Page 18-06 again: Flaperon Klöster Föken

Or, alternatively, Why I'm Re-ordering the Flaperon Skin

On several Van's designs including the RV-7 and RV-12 (unfortunately), a folded trailing edge skin is used for the control surfaces.  In general airfoil design, in order to facilitate the Kutta condition, the trailing edge should be sharp (not practical in the real world), squared off as is done with fast glass aircraft, or rounded, provided the radius of the trailing edge is less than 5% of the wing chord. I've

always considered rounded trailing edges to be an aerodynamic abomination no matter what the radius.  It offends me.  Van's did it right on the RV-9 which features a riveted trailing edge.  Coincidentally, the -9 is considered by many to be the sweetest-flying RV and it was the target for the handling on the -12.  Regardless of this complaint, having flown an RV-12 four times I can happily say that the -12 is the sweetest-flying airplane I've personally flown.  However, that doesn't mean it couldn't be better.

As discussed in the previous post, the build manual says to flatten the skins on the aft portion of the trailing edges using the hands, presumably thumbs, so that the skin is perfectly flat and parallel to the ribs.  I attempted this just prior to my annual week in Oshkosh (my 28th trip!) with limited success. Once I got to the show, I cornered an engineer from the Mothership (young guy whose name I can't remember) and asked him what I was doing wrong.  He explained to me that the way they do it at Van's is by using two blocks of wood, one on the upper and one on the lower surface, and squeezing them together with a large pair of Channel Locks.  This seemed to make sense, so upon returning to my NC home I attempted it.  The problem, and it turned

out to be a massive one, was that I misinterpreted the procedure he described to me.  I assumed that the wood blocks should be placed on the upper and lower surfaces just forward of the trailing edges, approximately where one might apply thumb pressure. Wrong.  The blocks should be placed on the trailing edge with the aft portion of the wood behind the trailing edge, hanging out in the air.  I found this out on another blog post, which re-posted the procedure from somewhere on Van's site, complete with pictures.  It turns out that this is the same procedure used to fix a "heavy" wing during flight test.  It was my mistake, but I wish Van's would publish a link to this procedure in the build manual, rather than just saying "flatten it with your hands."

Turns out I made several creases in the skin.  It wouldn't have hurt performance, and I tried to tell myself that I could build on, fly it without paint for a year or so as most people do, then build another left flaperon.  I was so unhappy with it that I drilled out several hundred rivets and removed the offending skin.  After (expletive redacted) up the left flaperon, I did the right flaperon with my thumbs (hadn't discovered the correct use of the blocks yet), and it turned out OK.  I may touch it up with the blocks later.  The new skins are on order.  They were surprisingly cheap.  

Update

It turns out that the aft flaperon skin can, indeed, be replaced while leaving the forward skin and aft ribs in place.  Upon completion of this, I discovered that the aft skins on each flaperon weren't lying flat as prescribed in the build manual.  I therefore constructed the tool shown to squeeze the trailing

edges, thereby flattening the skins.  I first practiced on one of the aft skins I had previously damaged, then proceeded to squeeze the trailing edges on both flaperons.  Success at last.

The "springs" connecting the two wood blocks are from a large binder clip.  The wood blocks have rounded edges with a large radius of curvature in order to prevent creasing the skin.