on my 6 (2010) I looked at an RV-4 that a friend owned. That's how that fuselage skin was attached to his plane. I asked if it was a mod and he said that that is the way the plans showed it. I called Van's and asked if I could do that with my 6 and was told yes. So I did. Makes it easy to access the back of the panel.

Dave
RV-6
230 hours
 
What you are referring to in your concern is known as "tear out". The same effect that allows you to tear a bank check out of it's check book. The perforations on the paper make a stressed area in a line that allows the clean tear out. A similar condition could exist on an aircraft skin if a designer or builder puts a row of fasteners too close together. The fastener minimum pitch value to mitigate concerns over tear-out failure depend on the hole diameter, stack up and material thickness. But it is usually very extreme, like the holes are almost look like they are touching. Take a look at a Luscombe horizontal stabilizer closely if you want to see some very close rivets that have performed well for decades. So I doubt the addition of the nut plate attach holes is a problem.

The nut plate fasteners are "hole filling" fasteners and when bucked expand in the holes providing beneficial induced hoop stress on the holes. Thus the riveted nut plate holes are not as weak or stressed as an actual open hole. Additionally each nut plate itself is a kind of small steel doubler as it is attached to the structure by the center fastener and opposite nut plate rivet.

So I would be surprised if there was a real problem here. As mentioned in previous posts this is how the RV-3 and RV-4 along with other makes and models have their boot cowl attached.

I too an an aero engineer but the above is only an educated guess eyeballing things. The final say will come from Vans since they have the stress analysis and loads from the initial design they can use to analyze your particular configuration if necessary. It is common for the aircraft manufacturers to provide engineering assistance to customers in the field by approving a proposed repair or helping to design an appropriate one for a damaged or modified area. And while Vans is in reality a component supplier to support a set of drawings and not a big aircraft manufacturer they know their stuff.

Jim
 
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On my own

I heard back from Van's today, and was advised that this is a user modification that is beyond what they would be willing to advise me on. So, I am on my own here.:eek:
 
To the people who say they've done "exactly this", how "exactly" are you talking about?

Sam said he replaced *every* rivet with a nutplate and screw. Every RV-3, RV-4, and Rocket I've seen with screw-installed forward panels, the spacing has been about where every *second* rivet (or so) would be. So there would be less swiss-cheeseing of the longeron and skin on the fuselage. There would be room to align all of the nutplates with the longeron so all the holes were in-line and closer to the neutral axis of the flange.

As an aeronautical engineer, I will say that the photos made me cringe, and I was hoping Van would give you better news. I suspect they were finding a polite way to say that the photos made them cringe too. :(
 
I heard back from Van's today, and was advised that this is a user modification that is beyond what they would be willing to advise me on. So, I am on my own here.:eek:

I'm curious about this. In the latest Sports Aviation Magazine that came out this month, there is an article that speaks about "departures from the factory plans," saying that a DAR might not allow such a thing without buy-off from the factory. So, is the DAR responsible to determine whether a departure from the plans is airworthy or not? Is the experimental builder allowed to "experiment" without stamped, signed, engineering calculations and/or factory blessings?
 
I'm curious about this. In the latest Sports Aviation Magazine that came out this month, there is an article that speaks about "departures from the factory plans," saying that a DAR might not allow such a thing without buy-off from the factory. So, is the DAR responsible to determine whether a departure from the plans is airworthy or not? Is the experimental builder allowed to "experiment" without stamped, signed, engineering calculations and/or factory blessings?

A DAR may deny airworthiness to an aircraft if he/she determines that a modification may deem the aircraft not "In a Condition for Safe Operation".

Typically this will not happen unless he/she is very uncomfortable with the mod. Our goal is to get you flying, but not at the expense of safety.
 
This may be anecdotal, but many moons ago when I was being trained to work on Uncle Sam's aircraft, part of our final exam in A school was the instructor would place a piece of a longeron in a vice, take a hack saw and cut it into two pieces, then we built up a doubler. I have seen USMC aircraft with longerons that were completely sliced in two repaired with doublers.

I also saw at least one RV that had seriously misplaced and additional holes in the longerons near the attach point for the horizontal stab, and Van's advice was add a doubler about a foot fore and aft of the place in the longeron with two many holes to carry the load.

I think you have options here.
 
was, as you Sam, awaiting Van's answer with anticipation and curiosity... not that I'm directly concerned.

