Gliders: How They Fly

Gliders are optimized to stay in the air as long as possible because they have no motor. After building lots and lots of model gliders I have learned a lot about them and have come to appreciate them even more because of how complicated and elegant their design is.(They are plane simple aka complicated(pun intended :) )

Balancing a Plane

My first plane was a pringles can with wings. It was not even close to balancing. My next glider looked more like a plane, but also did not fly. After several more unsuccessful planes I knew that if there is too much weight in the front the plane will nose dive and if there is too much weight in back it will back flip. Then I got lucky and randomly happened to build a glider which I called Wind that balanced perfectly. I was happy that I finally had a plane that could fly, but I was not able to replicate it because I did not really understand why and how it was properly balanced. So instead of just winging it(pun intended :) ) I googled how to build a good glider and found out that to be properly balance the center of gravity should be towards the front of the wings, at about 30-25% of wing area(not including ailerons) where 0% is the front of the wing. Like this the plane will be slightly front heavy so that it will tend to fly in that direction.

I tried to test if this was true by building Jet, but initillay failed because I did not understand that for a glider to fly it needs to be balanced relative to the wings not relative to the body. Meaning that if you move the wings you also have to shift the center of gravity(balancing point). For a long time I thought the plane needed to be balance relative to the body, but its relative to the wings because thats what creates lift, holds the plane in the air.

Only after I started making the wings and tail separate from the body and attaching it with rubberbands did I really get a feel for how to balance a plane since now I did not have to add weight, but could simply move the components relative to each other.

Crash Proof

If you make your plane out of cardboard, wood or other non-elastic material you need to build it such that the different parts of the plane(wing, tail and body) can move relative to each other. Otherwise, it will be more likely to break because the nose hits the ground first(most of the time) stopping the body from moving, but because of inertia the wings tend to stay in motion. So the wings want to continue to go forward and only the glue is stopping this causing the glue to give in after a handful of crash lands.
From my experience with cardboard, the main wing is very likely to break off from the body damaging the body or wings in the process while the tail does not have this problem as often. That's because the tail is usually less heavy since it is smaller, but made of similar material as the wings. Objects with more mass are harder to slow down. You can test this by attaching 3 different small weights onto a stick. Attach the heaviest in the front so that your "plane" is front heavy and so will land on its nose and attach it with a rubber band so it can move. Attach the other 2 with tape where you would have the wing and tail. Then throw the stick into the ground until one weight falls off. If the tail has the heavier weight it will come of first otherwise the wing weight will.

So, to make my planes crash proof I attached the wings and tail to the body with rubberbands. This also allowed me to adjust the position of the wings to balance it properly and to try different wings and tails with the same body. The only drawback is that because things always move upon landing I never know exactly where I positioned my wings in a good run.


Body Stability

No matter how well-balanced the plane is, if it's floppy it doesnt fly. This is because floppiness makes more drag and so there is no nice airflow. At first, I did not glue the sticks, which made up the main body, together because the rubberbands that attached the front weight, wing and tail were holding them together pretty well. But this made the sticks clump together under the forces of the rubberbands which allowed the wing and tail to wiggle left and right during flight which did not give the best flying results. By gluing the sticks in a row the wing and tail can't move sideways as much making the glider a lot more stable.
However, my bodies were still quite narrow so small wings flew better than long wings because the longer the wing the more mass is further from the plane so there is more lever arm so more torque to turn the plane sideways. That's why I later also glued popsicle sticks onto the body to make it wider. This further supports the wing reducing the wings and tails tendency to wiggle left and right. I put cardboard ontop of the popsicle sticks on both sides to better hold them together because otherwise they would easily disattach by accident.
My plane bodies also got longer over time because then I had more room to adjust the balance of the plane. At the beginning I could not make very wide wings because then I could not move them forward or backward very much.

