Antenna stress and DTA example 2

[sb1786]

Hello All,

Can someone provide me with a manual, tutorial, or actual example of a well detailed (with references to formulas, assumptions) DTA and stress report for installing a small antenna on fuselage crown of a transport category a/c.

i have this project coming up and need to familiarise with the subject and an example would be very useful.

I have already started with Niu Brahn, Chicago ACO, Swift DTA. Actually the best would be to have a project that followed Chicago ACO as a guidline.

Thanks for the help in advance.

 

[rb1957]

humm, that's a bit odd (isn't it) ... tom's calc is pretty straight-forward. there maybe some fussing due the nett area of the hole.

however, it is the outer rows that are critical and define the inspection program for the patch; the inner rows see much less load transfer and are working (on the fuselage side) at a much lower stress level (as some load has been sheared into the dblr) so it's hard to see that they'll be a problem.

 

[GBor]

You may want to look into a paper by Huth that talks about fastener stiffness. There is also a section in Niu's Airframe Stress Analysis and Sizing

Garland E. Borowski, PE
Engineering Manager
Star Aviation

 

[sb1786]

rb1957 the fasteners are not the problem... The outer rows are critical for the fasteners clearly and the load reduces as you get to the center of the doubler.

it seems from the report that the sum of the fastner loads from the first three rows gets transfered to the doubler and this load is considered critical for the doubler?

 

[bf109g]

You can easily do Swift's spring model in Excel. I punched it in one night waiting for the load crews to hit something.

 

[rb1957]

"it seems from the report that the sum of the fastner loads from the first three rows gets transfered to the doubler and this load is considered critical for the doubler?"

i wouldn't've used "critical" ... rather the 1st three rows are considered to be effective in shearing load into the doubler. there are several fudges that can by applied to this last effective row, i say that the spacing for this last row is all the way to the dblr CL, so you can draw a nice free body of the dblr.

 

[sb1786]

I did it on excel and by hand. Just to clarify... From swift's formula:

@ Fastener 1
For doubler displacement use P=R1= load fastener 1
For skin displacement use P= load in strip - R1
For rivet displacement use F= R1

than plug all other values in formula and use strain compatibility to isolate and solve for R1:

skin displacement = doubler displacement + rivet displacement


@ Fastener 2 and so on same idea is used:
For doubler displacement use P=R1+ R2= load fastener 1
For skin displacement use P= load in strip - R1 -R2
For rivet displacement use F= R2

than plug all other values in formula and use strain compatibility to isolate and solve for R2




That is the correct way right?

 

[rb1957]

fine, the basis of the compliance model is that the displacements are equal. there are two displacement equalities, (1) between rows 1 and 2, and (2) between row 2 and the CL.
for (1) the basis of displacment in the skin and dblr is the spacing between the rivet rows.
for (2) it is the distance between row2 and the CL.

 

[msm1981]

@ Fastener 1
For doubler displacement use P=R1= load fastener 1
For skin displacement use P= load in strip - R1
For rivet displacement use F= R1

'''''than plug all other values in formula and use strain compatibility to isolate and solve for R1:

skin displacement = doubler displacement + rivet displacement


@ Fastener 2 and so on same idea is used:
For doubler displacement use P=R1+ R2= load fastener 1
For skin displacement use P= load in strip - R1 -R2
For rivet displacement use F= R2'''''

how to calculate P, R1 and load fastener?????

 

[rb1957]

you have two compatability equations, compatability of displacements between the outer and inner rows of fasteners, and compatability of displacements between the inner row and the CL of the patch.

connsider the 2nd equation 1st ... the displacement in the skin = the displacement of the inner fastener + the displacement of the doubler (the load in the skin is P-R1-R2, the load in the inner fastener is R2, and the load in the doubler is R1+R2).

the outer compatability equation is the displacement of the skin+the displacemnt of the inner fastener = the displacement of the outer fastener + the displacement of the doubler (the load in the skin is P-R1, the load in the outer fastener is R1, the load in the doubler is R1, the load in the inner fastener is R2).

This is described in Michael Niu (pp234) and derived in NACA TN1051 (which is available on-line).

practically two rows of fasteners are sufficient, but you can analyze more if you want (three ros of fasteners (requiring three fastener forces) yield three compatability equations.