Saturday, March 3, 2012

Mystery of the unbalanced elevator

As I near the end of the RV-7A project, it seems as though the mysteries grow a bit deeper.

Here's one I had today:

A few weeks ago, when Tom Berge and Doug Weiler came by to do the pre-inspection inspection, they pointed out the unbalanced elevators, which have lead counterweights in them so that they default to level with the horizontal stabilizer.

I, of course, had noticed this too and it was a bit puzzling since long ago I put the horizontal stabilizer on the workbench and attached the elevators and made sure they balanced perfectly. But, as you can see, now they don't.

We kind of figured that because the lead counterweight wasn't fully mounted forward on the "counterbalance arms" on the right elevator, that this was the problem. So I ordered a 25 pound bag of lead shot and planned to mix up an epoxy slurry to fill in the gap, adding weight to make it balance perfectly.

Then I unbolted the elevators from each other and from the pushrod to the elevator bellcrank (which goes forward to the control sticks, which, by the way, are all perfectly aligned).

Ruh roh...

It balances perfectly, so that's not our problem.

Over on the left side, we see the offending elevator.

But that elevator, too, had been balanced on the workbench. So we figure it has to be the very thick and rigid trim cable. I'm one of the few people building an RV, I think, that didn't use the optional electric motor trim. But I installed it per the plans. It attaches at the bottom of the elevator after snaking its way from the tail, into the horizontal stabilizer and out to the elevator in an "S" pattern. I don't have an image of it but it's connected to the trim tab, which hangs on the elevator.

If you enlarge this image, you can see down the forward end of the elevator where the cable exits the horizontal stablizer and goes out through the elevator. There's no play in the cable because it's clamped in the fuselage.

As we follow the cable forward, this is where it exits (or enters depending on which way you're going) the horizontal stabilizer through the horizontal stabilizer spar...

And down into the fuselage. Note the adel clamp to prevent chafing and movement.

Inside that green shroud is the small cable which control the trim tab. We checked it at both extreme settings to see if that also changed the balance, and it did not.

Everything here, as near as we can tell, is per the plans of the aircraft, and yet it overrides the proper balance of the elevators. We're writing to Van's Aircraft support to see what we should do.

I haven't pulled the cable off the stabilizer yet to see what difference that would make because to fully know, I'd have to drill rivets out of the "inspection panel" on the bottom of the elevator and pull the cable forward. It's a real pain in the neck to do that and, even more so, to put it back.

There's one possibility that I haven't checked, it's that I didn't leave enough cable inside the horizontal stabilizer, and with it clamped on the turtle deck of the fuselage, perhaps it's "pulling" on the trim tab connection a little bit, forcing the whole operation down. It's a shot in the dark, but perhaps loosening that Adel clamp and pushing more cable from the fuselage into the HS to see if it pushes the elevators back into position would be a good start to solving the mystery.

By the way, here's another reason why keeping a good builder log is important. I scrolled back through it to be sure that I had the trim tab on the left elevator when I balanced everything originally, and that it was balanced.

One last piece of trivia: While doing this yesterday, I passed 2,900 hours of work on the project.

Update 3/5/12 9:16 a.m. I messed around with the amount of slack in the HS to no significant difference.

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