External Doubler, Antenna Installation

[CAD182T]

Good evening all,

I have a requirement to install a low profile antenna to a pressurised CS23 aircraft. I have designed many small low profile antenna installations and a few blade types and have always been able to utilise internal doublers.
The situation I have now is that the antenna footprint is relatively large with respect to the bays formed by the frames and stringers on this aircraft and such the connector and mounting screw penetration holes are not contained within a single bay.

The antenna is approximately 18” long x 7.75” wide. The penetrations themselves are nothing out of the ordinary with 2 TNC size penetrations approximately central and 4 off ¼” screw holes spaced out to suit the footprint. The pattern of these required holes with respect to the frames and stringers makes the use of a single internal doubler impractical. The use of multiple internal doublers is also complicated as again it is impractical to accommodate double rivet rows with satisfactory spacings and edge distances, and doesn’t seem like the right way to go in my opinion.

My attention is now turned to the use of an external doubler which would allow restoration of all penetrations in one continuous doubler. This is the first time I have had to consider the use of an external doubler and such would like to ask for a second opinion on its implementation.

The antenna would be seated on top of this external doubler, meaning that the ruling dimensions are driven by the overall antenna footprint which such introduces much more material than is needed to restore the cross section removed by the skin holes. When matching typical rivet spacing in this area (cabin roof) around the perimeter of the doubler the amount of rivets required gets quite high, in the order of 100 or so when 2 rows are employed, and that is not counting field rivets. To me this seems disproportionate compared to the actual strength removed from the skin by holes required for the antenna.

My second concern is that the doubler will then be covering a length of the centreline stringer and a width of frame. The rivets used in this area are 1/8th countersunk. In the aircraft SRM pertaining to patch repairs there isn’t any guidance on whether to cover these existing rivets or to pick them up through the doubler. To pick them up through the doubler would then require the use of an aluminium countersink filler piece or compound to fill the void left by the removed head. I have read about this but never had to specify it myself.

My concern is the inspectability of the existing rivet holes covered by the doubler as if they are picked up the skin still remains critical below or if they are covered they cannot be readily inspected from the inside or outside.

On observation of photos of special role fit versions of this type installing interesting antennas they almost always utilise an external doubler, but I have no data on actually how these are implemented.

In summary I would be most grateful for a second opinion on the following:

• The external doubler and riveting required becomes much larger than it needs to be restore the holes made in the skin, simply due to being driven by the footprint of the antenna. Should I follow standard rivet spacing guidelines driven by the geometry instead of the structural capacity requirements (which will be greatly exceeded)? I am concerned over making the reinforcement too hard and risking a low fatigue life.

• Picking up or covering existing countersunk rivets. If picking up what is the recommended best practice for filling the original countersinks?

• Can anyone advise any guidance material or reference installations for good practice when working with external doublers?


Thank you for your time,

 

[verymadmac]

In pressurized aircraft skins, the doubler rivet count is also driven by the requirement to make the doubler strain at the same rate as the skin. So copying lap joint detail is always a good start, cheaper to have too many rivets at this stage than a couple more inspections over its life. Although FAR / CS 23 aircraft seen to vary greatly in the quality of the way they address fatigue considerations (j32's look like they will make 90000 hours with little effect while metros seem barely able to make 35000 hours).

Don't cover the existing rivets, you want to use countersink fillers or if the doubler is sufficiently thin, dimple it (not great fatigue properties but better chance of correct assembly). there are several threads on countersink fillers on eng tips.

http://www.eng-tips.com/viewthread.cfm?qid=101047

There is always plenty of Boeing 737 SRM's on line, the SRM sections 51 and 53 have numerous fuse skin repairs.
[tt][/tt]

 

[der8110]

Good evening,

My first suggestion would be to use an internal doubler for the connectors, then separate internal doublers (nutplate strips) for the attachment screws.

But knowing nothing about your geometry, let's assume your hands are tied so then allow me to answer your questions directly.

