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Moving on — Horizontal Stabilizer

March 10, 2011

Ok, well I am moving on.  Cant dwell on the past.  I took the two elevators up to the attic and got them out of my sight.  Now that I have new parts for the rudder and the HS I can re-engage with them.  I decided to get on with the HS and try to knock it out.

When last we left our HS we had been trying to build the front spar.  The problem was that one of the sides of the spar was scratched really badly.  Vans replaced that spar and I got to it.  First thing I had to do was make some bends in the left and right spars.  Lots of people report to be nervous about this, I just took my time and didn’t try to over engineer anything.

6 degree bend. Lots of guys sweat this step. It was pretty easy, just take your time.

The way I did it, I took a scrap piece of MDF I had laying around and clamped it into the channel between the flanges.  I made the edge of the wood level with the bend line.  Then I just laid the part that needed to bend on my workbench and slowly bent it.  It took 5 or 6 attempts until it was the requisite 6-degrees.

This spar arrangement was originally designed for the RV-8, so to make it work on the -7 you have to make some cuts to accommodate things.  Starting from the inboard end of each spar you have to cut the flange back a few inches, then you bore some 1/4″ holes and then shape them into relief notches.

You have to make some cut in the original spar so it will be ready for the RV-7

This is a relief notch rough cut into the spar

The blue line is where the bend I just described is.  The relief notch keeps the aluminum from cracking right here.  One thing you learn early in the build is that every corner (inside and outside) have to be rounded off into a nice radius.  I’m not an engineer, but apparently vibrations concentrate stress at areas where there are sharp corners.  These stresses cause cracks to develop over time.  This is why this notch is kind of an odd shape.

Other things I got to do was  trim a few ribs so they will fit properly.

Rough cut of the rib

Why doesn’t Vans just do this for us?  Well simple, its the 51% rule.  The FAA wants to see that we fabricated things on the plane, not just put together a bunch of parts like a model airplane you got as a kid.  This cut is tricky.  The measurements are in 1/32″.  I have a rule that measures that small, but the tip of my ultra fine Sharpie is bigger than 1/32″.  So I rough cut it like this, and when I go to size it up, I’ll be able to see how much more needs to be trimmed.

I’d like to get all the HS parts primed tonight so I can get it put together tomorrow.  I am not going to be able to build at all next week, so I want to get as much done by Sunday as I can.

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