airguy

airguy

Unrepentant fanboy
Sponsor
I'm getting close to buttoning up the rear top skins on my 9A, and I'm planning on running Dynon glass in the panel along with the Dynon transponder. I'd like to mount the transponder box (about 1 pound) aft of the baggage compartment rather than the avionics bay up front, I'm going to be a bit nose-heavy as it is and I'm moving any equipment aft that I can in order to keep nose-wheel weight reasonable. The battery and strobe power pack will be just aft of the baggage compartment as well.

Soliciting opinions here - any comments/complaints/suggestions with putting the transponder box back there? I have my ADAHRS unit mounted just barely forward of the tail fairing, so the transponder box will be well forward of that, I don't think I'll get any interference with it at least 3 feet away. Any experience or direction from others that have already poked the bear in this area? I'll certainly mount it so that I can just pull the baggage compartment wall and have access to it for maintenance.
 
Last edited:
That is what I'm planning on doing. I will be mounting it behind the AP pitch servo bracket, and down under the aft control tube on the rib.

Here is a picture from Jason Beaver's web site that shows where he put the transponder (gold box under the servo).
IMG_9626.JPG


I'm mounting mine on the other side of that center rib. It will be well back of the AP servo and under the control tube.
 
I'm thinking I would want mine a little further away from the servo than that, the transponder radio pulse is pretty strong, wouldn't want the servo "twitching" each time it fired from interference.
 
Buggsy2 said:
I'm building a 9A also and this got my interest. How do you know you'll be nose-heavy, and what do you think is the cause?
Click to expand...

I'm putting an IO-360 up front. That automatically adds 17 pounds forward of the firewall. To counter that I'm going to install the lightweight Whirlwind 200 RV constant-speed prop and move some items aft to try to keep the firewall-forward weight down. ADAHRS, strobe power pack, battery, ELT, transponder, maybe other stuff too - all going behind the baggage compartment.

I want to be able to run 92-octane mogas, which means I'm realistically limited to 8.7:1 compression on the pistons, which means I can't use the stroked IO340 which would give more horsepower on the same weight as the 320 block. If I want both 180hp and Mogas, I have to run the IO360 and accept (and deal with) the forward weight penalty.
 
Last edited:
Greg,
Mount it where you proposed. Just did this in a friends RV 8 that way and it works fine. Doing it that way in my RV8 with a Trig. Go for it and build on!
 
Greg, I recently finished my RV9A and feel that it is a bit nose heavy (even if CG is within Van's limits). O-320, Hartzell C/S, alternator 60 amp, forward mounted Odyssey battery and a lot of avionics. Of course I placed strobe pack and ELT unit in the tail. This is not enough.

I approve your idea of placing the battery in the tail cone. I have to move mine from the FF to the tail. Van's has an old drawing valid for the RV7 (but it is OK for the 9). I am only guessing where to add the hole which passes through the main longeron assembly: I plan to add a hole just above one of the existing holes at the end of the tunnel and then passing the cable under the front cabin cover (where the fuel and brake tubes pass, too).
 
A forward centre of gravity makes the aircraft more stable, I would have thought that this was not a problem on a 9, different story on a 7 where aeros might be performed??
 
Forward C/G IS more stable. However too far forward can cause you not to be able to "flare" or "rotate" properly.
This will be even more of a problem if you have the little wheel up front.
 
Last edited:
Mel said:
Forward C/G IS more stable. However too far forward can cause you not to be able to "flare" or "rotate" properly.
This will be even more of a problem if you have the little wheel up front.
Click to expand...

Exactly.

I'm not interested in trying to get to a less-stable "neutral" position on the CG - I'm simply moving some items aft that can be moved, to counter the additional weight that I know will be installed up front. If my planning numbers are correct I'll have right at the same amount of noseweight as a standard O-320 with FP prop when it's done, just like Van had in mind. I'm just being cautious about the possibility of becoming the next pogo-stick victim, and trying to keep the nose wheel in the light-duty category (and yes, I already have the Anti-Splat Aero device).
 
Last edited:
Interesting location. Are you going to put the antenna in the belly, close to the box? With the limited holes through the spar, running the big cable forward might be a challenge.

If you want to put that back there, buy the strobe brackets Van?s now sells and mount it on the side.

Has anyone mounted the transponder under the panel? I?m getting ready to redo my panel and have been trying to figure out where to put all the odd sized boxes that come with the SkyView?
 
