Antennas for aircraft 2

[Higgler]

As an antenna engineer, our company is starting to make antennas for air vehicles. Is anyone out there familiar with some of the requirements for attaching antennas on aircraft,
Example, adding a new Anti-Jam GPS antenna to commercial aircraft is one product that we are producing;

1) Mounting techniques, a)screws with environmental seal, versus b) epoxy resin/glue attachement. Is there normally a cable with a service loop to attach to the antenna before feeding it back in for mounting. Or is blind mate common? What standard connector is most common, type N-female? Are the connectors loctite attached to prevent loosening of the cable attachement?

2) Vibration requirements, I know there is a wide range of requirements depending on location, some general info such as, fuselage of aircraft easy vibration, tail mount is high stress, wings are mid stress, etc.

3) Rain Erosion; testing, I've been involved in some. Is it required for all, or can you cite the material as "good for rain erosion" and skip testing.
Which materials are most common for aircraft mounted antennas. I've used Tefzel and PEEK with proven results, any other common ones. I realize sometimes rain proofing bad materials using a rubber like paint is common, like the forward radomes on commercial aircraft. What is the most common erosion proof paint out there.

4) Air Worthy Certificates, I once worked on a program whereby materials that burn toxic fumes are not allowed on military aircraft, is this true for all aircraft mounted antenna. I assume missiles could use anything, since they tend to burn anyhow. Is there reference material for this.

5) Who are the leading manufacturers of aircraft antennas and radomes to possibly build our hardware?

6) What questions/areas have I overlooked? testability to prove the antenna works? repainting after years of use? structural inspection criteria? Worthiness testing that it doesn't shred apart and enter an engine? Ballpark prices common for a GPS or other antennas? Power required by our antenna - is it easily supplied thru the center conductor of the coax? Overload protection to powerful radars hitting it that might blow out our electronics? EMI/EMC test requirements? Is color important? weight? CG?

I'm full of unknowing, any help would be appreciated,
Thanks,
KCH.

 

[Sparweb]

Two things for you to check out:

thread2-67807

and

forum247

Antennas mounted on pressurized aircraft give the engineers that certify them issues like fatigue life and crack propagation to deal with. When I have more time, I can give you a few more resources on this topic to help the engineers installing your antennas. I have also had a few conversations with antenna designers like yourself that illuminated a few things, which I don't mind sharing.


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[joekm]

I'm stricly a structures guy, but our consulting firm has plenty of electrical/systems types as well that could probably help you with this. However, this is right in line with what our group does so, rather than risk breaking this forum's policy, I'll just ask you to e-mail me directly if you'd like more info (follow the link in my signature block).

Regards,

--
Joseph K. Mooney
Director, Airframe Structures - FAA DER
Delta Engineering Corporation

 

[Stache]

The requirement to mount antenna is spelled out in AC 43-13-2A Acceptable methods, techniques, and practices, Revised 1977, chapter 3. If you need a copy the AC is located on the FAA web site or contact me I will send you a copy.

Denny

 

[Sparweb]

But please,

DO NOT use AC 43.13-2A to mount antennas on pressurized aircraft!


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[joekm]

I'll second SparWebs comment



--
Joseph K. Mooney
Director, Airframe Structures - FAA DER
Delta Engineering Corporation

 

[Aeromark]

You might want to beware that the FAA is taking a second look at what antennas are determined to be "large" and thus significantly change the handling qualities of the aircraft. This may result in a requirement for dive testing to satisfy 14 CFR §25.251 and §25.253.

I have an issue paper that is currently making its way through TAD on this.

 

[Sparweb]

Just last week I got another phone call about the antennas referred to in Thread2-67807, tearing its way out of a Hawker Siddley 748.


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[crackman]

Hi All

Antenna installations are one of my pet peeves. When designed appropriately (to take into account all effects - fatigue, buffeting, etc.) they work very well. Unfortunately, the current industry work force and in particular the design groups are not very experienced (and OEMs are not willing to train or mentor them) and therefore antenna installations are trivialized and thereby problems result.

In the past, I have spent quite some time analyzing and fatigue testing antenna installations on fuselage structure. In fact, several years ago, to demonstrate (to some hardheaded individuals)and validate the fatigue life of some antenna installations, I conducted a component fatigue test of a 4 frame bay / 9 stringer bay pressurized fuselage panel with an antenna installation for a large antenna (approx 18"). The first cracks occurred in the intercostals (they were not designed stiff enough) and then cracks developed in the holes for the attach screws of the antennas. Also as a note, the worst stress levels were seen while not under full fuselage pressure. One needs to be careful to investigate which is worst: full antenna load under full fuselage pressure; or full antenna load under partial or no pressure.

I have seen similar tests (done many decades ago when engineers were not so arrogant as to believe that they knew everything and therefore all designs were tested to some extent) which demonstrated the need to ensure the proper stiffness and load path backing up large antennas. Unfortunately today, short sided management rarely allows for researching this past data and mistakes are thus made.