Disappointment is what you must feel :(
I'm disappointed that none of those titled aero engineers, Van's and some of the posters here, can't give you a definitive answer. Especially since, as already mentioned a few times, the -3 and the -4 use exactly this principle.

I'm no engineer for sure, but is something like this sooo difficult to verify and stress analyse?
 
I would go to an aircraft structural maintenance guy, these are the guys who are qualified to sign off repairs to primary structure, and have them devise a repair for you. They don't do a lot of calcs, they just use established practices from AC43.13. He might not want to sign anything, because it is a homebuilt, but if you find the right guy, he will show you a beefup you can do and you can show the DAR that you have addressed the issue.

But the first thing i would do is calculate the cross sectional area of that member at the location where there are the largest holes called for in the plans, then calculate it with at a location with one of the holes you drilled and and see what the difference in cross sectional area is. It might be that you have not reduced the minimum cross sectional area at all so maybe you can convince the DAR to let it go.

I think the critical design load in that member is tension, when you do a max G pull up. The down force on the engine/prop will be something around 1500 lbs at 6G, assuming a 250 lb engine (Wt numbers pulled from a hat) so the engine mount would pull down like crazy on that longeron. So the stress on that member is the tension load divided by the area. This is oversimplified, because the member is riveted to some sturdy skin, but you get the idea. IF you have swiss cheezed that member too much then it won't be as strong. But increasing the hole size by a bit might in certain locations not make much difference. On my RV4 I have a row of #8 holes along there that hold on the top cover, by design. And I have an O320, same engine as you probably so same loads. And there are some -3 AN bolts in the top of that angle where the longerons attach to the flange where the engine mount loads go through, so there is quite a bit of area reduction there.

Frankly I don't see this as a problem. The issue is convincing a DAR of that. DARs are generally A&Ps, not engineers and they won't want to stick their neck out, which is understandable.

Homebuilding is all about solving 100 problems at a time, as you know. This is another one, but there is a way forward.
 
I heard back from Van's today, and was advised that this is a user modification that is beyond what they would be willing to advise me on. So, I am on my own here.:eek:

I've seen many RV's done this way, including my own RV-6. I wouldn't have a problem with it as long as the edge distance is ok on the holes. It looks OK from the pictures. Vic

Sam, you’re going to be fine. Vic’s opinion alone is enough for me. Your plane is not going to suddenly “fall out of the sky”. If you would have screwed up the main spar or the horizontal spar, that would be different. I could see a scenario where you develop cracks sooner in that area than maybe others and therefore need to pay more attention during your annuals. But beyond that...Folks on this site tend to error on the extreme side of safety which is not a bad thing, but sometimes it is unwarranted.
 
Especially since, as already mentioned a few times, the -3 and the -4 use exactly this principle.

I agree they have a similar design but it is definitely not exactly the same.

The corner firewall bracket side flange is closely spaced 1/8 rivets except for one that gets replaced with a screw and nutplate when the top skin is fitted.
The top flange of the firewall bracket uses AN3 bolts.
The rest of the longeron, aft of the firewall bracket and common to the front top skin, has 1/8" rivets filling in the gaps between the screws that are on 2 1/2" spacing.
 

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I'm disappointed that none of those titled aero engineers, Van's and some of the posters here, can't give you a definitive answer. Especially since, as already mentioned a few times, the -3 and the -4 use exactly this principle.
As shown by Scott above, it's not "exactly the same." That's why I asked earlier what the exact configuration is on the -3, -4, and -6 that people reported were built "exactly the same." I would venture that none of them were built with *every* rivet hole replaced with a screw.

As engineers, we can't make a definitive statement on this, without exact details to analyze. Van would be the best source for this, as they have access to the original design calculations that would have been done with the entirely riveted structure. But even with that info, setting up the analysis is a lot of work... Time and money to help someone who went off-script.

I'm no engineer for sure, but is something like this sooo difficult to verify and stress analyse?
Difficult, not really for someone doing aircraft structure analysis regularly. Time consuming? Yes.
 
Solution found.

Gentlemen -
First, I thank everyone who has posted here. Many well-thought out suggestions and thoughts. In discussions with a DAR, whose knowledge I trust, and others, we have decided that I will make another panel and extend the sides down far enough to place another row of fasteners with an appropriate backing plate. Thanks again to all who contributed to the solution of my dilemma!:)
 
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