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Nutplates for Non-Accessible Areas

Nutplates for Non-Accessible Areas

Nutplates for Non-Accessible Areas

(OP)
Hello all,

When I investigated the fighter jets which are F-16, F-35 designed by Lockheed Martin, I saw some details for fastener selections at non-accessible areas.
At F-16 wingbox, elliptical nuts (NAS 1734) are used to attach upper skin to under structures.
At F-35 wingbox, adhesive bonded nutplates are used to attach upper skin to under structures.
At Eurofighter, nutplates attached with blind rivets are used to attach upper skin to under structures.

American fighter jets don't prefer usage of nutplates having rivets so that they reduce the hole numbers on spar and ribs. On the other side, Gripen and Eurofighter uses traditional nutplates having rivets (NAS143, NAS1474 etc).
Also, KF-X fighter jet uses rivetless nutplates for radome attachment. I'm not sure whether they are adhesive bonded nutplates or not. They can be Forcetec rivetless nutplates too.

I have some qestions&concerns about these different solutions. Why do Europen fighter jet designers still use these traditional nutplates? Or do they not trust the these new nutplate designs (FTI rivetless nutplate, Clickbond adhesive bonded nutplate)? Or do they have no idea about these solutions yet? Alright, which nutplates do you prefer for non-accessible areas having fuel inside for composite skin and aluminum spar attachments?

Thanks in advance.




RE: Nutplates for Non-Accessible Areas

Just focusing first on the location you mentioned: the internal (wet) side of a fuel tank, here are some questions to ask when evaluating different fasteners:
  • Potential for fuel leakage through nutplate mounting rivets
  • Potential for fuel leakage around elliptical nutplate in circular hole
  • Potential for pressurized inerting gas to leak around nutplate
  • Potential for fuel to soften/dissolve the adhesive holding the nutplate
    - enough to release the nutplate
    - enough to prevent torquing the fastener
  • Adequate Electrical bonding contact to reduce hot-spots during lightning strike
  • Procedure to replace a nutplate with stripped thread
  • and you will get along better with your boss if you never forget: Cost, lead time and lifetime availability of a specialty fastener

www.sparweb.ca

RE: Nutplates for Non-Accessible Areas

Don't forget the age of the airframe design, the F16 first flew in the 70's, Eurofighter and Gripen in the 80's, F35 in the 00'S, while the KFX hasn't flown yet. Updating the hardware is very unlikely unless there are issues or other drivers like politics or major airframe revision, just because they maybe better isn't a sufficient driver for change.

In addition to sparwebs comments, the stock system (or IPC for those doing modifications to airframes) is a good place to start looking for fastener, everyone is happy when the new fastener already used somewhere else.

RE: Nutplates for Non-Accessible Areas

OEMs stick with techniques that work for them .. and their employees (& repair facilities) are confident in using.
Changing hardware for marginal benefit & later discovering an issue really hurts the OEM with customer confidence, warranty, etc ..
- (from Regional Jet perspective) - at one time bonded nut plates were the fad - until it was realized adhesion was only as good as the primer underneath.
.... and worse yet - non-OEM applied primer failures.
- Blind fasteners (multi-alloy composition) are much more susceptible to corrosion or initiating corrosion to adjacent structure.
- Myself, Being involved with a MRO - replacement "advanced technology" parts are almost non-existent to locate. Salesmen convince the OEMs in purchasing sufficient lots for production. But the Parts Mfr facility has no interest in sales of less than 1000 items outside the OEM (& they do not keep expensive inventory) .. and in a time frame that is practical to the MRO. MROs with tight out date schedules - just go 'ballistic' with the OEM who does not have the part & cannot obtain the item.
- Stick to what is common practice, works & is available. Consider the additional Tooling & training costs to the MRO when replacing "advanced technology" fasteners. Everything will eventually get replaced - why handicap yourself for Marginal benefit.

RE: Nutplates for Non-Accessible Areas

I am not a big fan of glued nut plates, more of the traditional blind Revit onto the skin.

RE: Nutplates for Non-Accessible Areas

F-15 used long strips of gang-channel nutplates for upper wing skin attachments... more efficient than individual nutplates... a little more effort to seal... but fewer attachment rivets... and the holes are 'close-reamed' tolerance to nominal diameter [floating/sealed nut-elements]

With the exception of the adhesive bonded nuts, the 'rivetless nutplates' mentioned all require a larger diameter bolt-hole [area-out] to accommodate the nut-shank that is 'locked-into the hole.

I have NO real working experience with bonded-on nut-fasteners. I am unsure, but the adhesive bonded nuts, probably adapt well to mild contours. Like everything adhesive-bonded, however... cleanliness is next to godliness... when it comes to durable adhesion... and leak resistance.

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Nutplates for Non-Accessible Areas

In my experience whenever the idea of click-bond nutplates has come up, people have expressed concern about the effect of removal/reinstallation on the integrity of the bond. This seems reasonable for a removable panel or access cover. But for a skin panel which may only ever get removed in very exceptional circumstances, there may be a stronger case?
They certainly save drilling/filling a lot of holes.

