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.

 

[der8110]

Sure! How about sending you a laptop with all the spreadsheets and software already loaded?

 

[GBor]

Polysim,

In case you missed the sarcasm, you are asking for people that do this for a living to potentially violate proprietary agreements and send you a complicated analysis from which they make their living.

Pay someone to do the first one, follow their example in the future, but continue to work with a DTA DER until they are prepared to support your own application. Through the Chicago ACO, I would recommend that you search for Kamala Meader.

Garland E. Borowski, PE
Engineering Manager
Star Aviation

 

[737eng]

U of W is offering a 1 week course which focuses on just this, DTA for antenna installs. Contact the SEA ACO or the U of W extension to get details and go. It is a great class and believe me you won't have any time to do any sight seeing while you're there!!
Also recommend utilizing Kamy. She is based in Ann Arbor Michingan. Look her up on the DER consultant directory.

 

[sb1786]

Thanks for the references. Actually that is the route I was taking. I did source out the DTA to a DER and am waiting for the report. I am also looking into the U of W course but it is quite far and time consuming... 737eng it looks like you took the class? I got some of the slides from Patrick Safarian and some of the material but it is not all...

Since I am not all that familiar with this sort of report I wanted to acquire more knowledge. As for the proprietary info I can understand that however, I am sure some of you came across some manual that clearly shows the steps with an example at the end... Erasing the proprietary info from a doc is not that hard also... Anyway, I understand some are doing this for a living and this is the route I would like to take some day... How did you do it? Remember that the cheese is big enough for everyone! More than enough projects for others to join the group! In the mean time I will use Mr. Borowski method which is to get an example and try to follow it. That is if I understand the concepts and assumptions as well as calculations and software...

Thanks again

 

[rb1957]

the analysis involved are easy (and not trade secret !) ...

like an DTA ...
spectrum loads ... the Chicago ACO doc provides a method to derive the flight spectrum. i personally think that it's overly complicated; whatever you do, your local cert people have to buy into it. presumably you're inside the pressure cabin.
crack geometry ... crack at the antenna hole, crack at the load transfer rivets (assuming there's an external doubler).
material crack growth data ... lots of open source data
crack growth algorithm ... AFGROW, FLAGRO, or a simple s/sheet.
inspection "issues" ... remember if your installation means a different inspection technique, that's a new inspection task. i typically use a 1 rivet pitch crack under the external doubler at the load transfer rivets as being detectable with LFEC.

good luck. btw, there are many other aspects to an antenna installation, most notably vibration concerns ... this is a problem that can go many ways ... if the installation is too flexible that's a problem, if the aero driving function (a wake?) is very strong then that'll cause a problem with a very stiff installation.

 

[sb1786]

i actually think my case is simpler.

The antenna is GPS size; quite small and flat.

I would install an internal doubler.

Thanks RB I will find some more info on each of those subjects above.

 

[737eng]

I took the class, it's worth the trip, don't plan on trying to keep up with work back at the office when you are there.

Method taught to determine the stresses is a bit conservative in my opinion, but it is valid method that will provide a very conservative inspection threshold and interval that you know it will be approved. Method taught to determine threshold and interval is very similar to what I used already. Take the class to get the basics and then the key is to work with or under someone who is experienced and learn from them. In my opinion, DTA is not a boiler plate type analysis, it is an analysis that needs tweaked for each individual case in order to ensure proper conservatism. The ability to "properly" tweak comes from experience. If you can find a mentor that not only has done modeling and analysis, but has been involved in testing and validation which gave them the experience to be able to refine their models, beta factors, and analysis, latch on and don't let go!!

With a GPS antenna, there will be very little bending (if any) induced by the antenna, but don't forget to include your bearing stress and your bending stress due to the eccentricity induced by your doubler at the outer row. I think that is a common omission in the reports that I have reviewed.

 

[rb1957]

GPS antennas usually don't present the same problems as blade antennas, like i was thinking of.

a little surprised you're using an internal doubler, in my experience the main reason for the doubler is to hide the CSKs of the rivets. maybe as a sort of a/nut plate, but then attach to the fuselage so it's not structural, maybe?

maybe you're using it to reinforce the 1" dia hole (for the antenna) ... probably not really worth it for this reason, it's not fully effective (however you rivet it on) and an inspection program for a crack at a 1" hole shouldn't pose problems for operators.

just my 2c ...

