Matt's RV-7 Project

A forum thread convinced me to buy and install a traditional cat whisker VOR/ILS antenna on the bottom of the fuselage, rather than the hidden wingtip type often seen on these aircraft. I'm happy to give up half a knot for reliable navigation performance.

I decided to mount the VOR antenna just aft of the F-710 bulkhead, which is just barely accessible when the empennage is attached. A doubler ties into the bulkhead and the F-779 bottom tail skin.

The doubler is made out of 0.063" alclad. Here it's drying after having alodine applied, although in retrospect I'm not exactly sure why I bothered to do this – the "puck" part of the antenna is plastic, and the mounting fasteners don't make electrical contact with anything in there. Oh well, at least it won't corrode.

Here it is riveted in place with the antenna attached via nutplates. The brown stain is alodine that ran downhill while it was drying.

It would be pretty easy to use driven rivets here during the initial build of the tailcone, but on a nearly finished fuselage with the empennage installed it's essentially impossible. Cherry Max rivets to the rescue.

Test-fitting the VOR antenna using some random bolts… when I go to install it for good I'll use AN525 screws instead:

When the rudder and elevators are installed, you won't be able to step on the VOR antenna, so no danger of tripping over it while walking around the back of the airplane.

Yeah, it's hanging out in the breeze, but the airflow down there will be pretty disturbed anyway, so it shouldn't cause too much drag (he said, despite having only a journeyman's understanding of aerodynamics). At least the nav radio reception should be pretty good!

Now that all seven of the antennas on the fuselage are mechanically installed, it's time to move on to wiring them… but that will have to wait for a future work session.

My airplane needs a bunch of external antennas on top of the fuselage… one WAAS GPS antenna for each 430W, and a combined GPS/XM antenna for the G3X system. I cut up some 0.063" alclad and made a doubler for each antenna:

I chose to put the two WAAS GPS antennas on either side of the canopy track, about halfway between the F-706 and F-707 bulkheads. As long as they're not placed too far forward, the canopy will pass over the top of them as it lifts up and slides aft. I couldn't put them all the way at the aft end of the canopy track, since the upper fuselage stringers taper together and make it too narrow to for the antenna doublers to fit. I used a yardstick as a spacer to make sure I drilled the doublers parallel to the fuselage centerline, and to ensure adequate space for the canopy slider block to pass by.

Lots of big and small holes drilled in the top of the fuselage… no turning back now! The doublers will be riveted to the inside of the skin, of course, but it's way easier to drill them from the outside. I also had to shape them a bit with hand seamers, since a perfectly flat doubler won't quite lay down on the curved fuselage skin.

The third antenna doubler is located just aft of the F-707 bulkhead. There's plenty of room for the canopy slider block to fit between the antenna and the end of the track, so there should be no problem removing/reinstalling the canopy.

I alodined the doublers and fuselage skin for good electrical bonding, and riveted on floating nutplates.

John came over to help buck the rivets, while I crawled back into the tailcone to drive them from the inside.

The actual riveting went pretty quickly:

A shot of the finished rear antenna doubler:

Now the fuselage has sprouted a crop of antennas:

Victory! Thanks to John for the help.

How was your labor day weekend? I drilled two big holes through my airplane's wing spar:

Relax, I got approval from the factory first:

Matt,

Yes, you can provide additional holes within reason. Use the same diameter as we have, and also keep in mind the proper edge distance (2D). Also keep clear of other structure such as seat ribs, spar bars, flanges, brackets, etc.

Joe Blank
Builder Support
Vans Aircraft Inc.

Since doing this incorrectly would basically ruin the airplane (or at least be extremely time-consuming and painful to repair) I spent a lot of time measuring and contemplating various locations. I eventually picked a spot outboard of the F-716 seat ribs and inboard from the F-783 cover support ribs.

Vertically, the new holes are equidistant from the upper and lower spar reinforcing bars, and have more than enough distance from the existing holes I've been running wires through. I had to pull the wire bundles out of the way to drill the new holes, and I had to use an extension to keep the drill chuck from grinding up the ribs, but it all went just fine.

You can bet I deburred the heck out of these holes.

I wasted no time putting the new wiring holes to use… this is the coax for the transponder antenna, which runs down the right side of the forward cabin floor, goes up through one of the new holes, and then turns inboard before running down the right side of the center tunnel.

After passing through the rear spar, the transponder coax jogs outboard again and runs down the F-727 rib before passing through a bushing in the baggage bulkhead.

This is as far as I got before I had to go mow the lawn. Later on I'll figure out a way to secure the coax to the floor, and put on the BNC connector. By the way, this particular piece of coax is actually the same one I had previously cut and run to go to the nav antenna connection in the left wing root. Subsequently, this thread on VAF convinced me that it would be a better idea to instead use a V-shaped dipole antenna under the tail instead, so I ripped that cable back out. By happy coincidence, it was the perfect length to use for the transponder, so nothing is wasted.

I ran a few other wires this weekend too. The tail and each wing got power wires for the nav and strobe lights, plus a wire to synchronize all three strobes. I have decided to go with LED nav/strobe lights, although since I am an inveterate bet-hedger I made sure to size the wires for the strobe lights so they'd each be sufficient to run a single-output power supply for a more traditional certified light head, should I have a change of heart and decide to go that way instead. I also ran a spare wire to the right wing with an eye towards eventually using it to power an APRS tracker (someday).

Many feet of wire are ready to be run back to the tail too, although not everything is in place yet.

New game! Whenever you drill a hole in your spar, you have to take a drink. Tonight's beverage was a summer seasonal from Blue Moon, which was not half bad.

Al Stuber put his CH 750 project on hold long enough to drop by and hold the bucking bar against the belly skin while I crawled inside and backriveted the transponder doubler. Thanks Al.

The brown discoloration in the above photo is alodine, by the way. Don't want any surface corrosion forming here and messing up my antenna ground plane.

Here's a shot of the transponder antenna bolted to the bottom of the fuselage… nothing to it, just a plastic shark fin thingy:

One more thing checked off the to-do list.