Avionics question

[Kitfox Pilot]

Remember I know nothing much on this subject and am laying out my garmin supplies in the plane.
I need aft weigh and my question is what items should I not put in the back, behind the seat area, of the plane?
Below is the list of items to mount.
Autopilot servos are all ready in and the gmu 11 is out in the wing and g3x and 507 are on the panel.
Battery is going behind the baggage compartment and I'd like to mount the vp-x behind the seat??


Left to mount is gsu 25, gtr20 remote com radio, gdl 50 adsb in, gea 24 engine monitor. Any of these need to be up front?


I'm not mounting a transponder or adsb out at this time. Thanks

[Delta Whisky]

Harlan - I don't know any specifics about the GEA 24 but if thermocouple wires go direct to it, you probably would be better off keeping it closer to the firewall then behind the seats. Maybe a Garmin guy will weigh in.

[jiott]

If you have not yet decided on engine/prop, I don't see how you can possibly decide where to mount the items you mentioned for proper cg location. Maybe I missed it and you have settled on engine/prop?

[Kitfox Pilot]

915 engine , airmaster prop.

[efwd]

Keep the GEA 24 up front. It doesn't weight anything, so no significant penalty there. The GSU 25 has mounting holes right in the back of the G3X if you want. Downfall to that is you have to run pitot tube hose all the way to the firewall and up to the back of the panel. Just gets crowded that's all.

[PapuaPilot]

I have a G3X and would recommend mounting the GEA-24 between the firewall and instrument panel. It is the unit that all the sensors & inputs are connected to and it only weighs 1.6#. If you put it into the tail then you have to run all of your engine sensor wiring to the tail, which adds uneeded weight.

The only other things (other than the A/P servos) that you can put in the tail would be the GTR-20 remote COM, the GDL-50 ADSB and GA-35 antenna. I put my GSU-25 in the tail to keep the static port runs to a minimum.

I don't see a transponder on your list. Are you planning on one and ADSB-In? There are remote transponders that could be mounted in the tail, which can be controlled by the G3X.

Also, the ELT can be mounted in the tail.

[Kitfox Pilot]

Phil, I'm not putting a transponder or adsb out in at this time. Planning on putting the elt on the elt mount because I assume I need to reach it.

Eddie, do you think the vp-x is ok behind the seat?

Thanks Guys.

[Shadowrider]

Harlan,

Seems like there is always something you don't think about and I am trying to think of issues you may have. One is you may have to get creative to get all your wiring tucked away neatly. Also you may want to try and keep power away from the ELT as much as possible. I know some have had problems with the ELT inadvertently going off? Not sure if the exact cause was found, but I think they are thinking you don't want power running close to the ELT remote wires. Maybe you are going to run the artex instead of the ACK. Doing it again I think I would switch. Wondering if it would be worth moving VPX to the back because you have more wiring to run and its not that heavy? Of course power to the avionics that are remote mounted is a shortened distance, but you do have a longer distance for the ground and power supplies to the VPX. Just thinking out lot. I have sometimes wondered if moving the header tank and enlarging it would be a good option for others going 915. I know your covered so it not something you want to do.

[avidflyer]

As much as a guy hates to think about adding weight to the tail, this is something to consider as well. 2 lbs of weight at the tailwheel equal about 10 lbs of weight just behind the seat for CG purposes. Maybe it comes down to adding a heavier tailwheel and you save a bunch of other issues that may complicate the build. Not saying this is the way to go, but putting it out there as something else to consider. JImChuk

[Shadowrider]

Thanks Jimchuk. Basically that is what I was trying to say. Maybe easier to just add weight to the tail!

[tracstarr]

I've got all garmin and a VPX. Keep the VPX closest to most electronics (if that's dash or behind seats). Although if you'd planning on physical buttons/switches on the dash, probably best to keep it under the dash. It's A LOT of wiring. If i did it again I would put the gsu behind the seats just to keep from having to run static, pitot and AOA to the dash. You could probably put the gtr20 behind the seats too. Would also make it easier to keep separation between coax and electrical. Gea24 is best under the dash.

