aircraft antenna mounting

[Higgler]

Has anyone out there added antennas to jet aircraft.
I'd like to find out what hoops one has to jump thru to add antennas to high speed aircraft (military or commercial)? this will probably be a large (36"x36") thin (0.5") antenna added to the belly and top surfaces.
And what are the challenges to adding large antennas directly applied to the surface (I realize that aircraft skin expansion as aircraft heat up is a biggie).
Just a reference would help.
Thanks,
kch

 

[rb1957]

higgler,

i imagine your antenna is conformal (rather than perpendicular to the airplane skin).

i don't have much experience with military jets, but i'd be surprised if skin friction was a big problem.

i have installed a large conformal antenna on a large jet transport. we considered aero-suction (or equally cabin depressurisation) as the static load case. then you've also got fatigue loads, principally pressuriation for a transport or maybe flight loads if a fighter.

good luck

 

[Higgler]

rb1957
what size was your array? Was it one piece or multiple pieces. Did you have to match the coeficient of expansion precisely to the aircraft materials.

kch

 

[Nigel]

Depends on the aircraft, civil or military. Structural things to check are or hoop and longitudinal static strength of the installtion, also fatigue and damage tolerance, plus aerodynamic loads transfered into the structure. For civil aircraft, the installation will need to be approved by your local civil aviation authority (FAA in the USA)

Nigel Waterhouse B Eng (Hon's)
Can-Am Aerospace,LLC, Canadian Aircraft Certification Centre

http://www.canamaero.com

 

[rb1957]

higgler,

our antenna was a little smaller than yours, about 32" x 16", sinlge piece structure. we never considered thermal expansion, for our large transport i would have thought that we might have a bigger concern with thermal contraction (the hull would get cold during cruise).

whichever, this would be much smaller than allowing a pressure differential (we used 2psi) under the antenna (as though the cabin pressure was leaking underneath the antenna) or aerosuction.

we did a fatigue calc (on the feeed-thrus, which had local dblrs), and recommended annual removal of the antenna to check the structure.

 

[Higgler]

rb1957,
Thanks for the info.
Can you provide any more generic details on the mechanical (maybe a generic press release?). Did you need anti-static paint on the antenna radome?

16"x32" is pretty large, I was guessing at my future size, it may end up being similar.

A fast aircraft would have both a cold and hot skin (100 C)capability, so the expansion of the aircraft skin might be more of a problem.
What if you lost cabin pressure? does your antenna fly away?

Kevin

 

[rb1957]

kevin

i can't say too much about the installation ... the airplane isn't "black" but it is very dark grey, if you get my drift. i think there should be commerical projects out there, boeing and airbus i think offer in-flight internet access, "connexions" i think, using these antennas.

we assumed that the antenna would support a 2psi differential pressure, cabin pressure leaking under the antenna and leaking out the sides, or aero-suction. the mass of these is small, so inertia loads are small. nigel is right about the damage tolerance issues (inspectability) on commercial planes.

i don't think thermal effects are significant unless you're going REALLY fast. most likely the antenna will have a higher coefficient of expansion than the airplane skin.


good luck

 

[Sparweb]

Is this aircraft pressurized? I expect it is.

Hoop stress in a 30" round fuselage under 6 psi pressurization is anywhere from 3 - 6 ksi. That much stress makes the skin stretch, by 5 to 10 thousandths of an inch, per foot of circumference. Your antenna is 36" across, so let's say 0.015" - 0.030". The antenna must not be bonded or fastened in place so as to prevent this stretching. Otherwise, the antenna will be a "hard" point with higher stiffness than the surrounding skin. What is the antenna made of? Are the materials stretchy?

Stresses due to landing loads can magnify stress in the skin, too.

Steven Fahey, CET

 

[Higgler]

Steven SparWeb,
the aircraft I expect to place this on is pressurized.
I plan on a multi-layer laminate type antenna using Rogers Corporation Dielectric material. Probably Rogers 4003 or 6002 dielectric material, about 4 layers x 0.060" per layer. Alternatively I could use Hexcel or Foam cores with 5-20 mil skins.
The antenna will have some active electronics built inside it.
The antenna could be segmented to reduce its' size, but only after proof of severe problems in the 36" dimension.
kchiggins,
Toyon Research Corporation.

 

[Sparweb]

I attempted to find some mechanical properties of the Rogers materials that you will use, but came up blank. I'm obviously not going to figure out how the antenna works, even if you do try to explain it to me, so I don't know what constraints apply to the electronics.

