A little balsa glider with a good yield
To build a paper, cardboard or balsa glider is not difficult. Many
Web pages explain the basic aerodynamic rules you have to follow.
You will most often succeed to make a glider that really flies.
Worst case, ask an experienced friend to tune it. But such a glider
generally does not fly very far. It rather gracefully falls to the
ground. Some rocket-shaped paper gliders seem to fly straight in the
air but only on a short distance. They behave like an arrow; once
their initial impulse is over, they will gracefully fall to the
ground too. It took me years to build a little balsa glider that
flies straight through my room, seemingly on a horizontal path. This
page proposes the plans of that glider. I will try to explain the
rules I followed to build it. The exact shape of the glider doesn't
matter. What matters is to understand the rules and reasoning behind
and to be able to adapt them to other shapes and purposes. The
explanations assume you have an elementary knowledge
of toy gliders aerodynamics
. If you want some insight in
flight physics then maybe try this page: Basics of toy glider physics
Here are drawings of the glider:
The Wing span is 300 mm. The Wing chord is 50 mm. The wing is made
out of a 2 mm balsa plate (fibers parallel with the span). The wing
incidence is about 7°. The leading edge of the wing is made round
while the upper side of the trailing edge is sandpapered:
The front of the vertical mast has a height of 10 mm. The rear of
the vertical mast has a height of 3 mm (which leads to the 7°
incidence of the wing). It is made out of 2 mm balsa plate (fibers
vertical). The front of the vertical mast is situated 70 mm from the
front of the body. (Jeffrey Reed pointed out that these measures
actually yield an incidence of 7.97°, which is 8°. You will be
closer to 7° if you use a height of 4 mm instead of 3 mm at the rear
of the vertical mast. The ideal height would be 3.86 mm...)
The body is a 250 mm rod of 3 x
3 mm pine
wood. The front end is a little hemisphere of hot glue. The rear end
has been cut out a little bit at the bottom. The front weight is
made out of say 1 mm diameter solder wrapped around the rod. The
exact weight doesn't matter. What matters is that the center of
gravity be situated 1/4 chord behind the leading edge, that is in
this case about 12 mm from the leading edge, like drawn below. The
best way to get the center of gravity at the right place is to add
solder carefully. Start wrapping a fair length of solder at the
rod's front end. (Possibly now add the hemisphere of glue at the
top.) Test the center of gravity and remove or add little lengths of
solder. Till the center of gravity shows to be at the right place:
What really matters is that the glider flies well. If when you throw
it, it clearly shows to need some more or some less front weight,
just do that.
The rear stabilizers are made of 0.3 mm cardboard (from a pack of
organic rice wafers). The vertical stabilizer has a height of 35 mm
and a chord of 25 mm. The horizontal stabilizer is so to say made
out of two vertical stabilizers.
The glider's weight is about 10 grams. A much heavier glider will
fly perfectly yet faster (provided it is rigid enough to keep its
shape). A much lighter glider may fail to fly correctly because it
will fly too slowly. Indeed, a too low airspeed changes the way the
air moves around the wing (Reynolds number...) Solutions for low
speeds can be to use turbulators, to increase the wing chord and/or
to use flatter wings. A friend was afraid to throw the glider so I
made a copy of it for him, much lighter, out of very thin balsa
sheets. It flies correctly but clearly I reached the limit. A
lighter version would need either turbulators or a longer wing
chord. Note that a heavier-and-faster version of this glider is not
supposed to fly farther than a lighter-and-slower version. Speed
does not imply distance. There will
be differences in flight distance between faster and slower versions
but they are not supposed to be tremendous. What will change is the
duration of the flight. A light and slow glider will stay much
longer in the air.
To download a printable plan of the glider, a template of the wing
shape and the drawings of the glider in vector format, click here
. The format is
OpenOffice.org 1.x Draw. Open
is free software, available for most computer operating
systems. To get the same drawings in Open Document format, click here
. To get just the
plan and the wing template in standard printable Poscript format, click here
Why a flat wing profile?