N941WR said:
Interesting location. Are you going to put the antenna in the belly, close to the box?
Click to expand...

That was my plan, yes. I'm (currently) thinking I'm going to mount the transponder, strobe power pack, and the ELT on the floor of the aircraft (I'll build a little mounting shelf) just aft of the elevator servo, and run the antannae for them in that immediate real estate so I don't have to feed COAX a long way fore/aft. I want to keep these electrically noisy items relatively isolated from other electronics, while at the same time keeping them aft for CG purposes but far enough forward that they don't interfere with the ADAHRS unit I have mounted just immediately forward of the tail fairing.

The ELT I'm not worried about - for normal flying it won't be emitting anything to concern me, and if it IS emitting, I'm not concerned about interference anymore, so get after it. I'm going to try to shield the high voltage wires from the strobe power pack to the bulbs in the wingtips and tail, and the transponder will get the best coax I can find and length-matching to keep the VSWR low to avoid excess emissions inside the metal fuselage. There is not exactly a plethora of real estate for noisy electronics in the aft fuse if you've got the ADAHRS mounted in the tail and you're trying to keep it happy.
 
Thanks Greg!

My T-sponder antenna is right behind the firewall, on the right side of the cockpit floor. Since I?m working with pre-existing installation, I?ll mount the T-sponder box up under the panel someplace.
 
N941WR said:
I?m getting ready to redo my panel and have been trying to figure out where to put all the odd sized boxes that come with the SkyView?
Click to expand...

I'll never keep up! I finally go the tail dragger, and then you throw in a Skyview..... :(

Next, you'll be getting one of those Whirlwind props .....

L.Adamson
 
Is there an RF Interference advantage?

I have my RV8 all wired and antenna wire routed to mount the transponder behind the panel. However, I considering mounting it behind the pitch servo and rear battery, also where the antenna is mounted.

It's more do over work, but if there is a RFI advantage to having about 1-2 feet of antenna coax that's not running parallel to other wires, I may do it over. Since most transponders have to be mounted in the panel, it seems that running coax to the remote antenna must work fine
 
You can expect a about 2db loss with a 15 foot cable vs a 1 ft cable. This equals a 36% power loss. A 250 watt transponder will lose about 90 watts with a 15 ft cable vs a 1 ft cable or 160 watts vs 244.5 at the antenna with the 1 foot cable :eek:

There is not "usually" an interference issue from transponder cables but I would not run it parallel (in the same bundle) with GPS antenna cables.
 
Last edited:
One additional caution.....

Don't make the transponder cable too short.

We had a Pitts with about a 6" cable that constantly "interrogated" itself apparently caused by SWR. Took some time to find the problem. Finally added about 2" of cable and the problem cleared up.
 
N941WR said:
Has anyone mounted the transponder under the panel? I?m getting ready to redo my panel and have been trying to figure out where to put all the odd sized boxes that come with the SkyView?
Click to expand...

Bill,

I mounted my Dynon transponder on the left side of the F-7108A center Forward Fuselage Rib forward of the sub panel. Of course, I am not flying yet so we'll see how it works out there. This location made for a quick run down the conduit located per Vans plans to the antenna mounted under the fuel valve area on the belly. Meets Dynon's criteria for the transponder install.
 
This past weekend I finally figured out where I'm going to mount that Transponder. It will be bolted directly to the aft side of sub-panel, above the radio and behind the 496. If I were building from scratch, rather than installing this in an existing aircraft, I might make a flip down panel to hold all of this stuff.

BTW, Dynon's upcoming SkyView radio will be the exact same size and use the same mounting tray as the transponder. So make some room for that, if you plan on using it as well.

(Dynon, please correct me, if I'm wrong.)

Also, in re-reading this thread, those of you building an "A" should work on getting everything as far forward as you can. The RV-xA's tend to be a bit tail heavy, what with the main gear behind the CG and all.
 
L.Adamson said:
I'll never keep up! I finally go the tail dragger, and then you throw in a Skyview..... :(

Next, you'll be getting one of those Whirlwind props .....

L.Adamson
Click to expand...

If I were to install a CS prop, that Whirlwind is the only one I would consider. My "light weight" -9 is getting heavier by the day.

Oh, and when I'm finished I won't have a single steam gauge in the plane.
 
Changing to a rear mount

Thanks for the input Walt. I'll do the change out and mount my new Dynon TPX aft with a short ~2" antenna cable. Also makes room for a back up Dynon radio some day.
 
You must log in or register to reply here.