Antenna installation stress issues 1

[SteveAero]

Hi,

This is a follow on to thread2-122412.

I am installing a SATCOM antenna on the top fuselage of a pressurized commercial jet. I am relatively new to this field, and have the following questions that I hope the experts can advise me on:

1. The doubler used to reinforce the skin, shown in the Eastin report attached by SparWeb in the abovementioned post, seems to be in the middle of the skin panel, not picking up on frames/stringers. Is this to avoid picking up global loading from these items? What will this sudden increase in stiffness do to the pressurized panel when it flexes?

2. Also, what about the out-of-plane loads induced by suction pressure on the radar assembly? This can be quite large, for example for a passenger jet travelling at altitude, and using the formula F= 0.5 x rho x v^2 x Cd x S. These out-of-plane loads are now being reacted in bending by the skin/doubler combo via an ineffective load path. In your experience, did such 'flat-ish' SATCOM doublers require more backup structure than a 'simple' doubler?

3. I was thinking of installing local intercostals between stringers to take the out-of-plane loads. I'm concerned however about the stiffness of such a structure. How will this affect the pressurized skin; will it act as a mini frame, splitting the pressurized panel in half, or will it just flex with the skin?

Looking forward to your responses.

 

[Sparweb]

Hi Steve,

1)
Eastin only provides the basics with simplified examples. Combining the doubler with the stringer or frame, or spanning 2 bays, would obviously complicate the load paths.

2)
Out of plane loads must be considered. Have you read FAR 25.571? There are specific requirements. There is also some information about them in the Eastin presentation.

3)
They are needed for some types of antennas, not needed for others. Your first place of investigation is always on the aircraft itself. What did the OEM do when they installed similar types of antennas on the subject aircraft?

2. Also, what about the out-of-plane loads induced by suction pressure on the radar assembly? This can be quite large, for example for a passenger jet travelling at altitude, and using the formula F= 0.5 x rho x v^2 x Cd x S. These out-of-plane loads are now being reacted in bending by the skin/doubler combo via an ineffective load path. In your experience, did such 'flat-ish' SATCOM doublers require more backup structure than a 'simple' doubler?

{emphasis added in red}

In your questions above, you have referred to SATCOM antennas and RADAR antennas. These are different species of animal. They must be treated differently.


No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[rb1957]

my 2c …

1) There are two schools for dblr design … those that pick up the surrounding elements (stringers, frames) and those that don't. If you don't pickup these then you need to consider the secondary bending in the skin, due to the load transfer into the dblr. For typical structure this can be something like 40% of hoop stress. If you pick-up these elements, then the stiffness of these elements is such that secondary bending is mitigated. If you pick up these elements then you should think about their "day job" .. the loads they carry during normal flight, and the impact your doubler will have on these (my opinion is this is minimal, as these structures aren't designed for anything near Ftu. If you pick up these elements you will need to NDI the dblr perimeter … usually not a big deal as you should be able to find a repair dblr inspection that does the same job. If you don't then it's either external NDI of an internal visual inspection (removing interior components). For me the secondary bending issue is the biggest concern.

2) yes, you need to include the effect of external suction of the dblr. Adding it to the cabin pressure (in calculating hoop stress) is conservative.

3) If it a low profile antenna you shouldn't need much internal reinforcement. If it a VHF blade you should IMO use a U-channel to beam the loads to the frames.

There are many other issues you haven't asked about …

a) internal or external dblr ?
b) vibration and buffet ? obviously not much of a problem for low profile antennas.
c) loads ? In my experience aero and inertia loads are usually trivial. To create a structure free from vibration and buffet, I use an abuse load of 200 lbs (call it limit if you want to be more conservative).
d) fuselage penetrations ? you need to be careful of penetrations in the pressure cabin … to prevent moisture getting between the skin and the dblr we put a ring of rivets coles (2D) from the edge of the penetration.
e) effective load transfer rivets … count on having two rows of rivets around the perimeter for load transfer.
f) dblr thickness …

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