One other point and then I will get off my soapbox. I am aware that Fokker did some significant testing several years ago (prior to their demise) and collected fatigue loads data regarding the effects of buffet. Basically, whatever you do, DO NOT put antennas in locations which may see buffeting loads such as: lwr fuselage directly behind the NLG, lwr fuselage directly behind the MLG, and behind any other pretrubance which may develop turbulant flow behind it. There is NO WAY to possibly design an antenna installation to sustain buffeting loads without eventually cracking and the only way to obtain the buffet loads is flight testing - very expensive. There are way too many cycles involved to do this properly.

Good luck

James Burd
FAA DER - Structures / Fatigue and Damage Tolerance

 

[Higgler]

Thanks for all the inputs, they're informatively scary.

Is there a general guideline for antenna height versus problems expected (I realize it's speed, altitude, location dependent). Seems like 4" would be green, 8 inches yellow and 12 inches red, if one were to put a fear factor on antenna heights (using the US traffic light conventions).

Most of our Toyon antennas are low profile, but requirements for low frequency antennas require larger size.

I guess another way to ask that question is, "what are the sizes of historically problem antennas and what type aircraft were they placed on".

Who are the recognized consultants who help get our products designed properly for air vehicle mounting?
thanks,
kch

 

[BrianR]

Having been involved with many aircraft antenna installations I strongly agree with comments made about buffet and fatigue.

However from an electrical perspective, female N-Type connectors are best with TNC also used for this application. Grounding for RF and lightning is an issue. Commonly a chemical conversion coating (alodine) is applied to the faying surfaces and the mounted antenna is sealed around the edges with goop like PR1422 or one of the equivalents.

This method is excellent for grounding but alodine is not that durable and there is a continuing maintenance activity to inspect/treat corrosion under the antenna. Painting the skin with epoxy or polyeurethane eliminates that corrosion issue, but how about grounding. Some RF grounding is achieved through capacitive coupling between the antenna base and skin and lightning grounding via the mounting bolts providing the captive nuts are grounded, (this also aids RF grounding).

I have installed TACAN beacon antennas on top of aircraft and they are a similar sized pie dish shape to your GPS antenna. We installed additional lightning protection in the form of a segmented diverter strip across the radome as while this area is not where lightning usually attaches, it is swept by the discharge as it is dragged towards the tail. I think though that this is a value-judgement type of call.

Radome material: If you can afford PEEK then it is ideal, otherwise epoxy covered glass matrix works too. A rain ablative coating should not be needed in this application.

 

[joekm]

Let me hop up on that soapbox with you crackman:

You're opinions reflect my own and I think there are two things driving this. One is the increasing availability of very user-friendly analysis software, the second is that nobody seems to want to "mentor" anymore. I believe that these two issues are, to some extent, related. It's just easier to teach someone how to use a program then assist them in developing as an engineer. Heck, we don't even do that anymore. Nowadays, we just look for people who are already proficient in Nastran, Cosmos, FEMAP, whatever......

I've had some discussions with the local FAA over this very thing during which I've voiced the opinion that someday we're going to see a major air catastrophe that will be attributable to this mentality.

This is also why I strive not to so much answer questions on this forum as much as try to point people to where the answer can be found. After all, you all don't know me from Adam and "because Joe said so" is a lousy reference.

Right now, I'm in the middle of evaluating data from a company that wants to substantiate a large and complex cabin monument using FEA only. So, this is kinda on my mind right now as you can imagine.

--
Joseph K. Mooney
Director, Airframe Structures - FAA DER
Delta Engineering Corporation

 

[BrianR]

jokem,

You have made some good points especially about the (unknown) quality of anonymous advice. The onus is certainly on the reader as to whether to accept it or not. In this field I frankly don't know where you'd go to for literature on antenna installations that combines the issues of class of vehicle, (speed, altitude, and so on), frequency, power and loading effects on structure.

As you pointed out it is back to mentoring. The company I work in is perhaps unusual in that it places high emphasis on exactly that and we have a good share of angry old b's whose job it is to provide mentoring.

 

[Higgler]

For Radome materials, I've used PEEK, it is a good material, especially for Rain Erosion. It's not a "controlled dielectric" having data from manufacturers for electrical performance versus temperature and frequency, ala the standard Rogers Corporation data sheet type info. Does anyone know of a company that tests and regulates PEEK for its electrical data? We tested it for rain erosion, both in pure form and partly filled with other binders, pure form was best in a rain erosion test machine. While at my former employer we found that Tefzel is good also (rain erosion tests) though much weaker and cheaper (easily cast) than PEEK.

Lightning protection is important, but I hadn't thought too much about it. Fully designing an antenna, then being told to add some lightning protection to it's outer surface would be embarrassing if it required an antenna redesign, which it probably would if you needed continuous metal strips on the radome surface.

I recall my former company adding components in the antenna connecting cable to prevent the lightning energy from coming into the receiver from the antenna. The items preventing this were discharge gap thingy's -MOS's?, surge arresting diodes, etc. Question: do all antennas on aircraft have some component attached close to their mounting RF connectors to prevent lightning from entering the aircraft.

kch

 

[Sparweb]

BrianR,
Working for "angry old b's" is certainly interesting, and can put you on the right foot for the rest of your career, if you're lucky.

