Extrusion Floor Rail

[edmeister]

Involved with a Rush job that requires installation of a 100+ feet of Longitudinal Floor rail + Brown track to secure custom seating & monuments. A typical procedure would be to duplicate the existing OEM rail installation adjacent to the proposed installation butt-line. The OEM rail installation being an extrusion (I-Channel 4" height) with integral Brown track. All is good & well except Long sections (& amounts) of extrusion (or alternate shapes) are not readily available at this time. There is the typical 33 week wait period for the Mill production upon order. As is the case - reality never holds back a client who has deep pockets & a high school level of engineering. So Forward we go!
- I suggested that duplication of the I-Beam extrusion be accomplished with a pair of C-Channel Formed angles. C-Channels back to back with a flat strap along the top. (strap used to secure the Brown track to the I-channels via center-located fastener). Rivet row along the upper web & also along the lower web as well as the strap riveted to the opposite upper flanges of the C-channels. Lots of labor - but a viable substitute - given no extrusion available. (at least in my opinion)
- Approving authority declined the substitution based on extrusion properties superior to flat sheet properties etc .. (whatever that means) Intention was to replace 7050-T76511 [Ftu 79ksi] with 7075-T6 [Ftu 78ksi] & additional thickness to compensate for lessor strength.
(secondary issues for those with deeper insight) OEM Rail notched at each xFrame - thus installation is more of a series of installed intercostals then a continuous span of beam. Clips installed at each Rail / xFrame intersection.
## Question - can anyone think of reasons why an extrusion would be superior to formed angles? I can think of efficiency in shape / possibly DTA due to fewer fastener holes. anything else come to mind??? Any reasons why a formed shape (combo shapes) with equiv Ixx to the OEM extrusion would not be a viable substitute??

 

[rb1957]

add a strap between the lower flanges.

I think the biggest difference between s/m and extrusion is the thickness ... but you've no doubt looking at that.

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

 

[3DDave]

Any chance of a boneyard picking?

 

[edmeister]

Boneyard not feasible .. Existing holes will not coincide with new location - plugging holes will result in unwanted DTA nightmares ..
- beside .. removing an old rail is lots of work ..
2/ Strap on bottom .. im 50/50 here Since the rail is notched at each xFrame - there will be a strap across each gap under each xFrame.
I suppose its a question of strap entire length or partial .. Did not mention previous - too much info & all that

 

[rb1957]

how about material ? s/m would be 2024T3 ? extrusion 7075T6 maybe ?

attach seat track to upper plate before attaching plate to C-sections.

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

 

[verymadmac]

The build up has a much longer load path under vertical loads so is going to have a greater defection upward (is this subject to dynamic loads).

A radical suggestion
How about welding a 4130 (or Ti) Tee chord, you could add notches so you can attach the selected floor track with anchor nuts.

 

[SWComposites]

Yes, under vertical load the track pulls up the top plate, which then pulls on the C flanges. Much more flexible than the OEM one piece extrusion. Maybe some sort of vertical gussets/stiffeners would help.

You also need to check buckling of the web sheets, and crippling of the C flanges and top plate edges.

And a bottom plate would be a good idea.

Think you need to model the OEM beam track and proposed built up beam track with the same crash loads and demonstrate equivalent stiffness and strength.

 

[SWComposites]

And show equivalent DTA.

 

[Stress_Eng]

What are the fatigue requirements? You would think an extrusion would give a better fatigue life (less stress raisers due to number of rivet joints). And what about damage tolerance (more locations for crack initiation / small flaws)?

 

[SWComposites]

I would expect the seat tracks to be sized by crash loads, not fatigue. Depends on whether the intecostals pick up much fuselage bending loads during normal operation.

 

[Sparweb]

I like where you're going, but I can see the point of view from the "approving authority" that there's a lot at play, here. A few more considerations:
[ul]
[li] Overturning moment, and point reaction loads applied to the track by the special monuments. Are they greater?[/li]
[li] Fuselage stiffness overall. If you're introducing numerous segments with many connections where there once was a continuous "very long" extrusion then this may redistribute stresses in the fuselage as a whole.[/li]
[li] Cross-section stability may be different when composed of sheet. Make up for it with thickness to excess.[/li]
[li] Extra instability modes are present because faces fastened together don't work together as a "total" unless extremely well fastened.[/li]
[ul]
[li] Case in point, your double web faces have stability based on 2*(t)^2, and NOT based on (2*t)^2.[/li]
[/ul]
[li] More assembly means more pathways for moisture/contamination to enter. Consider corrosion.[/li]
[li] Vertical load on the seat track beam is reacted through fasteners in tension NOT merely through-section transverse shear (as the extrusion did)[/li]
[ul][li] This tension load path is applied to flanges in bending. Think Lockheed Stress Memo 88.[/li]
[/ul]
[li] Are you using fasteners that have a tension allowable from AR-MMPDS? I see CSK heads so this is doubtful unless you have an OEM's stress book to refer to.[/li]
[/ul]

Since you're in a rush job I can understand this stuff begin missed. If you're getting rejection from the authority but not hearing the grounds for their rejection, then try to open up the communication lines a bit better. They may have other reservations, not just items I've thought of (in just 5 minutes, so I am also giving you a "rush" answer!)

 

[Ng2020]

Dta? Really? Are the existing seat tracks PSE's?
You will come unstuck in rehards to fastener tension.
Good thing the authorities exist to reject rush jobs, keeps us all honest.

 

[WKTaylor]

You ARE using standard Acft seat track rails, thus...?

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Regards, Wil Taylor
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