Matt's RV-7 Project

Prop hub and spinner backplate

January 17th, 2010

Another month without much work on the airplane. I have an excuse, though: it's been really cold in my garage!

I have a whole list of wiring chores left to complete, but I need to take a break from running wires through the fuselage before I go cross-eyed. I thought I might work on some firewall-forward tasks for a while, which will actually help with wiring in a roundabout way; before you can do much wiring forward of the firewall, you need to finish the baffles, which means you need to fit the cowl, which means you need to fit the spinner (the pointy bit on the very front of the airplane), which means you need to fit the propeller. Plenty of work to do there.

The plans suggest making a spacer out of wood or PVC to stand in for the propeller, but since the consequences of getting the cowl/spinner gap wrong are so ugly to contemplate, lots of folks end up using a real prop instead. However, instead of using my very expensive propeller as a fitting template, I had the idea to use a junked prop hub instead – that way there's no questions that the dimensions will be right.

I put out a call for an unairworthy Hartzell prop hub to use for fitting the cowl and spinner, and received the following hunk of grungy aluminum from a generous forum member (thanks, Joe!). It's only the rear half of the hub, but that's all I really need anyway. It is the older C2YK model instead of the C2YR that I have, but the critical dimensions for spinner-fitting are the same.

A little solvent and elbow grease cleaned it up pretty well. The key metric here is that we want to avoid leaving a big grimy smear on the clothing of any wives who happen to be walking through the garage. It just seems like everything will go so much more smoothly that way.

Digging into the pile of remaining airplane parts, I pulled out the rear spinner bulkhead and its associated doubler ring.

As I expected, the spinner bulkhead is noticeably dished, although it appears to be otherwise true and symmetrical. This appears to be a common occurrence, so I will probably just ignore it.

With some hardware-store bolts, I temporarily attached the doubler to the bulkhead:

…then drilled and clecoed the holes for the rivets that will eventually attach these two pieces together:

If you have a Hartzell prop like I do, you need to cut out a significant portion of the spinner bulkhead. I transferred the shape of the hole in the doubler ring:

Initial cuts were made with my air nibbler, which is great for this kind of work. Going slowly and carefully, I was able to get within 1/8" of the line.

Then it was time to file, file, file. The corners look lumpy here, but that's how the cutout needs to be in order to clear the prop hub.

More filing and finish work… I made plenty of aluminum shavings.

I spent a lot of time making sure all the edges were well and truly deburred.

I need to prime these parts before I rivet them, but it's way too cold and dark outside to do that, so it will have to wait on the weather.

Also: What are these?! I haven't looked at the plans for over a year, I think… ever since I got to the step that said something like "Now might be a good time to think about wiring and an electrical system, if you want one."

And speaking of the spinner backplate… I realized that this piece will literally be moving the fastest of any of the parts that I've fabricated: in addition to however fast the plane is moving through the air, the perimeter of the 13" diameter spinner will also be doing about another 100 mph as the crankshaft is turning at 2700 RPM. Neat.

Posted in Propeller

Antenna wiring

December 12th, 2009

Good grief, has it been a month since I updated this thing? I've been trying to work on the airplane a little bit here and there, but between holiday travel, work projects, and cold weather, things have been pretty slow in the airplane factory.

I have, however, managed to get the previously installed antennas on top of the fuselage all wired up. Four lengths of RG-400 coax run down from the panel, through the spar, and back under the floorboards. They penetrate the F-706 baggage bulkhead via snap bushings, then run back and up the F-707 bulkhead protected inside plastic conduit.

I drilled small holes in the J-stringers so I could attach the conduit with tie wraps. On the left side of the fuselage, I used a piece of rubber tubing and some tie wraps to prevent the conduit from chafing on the static line.

The conduits run up either side of the F-707 bulkhead – more small holes and tie wraps – and then the coax emerges to connect to the antennas.

Two wires go forward to connect to the WAAS GPS antennas – one for each GNS 430W. I riveted little tabs of scrap aluminum to the lower flange of the F-787 stiffener, so I could secure the wires with adel clamps. Meanwhile, two more runs of coax go through grommets in the bulkhead and connect to the combo GPS/XM antenna that's just aft of F-707.

Here's another view, looking up from [my very uncomfortable position on] the floor. In this picture, the nose of the airplane is to the left.

Up at the forward end, the primary display unit gets a GPS connection, and the multi-function display gets an XM antenna hookup. Each 430W gets a GPS antenna connection too, although I forgot to take a picture of that.

Not surprisingly, the G3X GPS receiver can get a fix even inside my garage with the doors closed. With a good antenna and a strong receiver, you can't lose.

This doesn't look like much, but it took two days' worth of crawling in and out of the airplane to finish.

Posted in Electrical/Panel

Map lights

November 14th, 2009

I already mounted the dimmers for my cockpit map lights, but I didn't get around to wiring them until today. So here's what one of the LED map lights looks like:

I discovered that the inside diameter of the mounting base is exactly the same size as a cleco, which is really handy for drilling the mounting holes.

I dug the rollbar out of storage, clecoed it to the fuselage, marked and drilled pilot holes for map lights, and clecoed a light mounting base to each of the trangular gussets.

I match drilled the three mounting holes to the rollbar, then enlarged the center hole to 1/4" and drilled the mounting holes for miniature nutplates. I'll rivet these to the rollbar gusset once it's been re-powder-coated.

I like this location… it's out of the way and not likely to get broken off by passengers' feet, but still in a good location to be useful for reading a chart or for illuminating dark corners of the cockpit.

I used a very small grommet to pass the wires through the rollbar gusset:

The dimmer modules came with some miniature potentiometers. I soldered leads to the terminals:

…then used heatshrink over the joints:

…and finally put a piece of bigger heatshrink over the entire component, which should help prevent the wires from breaking off.

I drilled holes in the F-721 canopy decks to mount the dimmer pots:

I used some anodized aluminum knobs that match the ones I used for the panel light dimmer knobs. Eventually I'll make little placards to identify the function of the knobs, which will have the added side effect of covering up the holes for the anti-rotation tabs.

I finished up the wiring for the little dimmer modules and closed up the connectors. The power and ground wires that go to the lights themselves are not connected permanently yet, since I need to do a little further work to route them properly.

However, I did hook up each lamp with alligator clips just to make sure everything was wired correctly. Yup, seems to work:

I have something special in mind for making the connection between the lamps and dimmers, but it's not ready to show off just yet.

Posted in Electrical/Panel

VOR antenna

November 1st, 2009

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.

Posted in Electrical/Panel, Fuselage

GPS antennas

October 31st, 2009

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.