This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
Flexies are very different to trim. I refer you to the master,
From: firstname.lastname@example.org (GCGassaway) Subject:Re: Newbee
Flex-Wing BG questions Date: 12 Jan 1999 06:43:05 GMT
There is a trick to making flex-wings have good pitch
stability without having to resort to other tricks such as
tails, canards, and weights.
About 20-25% back from the nose, use some very sticky tape
such as a strip cut from a band-aid (I use some good old
sticky adhesive mylar but that's not easy to find). Use the
tape strip to pull the plastic taut into the spar and another
piece to pull the plastic taut into the right spar. The
remainder behind should be allowed to drape loosely so it
billows with air when it glides.
Since you will need to experiment, do not attach the tape down
permanently until you get the tautness just right for a good
glide trim ( I usually only have a small portion lightly stuck
to the plastic, with the rest of the tape strip peeled up into
the air). After all, if you use the kind of thin plastic as I
use (1/4 mil dropcloth) once the tape is down good you will
rip the plastic before the tape will come up if you later want
to try to adjust the tautness and billow.
When the tautness and billowing are right, the taut front end
acts as a built-in canard, sort of like having a flying wing
that has elevators on the inboard leading edges instead of on
the trailing edges (not that I can think of any real flying
wings that were like that). I won't get into it any deeper
since it is mostly just a matter of trying it and adjusting it
until it glides stably. Once you get in the ballpark, you can
tell, though if stalling persists you might need to add a
little noseweight too (clay falls off too easily, I often glue
a scrap piece of spruce or something else, even a piece of
solder, to the nose center spar to move the CG a bit forward).
But you can experiment with clay at first to make sure it is a
CG problem before gluing anything. Sometimes to get a good
stable glide I've allowed one side to billow a bit more than
the other, making the flex-wing glide in a turn of about 10
feet in diameter. But too much billow on one side can make it
tend to spiral down. Again trial and error in learning will
teach far more than I could type about it.
Now, canards can do the job but they add extra complexity and
difficulty. Takes a lot less time to find out how to trim by
this nose tautness and rear billow method than do any actual
design changes....which you would also still have to learn how
to trim out anyway.
More flexie trimming information can be found at
http://www.wizvax.net/jvincent/sd/sd980102/fw1.html and in
NARTS publication NIRA4 "Glider How-To Articles from the Model
Rocketeer" for $3.50