For years I heard friends sweat and dream about wing profiles. They
exchange profile drawings like if they were treasures. They spend
weeks, even months, assembling wings that match the chosen profile
with a precision up to a tenth of a millimeter. After reading
through many texts I found out that a simple flat profile was the
best solution. It's far out the easiest to build. Its aerodynamic
yield is close to the maximum you can expect. Just give it an angle
of attack of about 7°. Apart from a good aerodynamic yield, flat
profiles have a simpler behavior in the air. They are easier to cope
with because they create less instability than curved/cambered
profiles. They are reliable and predictable. (Don't take me wrong.
An appropriate wing profile really can make a difference for an
airplane. But if in doubt, use a flat profile...)
The main advantage of a cambered profile, apart from the facts that
it has some more yield and can be more rigid, is that it requires
far less wing surface to create an equal lift. Conversely, a flat
wing will require much more chord to create the same lift than a
cambered wing (for a same span). Hence, and this quite important: a
flat profile can allow a glider to fly slower, because for a same
span and lift a flat profile implies more chord (Reynolds number...)
Why such a thin wing profile?
A balsa plate of 2 mm is enough to get a rigid and lasting wing. So
why use a thicker one? There are a few reasons not to do so. First,
a thicker profile would mean a heavier glider. It would fly the same
way yet faster. I prefer a slow speed, to have the time to enjoy the
glider's flight. Second, at low speed and short chord, thin profiles
are commonly a better choice (whether they be flat or cambered). You
get more easily the air to follow the wing shape neatly.
Basically, thin profiles allow for lower flight speeds (whether they
be flat or cambered). You need a higher flight speed to get the air
to move correctly around a thick profile. This is why the first
airplanes had very thin profiles. This too is one reason why they
were biplanes; because you can't build strength into a very thin
wing, so one solution is to assemble two superposed wings to get a
Advantages of thick wings in today airplanes is that they are more
rigid for a given weight and they can contain more fuel and
Why a wing incidence of 7°?
I read in papers that this angle provides the best aerodynamic yield
for real-size airplanes. So I tried it out on my glider. Maybe
another angle would be even better. Since I got the near horizontal
flight path I was seeking, I did no further experiment.
Many low-cost balsa gliders have a wing incidence that seems to be
0° (compared to the rear horizontal stabilizer). This doesn't mean
much since the horizontal stabilizer is not the only part of the
glider that participates into imposing the angle of attack in
flight. The weight at the front of the glider body matters too.
Furthermore the downwash created by the wings tends to push down
slightly the horizontal stabilizer, inducing an angle of attack on
the wings. Such low-cost gliders won't fly far anyway. Yet they
often fly correctly.
Update: look at this record-breaking glider: http://hosted.schnable.net/amaglider/assets/indoor-gliders/handlaunch-gliders/nifkin_by_bob_romash_plans.html
. At first glance my glider can be seen as a simplified version. But
there is at least one key difference: it has no wing incidence...
Try out the renowned FoilSim Java applet from NASA: http://www.grc.nasa.gov/WWW/K-12/airplane/foil3.html
. It yields that for a flat wing profile, the optimal angle of
attack is around 3°. This leads to think that, if my glider flies
optimally, then its nose is pitched down say 3° during flight, so
that with a built-in wing incidence of 7°, the angle of attack
during flight would be say 4°, like suggested in the drawing below.
The red line would be the flight path. The horizontal tail would be
pitched down a few degrees, which yields stability. Then the body
does not fly parallel to the airstream but this would have not too
much consequences because it is quite thin. This flight attitude was
not my intention and the glider would be optimal by chance.
On the contrary, maybe the Nifkin glider mentioned the paragraph
above, flies with its nose pitched upwards. Indeed its aim is not to
fly the furthest but to hang in the air the longest possible while.
Hence it needs to get the most possible lift out of its curved wing
profile, so long this causes not too much drag proportionally.
This is all why experienced model glider builders keep the
incidences tunable, of both the wings and the horizontal stabilizer.
You don't know in advance what the best wing angle of attack is,
neither what angle you must give to the horizontal stabilizer to get
that optimal angle of attack of the wings. If you also tune the
front weight (to displace the center of mass), then you get a hectic
three variables system.
Why such wide wings?
An airplane keeps flying because its wings are lifted by the air.
According to the laws of mechanics, the huger the volume of air the
wings ride on, the better the yield will be; the less energy the
airplane will have to spend to stay aloft. That's one reason why
gliders have very long wings; in order to span over a huge volume of
Why such short wings?
