During two months I made about a hundred trial gliders and read through a few books of aerodynamics to conceive a little balsa indoor glider with good performances. This is the final result. It has a span of 200 mm, a chord of 20 mm, made out of a 0.6 mm balsa sheet. It has a flying speed of about 3 m/s and a finesse of up to 7. The thin rod at the front is a 27 mm iron wire of 1 mm diameter:
The exact measures:
Most difficult part of the work is to sharpen the leading edge and the trailing edge so they become like razor blades. Best is to use a 10 mm wide strip of soft sand paper pinched between two fingers. Pass it gently along the edges till they are sharp:
If you don't sharpen the leading edge of the wing you can get a stable flight easily. This is because the rough square edge creates turbulences. Yet this breaks down the ability of the glider to fly far.
The 27 mm iron wire must be flattened with a tang on about 7 mm and bend to an angle of about 20°. Then the flat part is glued in the middle of the front of the wing, on the downside:
The two outwards triangles of the wing must be bend upwards so the rear of the left and right ends rises about 3 mm. There are several ways to achieve a neat bend. You can pass a few times on the bend line with a ball-point (on the upside, pushing hard). You can pass with a knife on the downside in order to cut halfway the thickness of the balsa sheet, bend the wing strongly, put a little quantity of glue in the wound and bend the wing back into place yet with a little angle. Too less bend result in poor flight capacity, too much bend makes the glider oscillate while flying. If the glider turns to the left or to the right while flying you can bend one edge a little higher or a little lower. (None of my gliders has both edges bend to the same angle.)
How should this glider be launched? One precision first: the iron wire is not the tail, it is the front part. Do not hold the glider by the iron wire, you would not have enough precision and the glue could break. Hold it by the front of the wing with your fingers pointing backwards.
Launching this glider implies some training. If it is launched correctly it will make a beautiful flight, like if it was catch by the air.
It is very unstable. It can fly on its back. This happens when it is launched too slowly. It plunges towards the ground then flies the opposite direction on its back. The flight on the back is not very stable. (This is an important concern for planes. One early french plane killed a few amateur pilots for that reason. It was conceived to make stable flights and could do no acrobatics. Yet a strong turbulence or a mistake could put it upside-down and make it fly on its back. It was very stable in that position, the pilot had no way to bring it back upright! Some aerodynamic study was made and lead to a simple change of the position of one wing. This made the flight on the back become unstable. So, whenever the plane got upside-down it came back upright quickly.)
The weakest turbulence breaks down the flight, it cannot fly outdoors even with almost no wind.
Simply hold its nose towards the ground and let it fall works: like many gliders it will fall, accelerate and bend its path towards a stable horizontal flight.
Like for any glider the position of the center of gravity must be tuned. One first way is to use a little longer wire then cut it shorter with a tang. Next method is to put a little drop of solder at the front of the wire (avoid the drop become a sharp point).
Like for any flying wing, the center of gravity is situated at about 1/4 chord of the leading edge of the wing.
I believe the glider does not wear enough energy so the iron wire could seriously hurt an eye, provided is it blunt shaped. Please do not try this out. (Neither make the wire become too thick. This makes the glider unstable. The wire needs to be thin. A piece of wood with the same length and weight does not make the job, because its aerodynamic effect makes the glider unstable.)
One thing you can try if you really don't manage to make it fly is to increase its weight. Indeed this glider flies very close to the Reynolds frontier. Increasing its weight increases its flight speed, pushing it a little away from the frontier. Glue some little piece of metal on the downside, at the place of the center of gravity (or at least a way that makes the center of gravity keep the same position). (Even if you kept the center of gravity at the same place you may have to tune it a little by changing slightly the weight at the front of the wire. Anyway, even if the position of the center of gravity is good you can want to change it to change the way the glider flies.)
What is the Reynolds frontier? The classical Reynolds formula is used for a liquid that flows through a tube. It involves some parameters amongst which the length of the tube and the average speed of the liquid. One calculates length times speed. If the result of the formula yields a number lower than a key number, the flow of the liquid will be laminar; fluent and straight (like the smoke of a cigarete just above the cigarete). If it yields a higher number the flow will become turbulent; it will wirl and wrap (like the smoke of a cigarete afther a given height above the cigarete). This is very important for the flow of air along a plane wing. One calculates the wing chord times the air speed (together with other parameters) to know if the flow will become turbulent or if it will stay laminar till the trailing edge. If the result of the formule is too little the air flow will for sure be laminar yet it will no more follow the wing shape. So the wing will no more be able to lift the plane. If the result of the formule is too high there will be a lot of turbulences that will brake the plane (because the plane must provide the energy to create those turbulences). There is an equilibrium: you need a little turbulence on the wing upper side in order to stay in the air, yet not too much because this brakes the plane:
- Wings of fast and big planes
are braked by too much turbulence, therefore tricks must be
used to decrease the turbulence: make the wing
surface be very smooth, used thin wings, use a little wing chord
(this is why gliders
have very long wings with a short chord: to prevent as much as
possible the air flow to become turbulent),
suck away the turbulent air layer on the upper side through holes
in the wing or make the wing go trough the air with the less possible
incidence.
One very good example of this is the
American World War II Mustang fighter.
