Crooked or Wavy Trailing Edges of Wings and Control Surfaces

A few weeks ago I sighted down the trailing edges of a newly-built aluminum
experimental aircraft with a respected following. The wing's trailing edges
would make you dizzy they were so uneven. It was a little shocking. The
phenomenon reminded me of a novice builder's wooden horizontal stabilizer he
brought to the EAA chapter meeting a few years ago. The stabilizer was a
reject, of course ---- it had been built by jigging it to a warped tabletop.
He could have pulled some string and used shims to keep the table's warp
from creeping into his project.

Meanwhile, the A&P who is following this otherwise attractive (and very
fast) aluminum aircraft says that the genre had best not be spun if the
pilot wants to live because each rendition, due to the unevenness in the
wings, performs according to its own caprices. And this is a squirrelly
airplane with teeth to bite you. One, he says he knows of, did not recover
from the spin. Thorpes are a little like that too because most T-18's are
not built in jigs and the wings always bear enough variations that they
don't perform the same from one aircraft to the other.

And today I read in a website as a builder fumed and groused over the
asymmetry in his RV kit.

Which brings me to the present subject of rebuilding two Cessna flaps. I
clecoed the trailing edge of one of the flaps together last night after
having drilled and deburred (whew!) all the new holes with the flaps in the
jig built here in the shop. All I could say was "Yuck. Pitiful." The
trailing edges looked like an attempt at a straight line by a drunk when I
sighted down them. Take note that I clecoed them together AFTER they were
out of the jig. Of course the jig, made of straight-and-true aluminum angle
gently but firmly squeezing both sides of the flap near the trailing edge,
will make it true when it is riveted together. Theoretically. And so I am
promised.

I looked at Tony Bingelis's indispensable writings on the subject, and he
says to rivet from the center of the surface outward to each end, and by
hops, then coming back and filling in the gaps in order to minimize the
stresses tending to warp the surface and deprive you of that wonderful
precise straight line so coveted on the trailing edge.

Another wise old salt from this neck of the woods, an IA who says the female
anatomy gave the contemplative sheet metal worker the inspiration for
stop-drilling, advised, "NEVER ever buck rivets on trailing edges. Always
squeeze them," he says, "and very gingerly."

Back at the airport they are smirking silently to themselves and whispering
to each other that I will return with two dog flaps. I intend to disappoint
them. While they were sleeping I was practicing.