Wings also leave little turbulences behind them, just like a rod
that you swing through the air. The swoosh noise that you can hear
is generated by the turbulences. Turbulences mean drag and loss of
energy. A wing is kind of a streamlined rod, that tries to generate
as few turbulences as possible. This is the second reason why
glider's wings are long and have a narrow chord: a longer chord
would generate more turbulences. But balsa gliders have the opposite
problem: their wings are so narrow (a few centimeters) and their
flight speed is so slow, that the air barely manages to follow the
wing profile correctly. The slower a glider flies, the more you have
to increase its wing chord, so the wings can wing properly.
Conversely, for a same wing surface, you get a shorter wing. (But,
if you would increase the chord far too much, then you would get
another problem: the wings now generate turbulences, which is the
problem of big gliders.)
Why those wing tips shaped like shark fins?
While the air shears towards and along the profile of the glider
wing, it will rise up, accelerate and decelerate (turn around the
wing profile and pull it upwards just like a stone in a swing pulls
on the cords), go into both huge and microscopic turbulences... What
matters is that behind the glider the air "closes down" and stays
immobile as much as possible. In other words: the air may do
whatever movement around the glider, provided it's fluent and has a
way to close down neatly behind the glider. The enemies to this are
the aerodynamic turbulences and instabilities. One cannot avoid them
but they can be lowered to a minimum.
The wing tip of a bare rectangular wing generates a strong local
turbulence. The air just before and especially behind the wing tip
will go on turning round in the turbulence for several seconds. That
turbulence carries away a part of the energy of the glider. Hence it
brakes the glider. To decrease that turbulence several solutions
- Use a
very wide yet narrow wing. That is a wide span and a
narrow chord. That's called a "high aspect ratio wing". In such
a wing, the wing tips are minute compared to the rest of the
wing surface. Hence the turbulences they generate are weak
compared to the glider size. Other good reasons exist to use
such wings. But I didn't, for two reasons. First of all such a
wing is difficult to manufacture. You cannot cut out a narrow
strip of balsa wood and expect it to be perfectly flat and
rigid. Second, a problem with little and low speed gliders is
that if the wing chord is too narrow, the air simply no more
travels properly around the wing. You get no more "wing" effect.
Thats due to the "Reynolds number". Below a given "Reynolds
number" the air no more follows the wing shape. To gain back the
wing effect and its lift force, you must either increase the
flight speed or the wing chord. (Turbulators can help too but
they add a little braking.) For my glider I was thus obliged to
use a comfortable chord and hence to limit the span. (On the
opposite, real gliders rely on a little chord to lower their
wing profile Reynolds number, because at very high Reynolds
numbers you get strong air turbulences all over the trailing
surface of the wing.)
- Use winggrids.
These are little wings, cleverly put at the wing tips to lower
the turbulences. Some birds use this system. I heard it's a very
effective system. It allows to build compact wings, with a low
aspect ratio, yet with a high aerodynamic yield. I didn't choose
this solutions for several reasons. First, I needed to make
experiments to check out this solution. Second, on my glider the
chord of the little wings would be below the Reynolds limit
(turbulators would help...) Third, a winggrid on a little glider
would both take a lot of time to assemble and be quite fragile,
hence the lifetime of the glider would be very short.
- Use a biplane or triplane
wing structure. I you intend to build a low-speed and big
airplane, a biplane structure is an excellent solution. It
allows to build a very lightweight, wide span and rigid wing
structure. It can even have both a rather short span and a good
aerodynamic yield, provided you put the lower wing behind the
upper wing. That way you get sort of a huge winggrid, like in
the Flying Flie. I
didn't use a biplane or triplane structure because my 2 mm balsa
wing was already rigid enough and because I feared some
intellectual aerodynamic and structural fuss building a biplane.
Besides, why try to make an even more lightweight wing? To allow
the glider to fly slower? I'm already close to the lower speed
- Use a trapezoidal
wing shape. So the wing has a strong chord at the middle
and a little chord at the tips. Hence the turbulences generated
at the tips are lower. This solution is used on quite a lot of
big airplanes. I didn't use it because The angle of the trapeze
has to be under 10°. On a wing with a short span like that of my
glider, this does not allow for a much shorter chord at the
tips. So the benefit would be real yet not extraordinary.