[efwd]

The VPX can go wherever you like but there are consequences. I suspended my VPX from the two braces that support the instrument panel immediately forward of my radio stack. You might say I have violated some guidance provided by some here. I have antenna and ELT in close proximity to one another, No ground planes, GPS antenna very near my radios etc etc. I have not had an ounce of trouble with regard to any of it. You will need to run a large gauge cable from the Battery to the VPX and there are obviously the many ground and power leads that need to be attached as well. I think I would have been cussing a lot had I needed to run more wires through the center tunnel. I had enough just feeding the Landing/Nav lights, AP servos, ELT, trim motor and the power boost required to supply the energy for the trim motor. Oh yeah, and the OAT cable. Wherever you choose, make sure whatever you install on the two avionics shelves forward of your instrument panel, remains clear of the NACA vents you will likely be installing in the boot cowl to allow ventilation hose to reach your eyeball vents in the panel. This is how I came to mounting the VPX in the center after finding out the original location blocked my NACA vent.

[Kitfox Pilot]

Ok this is the kind of info I need because I've never done it.
Thanks for all the info.
Would you keep the battery behind the baggage compartment? I never did like long battery cables from batteries to the engine. The earth x batteries are lite but still the heaviest thing and longest arm from cg.
But If a 5 lb weight on the tailwheel saves me a lot of trouble, that's what I'll do.


One more thing , does the ground bar attach to the battery ground? I'm just wondering where all the grounds go if my battery is in the back?

[PapuaPilot]

It is really best to keep your heavy battery cables as short as possible, which would mean putting it by the firewall. Long cables loose voltage, especially when starting the engine. I used to have an Odyssey battery in the tail and always had a hard time starting. Now I have an EarthX behind my firewall with only 1' runs to the battery contactor, starter contactor and airframe ground. I could not believe the difference it made is starting the engine.

[Kitfox Pilot]

I had a Grumman tr2 for a first plane with the battery under the hood and you could almost taxi on the starter. Piper archer for the last 27 years and never has cranked good with the battery in the back. we changed out the aluminum cables for copper a couple years ago and it helped but still nothing like having battery close. We run into the same problem on some farm machinery with long cables.
I hoped rotax didn't pull as much amps to start as what I am used to because I know there are lots of batteries in the back of the kitfoxes.
I know close is better on batteries but I know I need weight in the rear too.

I also noticed the earth x battery was only 4 lbs so not near as much gain moving it around either.

[Eric Page]

One more thing, does the ground bar attach to the battery ground? I'm just wondering where all the grounds go if my battery is in the back?
I have to agree with Phil. Unless you use thick (heavy) wires, you'll be a lot happier with starter performance if the battery is up front.

Regarding VP-X location, putting it behind the seat opens a can of worms. Where would items powered by the VP-X be grounded? Ideally there would be a ground bus bar near the VP-X, but that would mean a second wire run to the negative post on the battery, or another long thick wire from the firewall to the aft ground point. Then you'd have two major ground points in the plane and depending on currents in those wires, there could be a voltage difference between them.

That can cause sensor inaccuracy if the engine monitor box has a different ground point than the EFIS. Grounding the engine monitor in the back with the EFIS could solve the problem, but that scheme would be defeated by any sensor whose metal case is internally connected to ground and also touches the engine. Grounding the EFIS up front would fix the problem too, but that might cause data errors if remote devices that communicate with the EFIS are grounded in the tail. Those errors might only happen with certain high-current devices turned on, making them very hard to diagnose. Imagine a varying difference in ground potential as wig-wag landing lights attached to the aft ground point blink on and off.

Generally speaking, I would follow three guidelines for grounding:


For any device you install, provide only one path to ground.
If a device communicates with another device, ground both in the same location.
Shielded cables should have their shields grounded at only one end.

#3 applies to all shielded wires, whether you're talking about control lines to a remote radio, GPS position data to a transponder/ADS-B, control signals to a servo, microphone/headphone jacks or an intercom's music input. Ground the shield only at one end to avoid ground loops and to protect devices from high currents in the shield in the event of a broken ground wire.

My advice -- which is worth what you're paying for it! -- would be to put the VP-X behind the instrument panel and use a single-point firewall ground. With the VP-X up front you'll save weight and considerable complexity in wiring, and you'll avoid sensor and data-comm issues due to unequal ground potentials. Plus, adding a little weight way back in the tail will be easier, cheaper -- and my guess: less total weight -- than all of the extra wiring involved in putting the VP-X behind the seat.

You might want to call Kitfox and talk to John or Brandon about these two issues. They've done 915 installations, so they probably have a pretty good idea how the balance works out.