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This may be relevant to all Lancairs, as it relates to measuring a fast-build
tail to determine whether there is a twist which needs to be removed during
closing.
I was all set to Bondo the HTail (at least the stabilizer) to my assembly jig
when I noticed that there are significant unsupported sections. In other
words, the 3/4 inch particle board that I band-sawed to match the airfoil
contour don't line up perfectly, and that one can deform the tail to fit the
"template" by applying weight to the top.
Which is correct, the tail or the template? In other words, is the tail
still untwisted after 8 years of storage, or is my cheap, crappy jig ... err
... cheap and crappy? The jig is not really that bad -- I used 3/4 inch
thick birch plywood (the really good stuff) and have two layers of that
sandwiching several milled 2x2's. Not just rough sawn 2x2's but milled.
It's really good stuff. The resulting base is flat, straight and stiff.
(Not to mention expensive.) Attached to this base are particle boards
standing on end, bandsawed to a template provided by Lancair. There are five
such "ribs", one in the center, one on each end, and one midspan each side.
Recall that the tail is carbon fiber with the spars and ribs installed on one
side. The other side (face?) is more flexible, but with honeycomb-cored
graphite it's still pretty stiff. Therefore I had reason to believe that the
tail was untwisted, unlike many of the other large, floppy fiberglass parts
that inevitably change shape a little if stored unconnected for a long time.
So I got airfoil contours from Abbott and Von Dohnhoff for the NACA 0012
airfoil, which is what the Mark II tail is. I found a pair of points that
were the same height-above-centerline (y/c for you aero types), one on each
side of the "crest" of the airfoil. I then drew marks on the tail at the
center and at midspan for the corresponding x/c locations. What we have now
is a pair of dots (at each rib location) that should be at the same height
above the centerline. The dots are closer to each other on the shorter ribs,
but their percentage-of-chord-back-from-the-leading-edge are the same.
Why do this?
The problem is, the airfoil is humped (as we know, that's how they work) so
my digital level won't sit flat anywhere. So I took two pieces of hinge pin
rod (piano wire) and set those up spanwise at the two dots, and laid my
digital level on the two piano wires. Note that the readings were not zero
since the table on which I put the jig is not level -- that doesn't matter.
What's important is that I get the same readings for all rib locations.
In my case, the readings were identical between the centerline measurement
and the midspan measurement. (All were 0.7 degrees nose down, so my
workbench isn't all that bad either) I could not take a reading at the tip
since the elevator counterbalance arm is too far forward, but if I wanted to
I could take another measurement about 5 inches in from the tip. The
resolution on my digital level is 0.1 degrees.
So in conclusion, my tail is not twisted. I can goop up the jig ribs with
Bondo and gently lay the tail onto the Bondo rather than have to "prewarp"
the tail when attaching it to the jig (as I had to do with the wings to get
the tiny amount of twist out). The Bondo will smooth out the irregularities
in the jig ribs so that the tail does not warp when closeout weights are
applied.
According to Mark at Lancair, I also need metal angles to keep the tail from
sagging between the jig ribs. He told me that I could use them either above
the tail (put the weight on the angles) or below (cut notches in the jig ribs
for the angles and lay the tail on the angles). I have elected to lay the
weights on the angles, except for the elevator trailing edges, where I have
an angle underneath and one on top so I can use "cleco clamps" (side grip
fasteners) to clamp the trailing edge between two angles to prevent waviness.
That's my Thanksgiving weekend project. The tail will be closed before the
turkey leftovers are gone!
- Rob Wolf
LNC2 50% IO-360
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