- Use wing tips salmons.
This wing tip shape is known to be quite effective to lower the
turbulence. It is widely used on RC planes and gliders, often
associated to a trapezoidal wing shape. I didn't use standard
salmons because I believed their effect would not be sufficient
for my aims. Though we are coming close to the solution I
finally choose. The wing tips of my glider can be seen as "some
sort of huge salmons" or "a short and fluent transition between
trapezoïdal wings and salmons".
- Use an elliptical wing shape. That is the wing shape of the
legendary WW II Spitfire
fighter plane. It is seen as sort of "optimized to the extreme":
the air pressure and angle of attack is the same all over the
wing span, the air streams follow quite parallel paths all over
the wing span, the wing tips turbulences are kept to the
minimum, the wing span is short enough to get a swiftly
maneuverable aircraft... it is smooth, homogeneous and optimal.
Yet it has one big disadvantage: when the aircraft makes steep
turns the lift force may disappear suddenly all over the wing
inside the turn path. (The wing profile was actually slightly
changed so the pilot got warning vibrations before the real
stall would occur.) (Critics say the wing of the Messerchmitt
BF-109 were as effective as that of the Spitfire but it had a
more conventional shape and it was far easier to build.) Now my
glider is not meant to make steep turns. It's not even meant to
turn at all. So why not use this ideal shape? Well, the shape I
choose is quite close to the ellipse. You may argue it's closer
to an ellipse cut in two. Actually what matters is the chord
length. The chord length over the span has to obey the formula
of the ellipse. No matter those chords are pushed backwards to
align with a straight trailing edge or pushed forwards to align
with a straight leading edge. In the Spitfire wing, the chords
are put just a little bit forwards (this has some structural and
aerodynamic justifications). The four shapes beneath would all
be called elliptical wings. The upper left one is a mathematical
ellipse. The upper right one approaches the wing of the
Spitfire. The lower left one approaches the wing of my glider
(so I was quite confident in its qualities). The lower right one
feels to me to be an aerodynamic nightmare. I managed to make
some little balsa gliders fly correctly with that wing shape.
Anyway it was clear this is not an optimal shape (see the end of
the text for counterexamples).
- Copy a bird. That's what I did (go to the end of this text to
read why I actually didn't) (I left the rest of this paragraph
intact so you get a good example of a wrong reasoning, though
the last part, about the vibrations, is excellent). I used the
shape of a seagull
wing, just a little shorter (lower aspect ratio). A seagull wing
is a very complicated structure made of bones, muscles, tendons,
articulations, skin and feathers. So simply using its rough main
shape on a flat wing is not guaranteed to yield an optimal
result. But I was sure I wouldn't get a bad result either, since
the seagull wing shape was close to the elliptical wing shape of
the Spitfire and to the trapezoid + wingtip salmon shape of good
RC planes and gliders. The main difference is that the central
part of the wing has parallel borders. I inclined for such
parallel borders because I read a page
from a top balsa glider builder who explains that he simply
hates the elliptical shape. He prefers parallel borders and wide
rounded wing tips. Like the Westerner 2 RC model near the bottom
of this page.
Furthermore I thought this was a mathematical best choice in
order to keep the wing chord to a maximum nearly all over the
span (remember at low speeds the chord needs to be as wide as
possible). Next, why stick to the seagull shape even for the
shark fin-shaped wing tips? For three reasons. The first one is
not very important: because it is the closest one to the
successful salmon wing tip shape. The second reason is that I
read in books that this shark fin shape generates the least
braking and turbulences. It's the shape that most suits a fluent
air circulation. The third reason is really and excellent one: I
tried out and checked that this shape really yields the least
turbulence. I made two wings, nearly identical: one with the
shark fin tip shape and the other with rounded tip shape (close
to the Westerner
2 wing shape mentioned above). I hold them in my hand and
sheared them in the air around me. The wing with the rounded
wing tips produced weak vibrations I could feel in my hand. The
wing with the shark fin-shaped tips produced no vibrations at
all. It did glide through the air like if there was no air at
all except for the lift force. So the seagull wing shape was
- Use winglets.
