DTA (Damage Tolerance Analysis) for Aircraft antenna installation 1

[AeronauticalBG]

I was doing DTA analysis on antenna installation on an A/C and have used conservatively nx = 2.5 (max design limil load factor). if anyone was involved in that type of calculations, I would appreciate any hints, how I can icrease that number (as per CFR 25.337(b) nz is between 2.5 and 3.8), so the bigger, the better, since nz is part of sigma 1g max denominator equation.
I am new in that forum and in Eng-Tips in general, so stil doesn't know what the rules are here...

 

[SAITAETGrad]

Take this course. You will learn what you need to know:

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[AeronauticalBG]

Thank you SAITAETGrad, I know about that course, and actually I highly recommend it to everybody in that area...

 

[rb1957]

personally i use an equivalent once-per-flt stress (a GAG cycle) of 1.5*hoop stress.

if your support strcuture is designed for a reasonable handling load (say 200 lbs applied to the tip) then there should be any problems.

pretty much you only need to do DTA if you have a doubler attached to the fuselage skin (so you're changing the local inspection technique and/or interval). if you haven't changed the OML then there shouldn't be much of a problem ... yes, you can look at a crack growing out of the open antenna feedthru but that rarely gives rise to anything much.

we're getting a lot of "ear ache" from a national authority (and probably a 2nd) about the effects of vibration and showing the supportstructure is good to Vd.

 

[AeronauticalBG]

Thank you rb1957 for your espond. Yes I have a doubler (external one, under the antenna) and in my case, critical is not hoop, but longitudinal stress, since the antenna location is very close to the front wing spar STA location, top crown skin fuselage. And that is waht is nocking me down my final Threshold / Repeat interval cycle inspection...
Thanks again for your respond,
Cheers,

 

[rb1957]

a bit odd that ...

i assume you're using the Chicago Office method ? sorry but it seems nonsense to me that a higher nz creates less stress; but then that's me. i mean you have hoop stress acting on one edge of the dblr, and less than 1/2 on the other which also sees the fuselage bending (which is concentrated in the stringers). if you'd said rear spar i would have been less cranky !

what is the existing inspection technique ? external visual ? your dblr needs LFEC, yes ?

what plane ? i'm guessing large transport.

 

[AeronauticalBG]

...yes, I am using Chicago DTA for antenna instl, and as you can know long. stress increase linearly up to the front spar and stys const. after that. Conservatively for nz you have to use 2.5, however, if you can validate this number from OEM CFR 25.337(b) says that you can go from 2.5 till 3.8. I went to LFEC plus external visual (CRJ-100)...and as I said before, my hoop is less than long stress, so long stress is the restrictin one, since antenna location is further back away from the cockpit...

 

[rb1957]

ok, i won't labour you with my feelings about the Chicago method !

use the higher stress, but it does seem daft that if Nz limit increases the effect on fatigue stresses decreases. yes, i know know why (they assume fty at limit, which is equally daft = hopelessly conservative ... but now i start my rant ... sorry). there is an equation in FAR for limit Nz, a function of weight, so you might justify a claim of the higher Nz.

 

[AeronauticalBG]

Thank you rb1957, I am working on it...
Cheers,

 

[Sparweb]

I had to re-read this a couple of times, too, before I figured out how Nz=2.5 could be more conservative than 3.8.
I would deviate from the "Chicago method" in that area. As RB1957 pointed out, it makes no sense that the skin would be stressed to yield only at maneuvering load factor + pressurization. I remember that document but I haven't used it in years.

Consider the statement in 25.301 "... the specified air, ground, and water loads must be placed in equilibrieum with inertia forces..." That, plus some other information should be enough to justify some assumptions that are more sensible. It will put you in a much lower stress range, and hence the calculated life of the panel will improve.

The LAST thing you want to do is to create a new inspection procedure or schedule just on the basis of an antenna (except for unusually large antennas I suppose). Putting the doubler on the outside, though, can render external skin much more difficult to inspect. Is there a practical way to install the doubler on the inner side of the skin?

There are differing opinions on the subject.


Steven Fahey, CET

 

[AeronauticalBG]

Thank you Steven for your comments, I think you've put some nice thoughts which can be considered. We went to external doubler, since we are accomodating two small antennas in two adjacent pockets, top fuselage crown and that was more practical instl. Thanks again for your comments,
cheers,

 

[AeronauticalBG]

Steven, futhermore to your reply, nz of 2.5 is a conservative, since is a denominator in an equation for sigma 1g max-longitudinal stress. So, the bigger nz, the less sigma 1g stress and therefore less equivalent longitudinal operating stress, which affects big times the final AFGROW results as a Threshold / Repeats intervals of the skin panel where antenna is installed...
Cheers,