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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 reasonings 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. 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-faster version of this glider is not supposed to fly farther than a lighter-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.

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 Office 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 their 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.

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.

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 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 rigid structure. I was told early airplane builders could have used thick profiles but I have no proof thereof. I'm a bit puzzled by this. Ultralight aircrafts exist that use quite thick wing profiles. Their slowest flight speed matches the highest speed of the Wright Brother's Flyer... On the other hand my own little gliders with thick wings didn't fly very well... Maybe the first people experimenting with aerodynamics noticed that they got good performances only with thin profiles, because they were using little experimental wings and very slow airspeeds. But the much bigger airplanes they were building could have used thicker profiles... I've no idea.

Advantages of thick wings in today airplanes is that they are more rigid for a given weight, they can contain more fuel and hardware...

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 for, 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 constantly blow air downwards. Just like an helicopter does. Yet an helicopter stays at the same place and moves its wings/propeller through the air by rotating them, while an airplane passes its wing through the air by moving itself through the air. The air that an airplane's wings blow downwards does not travel all the way towards the ground. Instead it rotates sideways and you get two slow and huge turbulences/vortexes; one generated by each wing. This is kind of an invisible trail left by the airplane. Now, according to the laws of mechanics, the more volume of air the wings make rotate every second, the slower that air will move and the less energy the airplane will have to spend. That's one reason why gliders have very long wings; in order to span over a huge volume of air. The least the air moves once a glider went through it, the better that glider's yield is.

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 past the glider wings 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 more you increase the span of the wings of an airplane, the huger become the two main vortexes, the slower they turn and the better the airplane's yield.

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 exist:

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 easier.

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).

The pitch 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 less stable.

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 vertical stabilizer.

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 wings horizontal:
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 downwards).

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 this text.)


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 4 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 turbulators.

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 white glue:





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 stabilizer.





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 reasonings 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 reasonings were inaccurate. Globally they were good since the glider glides. For sure the reasonings 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 I shaped a bird's wing. That is totally wrong:



Eric Brasseur  -  November 17 2003  till  January 5 2011