Tension Fitting (Angle and Channel) understrength mitigation

[mangfpt]

This is a question for aero stress engineers.

We have a few locations showing negative MS where an external load (modification) is attached to a fuselage through the skin into a new reinforced channel or angle type fitting. The stack-up of parts on the fuselage side consists of skin double (0.10in), original aircraft skin (varies but 0.085in typ), intercostal (0.063in) and the new reinforcement channel or angle (tension fitting).

The fastener tension analysis is conducted using 100% of the tension load on the end pad of the channel of angle fitting. BA methods (BM7024.01.03.03 Rev B) is conducted and the fitting fails in end pad bending. Typical margin is MS= -0.06.

Given the significant stack of material/parts which is there to assist in reacting the tension load l believe a valid mitigation is to show a load path for the excess tension loads. This can be done via an increase in the effective end pad thickness (lets call it tb(eff)) much in the same way a stress pad (contour pad or throat pad, depending on which version of English you speak) provides additional support.

I need some input into the viability of this mitigation approach and, if possible, a reference to quote to support its use.

Many thanks,

 

[Sparweb]

Sure, a thicker pad gets a higher MS. But that's obvious in the BA methodology. Somehow I suspect this isn't the direction you want to go.

Showing the positive MS on paper is one thing. Being certain that all variables have been well bounded and assumptions are valid is another. Beefing up a low-MS intercostal flange to secure an external mod's load seems to be tempting fate. Why aren't you proposing to make more substantial fittings mounted to this intercostal? Fittings like the ones shown in the first 3 figures of the BA reference you cite are suitable in some cases - have they been ruled out?

External mods have a lot of load cases. Which load case is driving the low MS?
Have you accounted for high angles of attack? icing? bird strikes? handling and abuse?

STF

 

[rb1957]

Are you using Fitting Factor ?

Certainly you can increase your tension fitting flange (or endpad) thickness. Personally I wouldn't count on the skin and doubler to carry out-of-plane load. How are you managing the torsion that I assume appears in the member supporting the tension fitting (frame) ?

another day in paradise, or is paradise one day closer ?