Large Antenna Install Question

[ddav]

Guys,

I have to install a fairly large Antenna (spec sheet attached) to a Far 23 aircraft (pressurised area most likely).

I'm thinking intercostals and shear ties to transfer loads into stringers and frames. I'd like some opinions/things to watch out for?

Would it be crucial to cold work fastener holes?

Would the antenna manufacturer provide me with a Cd value to be able to calculate max side loads? Does anyone know what the Side Load shown in the Environmental Section of the spec sheet means?

Also, how would I know if the loads getting transferred from the Antenna installation into the a/c will not overload the existing a/c structure?

I've only been in the field for 2.5 years so I have lots to learn.

 

[FastMouse]

Hi ddav. I don't have complete answers for you and maybe you've already got this stuff in your file, but here's my 2 cents worth.

> I'm thinking intercostals and shear ties to transfer loads into stringers and frames.

Sounds good.

> I'd like some opinions/things to watch out for?

Non-exclusive list:
(1) Ensure that new holes & fasteners are installed to the same spec.s as existing, adjacent features to assist pass-by-comparison rationale.
(2) Ensure that the loads introduced to the frame do not adversely affect the buckling properties of the web. Easy to say, harder to do.
(3) Don't forget abuse loads.

> Would it be crucial to cold work fastener holes?

Advise looking at the SRM as a starting point.

> Does anyone know what the Side Load shown in the Environmental Section of the spec sheet means?

My *** GUESS *** is that it means that the antenna housing can withstand a pressure differential of 6 psi on the sides of the housing, within the environmental envelope. In operation, any pressure differential will be non-uniform, so I guess that the 6 psi is some kind of equivalent. Guesses are worth nothing, however, so I suggest a call or email the manufacturer!

> Also, how would I know if the loads getting transferred from the Antenna installation into the a/c will not overload the existing a/c structure?

It sounds like there is no OEM support available for this mod. I am currently wrestling with the same issue. Given that the load in the as-manufactured structure is unknown, how can it be stated that the incremental load due to the mod is acceptable? I regret that I don't have an answer yet and I would love to hear others' thoughts. There must be a zillion mods certified that locally introduce additional load, so the answers exist.

 

[rb1957]

don't bother cold working the holes.

you could use a pair of intercostals, each picking up four antenna screws, or you can use a large U-channel and pick up all eight screws. attach intercostals/U-channel to the fusealge independent of the antenna screws.

i expect you're not planning on using protruding head rivets thru the fuselage skin, and the fuselage skin is not going to be thick enough to accept the CSK. I'd recommend an external doubler, 0.05" thk, with LZ-4 rivets. a 0.04" skin is border-line knife-edge for LZ-4 rivets, but these are probably "standard of type", so you might be able to avoid the extenral dblr, if the installers are careful

loads ... forget drag, it'll be really small ... investigate it to satisfy yourself. from above ""(3) Don't forget abuse loads." the key with antenna design is to make the installation stiff so it doesn't vibrate. typically apply an abuse load normal to the plane of the antenna (applying a moment onto the screws, which react with a couple). the load is whatever you want it to be ... i use 200 lbs ULT, i've seen 100 lbs LMT or 200 lbs LMT.

if you use the external dblr, remember damage tolerance ... you'll have changed the inspection of this part of the fuselage. some will say that it's only a small area so who cares. some will say that there's no structural inspections required so who cares. these are defenable positions once you've investigated them. the fatigue stress (hoop stress from pressure) should be very low. the area of interest is the load transfer rivets along the edge of the doubler; this is more severe than the antenna fastener holes or the antenna cable feed-thru. Niu's book has a write up, a fatigue stroy might be sufficient for a part23 plane.

one thing, FAA has been really strict requiring flight test to prove no flutter/vibration/buffet from the antenna. They may require flt test to Vd, they might be argued down to Vmo.

 

[Sparweb]

Would the antenna manufacturer provide me with a Cd value to be able to calculate max side loads? Does anyone know what the Side Load shown in the Environmental Section of the spec sheet means?

