HELP!!! Clip-Angle Connection Strength Calculation 3

[jeffhmc]

Hi, I'm involved with an analysis project on a rack for an aircraft for my internship this summer. While I've completed most of the other parts of the dynamic analysis, I'm unfamiliar with this particular aspect of the analysis so I appologize in advance if any of these questions seem stupid.

I need to determine if the hollow aluminum channel material around a bolted connection would yield under loading. The bolt itself is inserted into a spacer, which is welded into square aluminum channels of which the rack is composed of. The rack has a base of 19in x 33in and a height of 55 in. It is basically supported by four aluminum channels used as columns and crossbars in between.
The documentation I fould most applicable to this situation are clip-angle connection calculations for top-and-seat connections with T-stubs, but the rack I'm analyzing has the bolts attaching the rack to the plane at the four bottom corners of the rack.

How would I go about analyzing this? Could you direct me to any websites or references? Thanks in advance! Help a budding young engineer out!

~Jeff

 

[UcfSE]

Internship for the summer sounds like you are a student. Is that the case?

Either way, if you are interning you should be there to learn how to answer these questions for yourself or to communicate with the engineers with whom you work for assistance.

 

[CoryPad]

Bolted joints are not taught in very good detail at universities.

As far as Internet sites, you should look at these:

http://www.boltscience.com

http://www.bossard.com/techinfo/en/index.html

The best information sources are books - look here: faq725-600

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Heckler]

Jeff,

I'm assuming you have resolved the forces at these joints taking into consideration the items that will populate the rack. Is this a commerical plane or a military plane that is governed under FAA rules. If so, you will need to research FAR 25 Airworthiness Standard and subsection FAR 25.562 Emergency Landing Dynamics. Your analysis will have to follow these guidlines if you expect a FAA DER to sign his/her name to this design. I've done a similiar analysis package that had to be signed off by a DER.

http://www.flightsimaviation.com/data/FARS/part_25.html

Best of Luck,

Vince

 

[jeffhmc]

Thanks Vince and Cory,

To answer your question Vince, the rack is to be used in a Coast Guard aircraft- so that should be Military not FAA. And yes, all the static and dynamic analysis for the rack and it's payload have been done. It's just that the rack was overweight (2x actually) and the walls of the aluminum channels used were reduced to 0.09" to save weight (not my idea!) so there's concern that these channels might not be strong enough around the bolt connections.

Cory- it's true that the info on bolt connections is very disorganized. I've checked out boltscience.com before and not sure they have quite what I was looking for. I am not a structural engineer in the traditional sense but more of a structural systems engineer so I'm learning some of this on the fly so I really appreciate all your suggestions. Thanks again.

~Jeff

 

[Heckler]

Jeff,

Just so you know some military planes fall under FAA control....Air Force 1, 2, & 3 and some King 200Bs the Air Force fly for unmarked reconn. So is this plane a C130? In you case I would be more concerned with the colum strength of the rack itself verses the mounting points. But without seeing the bolt connection/channel interface it's hard to predict.

-v

 

[Sparweb]

I have the strangest feeling that I've seen this rack myself before. A few years ago I handled racks in a Twin Otter that match your general description. If there is also a hole in the floor of the airplane in question, then I've a good guess at what you're doing.

This may be a disappointment, but I resorted to load testing the racks and placing upper bounds on the weight and height of the gear mounted in the rack.

Depending on the aircraft, the forward "g" factor will be 9, or perhaps higher for a very new aircraft. Assuming it's 9, then you could arrange to get 2000 pounds of lead shot in bags, mount the rack sideways on a welding table (or some other sturdy support). Put 1000 pounds of lead shot on the table and 1000 on the rack, which will give you a 110 pounds maximum weight for the rack and installed equipment. The rack is verrry tall (55"), so be sure to center the lead where the rack's CG will be.

Bear in mind that the locations to which you mount your rack must be able to support the reaction loads.



Steven Fahey, CET

 

[jeffhmc]

Hi Steven,

Thanks a lot for your suggestion. I'm beginning to think it is the same or similar rack as well. The rack is 55 or 54" tall and the foward (largest) load case is 9x. However, we are trying to do this analysis completely on paper and a simple finite element analysis (using SAP2000). I think they only want to manufacture this thing once so there won't be any available for testing. I was trying to find a similar "Back of the envelope" calculation model and go with a more conservative suggestion for the people building this thing. It's a fairly simple rack and the same company has build a lot of variations for other applications so I didn't think finding calculation methods would be so difficult.

We are also trying to do shear stress analysis on the thin honeycomb panels...anyone have suggestions on sources? Just to make UcfSE happy, I'm not trying to "use" anyone here- I am trying to find references on my own. I just figure more experinced engineers can point me in the right direction.

~Jeff

~Jeff

 

[Sparweb]

Get a copy of Roark's Formulas for Stress and Strain, and look up rectagular frames with side loads. The problems solve with linear algebra, so you'll be using the back of a 9"x12" envelope, but that should do the trick.

What's the point of using FEA on a one-off? You don't have to test the rack to destruction, either. The monster rack that I tested withstood hundreds of pounds without yielding.

Steven Fahey, CET

 

[Sparweb]

Stress Analysis of honeycomb panels: I've used a Hexcel Tech manual (#124) for calculations. The formulas are pretty easy to relate to basic principles, and cover all the major issues like point loads, bending in various axes, etc.


Steven Fahey, CET

 

[Heckler]

Jeff,

SparWeb is right on with his references....It sounds like we've all worked out equipment racks in planes. I will also check out Mil HDBK 5 (aerospace vehicle structures) and their is also a composite HDBK but I can't remember the title. If you're getting a DER to sign off on this with a 8110. I don't think you will be able to get around some mechanical testing. We were limited because Boeing wouldn't give out any design data on the seat tracks so we had to "over" engineer the mount points and validate with testing. I would be interested to know if you ran into the same.


Vince