Do follow standard rivet spacing, the "lost area" method is overly conservative anyway (compared to a rivet strip analysis). As long as the doubler is only one gauge thicker than the skin, you are not creating a hard point. For fatigue, it is the outer row of rivets that is critical anyway, whether there are two or 5 or 6 rows of rivets.

For filling redundant countersinks, my order of preference is: dimpling the doubler, washers made from rivet heads, then Hysol or Metlset.

Unfortunately I can't refer you to any reference material on antenna doubler design, it sounds like a good project for retirement.

 

[rb1957]

we fill redundant CSKs with a cherry rivet and drill out the shank.

In my very humble opinion, I'd Never dimple the doubler into the CSK.

DT, inspection of the fuselage pressure skin under the dbler, is an issue.

you'll be installing the dblr with CSK rivets ... LZ4s I suspect. Take care of the doubler thickness (1.5*rivet CSK depth is the general rule).



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

 

[CAD182T]

Thankyou all for your constructive responses.

I am due to get back onto this design next week, the more I think about it I would like avoid the requirement for disturbing and filling so many original rivet locations, both from creating an inspection burden and specifying that the guys will have to drill the heads off so many rivets!

I had a proposal that utilised an internal conventional doubler and individual fastener hole doublers, I think I may revisit this to see if this can be made to work and keeping the first fastener rows as visible as possible.

Again thank you all for your time, much appreciated.

 

[WKTaylor]

CAD182T

Every modification like what You are doing to this fuselage is essentially a 'repair'. Although a Mod is NOT for damage... but for a specific purpose... the outcome/intent is the same for analytical reasons: cutting-out, inserting and installing, etc.

One report You might find following document enlightening, this subject: REPAIRS TO DAMAGE TOLERANT AIRCRAFT, Authored by Tom Swift FAA.

http://www.dtic.mil/docs/citations/ADA225742

This report provides enough analytical information 'to-get-the-theory-across'; but then provides an excellent discussion with illustrations of the 'how-to, why-to, like-this, etc.

CAUTION.
Large aircraft I have dealt with have antennas (a) contoured to the fuselage curvature; or (b) have very small/flat bases; or (c) have wide/flat bases, and are installed-outside-of-contour [with built-up aero-structure-mount] to minimize penetrations/flattening of the of the primary pressure vessel from a 'round shape'. In all cases, the antenna is [obviously] installed on an unpressurized skin area: however, that installation must perform acceptably, even when the assembly it is mounted to is pressurized to ultimate P-load. The ultimate strain/stresses across the installation must be anticipated. OH yeah, skin temperature extremes must also be anticipated [it is possible that thermal strain due deep-cold at max /\P may be beneficial... or not]. Also, General practice ['here'] is that the antenna must only experience its on loads [drag, G, abuse, etc]... without strain/stress transfer from its mounting structure.

Also...be aware that antennas are electrically active and must be grounded-bonded to the airframe [very low electrical resistance between parts]. Corrosion protective coatings/sealants/gaskets must applied to antennas and their bases/doublers/etc CAREFULLY to ensure they do indeed provide corrosion resistance [including moisture/environment exclusion], while maintaining a good electrical ground/bond. What a pain that is to get right [durable]! Remember, that high strain rates and temperature swings [-65F +180F typical], are typical in pressurized structure, so creating a joint that will ALSO be environmentally/corrosion durable is a challenge!



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can b

 

[rb1957]

sure you can use an internal dblr, but you're limited by the skin thickness (CSK no more than 2/3rds thickness is the normal rule). With your aircraft I suspect the OEM banged in LZ4 rivets into 0.04" thk skin, in which case you can do the same.

You're making the installation easier, at the expense of your analysis and inspections in the future. Your internal dblr will create secondary bending on the skin, making the fatigue situation worse. You'll still need to do some F/DT analysis for the load transfer into the dblr. With your class of plane I suspect you can get away with a fatigue calc, though I see no problem to adding 2 or 3 inspections during the life of the plane to inspect this dblr ... at least the internal dblr allows for an external visual inspection.

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