I didn't even consider using winglets. That aerodynamic device
is difficult to design. A random design, even an aesthetic one,
is said to brake the plane more than using no winglets. Besides,
winglets are usually meant for the tip of narrow ending wings,
which is not my case. I once saw a prey bird gliding. The tip of
his wings was round shaped, with no winggrids. The tip of his
outermost feather made an upwards bend to form a quite little
yet beautiful winglet. So maybe little winglets can enhance a
little bit the yield of this glider. Maybe I should give it a
try but I know their effect will be negligible. I've no way to
verify their usefulness. I won't add to my glider something that
I cannot check to be worth the effort.
Why pretend that the wing profile is flat though only the upper
side of the trailing edge was sandpapered, which creates a very
slightly curved profile?
Well right, the profile is not perfectly flat. But I'm quite sure
this doesn't change much of the glider's flight characteristics.
Such a thin wing with the upper and downside trailing edge
sandpapered symmetrically won't change much to the glider's
behavior. Sandpapering only the upper side makes the manufacture
Note that if you sandpaper the downside of the trailing edge, you no
more need a tail horizontal stabilizer. This is a "flying wing"
shape. (Such a flying wing may benefit from a very little tail
horizontal stabilizer anyway, just to avoid pitch oscillations.)
Why such a little horizontal stabilizer?
This is a key feature. Many beginners (like me) boldly believe the
rear stabilizers of an airplane behave like the stabilizers of an
arrow. So the bigger the stabilizers, the more stable the glider's
flight will be... That's wrong.
In first approximation, a glider with a flat wing profile almost
does not need a horizontal stabilizer. I made such a tail-less
glider and it flies quite correctly (the build tip is to sandpaper
only the downside of the trailing edge; see data about flying wings
to know why).
stability of the glider arises first of all from the interaction
between the weight in front of the glider and the tendency of the
wing to flip upwards. It's sort of a battle between the two forces.
While in flight, the wing experiences an aerodynamic torque that
tends to make the glider pitch upwards. But the center of gravity of
the glider has been placed slightly frontwards, which tends to make
the plane pitch downwards. The two forces equilibrate and the
aircraft flights straight. Why such a fragile equilibrium between
two forces of different natures? Well because it is not fragile at
all. It's quite stable and the speed regulation of the glider relies
upon it. Indeed the weight in front of the glider is constant. It
doesn't change when the speed changes. It always tends to make the
glider pitch downwards with the same force. On the contrary, the
aerodynamic upwards torque on the wings depends on the glider's
speed. The faster, the more torque. The slower, the less torque. So,
should the glider fly too slowly, the upwards aerodynamic torque
decreases and the front weight makes the glider tend to dive, which
makes it gain speed. Should the glider fly too fast, the aerodynamic
torque will supersede the front weight, the glider will pitch
upwards and tend to climb, so it will loose speed. The result is
that the glider flies at a nearly constant speed. A huge rear horizontal
stabilizer would supersede this phenomenon and make the glider
Then, why put a little horizontal stabilizer anyway? For two
reasons. First, The regulation phenomenon explained above tends to
make the glider oscillate. That's often a problem you get, when two
forces battle and the system has some inertia. The tail-less glider
I made flies that way: oscillating. It's cute to see but I bet it
participates in the low yield of that glider. The horizontal rear
stabilizer tends to prevent the oscillation. It wears away the
energy of the oscillation, like the dampers of a car suspension.
Second, when I said this speed regulation system is stable I was
boasting a little bit. On a plane with a standard curved (cambered)
wing profile there is a clear instability. Indeed when the plane
pitches a little upwards, the upwards aerodynamic torque will
increase a little bit. So, if the plane pitches upwards it will tend
to pitch even more upwards. And if it pitches downwards it will tend
to pitch even more downwards. It diverges. The forces implied are
not very strong yet they make the plane unstable, unable to fly
straight. The purpose of the rear horizontal stabilizer is to
dominate this phenomenon and make the plane fly straight. But only a
quite little horizontal stabilizer is necessary for this... (As
written above, airplanes with a flat wing profile are quite neutral
in regard of this phenomenon. They don't tend to increase an upwards
or downwards pitch. They don't tend to stabilize it either. Only
special wing profiles intended for flying wings do.) Third, I have
few confidence in the wing to willingly adopt the 7° angle of attack
I choose. The horizontal stabilizer helps it conform to my wishes.
