Matt's RV-7 Project

Rudder cable fairings

March 18th, 2007

On Saturday I tripped going down the stairs with a big basket of laundry and wrenched my ankle rather smartly, so between icing down my swollen ankle and studying for the commercial written exam I didn't get a lot accomplished on the airplane this weekend.

I did however manage to get one little task mostly done, which was fabricating and installing the rudder cable exit fairings. The plans just have you punch the rudder cable fairings straight through the fuselage side, and there's probably nothing wrong with that, but it does seem like a small set of fairings would help to aerodynamically clean up that area. You can get pre-made fairings, but I wanted to do it the hard way and make them myself. So, first I drew a whole bunch of lines on a piece of 0.020" alclad scrap, using the dimensions given by Sam Buchanan as a starting point.

Then the two pieces were cut out:

A little work with the hand seamer, and I had a nice looking pair of fairings. All for the low low price of free.

I laid out holes along the flanges and drilled the fairings to the fuselage. They are aligned with the main longerons, which are level in cruise flight. Notice there are three holes on top and only two on the bottom – that's because the lower fuselage stiffener is in the way of where I would have put the third lower hole. Actually I did drill a hole in that location, which you can see in one of the other photos below, but it's too close to the stiffener to dimple so I just covered it up with the undrilled fairing.

I had several choices for how to attach the fairings to the fuselage. In order of increasing difficulty, they were: A) round-head pop rivets; B) flush pop rivets; C) flush screws and nutplates. If you've been reading this site for long you've already guessed that I picked option C. However, it wasn't just as simple as riveting nutplates to the fuselage skin, because it was too tight inside the already-riveted tailcone to have any kind of decent bucking access. So, after some head scratching and trial and error, I made a nut ring to support the nutplates on the back side of the skin. The odd shape lets it clear the cable slot and the various other structural elements back there.

The same screw that attaches the adel clamp to the fuselage side also holds the nut ring in position. The fairing and fuselage skin are dimpled, and the nut ring is countersunk to accept the skin dimples.

On the exterior side, flush stainless 4-40 screws hold the fairing to the skin. Since it's removable I was able to iteratively shape the edges with my hand seamer until the whole thing laid down tight against the skin with no gaps anywhere. Now I have custom, scratch-built rudder cable fairings that continue the cool attached-with-screws motif that I have going on with the other empennage parts. Hopefully at least one person at a fly-in somewhere will look under the stabilizer and go "woah, how'd he do that?". Plus if I ever need to replace the rudder cables I won't have to drill out any rivets and mess up my paint.

I still need to finish installing the other fairing, but now that I have a template to make the nut ring it should be no sweat.

Posted in Empennage

Empennage fairings part VII

March 11th, 2007

Still not too much time to work on the project lately. I did have a chance to reinforce the balsa ribs on the horizontal stabilizer tip fairings with a mixture of epoxy and flox, and to fit and install the 4-40 nutplates that will hold the fairings on.

I mixed up the usual batch of micro filler and smoothed out the transition between the tip fairings and the stabilizer, with an extra blob at the nose to allow a perfect fit after it's all sanded down later. I also put a big glob of filler on the outboard corners – a lot of this will get sanded off, but what remains will be the the first step of building up the tip fairings to match the contour of the elevator tips.

Posted in Empennage, Horizontal Stabilizer, Tip fairings

Empennage fairings part VI

March 4th, 2007

I was either in class or studying for half the weekend, but I got a little bit done on the empennage fairings. I ground down the elevator tip fairings until I got the shape pretty much how I wanted it – they turned out not bad at all. I sanded them down to 320 grit once I got the shape right.

This is why I hate working with fiberglass – dust everywhere.

I removed the horizontal stabilizer from the airplane and laid it out on the bench to receive its fairings.

Trimmed the stabilizer tip fairings to fit and drilled and clecoed them in place:

The plans call for a minimum gap of 1/8" between the elevator counterweight and the fairing – mine is more like 3/16" in the neutral position, or slightly less when the elevator moves through its range of travel. Good enough. I did open up the other side a bit so the gap would be even on both sides of the stabilizer.

You can also see where I'll need to build up the outboard part of the stabilizer fairing to match the contour of the elevator.

I made some ribs for the stabilizer tips out of 3/16" balsa sheet. This reminded me a lot of how I used to build model airplanes as a kid. Actually balsa is really nice to work with. Maybe I'll take up model building again someday… nah.

