Here's a top view of the glider sitting on its launcher in
its unfolded, airplane state. The "engine" in the photo is 3
Robinson Coupled 1 liter bottles for
testing (easy to rebuild after repeated crashes) but the real engine
is made of 5 one-liter bottles
spliced
together, for a total volume of 3.8 liters.
The wings were cut from a block of expanded polypropylene foam (the foam used in most full-contact radio control combat planes) with a hot wire cutter. They have an inset wooden spar and a balsa wood trailing edge. A strip of filament tape along the bottom also helps stiffen them.
The airfoil is flat bottomed (SD7055 thinned to 8%, for you flight junkies) and the outer panels fold in along the polyhedral crease, bottom to bottom, creating a fully symmetric airfoil when folded up. A conventional airspeed flap controls the trigger, causing the wings to unfold and snap into place at apogee.
Or at least that's the idea.
Here's a bottom view. The tail boom is made
from fluorescent light protector tubing, and the glider uses a
plastic garden hose quick-connect for a nozzle and trigger, just like
the long tail rockets detailed on another
page. In fact, that design was the result of needing a stiff tail
boom for this glider (originally I had a 1/4" diameter fiberglass
tube, but it was too whippy and flexible). So this thing launches
with a reduced nozzle, which I wanted: it's so draggy that I thought
keeping the velocity down would be a good thing.
The tail surfaces are balsa, in a conventional airplane configuration. The rudder is gently spring loaded to make the airplane fly wide circles: at launch, the high speed air flowing over the surface easily overcomes the spring tension, holding the rudder straight. The elevator can be "dialed in" with a plastic screw.
The two vertical white things above the wing are plastic shrouds that hold the wing closed, and cover the gap in the leading edges, when it's folded up. They spring up into this position at apogee, allowing the wings to unfold. (Awkward to have them just hanging there, I know, but I never thought of a good way to stow them, and it actually didn't seem to matter much, flight-wise.)
Here's the glider folded and ready for launch.
The outer panels of the wing are folded under, and the plastic
shrouds are lowered into place, holding the wing in the folded
position.
Note the symmetrical airfoil formed by the folded wing, and the fact that it hangs off one side of the rocket. This causes drag on one side of the rocket, which would normally cause the rocket to curve toward that side. But there's a little bit of "up elevator" dialed into the tail, so that compensates for it (or at least that's the theory, and it mostly seems to be true). That up elevator also helps the plane fly slow and stable once the wings are unfolded.
Here's a closeup of the center of the folded
wing, and the trigger mechanism. It's a little hard to see here how
the trigger works, but I'll try to explain.
The horizontal black piece (let's call it the "trigger bar") is fastened to the rocket body, and is free to rotate around the white dot (which is the head of a nylon screw). There's a piece of fishing line attached to its left end (you can just see it) that goes up and over the rocket's nose, and hooks onto the airspeed flap on the other side of the rocket. So as long as the airspeed flap is down, the trigger bar is prevented from rotating counter-clockwise by the fishing line.
The short vertical black piece (call it the "catch bar") has a post sticking straight up from it's surface (again, it's a nylon screw, with a spacer), and that post is caught under the right side of the trigger bar, which has a notch cut in it for that purpose. Attached to the bottom of the catch bar is more fishing line, which is attached to the plastic shrouds. The yellow thing is a long, strong rubber band, stretched and fastened to the nose of the rocket. So the rubber band is trying to pull the catch bar up (and thus the shrouds off the wings), but it can't because the trigger bar is holding it. The airspeed flap is, in turn, holding the trigger bar in place, via that piece of fishing line.
So at apogee, when the airspeed flap opens, it releases the fishing line attached to the trigger bar, which lets the trigger bar rotate conter-clockwise, which releases the catch bar, which pulls the plastic shrouds off the wings, which then spring open (they have strong rubber bands that pull them into place).
Now you can see why I started calling this thing the "Goldberg Glider" It's a nod to one of my idols, Rube Goldberg. If only I could have figured out a way to get a mouse and a cat in there...
Here's a nice action shot of a test launch. As
I've said, I never did get the glider to work the way I envisioned
(long straight launch, snappy deployment, immediate transition into a
nice glide without losing any altitude -- yeah, right). After a bit
of tuning, I got the deployment mechanism working fine. But it turns
out to be a really gnarly "trim" problem to set up the airplane so
that it will recover quickly from whatever attitude it finds itself
in, and then fly stable circles all the way to the ground.
My best flight to date was quite exciting, though it didn't last long. Launch was lovely, nice and straight to perhaps 100 or 150 feet (which is all I'd get for a draggy, 1 pound rocket at 75 psi). The rocket slowed, the wings snapped open, and the rocket (now a glider) stalled. It tipped over, and dove straight for the ground. Yikes! It accelerated rapidly, as falling things will, and only slooooowwwwly started to pull out of the dive. There were three of us there, and we were all screaming "pull up! pull up!" and little by little it did, but it didn't look like it would make it in time. It finally pulled out, at about 5 feet off the ground, going really fast. It flew a nice flat decelerationg half circle, and landed gently less than ten feet from the launcher. Pretty exciting, but hardly a good duration flight: it took maybe 12 seconds total.
I'll continue messing around with it, of course, and maybe one of these days it'll work. There are no metal parts in the thing, to conform to the Olympiad rules, but if that restriction is lifted in a future contest, I might make it radio controlled, so that I can fly it down myself.That would solve all my remaining problems :-)
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