Composite Reinforcement, beyond 43.13

[JQCF1]

Hi All,

This is not a repair but an access hole to be put in belly shell of a vintage Bell 206 helo. Say 4" x 6" for discussion purposes with radiused corners in flat honey comb floor and say 1" thick with .025 al facings on H'comb panel.
Current thoughts are to reinforce around opening with 1.75 wide,.040 2024-T3 top and bottom, apply doubler with CherryLocks and fill voids with Hysol/Loctite EA 934 epoxy. We will also use Delron plug/sleeve inserts to get 10-32 thds around hole for closure plate. Any thoughts on doubler thickness and analysis methods if required? Gut feel says this scheme is OK but the Feds may require more detail. 43.13 no help since this is not a repair of damaged h'comb.

Thanks for your comments. John Cragin

 

[Sparweb]

I think you need a DER for this kind of mod. You need one who is familiar with that type and can deal with the honeycomb in a reasonable manner.
Just about Everything in that helicopter is attached to the belly panel, so you have to provide a stress analysis (or test) that shows that your mod doesn't affect hard landing and crash conditions.
Where's the hole? I hope for your sake you've picked the passenger foot-well, because anywhere else is "busy" doing something else. Not too close to the cargo hook, though, I hope. Most attachments to the panel apply loads out-of-plane, causing local bending loads and core shear or compression. Other cases, like hard landings, imply bending of the entire fuselage. The way the 206's are designed, that may not be as bad as it sounds - I haven't done the math, but there's a lot of "straight-line-of-action" members between the engine and the landing gear.

Again, you really need a DER familiar with 206's to look over the project to tell you what should be done, by figuring out where the critical stress cases are. Hard to do over the 'net.

"Restricted Category" may also be an option to bear in mind, but that's a hit against the value of the helicopter for the rest of its life.

Good luck.


Steven Fahey, CET

 

[JQCF1]

To SparWeb..Thanks for your comments and suggestions.
This access hole is enlarging a 10 year old existing 3" dia access hole fwd and outbd of R/H aft cargo hook and is clear of the prohibited repair areas in the SRM. It is in the rh flat floor well. I have for ref a Bell dwg showing a 8"x 12" oval camera port in the same area.
BTW I am a structural DER but do not pretend to know it all. Never ran into this situation before. Any analysis suggestions without resorting to FEA? Thanks JQC

 

[Stache]

I ran into a similar access hole a couple years ago on a Bell-206. Bell has a repair for this, but it didn't come cheap and requires some well trained mechanics with the proper equipment and tooliling to install the doublers. As a mechanic I would recommend a good tap test of the area or eddy current inspection as many will be debonded in and around the hole area. Have the mechanic mark any debonded areas so you know what you really have to work with.

Stache

 

[thruthefence]

You say this is an enlargement of an existing access hole? Was this existing hole you speak of hole part of the original TC, or was it added later? (an alteration) If it was an alteration, is there any reference to approved data for that alteration? There should be a paper trail of some sort. An "alteration" always requires a 337 , as opposed to a "repair" where a CRS workorder document will serve.( this remains as part of the aircraft's permanent maintenance record ) If you can find reference in the logbooks, maybe you could get a clue where to go next. Is this a sure-enough "N" numbered aircraft, or is it a "public" aircraft? I ask because there are some "gray areas" when aircraft are operated by a government agency. While most operations adhere to the FAR's as a general rule, I think they have a lot of lattitude. Correct me, anyone?
regards, Steve

 

[rb1957]

enlarging a 3" dia hole (< 10in2) to a 4"x6" (24in2) hole needs more (IMHO) than a 0.04" doubler.

I assume that this is unpressurised (i'm not that familiar with the 206).

a 0.04" doubler riveted onto the skin really doesn't provide much reinforcement; and btw, why reinforce only one side of the original panel ?

i'd like to see an edge frame, which is probably a complicated piece to machine or assemble, with a fairly robust connection in the corners.

good luck

 

[Sparweb]

Going back to basics: You can either approach the problem with "What loads are applied and hence find the stresses in the structure" or, alternatively:
"How do I restore equivalent strength and stiffness of this material"? I doubt that a loads analysis will lead anywhere (now that I've had some time to think about it). So you're probably going with #2.