I epoxied the ribs into the fairings and left them to dry overnight.

Since it was fairly nice outside, I decided to mask off the elevator tips and shoot a coat of primer. This will help me find the low spots and imperfections.

Not too bad. I got the look I was going for, and they turned out pretty nice. I will probably come back and do some more filling and sanding in order to work out a few minor imperfections, but overall I'm pleased.

Before I turned in, I managed to get the tail fairing nutplates riveted to the stabilizer.

All this work on fiberglass fairings, and this guy made his out of aluminum. I can't even comprehend the amount of skill that takes.

Posted in Elevators, Empennage, Horizontal Stabilizer, Tip fairings

Engine arrived

February 26th, 2007

This morning I got a call from the freight company telling me that the engine was waiting at the dock, so after work John helped me bring it home using his pickup. Then, he and Scott helped me get the 400 pound crate into the garage. Behold:

Shiny:

More shiny:

Shiny here too:

Mattituck Red Gold engines are built from start to finish by one technician, who then signs his name in gold on the case. Good job, Pat.

Unfortunately the huge crate was taking up more than its fair share of the garage, so I broke it down and put my little card table over the top of it so I can use the area for plans and parts. The engine will hide under there till I need it.

I'm thinking of calling it the "Limited Edition Lycoming Executive Desk". Take that, MotoArt!

Thanks to John and Scott for all the help getting this thing into its new home.

Posted in Engine, Firewall Forward

Yet another trim tab

February 25th, 2007

So here's the deal.. I decided to build a new trim tab today. The reasons were many:

1. I was fed up with fiberglass and wanted to make something out of metal.
2. I had the left elevator out already.
3. I was already in posession of the necessary parts, having ordered them a while ago (see #4).
4. John brought his trim tab to work to show it off, and it was so much better than the other one I'd previously made that I was shamed into building a replacement in order to redeem myself.

Absolutely the first thing I did was cut off the little flaps on each end of the skin. This trim tab ain't gonna have no bent flaps, it'll have ribs like a real aircraft control surface.

To make the inboard rib, I simply took an extra E-709 rib I'd bought from Van's and cut off the back end.

Here the inboard rib after being shaped to fit and clecoed in place. I decided to make the flanges point outboard, even though it made riveting harder – having the flanges point inboard would have put the web in a position to interfere with the rivets holding the control horns to the skin.

The inboard rib has a couple of strategic divots to clear some of those previously mentioned control horn rivets.

To make the outboard rib, I first cut a block of wood to the correct shape. I then took a piece of 0.025" alclad, started the bends with my hand seamer, and clecoed it to the form block.

Using a flush set and with my rivet gun turned down low, I finished the bends by hammering the aluminum down onto the form block. This is, of course, the same technique that the plans have you use to fold the tabs on the end of the trim tab skin, but doing it this way is a lot easier and more precise. Plus, if you screw up you just throw it away and grab another piece of alclad.

Here's the outboard rib after being fitted and clecoed in place. Notice I staggered the rivet holes at the aft end for clearance.

I fitted the hinge to the elevator in the usual way:

Then, after deburring but before dimpling, I formed the bend in the lower skin. The plans have you do this after the entire trim tab is riveted together, but if you do it before the spar is in the way you can make a slightly nicer bend. However this gives you less clearance for riveting the hinge, so you have to think carefully about the order in which you rivet the various things together.

I put a nutplate on the trim tab spar to capture the hinge pin, the same way I did last time around.

Here's the finished product, all riveted together and ready to go:

And, installed in its home on the elevator:

The gap between the trim tab and elevator is nice and uniform. You can see that I had to use one blind rivet at the aft end of the smaller outboard rib – this is the only pop rivet on the (more visible) top surface of the new trim tab.

Here's the underside of the outboard rib – I used one pop rivet at the aft end here too.

On the inboard side, I was able to use all solid rivets on top.

On the bottom, I used all solid rivets except for along the spar – there I was forced to use a row MK-319's to close everything up. I think if I had to do it over again (oh no!) I could probably find a way to turn the inboard rib the other way 'round and build the whole trim tab using only two pop rivets, but oh well. Come to think of it, that could be a good game show: "I can build that trim tab with two pop rivets!" – I'd watch. Anyway, lots of builders use blind rivets on the bottom just like I did, and it's approved by Van's. Good enough.