Aircraft Shoring/Cribbing Design 2

[WKTaylor]

OK Guys... I need help needed desperately (this seems to be an industry “secret”) [for a military acft, field fabrication]

I need references/data for design/fabrication of acft [fuselage wing and stabilizer] shoring/cribbing fixtures for structural repairs.

Anyone with design/fabrication experience???

Areas needing data/references.

Shoring/cribbing design
Contour tolerances
Materials [plywood, lumber specs]
Fastening/joining with bolts/nails
Padding materials/thickness [neoprene, felt, etc]
Wood tooling fabrication [including fastening/gluing]
Steel detail fabrication/welding


Regards, Wil Taylor

 

[737eng]

Large Transport Aircraft that are provided with SRM's, have this data in SRM Chapter 51. I believe that it might be in the recovery manual as well. The SRM provides guidelines and some requirements but as far as actual details for wood joining,etc.....that is basically up to the operator. If you don't have access to a SRM, let me know and I will try and get you a sample. I have no clue as far as military goes, however, while the safety of the aircraft and the crew working the aircraft is a very important issue for shoring, the most important issue is not causing stress (or deformation) in the aircraft structure which may cause problems later (i.e. if you shore the aircraft at a certain body station and overload that frame and surrounding structure due to the weight of the aircraft, etc....) Your above post is not asking for shoring locations, so I assume that you have the correct shoring locations provided by the OEM and this will not be a concern.

 

[WKTaylor]

737eng...

Yeah... I know about the SRM chapters and am familiar with them. However... someone engineered the shoring/cribbing shown. What I need is a peek at the engineering [civil or construction] for this process.

I have to re-engineer an existing shoring design: everyone I've talked with just shrugs... and they just say "copy-the-old-design-with-your-improvements".

My question is more basic: what are basic design practices and pronciples of aircraft shoring/cribbing... and what are "good" detail- design practices.

The "better SRMs" have fairly basic design details and seem sort-of incomplete to me... IE, they lack: definition as to WHY the shoring is located/sized as-it-is(!?!); critical support dimension callouts (?); wood-type/grade callouts (plywood, finished lumber, etc]; glue-types; felt or rubber type/thickness; exact bolt/nut/washer definition; plate and angle-iron [steel?] material callouts; steel-weld process details; finishes for long-term protection; etc.

Regards, Wil Taylor

 

[SuperStress]

Wil,

I've been involved in a couple of shoring projects for large transport aircraft and I feel your pain. There isn't much in the way of reference material for the engineer.

A lot of the design is done by using "best shop practices". We tell them where to support the aircraft, and the shop just figures it out. Depending on the kind of work you are doing (how much you can allow things to shift during maintenance) your shoring might be as tight as every one or two frames (or ribs), or as loose as every 5 or 6.

One big thing to watch out for is overhanging (or sagging) moments. Make sure that your aft-most (or outboard-most) shorings are sufficiently far out to prevent a large moment that will cause structure to "spring" when it is disassembled, making reassembly nearly impossible. One SRM shoring procedure I reviewed would have resulted in a 2,000,000 in-lb. moment from the empennage since the aft-most shoring was forward of the CG of the horizontal and vertical tails.

Here are some of the things we've done in the past:

Contour tolerances aren't that big of a deal; that's why you use some neoprene rubber or dense closed-cell foam to redistribute load over the hot spots. I think we generally used the neoprene in 1/2" or 3/4" thickness, mostly because we couldn't use the foam anymore after our "hot wire" cutting was disallowed due to toxic off-gassing.

Commercial-grade 3/4" plywood is most often used, and doubled-up as necessary. A thinner, 1/8" or 1/4" piece may be contoured over and around the 3/4" in order to create a base to attach the rubber.

Fuselage forms are generally placed under the frames, and wing forms are placed under the ribs. Try to keep the width of the forms equal to the width of the frames/ribs since you don't want to put big out-of-plane loads on the surrounding skin. Make sure the structure can handle whatever load you apply - basic weight and balance information is usually sufficient to approximate your beam over multiple simple-supports.

For welded structures, my favorite reference is "Design of Welded Structures" by Omer Blodgett. It's a GREAT book. You can find it on Amazon.com or other resellers.

http://www.amazon.com/gp/product/9998474922/ref=nosim/002-2960692-2289638?n=283155

Steel plate is generally A36 steel, and tubing is generally A500. These are both very mild steel and very easily weldable. (Also cheap and available.)

I have some limited design information on allowables in wood structures, if you can figure out a way for me to get it to you. It's in .pdf format. Don't post an email though - the Eng-Tips gods frown upon that.

Hope that's helpful.

SuperStress

 

[berkshire]

Most of my dealings are with part 23 aircraft, which are a little simpler to retreive than part 25 aircraft.
However if you are looking for design allowables in commercial timber, the Forest Products Lab., Wood hand book will give you useful information. it can be downloaded at

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm

It is a rather large document and you may only need sections of it.
B.E.