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Thread: Avionics in Upper Rear Baggage?

  1. #1
    cubman's Avatar
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    Avionics in Upper Rear Baggage?

    I'm building a Javron cub and planning the avionics and panel now. Has anyone remotely mounted a comm radio, ADSB/transponder and ELT in the rear upper baggage area of an Experimental or Certified Supercub? Good or bad idea? What are the factors to consider, i.e., weight, wire route/resistance? Biggest advantages seem to be easier access for maintenance, shorter runs to antennas and less heat. What are the cons?

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    Biggest issue would be sufficient structure so they donít hit the back of your head in an accident. Figure a minimum of 9g impact, better if you design to 18g.


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  3. #3
    cubman's Avatar
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    Good thought. I have a headliner but could install a bulkhead between the back of the headliner and the rear upper baggage.

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    mvivion's Avatar
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    With such compact and light avionics now available, why would you even consider remotely mounted avionics? Look at Trig or any of a half dozen others that make 2 1/4 inch panel mounted comm and X-ponders. Then portable GPS mounted in panel. Only thing that needs to be aft is ELT. Unless you want an auto land approved autopilot....

    Itís a Cub....keep it simple and light.

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    I put my elt on a panel I made just forward of the tail in my pa12. It required making a small access door, no big deal. 3 advantages of it way back there: it helps with 12s nose heavy tendency, the antenna would be protected by the tail in case of an accident where you end up upside down, it is as far away from the fire source as you can get.

    I also installed my ADSB GDL 82 under the rear seat, easy to do, easy to access, easy to run wiring and cables. What's not to like about it
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  6. #6

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    I have the ADSB-in reciever back there (certified cub battery compartment) and it works really well via blue tooth and you don't have to put that Garmin flip-you-off antenna/thing on the dash but other than that why? Pierce put a shelf under my panel for the remote radio, transponder, and ADSB-OUT magic boxes which work really well.
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  7. #7
    www.SkupTech.com mike mcs repair's Avatar
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    what brands??

    ELT usually is back there, but that's all...

    everything else fits fine up by panel, might add a couple side access doors like i do to get at stuff behind dash easily...

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    Avionics in Upper Rear Baggage?

    Every component of my G3X is mounted on a shelf behind the panel. For access the boot cowl has easily removable panels to access the boxes and wiring and that's already come in handy a couple of times. If you use a conventional boot cowl you can make access through removable panels in the instrument panel. My old -12 had drop-out component panels. Also handy.

    These new electronic gizmos have a crapton of little wires running between them. I wouldn't want them to be any longer than they need to be.

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  9. #9
    wireweinie's Avatar
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    Structurally it should be fine. Remember that the original battery was located there in factory PA 18's. Like mounting anything else, you have to use proper fabrication techniques.

    But . . . . as said above, why? Boone don't get that hot (I grew up south of Lacrosse) and new avionics do not generate nearly as much heat as older models did. They are also smaller than they used to be. I have fabbed panels for certified Cubs, using the short factory panel, that had all instruments, com, and intercom. You can even fit a GPS using a ball mount. I'm assuming that you have no header tank? This means you have plenty of space between the firewall and the panel. IF YOU NEED TOO, you can mount components in this space. Some mount them on a horizontal shelf tied to the X brace behind the panel. I prefer a vertical panel tied to the structure on the backside of the firewall.

    Stay with small (2 1/4") instruments and small radios such as Trig or Becker (also 2 1/4"). They take up less space and don't need trays to be installed. If you put the battery under the seat, or somewhere in this forward area, put the instruments and avionics on the panel, along with the breakers and switches, you'll use the minimum length of wire for your harnesses. Shorter runs of wire = less weight and less noise issues.

    With the availability of ADS-B systems, for new installation, you shouldn't need to mount any of these items in the rear.

    The one item that really should be in the aft section is the ELT. It needs to be mounted where it has the best chance of surviving the crash, even if you don't.

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  10. #10
    Flyingde's Avatar
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    That sounds like a bad doctor's prognosis. "Why here is your problem boy. You have avionics in your upper rear baggage you see"......



    Sorry. I'm bored....
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  11. #11
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    My ELT and ADS-B (Status II, there are newer, better units) in are mounted on the battery shelf above the extended baggage. My ELT antenna is mounted there, too, using the extended baggage as a ground plane. It all seems to work pretty well.

    As others have pointed out, I can't see putting a remote com or transponder there. The remote radios are more expensive and require more wiring, and ultimately, more weight.
    Make friends, because life isnít fair.

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    cubman's Avatar
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    I have not decided which avionics option yet. Many are putting in a G3X screen with everything on one screen. The Garmin COM and ADSB can be mounted remotely -- I realize this doesn't necessarily make it a good idea. Another option I'm considering is exactly what MTV and Wireweinie suggest. The Trig COM and ADSB can also be mounted remotely with just the head in the panel. The ELT and antenna will go on the shelf above the baggage as Steve described. Based on this feedback, I'm rethinking where to locate the COM and ADSB.

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    The G3X Touch with remote comm and ADS-B txp works great and is very simple to use. I like mine a lot. ADS-B adds a waas GPS box, GDL, and antenna but that wasn't any problem.

  14. #14
    cubman's Avatar
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    Thanks for the information. Where did you mount the remote Garmin comm and ADSB?

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    Avionics in Upper Rear Baggage?

    On the far side of the shelf in the photo I posted a few days ago. The transponder hangs from the bottom of the shel so access by removing the boot cowl panel. That's not imperative, just how we did it. In this pic you can see the txp tray under the shelf.


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    Mike's done at least on more install since, in the Airframes Cub that was supposed to go to OshKosh, and I never got to see that one. Maybe he'll chime in about what he chose to do differently.

  17. #17
    wireweinie's Avatar
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    Quote Originally Posted by cubman View Post
    The ELT and antenna will go on the shelf above the baggage as Steve described. Based on this feedback, I'm rethinking where to locate the COM and ADSB.
    At the risk of starting a brush fire, don't put the antenna inside the fuselage frame. The ferrous tubing reduces the signal out. Also make sure to mount the antenna vertically. It's designed to transmit with the wire/rod straight up or straight down.

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    The upper baggage is not always the driest place on a plane, so if you do put a lot of electronic stuff in that area make sure it will stay dry. Before you put anything in the panel put on the boot cowl. Several cubs have been built with the boot cowl off only to latter realize that now getting to a screw or bolt is a major pain. 3/4 part boot cowls are worth the time and effort.
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  19. #19

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    One more comment re: post-installation access. In my case with the G3X, and I presume other systems are similar, you can’t fully test the instrument functions until you start the engine. In my case a couple of issues appeared. My boot panels had to come off so I could terminate the Can Bus, which hadn’t gotten done, and I had had one pair of GDL wires pinned into the wrong connector. That one was difficult to troubleshoot and it was a Garmin engineer who pointed me to the error. Access proved critical. Months after those issues I chose to add a fuel pressure sensor and once again got to open it up to access the proper connector to pin the leads and ground the shields. Point is, you’ll need to get to the connectors so plan for it.

    Don’t let that intimidate you. It isn’t that hard and the instrument is worth the effort. I’m just sharing lessons I learned.

    Re: modified boot cowl. See post #65 here. https://www.supercub.org/forum/showt...dcat-Cub/page2
    Last edited by stewartb; 03-30-2020 at 08:11 AM.

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