I'm quite sure the wing actually does not fly with an exact angle of
attack of 7°. Conversely I doubt the horizontal stabilizer flies
with an angle of attack of exactly 0° through the air. Anyway the
system seems to come globally rather close to my wishes.
So, the purpose of the horizontal stabilizer is not to impose the
flight attitude of the glider. It is rather to compensate for some
weak and secondary disorders and instabilities. Therefore it does
not need to have a huge surface. Actually it must not
have a huge surface, in
order not to dominate the main speed regulation phenomenon.
Why such a little vertical stabilizer?
The reasoning is quite close to that for the horizontal stabilizer.
But I don't master it. Specifically, with a huge vertical stabilizer
the glider gets two problems. First, it will tend to oscillate
sideways; it has a yaw
instability. Perhaps because the vertical stabilizer is caught
alternatively by the left and right main wing vortexes. Second, it
makes the glider gradually go in a spiral dive; the much hated
phenomenon that killed many pilots.
The first prototype of this glider had a quite bigger vertical
stabilizer and it didn't have a beautiful flight. The instabilities
made the glider loose a lot of energy while flying. It was falling
fast while flying. But I knew about the problem and I made a haircut
to it. Once the vertical stabilizer got little, the glider's flight
became wonderful. So smooth and straight. Especially, it flew much
more horizontally, like I wished it to.
I was told about an even better place to put the vertical stabilizer
and avoid the instabilities: below the plane body. A few airplanes
and drones have such an inverted tail. The problem is that this
makes the aircraft more fragile or even dangerous during take-off
and landing. Even on a little balsa glider this can yield problems
since a downwards vertical stabilizer will be hit more often and
hence be damaged. I would have adopted this layout anyway if it had
been necessary. But I got what I wanted just by using a little
Then again, why not simply remove the vertical stabilizer? I tried
it out and got the glider fly sideways, with obviously a lower
yield. A vertical stabilizer is needed.
I believe the seagull wing shape itself offers some stability to
avoid a sideways flight, anyway not enough.
Why not use a dihedral?
Because I didn't understand the dihedral. I did read the explanation
available in popular books but I felt it was wrong. So I preferred
to use no dihedral and that was OK.
There are two ways to stabilize the roll of a glider / keep the
- Put the center of mass below the wings.
Obviously my glider relies on the second method.
The lower the wings are implanted on the airplane body (hence the
higher the center of mass is situated above them), the more a
dihedral will be necessary to keep the airplane stable in flight.
Conversely, airplanes with wings implanted very high may be too
stable (or oscillate), then a slight anhedral will be necessary to
compensate (an anhedral is a reversed dihedral; the wings pointing
I want to adopt the seagull wing shape for my own gliders, with
different aspect ratios. What's the build seed?
Consider that the wing is made out of four equal parts. Then cut out
the two outer parts in the shape of a quarter of an ellipse:
Actually, sea bird wings have the inner rectangles maybe 50% wider.
Yet anyway we are not copying a seabird wing, since their wings have
a strong curvature in the inner part. (See further at the bottom of
Why use such a thin body?
The thinner, the less aerodynamic drag. Though this is not the most
optimized shape. I should have bend the body so that it follows the
upwards and downwards movement of the air around the wings. Maybe on
a next glider.
A lot of little balsa gliders use a
plate of balsa wood to be the body
. Clearly those gliders do
fly. But I would have been afraid to use that design. Because to me
such a body is sort of a giant vertical stabilizer smeared all along
the plane body. A conceptual nightmare.
Can I build a more lightweight exemplary anyway?
Yes. I build one two times more lightweight, using 1 mm balsa plate
for the wing and stabilizers and a 4 x
mm balsa rod for the body. But this is the limit. I made one even a
little more lightweight and it simply fell to the ground. I had to
increase its weight to get it flying properly.
If you want to build an even more lightweight glider, so it can fly
slower, then you must build a bigger one so the wing chord
increases, decrease the wing ratio (again to get more chord), or use
Some build tips and data?
I used standard hobby water-based white glue.
For some assemblings I put one or two loose drops of hot glue to
latch the two parts. During the few seconds the glue cools down I
hold the pieces in their correct positions. Then I smear a little
quantity of white glue all along between the two pieces. Hours
later, when the white glue is dry, I remove the hot glue drops.
