Transponder and Comm Rack in Tail

[OneTwoTree]

Please rate this set up! Give it to me straight. I had an acquaintance many years ago, who tragically lost his life due to a mobile GPS unit falling off the dash and getting jammed in the tail cone of his Extra. So I am trying to do my due diligence when it comes to this area even though it would be a different situation. I created this rack for the transponder and second radio. I understand the 14 is already tail heavy and if someone could share their weight and balance, that would be greatly appreciated. However, I have to run these two units in the tail. Any eagle eye criticism is welcomed!

 

[Walt]

I often put the xpdr in the back but never the com due to long audio wire runs (noise).

 

[planenutz]

I recently completed a very similar installation in our Monsun. I won't comment on the weight & balance side of things as I needed weight aft on that machine, but in terms of the physical assembly I have the following comments (since you asked):

1. What material thickness did you use? It looks very heavy. I used .063" for my baseplate and .040" for my sidewalls and that was more than enough.
2. It sure is solid.
3. I would recommend cutting some lightening holes in the sidewalls and baseplate. Not only would it make the assembly lighter, it would look better as well. For example I could see you putting three rows of 3 x 1" dia holes in the centre portion of the baseplate - so 9 holes in total.
4. You've got more rivets in that angle than a Cessna 152 Aerobat has in its entire wing. Nice work.
5. The portion of the baseplate aft of the rear angle appears to be superfluous... adding weight for no benefit. It's only there for the ride. Cut it off.
6. Because the angles are the primary structural load-bearing parts of your frame, it would have perhaps been better to place the mounting bolts through these angles (2 at the front and 2 at the back) to bolt the assembly to the airframe. No point changing it now, though.
7. That square hole is a pain to cut and trim, I know, but you can do better than that. Finish it off properly.
8. What strengthening have you added to the tunnel mounting points?
9. One screw on the front of the baseplate? It's not really doing much. You could have three across the front, or two (one at each outboard edge), but one is not really doing much. You could almost get rid of the front tab as well as the back tab but I assume there were reasons you left it there.

I'm not trying to be critical here, just offering some thoughts in the same way I would if a local builder had asked me to look at it.

Here is a photo of a similar installation (during a trial fit to check whether further modification is required) and you might be able to use this for inspiration on how to reduce the weight a little:

 

[Norman CYYJ]

Please rate this set up! Give it to me straight. I had an acquaintance many years ago, who tragically lost his life due to a mobile GPS unit falling off the dash and getting jammed in the tail cone of his Extra. So I am trying to do my due diligence when it comes to this area even though it would be a different situation. I created this rack for the transponder and second radio. I understand the 14 is already tail heavy and if someone could share their weight and balance, that would be greatly appreciated. However, I have to run these two units in the tail. Any eagle eye criticism is welcomed! View attachment 90675

My only comment is that it way over built, just to hold a couple of units.

 

[OneTwoTree]

I often put the xpdr in the back but never the com due to long audio wire runs (noise).

As an air traffic controller the one thing that drives me up a wall is bad coms on the ground. So I have one antenna on the belly and one on top right behind the rear window, so when I’m on the ground, comm 2 will be used transmitting and receiving from the top antenna. So the run will be VERY short.

 

[OneTwoTree]

I recently completed a very similar installation in our Monsun. I won't comment on the weight & balance side of things as I needed weight aft on that machine, but in terms of the physical assembly I have the following comments (since you asked):

1. What material thickness did you use? It looks very heavy. I used .063" for my baseplate and .040" for my sidewalls and that was more than enough.
2. It sure is solid.
3. I would recommend cutting some lightening holes in the sidewalls and baseplate. Not only would it make the assembly lighter, it would look better as well. For example I could see you putting three rows of 3 x 1" dia holes in the centre portion of the baseplate - so 9 holes in total.
4. You've got more rivets in that angle than a Cessna 152 Aerobat has in its entire wing. Nice work.
5. The portion of the baseplate aft of the rear angle appears to be superfluous... adding weight for no benefit. It's only there for the ride. Cut it off.
6. Because the angles are the primary structural load-bearing parts of your frame, it would have perhaps been better to place the mounting bolts through these angles (2 at the front and 2 at the back) to bolt the assembly to the airframe. No point changing it now, though.
7. That square hole is a pain to cut and trim, I know, but you can do better than that. Finish it off properly.
8. What strengthening have you added to the tunnel mounting points?
9. One screw on the front of the baseplate? It's not really doing much. You could have three across the front, or two (one at each outboard edge), but one is not really doing much. You could almost get rid of the front tab as well as the back tab but I assume there were reasons you left it there.