If you can come up with a way to bond it in place using a substance that allows the skin to stretch underneath, then you have won half the battle, because the antenna material won't pick up stress from the skin. (Not much, at least.)

Penetrating the skin is an issue that has to be solved concurrently. Usually, the holes are reinforced with an extra layer of aluminum around the hole. Pressurized aircraft see regularly repeated cycles of stress, therefore stress concentrations (like open holes) are invitations for cracks to begin and spread. Hidden under a conformal antenna, the crack could be ominously large before being discovered. A doubler on the outside of the skin makes for a lumpy surface to attach your antenna. A doubler on the inside is harder to install (sometimes) but leaves the skin smooth for the antenna.

When you try to put it into service on an aircraft, the owner/operator will need engineering & regulatory approval to do so. They won't like it if the task is complex or expensive.

I would suggest creating an array of smaller antennas that would allow inspection of the skin every 6-12", provided that each individual antenna element doesn't need its own connector hole drilled through the skin.



Steven Fahey, CET

 

[rb1957]

kchiggins,

you still haven't been very open about the intended plane. assuming that it is a commercial transport, i'd express some caution with steven's proposal of bonding the antenna in place. consider if you have to remove the antenna to inspect the skin underneath ... usually not very easily done with adhesive. another concern about adhesive is how well will it react the aero-suction that will be there.

as steven says we're not really up to speed with how the innards (yes, that's the technical term, from a structures prespective) will react to a bit of strain. however, i have difficulty imaging that a plastic antenna will either prevent the shell from expanding (under pressure) or drive large displacements into the fuselage (straining more than the expansion of the skin).

i would prefer to attach the antenna with screws, repair (local dblrs) the damage you've done to the plane, write up the required inspections ...

if this isn't a commercial transport, i think you have a bit more latitude with the inspections, but youstill have to do something.

ps ... i e-mailled your webmaster (attn kchiggins)

 

[Nigel]

I have a colleague that has Transport Canada approval for bonding antennas to the skin, not sure if it is on pressurised fuse' though.

Nigel Waterhouse B Eng (Hon's)
Can-Am Aerospace,LLC, Canadian Aircraft Certification Centre

http://www.canamaero.com

 

[Stache]

I have been working with the Western Pacific ACO installing antennas and have ran into a problem with some manufactures flight manual supplements. These supplements are spelling out certain stations locations to install antennas and not have any others antennas within a certain distance.

Needless to say this has placed a lot of work on others who want or need installs trying to meet the instructions for continued airworthiness (ICA) associated with previous installs and flight manual supplements.

Of course each make and model is differnt so you may want to keep this in mind when you decide to intall it.

Stache

 

[Higgler]

rb1957,
lets say the aircraft is very fast and grey in color.

If I summarize the above info;
1)adhering a large antenna to the fuselage may not be a good idea for inspection and skin stretch reasons. Small antenna adhering ok (Nigel, what size max. antenna is ok for bonding? au Canada, does he use 3M adhesive?)
2)screw mount the antenna if it's one large piece. RF gasket edge of antenna for good electrical contact(screw hole mounts give rise to crack propagation).
3) Sectioning the antenna to minimize it's largest size may allow for adhering a larger antenna, but problematic for inspections.
4)need for routine (yearly+) inspection under the antenna is critical to see if cracks are propagating (= problem when adhering the antenna).
5)keep antenna low mass to minimize inertial loads in high G maneuvers.



Stache,
Your example shows a bit of fear in the antenna manufacturer. What happens in a crowded antenna world with one antenna next to another. = The higher frequency antenna performance degrades more than the lower frequency antenna performance.

For example; if an antenna at f= 500 Mhz has instructions to leave 4 feet clearance (2 wavelengths). Placing a higher frequency antenna (say 6 Ghz quarter wave omni antenna) within one foot of this 500 Mhz antenna will not affect your 500 Mhz antenna at all. The new 6 ghz antenna is invisible at lower frequencies because it's too short to be resonant. The 6 ghz antenna will be affected if it's omni. Wings, fuselage and tails will affect that 500 Mhz antenna more than that added 6 Ghz antenna. Aircraft tend to produce a lot of ripple in antenna patterns for omni antennas.


