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9.4.h Strategy & Tactics: Flex Wings




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This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.

9.4.h Strategy & Tactics: Flex Wings

From George Gassaway:

(Note - This really belongs in the glider FAQs but over there, may end
up missing ;-)

>>>I'm not getting enough billowing in back (that is, the covering is
>>>not loose enough), with the result that the thing dives (not enough
>>>incidence between front and back). How do I cut and mount the
>>>covering in such a way that it's tight in front but loose in
>>>back?..."<<<

Well, ready to learn? :-)

I was fortunate that early on when I used to apply the plastic to
spars by stitching it on, something about the process made it tight in
front but billowed in the back. Later when using thinned contact
cement to attach the plastic to the spars, and laying the plastic down
tight across a building board, it didn't produce the same effect.

The trick I use to force the correct amount of billow is to use some
very sticky but thin and flexible tape. Originally that was the CMR
adhesive mylar, but that's been gone for a long time now (Ah yes,
there is adhesive mylar around the hobby but none that is as thin and
sticky as CMR's was). So I use strips cut from band-aids (also that's
what I use to apply shroud lines that I used to use CMR adhesive mylar
for).

I apply a small strip to the top of the plastic, near the edge of a
side spar, at around 1/4 of the distance back from the nose. So for a
flex-wing with 12" spars, that would be about 3". I use the tape to
pull the plastic taut, towards the spar, so that the front part of the
plastic is taut but the rear part billows. The amount of billow at the
rear depends on how much you make the front taut using this method.

BTW - first prepare the area for the tape, by wiping away from that
area any talcum powder you may already have applied to the plastic. I
do the left and right side, but do not stick the other side of the
tape down until after making some test glides to see if the billow is
about right. By the other side of the tape, what I mean is that once
you are sure the tautness is right, then you'll curl the tape
completely over the spar and stick the tape to the bottom side of the
plastic. But as I say, do not do that yet, as it is almost impossible
to peel the tape off to make adjustments later.

So, just test glide it indoors. If it stalls a lot, try adding a bit
of noseweight. However usualy a LOT of stalling indicates there is too
much billow, so you'll need to adjust the tautness. If it glides OK,
try the acid test. Hold it almost straight down and let go. If it
starts to pull out of a dive, then you're OK. If it flutters and dives
down, then you'll have to try increasing the tautness.

Keep making adjustments until "you learn how to do it, or something
like that".

When it is right, then curl the tape around the spar and stick it to
the bottom of the plastic too. And give it all another dusting of
talcum powder so that any stray adhesive won't stick to anything it
should not be sticking to.

 

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