DTA (Damage Tolerance Analysis) for Aircraft antenna installation
DTA (Damage Tolerance Analysis) for Aircraft antenna installation
(OP)
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...
I am new in that forum and in Eng-Tips in general, so stil doesn't know what the rules are here...





RE: DTA (Damage Tolerance Analysis) for Aircraft antenna installation
http://fdtcourse.com/
RE: DTA (Damage Tolerance Analysis) for Aircraft antenna installation
RE: DTA (Damage Tolerance Analysis) for Aircraft antenna installation
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.
RE: DTA (Damage Tolerance Analysis) for Aircraft antenna installation
Thanks again for your respond,
Cheers,
RE: DTA (Damage Tolerance Analysis) for Aircraft antenna installation
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.
RE: DTA (Damage Tolerance Analysis) for Aircraft antenna installation
RE: DTA (Damage Tolerance Analysis) for Aircraft antenna installation
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.
RE: DTA (Damage Tolerance Analysis) for Aircraft antenna installation
Cheers,
RE: DTA (Damage Tolerance Analysis) for Aircraft antenna installation
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
RE: DTA (Damage Tolerance Analysis) for Aircraft antenna installation
cheers,
RE: DTA (Damage Tolerance Analysis) for Aircraft antenna installation
Cheers,