Poking holes

[RoarkS]

So I've gotten pretty good at poking holes in planes. I have a few boilerplate reports I've gotten approved over the years and they are good to go.

So I got a call today and I have to poke a hole next to another hole.

I looked at the existing hole, counted rivets, measured pitch... so on... first hole is perfect. They did it right.

Now here's why I got called. Someone already got a hold of the actual aircraft manufacturer FEA model... they "poked" out the original hole and my proposed new hole.

think of your typical FEA definition of sheetmetal skin (CQUAD4), stringers (CBAR), and frames (Mixed as needed).

A bunch of adjacent "panels" failed. So we were told we needed a doubler that is absolutely massive covering both holes... like no big.

I reprogrammed what was given to me just to see and the existing hole fails in the same manner without the new hole. Yet there it was... on every aircraft on the floor.

Here I am at home and the thing that kills me... I see no evidence in the FEA of any sort of doubler around the existing hole or the new hole.

Am I crazy... of course it's going to fail. That's why we put doublers in... they didn't model doubler rings.

So the question... Am I the idiot or did a decent size engineering house just rip my customer off???

 

[rb1957]

I would ask to see the OEM analysis ... just 'cause an OEM did it, don't make it right ! (heresy)

There could be something happening that we don't expect, but I agree with you and your instinct ... adding a hole isn't going to break the plane.

But now "someone" has asked them their opinion, well that limits your options. Maybe if you can see the analysis and see something, you can talk them away from the edge. But I doubt it ... there's little up side for them.

If you can't get them to change their mind, I think you'll have little success going to the FAA (maybe a local "tame" DER might help). It's a hard seell ... "look the OEM is being overly conservative, you can see it here; and a less conservative analysis shows it good", and I'd expect the FAA to say "but the OEM says it's bad".

If you can't get them to change their mind, I'd move the antenna to somewhere else, explain to the customer his options are ... a big a$$ dblr, and a lot of work; or a small "sensible" dblr in a less than optimal location.

This is a static problem ? Could you improve the skin allowable by interpitching stringers and/or frames/formers ? yes, way more work than you'd expect, but way less than a large dblr. If this is a DT problem, which amendment level did the OEM use ? We in the mod world don't have to go above amdt 96.

Is it composite skin ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[SWComposites]

Am I the idiot or did a decent size engineering house just rip my customer off??? My $$ are on the latter. You would not believe the level and amount of FEA abuse and insanity that I have seen, even at large companies. And don't even get me started on the outsourced to "low cost engineering sites" stuff.

 

[SWComposites]

Is it composite skin ? dear god I hope not.

 

[LiftDivergence]

Someone already got a hold of the actual aircraft manufacturer FEA model... they "poked" out the original hole and my proposed new hole.

If this is an OEM hole on every production A/C, then why did whoever got hold of the OEM FEM has to make the hole in the model?

Likely the OEM doesn't put that level of fidelity in their model. They are probably using it as a loads model, and using some other analysis technique to substantiate the cutouts.

So if there is no OEM doubler there must be other internal substantiation you aren't privy to.

Sounds like whoever grabbed this OEM FEM is trying to use it outside the context it was developed for. When you say adjacent panels are failing, what do you mean? Peak point stress from the FEM? Joint shear flow? Net section yield?

If it's flying around like that, it must structurally acceptable (or at least, it's been approved). You should be able to substantiate your new installation by hand.

That being said... I'd need to see the details to know if what they are asking for is reasonable. It's very common to install these type of doublers, even large ones, when you have multiple details in proximity.

It can be challenging to substantiate net area losses when you have to compare back to OEM strength.

Keep em' Flying
//Fight Corrosion!

 

[RoarkS]

I have the OEM analysis and what this (I just looked up the author... looks like textbook low cost outsourced) it's based on removing skin elements bounded by frames and stringers.

I should have said... the OEM put the first hole in... It matches up with AC43.13 and Niu beautifully. @Liftdivergence I love what you said there... It's a beautifully executed hole that as far as I know should fully restore the existing strength. You're right the OEM data doesn't even cover that it exists. I don't see why one of my old school hand reports wouldn't cover it... especially if I just copy the OEM hole on the other side. We wanted it on the same side for... dumb reasons but I'm able to throw that out if it comes to it.

RB1957 That's exactly what I said... were you in the room lol... now this Cat is out of the bag... It's a whole STC program so finding a friendly DER is gonna be a tough sell to the ODA. Looked up the author just now hate to say it might not be hard to change their minds.

It failed on a gust and related maneuver load. Wish it was an antenna...Bigger hole than that. What makes the big doubler is their engineer wants to span to the next frame because one of the stringers fails up to the nearest frame. I would like to say I thought about throwing in a former but that seemed more invasive. Lots of stuff to move inside.

Nope good old AL2024.

Re Damage tolerance. I don't see any mention of DT or amendment used in anything from OEM or this engineer. That's admittedly a step above what I normally operate at. I'm definitely stepping up to this problem... but it feels like it's too easy ya know?

 

[rb1957]

oh, sweet merciful ... I missed the detail that "someone" else did this with the OEM model.

And they modelled a "small" hole by removing a skin panel. well that is about as "sensible" as ... I don't know what !

Have the confidence in your own analysis, that has been acceptable in past (though that is no guarantee that it is acceptable now or in the future).
If your customer wants to go ahead with the large repair, say "ok, but you do this on your own". I want no part of it.
It's a shame you can't find out who undercut you.

The real shame may be that future projects may look to this solution as being "appropriate".

Is it a composite skin ? pressurised I assume ??

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[RoarkS]

I'm digging through the OEM FEM result trying to make heads and tails of it as I write this. I haven't seen the back end of a model like this since university.

Trust me I have confidence, and my customer has good intuition to put brakes on this. I'm shocked its comes down to me to call BS.

Nope All Aluminum. There is a composite faring near by we were thinking about messing with, but I said hard no. I don't play with composites. In the tail, behind the pressure bulkhead.

 

[rb1957]

unpressurised Al ... not much more innocuous than that !

it is good that your customer trusts you, but beware you may not have heard the last of this. This guy could go behind your back to the FAA.

Do you happen to have OEM detail stresses in this area ? or "equivalent strength" argument (which should be easy to make) ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[WKTaylor]

Was this structure designed for 'safe-life' or to-be 'fail-safe' and/or was DADTA/fatigue spectrum actually performed.

AND/OR...

Was the structure static tested 'to-ultimate +/-G'... AND was it also fatigue tested to XXXXX-hrs [x2] or [x3]?



Regards, Wil Taylor
o Trust - But Verify!
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", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[rb1957]

I suspect the OEM is Part 23, probably safelife ... possibly without a full scale fatigue test (a GA airplane I suspect).

But however it was done, this small change shouldn't require a new evaluation (of anything more than the mod, and then analysis only).

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.