I'm not trying to be critical here, just offering some thoughts in the same way I would if a local builder had asked me to look at it.

Here is a photo of a similar installation (during a trial fit to check whether further modification is required) and you might be able to use this for inspiration on how to reduce the weight a little:

View attachment 90695

I asked for replies and I appreciate your time!!! The plate and walls weigh 1.9 and are built out of .088 because it’s all I could get my hands on. Let’s say I rebuild it using what you did AND do some lighting holes. I bet I save a pound. Is the squeeze worth the juice. I guess I should probably wait until I do a W&B. Again thanks so much for your help. I welcome all thoughts!!!

 

[rv8ch]

Please rate this set up! Give it to me straight.

Thinking about maintenance, how easy will it be to remove the radio/transponder when there is a problem and you have to send them back? I don't know much about the RV-14 tailcone area.

 

[planenutz]

I asked for replies and I appreciate your time!!! The plate and walls weigh 1.9 and are built out of .088 because it’s all I could get my hands on. Let’s say I rebuild it using what you did AND do some lighting holes. I bet I save a pound. Is the squeeze worth the juice. I guess I should probably wait until I do a W&B. Again thanks so much for your help. I welcome all thoughts!!!

Wow... 0.088". I've never seen that gage used before, but if it's all you could get then run with it.

No, I don't think the squeeze is worth the juice - not now that you've got it to the point where its ready to roll. From this point you could just make a couple of modifications and be done.

I've PM'd you.

 

[Jetmart]

I only mounted my transponder in the tail but screwed one end of the rack in directly and the other end made small brackets on the side. Is there any concern about keeping the racks open for cooling? I thought so at the time therefore kept the rack open.

 

[Walt]

As an air traffic controller the one thing that drives me up a wall is bad coms on the ground. So I have one antenna on the belly and one on top right behind the rear window, so when I’m on the ground, comm 2 will be used transmitting and receiving from the top antenna. So the run will be VERY short.

Antenna cable loss at com frequencies is very low so short antenna cable won’t really improve anything, the top mount of the antenna is key.
Cable loss at transponder freq is very high, so keeping that cable short is important.

 

[Walt]

I only mounted my transponder in the tail but screwed one end of the rack in directly and the other end made small brackets on the side. Is there any concern about keeping the racks open for cooling? I thought so at the time therefore kept the rack open.



View attachment 90737View attachment 90738

I do almost the same except use the 2 mounting screws on each side at the front of the rack attached to an angle, or one in front and one at the back attached to an angle. With your attachments the rack is able to rotate around that single screw.

 

[Jetmart]

I do almost the same except use the 2 mounting screws on each side at the front of the rack attached to an angle, or one in front and one at the back attached to an angle. With your attachments the rack is able to rotate around that single screw.

Walt,

Not sure I understand what you are suggesting by pivoting. I have four screws in total. Two screws at the front of the rack are bolted through the tailcone rib and two screws through the bracket I made. Am I missing something?

 

[Walt]

Walt,

Not sure I understand what you are suggesting by pivoting. I have four screws in total. Two screws at the front of the rack are bolted through the tailcone rib and two screws through the bracket I made. Am I missing something?

Sorry missed the 2 in the bottom of the rack, you must have modified the rack as those aren’t standard mounting holes.

 

[OneTwoTree]

Thinking about maintenance, how easy will it be to remove the radio/transponder when there is a problem and you have to send them back? I don't know much about the RV-14 tailcone area.

Hey! Thanks for the response! At 40, not too bad. 20 years from now is the great unknown! They both slide out easy with a hex screw.

 

[OneTwoTree]

Sorry missed the 2 in the bottom of the rack, you must have modified the rack as those aren’t standard mounting holes.

Yes Walt, I did. I used a biplane jig to make 4 more mounting locations along the ribs. And the .088 aluminum really stabilized the ribs along the top. I’ve tripled to “twist” it. There is no twist.