The Cd is drag, with is aft-ward, not side-ward. For a side-ward load you need the "normal" pressure - which is given as 6 psi in the datasheet. Treat it like a little "wing" with uniform pressure over the entire surface. Lift on an antenna blade would be caused by (a) yawed flight or (b) ice accretion, which also increased drag. The other numbers (mach 0.85 at 35,000 feet) can be deceptive. The air pressure at 35k feet altitude is much much lower than at sea level. Don't rely on the antenna maker to tell you what the loads are. Your aircraft may have different Vmo and Mmo figures and one is higher, in dynamic pressure, than the other.

You can go overboard on the loads, as they've said, but the lift is important enough that you usually do need intercostals to stabilize it. They should go laterally, so that the antenna's lift is a "couple" on the mid-section of a beam. Stiffest way to go, especially if you use 2 of these intercostals. Which ever screws line-up with the intercostals, those are the loaded ones, the other screws won't be load-bearing (in a conservative analysis) so treat them like they're there to just keep the rain out.

Don't forget to look for similar antennas on the airplane already. Solved problems for me in the past, to find OEM antenna installations on a Piper Malibu with no reinforcement at all, when I was analysing the structure and my assumptions led to big doublers and stiffeners for the same antenna. Then you can point to the OEM and say "see, see?" and you can back off some of your conservative assumptions.


STF

 

[rb1957]

i think you need to be careful in designing something like how the OEM did it. they have access to way more data on the a/c than we do so we're assuming stuff and being conservative. sure, you can be "overly conservative", if for example you figured you needed a 1/4" thk dblr. and truth be told, a riveted on dblr is very inefficient at reducing the stress concentration at the hole so why bother ? 'cause i think most people expect it, and question it's absence.

i suspect the fuselage is 0.04" thk which is enough to hide LZ-4 rivet CSK, and the hoop stress in the skin is also very low. the other advantage of this design is that it is the way the rest of the fuselage is put together. i'm used to a larger fuselage with a 0.032" thk skin and a higher hoop stress and an OEM design which doesn't predominately use LZ4s.

 

[ddav]

Folks,

I have contacted the Antenna Manufacturer for some assistance and will update.

 

[kontiki99]

I'm avionics not structures. I have been told by structures folks I work with that some long blade antennas (VHF antennas for part 25 aircraft) tend to vibrate like tuning forks, side to side) making it a constant source of problems for some aircraft installations.

In theory, there is no difference between theory and practice; experience suggests that in practice, there is.

 

[rb1957]

true enough, particularly an antenna in a vortex street; but then that's why we flight test these things

 

[WackoJ]

I have seen installation of these antennas which were not properly supported in the lateral direction, and yes they did vibrate leading to cracking in the skins. And if possible, I would always recommend installation to an area not on the pressure hull. Let avionics workout how to get the Coax there!!

 

[debodine]

WackoJ, there is a lot to be said for conferring with the avionics guru to see if what you suggest can be done. It is possible that RF shadowing, interference with other antenna signal paths, manufacturer's approved data instructions or system cable length requirements, etc., the location may end up dictated to the OP and not be negotiable.

But since the OP says "...(pressurized area most likely...)" it may be that consulting avionics and/or the manufacturer can produce an installation that will avoid using the pressure vessel, validating your recommendation to do just that. Personally when I have an antenna installation to develop, one of my first questions is, "Can I avoid the pressure vessel for the antenna?" Sounds like you and I have both been there, done that and have a faded T-shirt in the closet.

On the lighter side, when developing an antenna installation I always enjoyed asking the structures lead to accompany me to the aircraft. Inside the fuselage, while pointing to a piece of primary structure, I state, "The hole for the harness has to be right there" and watch their faces as they prepare to vociferously protest my plan. Without giving them a chance to explode, I tell them their structures manager said holes in primary structure were not that big a deal and they would typically realize instantly I was messin' with 'em and we would have a good laugh.

 

[Kwan]

The electrical guys always want holes through structures. Ask to put a hole in the wire and they get really upset!

I heard this from a structures type back when I was doing rack and stack. It still makes me laugh.