Higgler,
I only wish I could test the antenna installations I design. However, all of my customers have for the longest time put antennas where they wanted, how they wanted, and that was it. Bottom dollar owners of <12,500 pound aircraft are not interested in hearing about the damage tolerance issues with the doublers they install, and some have no fear of icing conditions, lightning, or buffeting. When we encountered problems with the aforementioned antennas, the owners came to us thinking we were at fault. We did our best to explain that antenna #1 is not a direct replacement for antenna #2, even if the manufacturer claims that it is. They weren't completely convinced.

Ever since then, I've been of the opinion that the onus should be on the manufacturer of an aviation antenna to provide installation instructions that can be adapted to various aircraft, speeds, and altitudes, plus considering icing, and lightning protection. Generally all I see is a reference to AC 43.13.


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[BrianR]

joekm, firstly I owe you an apology as I spelt your name incorrectly in my last reply. That was my dyslexia at wrork.

Higgler, segmented diverters are unlikely to have much affect on the antenna. The metal segments are about 2mm dia and they're mounted on a high resistance substrate strip that ensures the lightning follows that path. I have been looking for an old paper that's here somewhere describing tests carried to see how readily lightning penetrated a nose radome and what were the guidelines for placing divertsr strips.

The results showed that lightning was lazy and wanted an easy path to ground. In the example given (a typical nose radome), where the radar antenna came within 2cm of the radome, lightning would still attach to an external diverter strip within 15cm of the hit point but any further and it might blow through to the antenna. This is just an indication of ratios and it depends on dv/dt of the impulse.

The supply of qualifying data with a new antenna is fraught with risk I think. The manufacturer can qualify it for mounting in a clear location. How much ice do you allow for and you can never allow for the genius who mounts it in a turbulent area. There is no substitute for experience here.

 

[crackman]

joekm, I agree with you wholeheartedly.

I myself have a couple of young engineers I am training. It is very difficult in that management allows little to no time for any mentoring. I always try to stress the points of learning the basics and where ever possible to have them involved in testing. Testing as far as I am concerned is our sanity check. Once a design and method has been verified by testing, then and only then can designs be expanded upon through computerized methods. Unfortunately, these days, this industry is being run by a bunch of bean counters who wouldnt notice nor care less about the differences between manufacturing chair legs or airplanes.

As for general guidelines for designing antenna installations, without doing a brain dump and not pointing to any one actual design, the basics are to always make sure you have adequate stiffness in your backup structure for supporting the antenna loads. Never use the skin to transmit anything but in-plane shear loads. Typical designs that I have seen work well are those employing two intercostals spanning frame to frame and using extruded T clips on the end to tie into the frames. Depending on the mounting hole locations, placing two small channels between the intercostals to attach the antenna fully. To sum it up, make sure you have a good torque box. This of course is only speaking generally as each design may require its own unique details and placement along the fuselage may play a significant influence due to fuselage loading.

James Burd
FAA DER - Structures / Fatigue and Damage Tolerance

 

[joekm]

crackman:

We're I not prevented from discussing it, I could show you a large TV antenna install concept sent to me one time that would give you nightmares.

One of the little "visual cues" I used during one of our meetings was as follows:

"See that mini-van you all drove here in?, the aerodynamic lift alone of this behemoth is greater than the weight of that van. You're gonna have to do better than attach it to the skin."

At the time, they were concerned that the adaptor plate structure (around 30 lbs) was gonna be too heavy.

--
Joseph K. Mooney
Director, Airframe Structures - FAA DER
Delta Engineering Corporation

 

[Higgler]

I have often thought that designing a flushmount phased array antenna with thickness 0.1 inches and size somewhere between 12"x12" and 36"x36" square-ish could be attached to an antenna skin this way;
1) drill one small hole (0.5" diam) for the RF connector in the aircraft skin.
2) drill another small hole in the aircraft skin for control cable wires(0.5" diam). These wires adjust the antenna's pointing direction and power the electronics attached to each antenna in the phased array.
3) Epoxy the flat, bendable antenna to the skin of the aircraft.
4) Possibly pop rivet? the antenna edges to shore up the epoxy resin, in case the resin wants to let loose.

Is this a ridiculous idea? I'm thinking of the ease of installation for a flat,large antenna that follows the skin contour of the air vehicle. Our company makes such antennas and building them for mounting on aircraft requires some hint of how they can be mounted, which leads to how we should design them.

Thanks everyone for inputs.

I am a bit surprised that bean counters are even picky regarding life and death airworthiness decisions. Maybe it would be better if the only FAA allowed job title was "accountant-manager-test pilot". The decision making would take on a new importance.

kch

 

[BrianR]

Higgler,

A thin conformal antenna is a good idea as it should add little to the aero loads. I think you'll have difficulties bonding it though, due to changes in hull barrel diameter at altitude. Thumbnail calcs suggest that over a 36" part of the skin the local stretching could be from .05" to .15" with 8psi dP of according to barrel dia. This is outside my experience though.