More inputs would be appreciated,
kchiggins

 

[rb1957]

kc,

thx, understand your a/c isn't a large commercial transport. suspect your aircraft may have significant aero-suction effects (speeds greater than mach 1?) and pressurisation concerns maybe irrelevent (ie not mounted on a pressurised cabin). can you get Cp plots for the intended plane, or make conservative guesses/estimates ?

your antenna can be mounted with peripheral fasteners, or adhesive; i assume screws through the middle of the antenna are not going to happen. i prefer discrete fasteners around the edge as being more reliable (quantifable) than adhesive. also easier to remove, replace, repair, ... i suspect that the antenna is substantial enough to transfer the loads to the periphery. compare whatever you're thinking about with a Boeing "Connexions" antenna.

if you are going to cover up primary structure, then how are you going to inspect it ? mounting on an access door might be an idea ? again if you know the plane intended for this, then you'll be able to find out this info.

good luck

 

[Higgler]

rf1957,
The antenna may be flimsy and flexible to keep the weight minimal. If we screw attach it at the perimeter as you suggest, it seems that flexibly adhering it all along the surface will be neeeded.
Possibly we could remove the outer edge attachment points for inspection without removing all the adhesive on the interior portion, or is some type of Frame attachment at the perimeter ok? We'll need RF connectors and multi wire connectors to penetrate the aircraft skin, placing that under a separate frame would seem convenient (to a rookie like me).

This is a proposal and I want to show some effort in understanding the connection problem to the aircraft. I'll do some drawings and discuss the problems.


I had to google the Cp comment,
"airframe cross-flow pressure distribution (Cp) plots".
I won't have that data from the customer until after the program begins.

kchiggins

 

[rb1957]

kc

if you glue the middle of the antenna, you might as well glue it all ... and if the antenna is so flexible that you could bend it, then this may be the better approach. in an unpressurised plane, holes for electrical connectors are probably not that big of a problem ... perhaps repair the holes with localised doublers. if your equipment is anything significant in weight (more than 10 - 20 lbs) than i'd suggest a platform supported by the airplane's frames.

Cp is co-efficient of pressure ... plots of the local air pressure (ie aero-suction)

 

[Higgler]

rb1957
The antenna will be flexible, and contoured to the airframe, especially in the large 36" square size.
So we'd adhere it along the surface with some slightly thick adhesive that's removable (for inspection) and flexible(to compensate for temperature and pressure changes).
Add a metal or non metal frame (2" wide ish) on the perimeter to help mechanical attachment and provide rf gasket compression at the antenna edge. I assume the frame would taper from the antenna thickness of 0.5/1.0 inches down to 0.060" ish for aero considerations.
Add doublers for the hole penetration of the RF and Multi-wire connectors penetrating the fuselage to prevent crack propagations. These penetrations probably located under the frame.
For inspection, unscrew the frame to view it's mounting holes and the connector mounting holes. Maybe do a pull test to check the adhesion of the rest of the antenna, or inspect it with ultrasound? etc. Although that is difficult.

How's that sound?

kchiggins

 

[rb1957]

doesn't sound unreasonable ... there's always more than one way to skin a cat ...
a thought ... instead of bonding the antenna to the airplane skin, you could make a substrate for the antenna. either from forming a sheet (0.063"?) of 2024 to the fuselage contour, or if the geometry is difficult then make a pattern using a room temp cure glass fuber/resin composite, then make a mould, then make a structural composite graphite? substrate for your antenna. don't make it too substantial, you don't wnat it to be overly stiff (even if it isn't being mounted on a pressure skin).
i do like the idea of a peripheral frame, to secure the edge of the antenna ... you can rivet the substrate to the frame, and then the frame to the plane.

 

[Higgler]

Lets see if I understand your statement;
This 0.063" sheet, would that be just an added mount, or a base to attach the antenna to. Assuming this 0.063" thick 2024 matches the aircraft skin perfectly for expansion. Then you'd attach this antenna & 0.063" mounting plate to the aircraft with rivets every X inches, plus add the mounting frame on the perimeter? This way, the entire antenna isn't bonded to the aircraft, but attached with hard mounts, which when removed periodically allows for inspection (and antenna functional testing).
Or, this 0.063" skin is replaced with a molded composite equivalent?
I like the metal idea for creating good electrical contact to the aircraft skin for antenna ground plane effects.
I would think that installation and design of everything would be simpler if everything was flexible. I'd expect high G maneuvers must change the shape of things. If we put this on the underside of a wing, I think they change shape drastically. The frame being multi piece (4 pieces)may help the reshaping too?

kchiggins.