- Wings of little and slow planes lack turbulence. Possible
tricks to solve the problem are to create turbulences by
increasing the wing chord, a wire placed just in front
of the leading edge, little squary objects glued on the upside of the
leading edge (such devices are called vortex generators or
turbulators), make the upside of a balsa wing rough with coarse
sandpaper... In Nature the most little Reynolds number
is achieved by the wings of butterflies. Wings of bees and flies
are a lot more little yet they compensate by vibrating in the
air at high speed. (A lot of flying insects have a cute way to
create turbulences. They put their wings vertically above them,
touching each other. Then they push the wings downwards. The
air penetrating quickly between the two vertical separating wings
creates two big beautiful turbulence vortexes. While the wings
go further downwards the two vortexes separate and follow each
one the upper side of one wing towards the wing end. When the
wings touch each
other vertically below the insect, the vortexes meet again
and flow away towards the ground. One can say the insect wings
create two big wirling balls of air and throw those balls
towards the ground in order to stay hanging in the air. If the
insect wings didn't create those prehensible turbulences the
insect would fall slowly to the ground no matter its wings
flapping.)
- The wing profile matters: thick profiles are
stronger or more lightweight, can carry more fuel, have better
aerodynamic performances, can fly at stronger angles and give more
lift, yet they must be used far from the Reynolds
frontier. The profile that allows to approach closest to the frontier
is the basic flat and very thin profile, like a razor blade.
Flat profiles yet
slightly curved, like the wing profiles of early planes, allow to go
close to the frontier too yet have better lift performances than the
flat profile and allow to fly at a higher incidence angle.
- On another scale, the supersonic Concorde plane
uses turbulences too to be able to fly at a high angle for low-speed
take-off and landing. The problem is not the Reynolds frontier
yet the fact at a high angle a wing stalls. Strong vortex turbulences
at the upside of the leading edge force the air flow to follow
the shape of the Concorde wing towards the trailing edge. This prevents
the wing to stall and allows a strong upwards lift force at low
flying speed thanks to the high angle.
For most planes the solution that was preferred
is to allow air to come from the high pressure zone of the downside
of the wing and let it blow on the upper side, towards the back.
Why did I cut the front parts of the sides of the wing away and not the rear? This shape is considered to give best aerodynamic results. The reason why I choose to do so is this gave me the best flight. Some of my first prototypes had the rear of the wing cut away and flew correctly, yet not with the same yield.
Why did I use a lengthy wing? Indeed a much shorter wing with the same surface would have a longer chord and thus be further from the Reynolds frontier. This is because every wing creates two symmetrical huge slow turbulences made of air going globally sideways from the downside of the wing towards the upside. Those two turbulences are bigger than the wing size. The shorter the wing (thus the longer the chord), the stronger the turbulences. This is why commercial planes and a lot of birds have long wings (one says "high aspect ratio wings"), in order to use less fuel or less food to fly the same distance. Birds with short wings are most often birds who must fly through forest trees or be able, turn very quickly and take-off at once. This is also why a plane near the ground flies using less energy: because the presence of the ground hampers the turbulences. This is too why ekranoplanes (sort of flying boats that travel a few decimeters or a few meters above the water) consume less energy than normal planes.
Why are the side ends of the wing big triangles bend upwards along an oblique line? This oblique line is the synthesis of several considerations:
- This glider is a flying wing. (The reasons why I choose to make
a flying wing are that a tail is difficult to tune and it would
have to be very big and braking in order to stay behind the
Reynolds frontier.) Flying wings must use a particular wing
shape in order to be stable: the trailing edge must be a little
bit upwards. The bend line of a rudimentary trailing edge would be
perpendicular to the glider body.
- Bending the right and left ends of a wing upwards increases
stability. This is a most classical layout, thoroughly used for
simple gliders. The bend line is parallel to the glider body.
- A way to increase stability is to make the wing be twisted
along its span axis. The left and right ends are turned a few
degrees towards the ground, along the span axis (like the
blades of a lot of propellers). This allows the left and right
edge to stall later, in order to keep the plane stable. Perhaps it
also allows the edge turbulences to be weaker since the edges no
more lift the plane at a null angle.
I made gliders with the same shape yet two times smaller. They fly correctly yet they had to be proportionally heavier and fly two times faster. Once more because of the Reynolds frontier. Two of these little gliders were amazing. The first could be launched strongly towards the ground, it bounced back like if there was a big magnet repelling it, following a U-shaped path. It stabilized horizontally fluently at the end of the U path. The second one has a little vertical piece of balsa glued beneath the end of the iron wire, intended to look like the head of a bird. After a normal flight of a few meters it suddenly plunges towards the ground, like if it had seen a rabbit. Bigger gliders will certainly be allowed to fly slower.
I don't know if every phenomenon and aerodynamic law I mention does really matter for this glider. While conceiving it, several times I tried to follow a given law or hint and got no results or even bad results. I could not calculate the exact sizes of the parts of the glider to follow the laws at best. I had to try it out and improve step by step. For sure some laws or hint could have yielded good results but I tried them out with wrong sizes and positions. What matters for big planes does not necessarily matter for little planes, and reciprocally. But on the average I got the results I was seeking for. Improving further the performances of this glider shape could be another challenge, involving tools I didn't have, like a wind tunnel.
Thanks to my friends Didier Bizzarri, Jacques Donneux and Yves-Dominique Franck for their data and advice.
Jonathan Bosh send in these pictures. He made a few gliders and even got one thermaling away! Clic on the pictures to get them fullscreen.
Plan to make more than a few gliders. Use a template drawn on a piece of wood to quickly create the wing. Cyanoacrylate glue best stabilizes the crease, but epoxy is needed to attach the ballast wire. Have Fun!!!!
The glider sits on the shelf above my desk, along with a few other totems. I was intrigued by the minimalism of the design; it is lift in the simplest expression. It's purpose now is to remind me how simple solutions are often the most elegant!!!
This page proposes a glider. It has a rather close design yet it is easier to build:
http://www1.faa.gov/education/resource/glider.htm
Eric Brasseur
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March 15 2000 - September 7 2003
