Have you heard about the easy way to taper anything? Use different size rods and sand to the rods. Lets say you want to take a 1/4 inch flap and taper it to 1/8.. You need 3 rods (music wire works great), 1/4, 3/16 and 1/8. You put the 1/4 inch rod at the leading edge and the 3/16 rod at the trailing edge. Sand till your sanding block is flush with both rods along the entire length. Flip the piece over and put the 1/8 rod at the trailing edge leaving the 1/4 rod at the LE, sand till you are even with he rods and you have a perfectly tapered flap. For different thickness/tapers just figure out what size rods you need and get after it.

Even works on round parts like Ringmaster elevators, just bend the rod to match the part. Works every time and is fool proof. The only way you can screw up is by not sanding enough. 

  The rods have a tendency to wander a bit and have to be watched carefully.  Is there some trick to locking them in place?

I use masking tape along their length. It may sound less than ideal, but it works fine.

SK

The easiest way I've found is to just CyA the rods to the glass top. They'll come loose with a whack of a chisel or just a hammer when you're done. Then clean up the residue with a razor blade scraper.

When I whack glass with a chisel I usually end up cleaning up the residue with a broom and dustpan!

Seriously, I know you meant to whack it parallel to the glass surface.  CYA is very brittle in shear.

I use an old glass top dining room table.

When need to stick something down I use a hot melt glue gun.  I can get it off when I'm done with a combination of a heat gun and single edge razor blades.  Haven't broken the table yet.

ps: fixture block are glued from the side, not underneath.

Good idea to glue down the rods!

Next ones I cut, I'll tack them to the glass with thick CyA in enough places, and "kick" the CyA to make sure it's cured. Later, debonder to release the rods, then a razor or #11 blade edge to pop the CyA residue off.

I've occasionally had the balsa flap blanks slip, but then I remembered I have plenty of 2-sided "scotch" tape... Not so thick it can distort the taper, and holds very well if the wood and the glass are clean and dry. (Clean both when you flip the blank over...) 2-side tape doesn't peel off well, but a #11 blade makes it easy.

I'm also in the habit of tack gluing jigs and parts to my glass top building
surface with thin CA.. I use a razor scraper or safety razor blade to remove
things. On at least one occasion, I managed to chip a small area of the glass
surface, so I'm more careful now.

As for the rod sanding method, I founding taping them in place fairly useless
for me, and I let them float. The important thing to do is frequently remove the
sawdust, because it will easily build up under your part and allow it to be sanded
too thin..

I believe Joe Adamusko showed a special sanding tool he made for the elliptical arc
shapes on his big Spits and similar elliptical winged ships.

Here's my idea - build a sanding jig using the square solid brass rod, with the rods
glued into place, with proper curvature for your flap TE as required - one for top
surface and one for bottom surface taper. You'd need a decent flat strong surface,
and I have seen some fairly inexpensive pressed wood shelf stock with laminate
(Formica or similar) surfaces. (Not all shelf stock is reasonably flat, of course.)

In reality, I don't think the flat surface is so absolutely critical for the sanding, since
the tubing diameter limits sanding, and the flaps/elevators have a little compliance. But
I like accuracy for its own sake; it could be worthwhile to build a taper sanding jig for
your favorite model.

Here's a link for an online site where you can buy small quantities of square rod for
reasonable cost: http://www.onlinemetals.com/merchant.cfm?id=82&step=2&top_cat=79
I haven't actually done business with Online Metals, I just googled for square brass bar.

Just sayin'.. 

L.

"The policy of being too cautious is the greatest risk of all." -Jawaharlal Nehru