Empennage « Matt's RV-7 Project

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Yet another trim tab

Sunday, 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.

And finally – celebration.

Posted in Empennage, Trim Tab |

Empennage fairings part V

Saturday, February 24th, 2007

I decided on a new plan of attack for the elevator caps – I'll attach them permanently, but I'll make them look like they can come off. Then at least all the fairings on the tail of the airplane will have a consistent look, which is important if you're obsessive like me. First I rounded off the forward edges of the lead counterweight with a file, then I match drilled the untouched elevator cap and installed nutplates and reinforcement strips:

The two nutplates at the aft end are very close together. If I hadn't been forced to use the prepunched holes I would have staggered these a bit to avoid interference, as I did with the aftmost rivets holding the aluminum strips to the fairing. As it is I had to cut down some screws to keep one screw from trying to push the other one out.

Here's another thing that I'll do differently on my next airplane – when you dimple the attachment holes for the elevator caps, the rib flange gets between the dimple die and the skin and all your dimples end up with this little crease on the inboard side. You won't be able to see this once it's all painted, but if I'd known this I would have at least match drilled the fairings and dimpled these holes before riveting the elevators together.

Here's a test fit to make sure all the nutplates and holes are lined up:

I riveted a #10 nutplate to the outboard side of the counterbalance arm, using the 3/16" tooling hole that was already there. The reason for putting a nutplate here is that I can use it to bolt on a wide-area washer or two if it turns out that my elevator needs a little bit more weight up front… call it balancing insurance. Also, this photo is proof that I checked the torque on the outboard counterweight bolt on the right elevator – it will be inaccessable once the fairing is on for good.

Okay, I wasn't able to take pictures of how I performed the next series of steps, but I'll describe it and show the result. I mixed up a slurry of epoxy and flox, and applied a thick layer between the lead counterweight and the inside face of the tip fairing. This should glue the fairing to the lead and also fill in the gaps between them. Then I put in all the screws, and covered the edge of the skin with a layer of electrical tape. Next I mixed up a big batch of epoxy and micro, and used it to fill the gap between the skin and fairing on both sides of the elevator. I also applied a generous helping of filler to cover over the forward face of the counterweight, and the corners with the fairing and the counterweight come together. Most of it will get sanded off later, but this should hopefully provide the basis for a nice looking fairing closeout.

After all of the above was finished, I figured the right elevator would be out of commission for the rest of the day before the goo was cured, so I started thinking about the left elevator. It turns out that a replacement elevator cap is thirty bucks from Van's, so instead of throwing away the one I'd previously tried to put glass on, I spend some time sanding off my mistakes and restoring it to nearly-new condition.

Then I performed the same series of steps – match drilling, installing nutplates, and so forth. This photo is proof that I installed the spare nutplate and checked the torque on the inaccessable bolt on the left elevator

After all that (a couple hours of work) the left elevator cap got the same filler treatment:

Once both elevators are dry I'll start sanding them down. For now, though, I'm sick of composites.

Posted in Elevators, Empennage, Tip fairings |

Empennage fairings part IV

Wednesday, February 21st, 2007

One more application of filler to the rudder cap – hopefully the last one, save for the inevitable pinhole filling. I'm really happy with the way the rudder cap has turned out.

I'm much less happy with the elevator tip I did the other day. I will probably throw this away and start over with a new one from Van's. The stupid counterweight is totally in the way, and it's looking less likely with every try that I'll be able to have removable tips on the elevators. Darn.

Here's the other, mostly untouched tip fairing, just to illustrate the situation.

Once again, I'm really glad the whole airplane isn't made of this crap.

Posted in Elevators, Empennage, Rudder, Tip fairings |

Empennage fairings part III

Monday, February 19th, 2007

Okay, so the first try at closing up the elevator tip fairing didn't turn out so well. I forgot to take a picture, but basically the fiberglass had nothing to adhere to (the lead was coated with tape to prevent sticking) so it was very wavy and uneven. Also, since I only used three plies of 1 oz cloth it was very flimsy.

One of the nice things about fiberglass is that if you screw up you can usually grind off your mistake and try again. This time I'm going to try using a piece of aluminum to provide the structural strength, and the fiberglass will just be there to hold it all together. I made a forward bulkhead out of some scrap alclad (0.040" I think), scuffed it up so the epoxy would stick, and temporarily attached it to the counterweight with double-sided tape.

Then I laid up three layers of 5 oz cloth, with a single layer of 1 oz cloth over the top to help fill in the weave. We'll see how this turns out.

I must say, the new style of elevator counterweight is a real pain. The old arrangement seems like it would allow you to spend a lot less time messing around with fiberglass to get the tip fairing to come out just right. But what do I know…

Back to the rudder, I mixed up another batch of micro and applied it here and there. This photo is kind of hard to interpret, but what you're looking at is a glob of micro applied to the front face of the rudder cap, with a piece of scrap alclad (covered with duct tape to make it non-stick) taped in place to make it cure with a nice smooth, even surface.

My cat wanted to help write this entry:

Posted in Elevators, Empennage, Rudder, Tip fairings |

More empennage fairings

Sunday, February 18th, 2007

Today was an all-fiberglass day. Sorry, no action shots – hard to hold a camera with sticky fingers.

I sanded most of last night's primer off of the top rudder fairing, then put another glob of filler on the front where it still needs to be built up. My favorite tool for this kind of sanding is a 3" red scotchbrite disc in a die grinder turned down to low pressure. I also use a 60 grit sanding block where necessary, but power tools make the job go faster.

While the rudder fairing was curing, I got out one of the elevator tip fairings and started playing with it. The lead counterweight at the front is going to throw some sand in the gears of my plan to make removable empennage tips. It is tempting to permanently attach the elevator tips and just glass over the counterweights, but I'm still going to try to make them removable because I love a challenge. (Mary would probably say that I'm just plain stubborn)

I laid up three layers of lightweight cloth, sufficient to cover over the open end of the fairing where the counterweight lives. It's not much to look at right now because the peel ply is blocking the view. Underneath is a layer of electrical tape to prevent the fiberglass from sticking to the lead. With any luck this will form the starting point for a closed-end fairing that will cover the counterweight but will still be removable – we'll see tomorrow.

Hours later, the second coat of filler on the rudder was dry, so I proceeded to sand away at it. The shape is almost there now – just needs one or two more layers.

After shooting a coat of primer to help me check the finish, I put more filler on a few spots to fix some of the more egregious pinholes. My goal is just to get the basic contour correct – I'll leave the detailed surface finish work to the painter.

In the middle of all this, I bought and put together this particle board file cabinet for Mary. I found myself thinking how awesome it would be if airplanes were put together with quarter-turn fasteners. I'd already be flying!

Posted in Elevators, Empennage, Rudder, Tip fairings |