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Belly Antenna Alignment

1001001

Well Known Member
A question on aligning belly-mounted antennas (bent whips, blades, etc.):

What's a good reference for aligning these antennas with the airstream and/or the aircraft axis? It looks like, but I can't be certain, that the ribs under the rear seats are aligned with the longitudinal axis? Or are they angled slightly, like, but to a lesser degree than, the baggage floor ribs?

I'm looking to install my antenna doublers under the rear seats and it would be easiest to use the ribs for alignment purposes.
 
A question on aligning belly-mounted antennas (bent whips, blades, etc.):

What's a good reference for aligning these antennas with the airstream and/or the aircraft axis? It looks like, but I can't be certain, that the ribs under the rear seats are aligned with the longitudinal axis? Or are they angled slightly, like, but to a lesser degree than, the baggage floor ribs?

I'm looking to install my antenna doublers under the rear seats and it would be easiest to use the ribs for alignment purposes.

The F-1017 ribs under the rear seats do not taper aft, so you could use those for alignment. If you're mounting under the rear seats, the F-1005A bulkhead is a good reference to square from.

I mounted my COM antennas just forward of the landing gear mounts each side under the crew seats and used the main spar for reference, which is the base lateral reference point for the airframe.
 
A question on aligning belly-mounted antennas (bent whips, blades, etc.):

What's a good reference for aligning these antennas with the airstream and/or the aircraft axis? It looks like, but I can't be certain, that the ribs under the rear seats are aligned with the longitudinal axis? Or are they angled slightly, like, but to a lesser degree than, the baggage floor ribs?

I'm looking to install my antenna doublers under the rear seats and it would be easiest to use the ribs for alignment purposes.

I made a template, which later became a doubler. It was aligned with a rib under the rear seat. It may be off a degree or two, but doesn’t seemed to have an impact.
 
I picked two spots on the fuse and ran a level perpendicular to the ground down the sides (ensure door bottoms are level first) and found the belly center by measuring inward to find the width, then half the width from the levels in that spot.. Run a string between those points and you gt th bellies center line. Very easy to use a tape measure to then make lines parallel to the centerline.

With one level, measure in a fixed distance from the level on one side and then repeat on the opposite side, same distance back from the firewall. Very easy to use a tape measure and math to find the center based upon those two marks.
 
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I made a template, which later became a doubler. It was aligned with a rib under the rear seat. It may be off a degree or two, but doesn’t seemed to have an impact.

Maybe just enough, if you're lucky, to compensate for the swirling prop wash that helps define the slipstream on the belly of a tractor airplane. :D

It has always bugged me that we don't allow for this perturbation when we set the incidence of gear leg fairings and other protuberances that live in close proximity to the fuse and inside the rearward projection of the propeller arc - you know it's real, but it's hard to quantify...
 
OK I will bite

Maybe just enough, if you're lucky, to compensate for the swirling prop wash that helps define the slipstream on the belly of a tractor airplane. :D

It has always bugged me that we don't allow for this perturbation when we set the incidence of gear leg fairings and other protuberances that live in close proximity to the fuse and inside the rearward projection of the propeller arc - you know it's real, but it's hard to quantify...

OK I would like to know if the prop wash is continuously rotating, or does it only rotate when the blade prop wash comes by?
 
Two pictures and a sketch to illustrate the phenomenon. Complex interaction between the freestream and the spiraling propwash.

55CKC.png


omiVT.jpg


main-qimg-57ef053dba294240c211bd8ec37cf879-c
 
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