"Any analysis suggestions without resorting to FEA"

Try hand analysis techniques first to get a feel for what the materials can withstand.
Sandwich panel analysis methods can be found in a variety of published data from honeycomb panel manufacturers, like Teklam and Gill. The panel you're working on is almost certianly a honeycomb cell core (don't know the density), bonded to the facings with (probably) Hysol. There's nothing fancy about it, although in some of their helis Bell has multiple facing laminations that build up at high stress areas (hence reducing core depth), plus inserts, plates, and cavities potted in at various locations to pick up on the attached structures.


Steven Fahey, CET

 

[JQCF1]

Hi All, Just got back to read all of your astute comments and suggestions...very helpful. I'll see if I can add more details of what we did on this helo. First I got the CD from OKC on this and a sister a/c operated by the same group. Both a/c have had a variety of round, rectangular access holes added over the years. You won't believe the history on the OKC CD!! In and out of Experimental and Restricted ops over the 25 years but never as a "public" a/c. These mods were all done via the 337 route signed by an IA, most cases no FSDO involvement with no stamp/signature in Block 3 of relevant 337. Based on the Bell dwg for a camera port in the same foot-well location that is twice the area of this hole, the ACO agreed with my approach. Especially since we are well away from the prohibited areas on this belly shell. In addition to the doublers (which ended up thicker and wider) we also added fore/aft angle stiffeners on the outside belly doubler edges to restore the stiffness of the cut-a-way aluminum h'comb. Thanks again for your collective input and BTW I'm also a card carrying DAR so I have my feet in both camps...JQC

 

[Sparweb]

"BTW I'm also a card carrying DAR so I have my feet in both camps...JQC
That just means you're crazy! :^)


Steven Fahey, CET

 

[JQCF1]

Steven, I've always accepted your comments as well considered and presented. Now I'm beginnng to question the veracity of some of your contributions!! Believe me I have trouble explaining the difference between a DER and a DAR to many people, in spite of the central word in each acronym. A CET is Canadian what??? Thanks JQC :-D

 

[Sparweb]

Sorry, didn't mean to give the false impression that I'm a DAR (or a DER). I work for one. I'm a Certified Engineering Technologist, and I wear the "CET" acronym on this forum as a subtle way of raising awareness among other engineers that there is another group of "engineers" who are "professionals", but are not allowed to wear the "professional engineer" title.
It's a Canadian accreditation - there's no equivalent in the US, as far as I know.

Steven Fahey, CET

 

[JQCF1]

Steven, Don't get the wrong idea...I was complimenting you on your contributions to this forum. I'm an Aero PE in Massachusetts (Only) and when I spent a couple of years in the UK I tried thru I Mech E to get a British "Chartered Engineer" label but the Brits wouldn't hear of it!! I guess it will always be us and them!! Do the Canadians use the PE label or Chartered label?
Thanks JQC

 

[Sparweb]

Funny, now that I re-read your previous post, I see the irony in your tone. I didn't the first time. It's a hard lesson to learn when reading forum comments.

Canadian engineers use "PE", when they qualify and have been given accreditiation by the local engineering professional organization. This is roughly equivalent to the american system, and most companies in Canada hiring PE's don't care if you got in the US or Canada.

It's when you try to run your own engineering business that "us vs. them" applies. To practice in Alberta, you must be registered with Alberta's PE organization.

Steven Fahey, CET

 

[JQCF1]

Steven, Thanks for the explanation of the Cdn system as regards PE. Many times I'll write a "To whom it may concern" letter discussing some a/c problem and sign the letter as a PE since I'm forbidden to sign anything but the FAA Form 8110-3 as DER. Clients accept that approach and the FAA seems to as well.