Gang channels would be my preference though: most of the drilling operations can be taken offline, you still have the benefit of positive retention, and no need for oversized holes. Small weight penalty however, and a few extra parts too.

RE: Nutplates for Non-Accessible Areas

Quote (Ng2020)

But for a skin panel which may only ever get removed in very exceptional circumstances, there may be a stronger case?

Then the stronger case would be for permanent fasteners. Rivets, Hi-lok, visu-lok, and the like.
The general philosophy is that any panel that comes off for maintenance needs removable fasteners with low chance of difficulty installing/reinstalling many times in the life of the aircraft, meanwhile any other panel is designed to not be removable except for the purpose of repair. There are no panels removable "just because it's interesting".

www.sparweb.ca

RE: Nutplates for Non-Accessible Areas

I've seen OEMs do that in production, just saying...

RE: Nutplates for Non-Accessible Areas

(OP)
If gang-channel nutplates are used for upper skin, how will they be repaired without accessing inside wingbox when the thread or nut got damaged? Same problem is available for anchor nuts. As far as I know, we need to remove all upper skin / panel. So the solid / blind rivets can be removed from the inner structure to renew the anchor nut. In my opinion, it's not sense to renew a few anchor nuts / gang channel nut removing nearly 1000 screws on the upper skin / panel. For NAS1734 on F16, you can drop it inside the wingbox and use blind rivet / bolt. If you can't take it from inside of the wingbox by vacuuming, it'll be FOD. In this case, it can be sealed to prevent move.

Here a photo from F-15 wingbox. Yeah there are gang-channel nutplates. They use it for lower skin also. Interesting. For lower skin, no need removable fasteners.

https://media.defense.gov/2020/Dec/09/2002550885/-...



RE: Nutplates for Non-Accessible Areas

Aviator9… Your photos looks like they were taken [recently] on the PDM/overhaul line at WR-ALC. I would NEVER publish an photo like this, since photography in these areas is strictly controlled and mostly prohibited... with specific/limited permission mandatory/written take ANY photograph in production areas for technical record purposes... where I work.

NOTE1. The initial photos You published appeared to be production-line photos of F-16 wing [and other] structure undergoing new Assy... which I had assumed were 'stock photos'. Now I wonder where those photos came from... and how you got them.

BTW the wing leading edge is 'down' in this photo, trailing-edge is 'up'. The dark shaded spars and integrally stiffened-skin [lower INBD, under the ribs] are bare titanium. The area You emphasize [lower photo] is a dry-bay, as I recall... but I do not recall the lower surfaces installed with flush-bolts into GSNs... I recall them being installed with HLs or solid rivets. That appears to be 'different'. Hmmmm NOT what I would recommend.

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Nutplates for Non-Accessible Areas

(OP)
All photos are from open sources. Here some links to access those photos or screenshots taken from video.

F-16 Detail Parts and Assy:
https://aero-com-tech.com/products/

F-35 Assembly Line Video:
https://www.youtube.com/watch?v=bXuZbLxWxYE&ab...

Eurofighter Assembly Line Documentary:
https://www.youtube.com/watch?v=rqOX2Zj3LJo&ab...

You can find to the photos of the inner structures of the fighter jets on google easily.
Thanks for the answer, Taylor.

RE: Nutplates for Non-Accessible Areas

I had no idea that airframes were perforated like that... a bunch of holes held together by a little metal. Impressive.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Nutplates for Non-Accessible Areas

Those photos are linked from DOD URLs.

We're all probably on a list now.

RE: Nutplates for Non-Accessible Areas

"I had no idea that airframes were perforated like that" ... yes, flanged lightening holes are very effective for sheet metal webs.

Unfortunately the machining equivalent, where we typically add a thickness around the hole, are often called "heavying holes" !

"Those photos are linked from DOD URLs." ... yes, the later ones are, but the earlier ones (including European a/c) ??

"We're all probably on a list now." ... I'm not ... oh, damn

another day in paradise, or is paradise one day closer ?

RE: Nutplates for Non-Accessible Areas

RB...

I have yet to be convinced about the origins of the F-15 Wing structure photos... under-going Depot maintenance at what looks like WR-ALC. Av9 did NOT identify the origins for those photos.

NOTE.
John Thorp sometimes referred to open-holes and opened/flanged-holes in sheet metal ribs, spars, frames, etc as 'lightening holes'... but often reminded-us [actually, warned us]... they were in-fact 'weakening holes'... and had to be carefully located/sized to ensure the structure was adequately 'strong/stiff-enough'... and worth cost/effort to take-out that little-bit of weight.

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Nutplates for Non-Accessible Areas

Quote (WKTaylor)

I have yet to be convinced about the origins of the F-15 Wing structure photos... under-going Depot maintenance at what looks like WR-ALC. Av9 did NOT identify the origins for those photos.

The link in his last photo appears to be the source... from 'media.defense.gov/XXXXXXXXXXXXXX'

I'm not convinced that's locate-able by a google search.

RE: Nutplates for Non-Accessible Areas

Just another list to add to my collection... What, me worry? with aplogies to Alfred...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Nutplates for Non-Accessible Areas

ah, a Mr Newman reference !

another day in paradise, or is paradise one day closer ?

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