Maybe best cut out a wider wing, sandpaper the trailing edge, then
cut out the wing shape:
The wing tips are fragile. So I "painted" them with a thin layer of
If the glider assembly lacks symmetry it will turn in flight.
Canonically the way to compensate for this is to make the outer rear
parts of the wings go a little bit upwards and downwards. This will
create a roll
force to compensate the asymmetry. But tuning the balsa wings may be
difficult and produce an ugly result. So if the asymmetry is not too
important and the tendency to turn is quite weak, best is to create
a roll force using the rear horizontal stabilizer. Bend it a little
bit so the outer rear part of one half goes a little bit upwards and
the outer rear part of the other half goes a little bit downwards.
Give it a smooth gradual curved bend. Do not bend the vertical
When assembling two perpendicular pieces of balsa, a common practice
is to spread a little glue on the edge of one of the pieces and
press the two together. This yields a fragile link. You get a far
better result by smearing a thick little joint of white glue along
the interface between the pieces. Say 2 up to 3 mm thick. This takes
at least half a day to dry up but you get a really strong result. Of
course to put glue on the edge of the piece is a good practice, this
will add to the resistance. But the main resistance will come from
the joints on the sides.
Did you invent this glider shape?
My pride is that I managed to understand most choices and
aerodynamic reasoning when conceiving this glider. Yet the result is
quite identical to the canonical airplane shape. This simply means I
got some understanding of this wonderful invention that took
centuries to grow in the successive brains of hundreds of inventors
and scientists. The fact that the glider flies quite well is the
ultimate and necessary proof.
This page shows the "Feather Shooter", a glider that resembles: http://xoomee.com/feather.htm
This page shows the "Epsilon 2M" glider: http://www.nesail.com/detail.php?productID=3066
A plane shape depends on its purpose. This page: http://www.richard-seaman.com/Aircraft/AirShows/index.html
shows quite a lot of different airplane shapes. All are near optimal
choices in regard of the plane's operational needs.
So your glider is optimal?
Reasonably. Yet this does not mean that a better glider cannot be
build. Of course it can. I've got some ideas to do that.
Part of my reasoning was inaccurate. Globally it was good since the
glider glides. For sure the reasoning behind the building of the
glider can be enhanced.
Roughly halfway the present text I compare four elliptical wing
shapes and explain that the one with the straight leading edge is
the worst. Well my friend Didier Bizzarri build a successful very
low speed indoor glider that approximately uses that kind of wing
shape. To view it click here
Also, flying fishes use nearly exactly the same wing shape as my
glider but reversed; with the longest edge forward. (If you check
for photographs of flying fishes on the Web, be careful to get
pictures of flying fishes in flight. Or at least pictures that show
the wings fully deployed. If a dead flying fish lays on a table or
is lousily held by hands, its wings show quite a different shape.) I
don't know if the "reversed" wing feels stable to the flying fish.
As the fish "pilots" its flight, a less stable wing is not a
problem. It is quite probable that the straight leading edge's
purpose is to resist impacts. The wings of the Wright Flyer tend to
end in this shape too.
Most prominent error till now is that I thought that I shaped a
bird's wing. That is totally wrong:
- While the shape viewed from above does match that of a sea
bird, a real sea bird wing is cambered at its root and the
camber decreases towards the tip. To get the same lift at the
root as such a cambered bird wing, my flat wing should
have had a much longer chord, maybe two times longer. Halfway
the bird wing, the camber is milder. So, to have a chance to
mimic the lift of a bird wing (with my flat wing) I should have
used kind of a delta wing or a trapezoidal wing. Actually, real
delta wings on military airplanes are quite flat... I suppose
this is not a coincidence. (Some of the best WWII fighters use a
strong camber at the root, down to flat wing tips.)
This condor on the other hand, does appear to have a wing with a
constant camber: https://en.wikipedia.org/wiki/File:Condor_flying_over_the_Colca_canyon_in_Peru.jpg
- The incidence of a bird wing changes along the span. Maybe to
adapt to the changes in airflow, maybe simply because a change
in curvature also changes the optimal angle of attack.
- The bird's tail adds itself to the chord of the bird's wing.
Eric Brasseur - November 17
2003 till September 9 2015