 

[sb1786]

the doubler is installed to compensate for the hole in the skin and is riveted to the skin.

The inspection will surely be on the first row... it takes the report to justify that!

i am looking into courses available and found a course that seems to have what I am looking for at Cranfield U in the UK. Location is a problem but at least its only 4 days.

In the mean time i am still after some course notes, class notes or magical company manuals that will have great tutorials and examples. Seems like mission impossible though.

 

[thruthefence]

Just did one of these,a Garmin GPS, & tried to "previously approved" data from an install on the same aircraft, ( a 601P Aerostar ) the feds would have none of it; I ended up contracting with a DER service I found on line, (googled "DER service") and for $600.00 got a wonderful set of drawings, & instant approval! Guess what? The drawings, materials, and hardware were nearly identical to that I submitted earlier!

 

[rb1957]

i know i'll get $hit for this but i can't resist ...

DER = licence to print money ?

 

[thruthefence]

Easier then fighting with the fsdo, I guess. By the way, the doubler was inside the aircraft. .040 2024T3, two flanges bent up on each side; domed, sealed nutplates shot to the doubler itself, installed 'wet' PRC 1422, NAS1097-4 rivets.

 

[crackman]

Just a couple words of advice.

One, the hardest part of doing the DTA is developing the spectrum. There are simple conservative methods out there proposed even by the ACO's. BUT, most customers, in particular large transport airlines are very keen as of late about DTA, Aging Safety, and inspection intervals. I can tell you from experience that it is pretty near impossible to get a 10,000 hour or more interval for even the smallest antenna install if its at the top of the fuselage crown in an area with with fuselage bending with one of these conservative methods. For areas without bending, you can get away with conservative methods. I have spent several years with my ACO having all of our full aircraft loads methods approved so that now it is pretty easy for us to develop realistic fuselage spectra at any point on the fuselage. We have done this for most large part 25 aircraft by now.

Second just be aware of the FAR 26 requirements and use DER's who have been delegated with this. The timeline crunch is going on right now so customers are in the thick of it and so most DER's are also tied up with this workload. I actually have several of my DTA folks doing nothing but supporting this.

Third, as for references, there are several but a couple little known include the FAA DTA Handbook (different than Swifts) and the USAF DTA Handbook. Both publicly available and quite technically detailed.

 

[FastMouse]

PolySim,

This will not entirely fulfill your original request, but maybe it's a start and it shows, at a general level, what things might need to be considered when performing a damage tolerance assessment of the type you describe.

http://www.pacsengineering.com/exmpl1.htm

crackman,

I must confess my ignorance here! I'm not sure what the FAA DTA Handbook is. Do you have the full title or a document reference to hand? Thanks in advance for any info that you're able to supply.

FastMouse

 

[sb1786]

FastMouse,

This is a great small general example. Actually if the report itself was available or even the references it would be gr8.

Thanks for the link.

The FTA DTA handbook precise reference would be appreciated.

 

[sb1786]

Crackman,

Maybe you mean the DTD handbook available from afgrow??

Link is:

http://www.afgrow.net/applications/DTDHandbook/pdfs.aspx

 

[sb1786]

So just an update and a question...

I finally decided to purchase the DTA from a DER however obviously not everything and every calculations are explained in it. I have read and understood most of the report, however I am stuck on the fastener loads calculation using Swift's displacement compatibility analysis. When I try to compute those for all 6 rows of rivet and the doubler, I just do not find the same values. I am not far but not exactly their. I am able to find the values for rows 1,6 and 2,5. For rows 3,4 which are in line with the hole of the doubler i get a result that is far off (25 pounds). Can someone provide me with an actual example with values of how to determine the load on the fasteners using swift's method. I have read a couple of threads on this in here at eng-tips including the FAQ but nothing is really shown. If anyone can help it would be much appreciated.

Thanks!

 

[bf109g]

6 rows? You mean 3 rows on each side of the cutout?

 

[sb1786]

Right bf109g

3 rows on each side