Drag, Air Flow and Stability In The Air

A plane swims through air so having more surface area in the back rather than the front helps the plane be stable in the air. Otherwise, it can more easily wiggle left and right.
So in other words the center of pressure(air pressure on surface)should be behind the center of mass so that if the plane goes a bit to one side the air rushing past will push more on the otherside in the back of the plane causing the glider to autocorrect and fly straight again. So, the tail rudder is the vertical stabilizer, it stabilizes the plane in the left-right direction.

The small wings on the tail are good for horizontal(up-down) stabilization. If the plane pitches upwards the horizontal tail wings will get hit by air more on the bottom pushing the back up again. The same goes for the plane pitching down where then the air pushes more on the top of the horizontal stabilizers and so brings the back down again making the whole plane straight.

If you reverse your plane such that the tail is in front and the wing is in back it will only fly if the vertical component is small(if it's big the plane will downward spiral) and you readjust the balance such that the center of mass is infront of the center of pressure. So in other words the tail becomes the wing and the wing becomes the tail.(This explains why planes need to be balance the way they are).

Tail Designs I Tested

At first, my tails were rather small. I tried different designs, but it did not seem to make much of a difference as long as they were not to heavy nor floppy.
Then I decided to try combining tails which greatly improved the flight for planes with bigger wings. For smaller wings I had to readjust the balance because the horizontal stabilization adds extra wing area.
Also using wings as a tail is doable if the wing is not too heavy. It is a little unstable in the left right direction because there is no vertical stabilization, but otherwise its pretty good. If the wing has airfoil, then it will act like ailerons directing air down making a bit more lift so the balance needs to be readjusted otherwise the plane will seem back heavy.
Then I built a tail with a big vertical stabilizer since all my other tails had a small vertical component. And because making the horizontal tail surface bigger by combining tails was easier than doing the same for the vertical stabilizer. But it turned out to not be a very good tail because I had made it with an airfoil which produced a lot of drag which destabilized the plane. Then I built another tail with a large but flat vertical component, but it also did not fly well both on super long wings and smaller wings.
I also built tails with very small or no horizontal surface and it turned out that for very big and large wings this was not bad since these already had so much wing area that they did not need the horizontal stabilization. But for smaller wings it was very unstable and did not fly. Only when I added a second tail with a vertical component did the plane fly.

At the beginning I used styrofoam which is light but breaks more easily or thin cardboard which is heavier but more crash proof. But I discovered that making the tails horizontal component like I make my airfoil wings(see below) just that the styrofoam is flat instead of curved, makes great tails that are both light and crash proof. For the vertical component keeping them thin cardboard minimizes drag but adding a small stick to the back stops it from bending and getting crooked over time.

So in short, a good tail is as light as possible while being as sturdy as possible. And producing the least amount of drag while having the most surface area to increase stability.

Wing Designs I Tested

At the beginning I used flat thin cardboard wings which were quit floppy.

Then I tried making wings out of rectangular foam bowls. I put some cardboard underneath to improve stability. This design seemed sturdy, but it did not fly. My theory is that the angle of the plate edge was to big, creating lots of drag instead of a nice airfoil. An airfoil is a wing that has a small bump and then a gradual slope down ward(looks like half a rain drop). This allows to push the air downward and create lift. If the bump is to steep however, the air will not make it over the bump and so the wing will push air up making the plane go down.

Then I went back to thin cardboard and put sticks on top to try to improve stability. I got lucky and got a good flight, but it was very difficult to repeat(one good flight and then 10 or more bad flights before the small adjustments in positioning were just right again for another good flight.) because the sticks did not completely remove the floppiness.

Then I tried making an airfoil with thin cardboard at the bottom, foam air foil strips glued on top and stiffer paper to make the wing surface(Initially I used not so stiff paper and that caused the surface to be quite bumpy/crumpled which is not good(makes drag and destabilizes the plane)(also, when gluing the stiff paper on, dont use to much glue because the areas with glue are usually a bit less smooth). This airfoil wing flew much better because it was a sturdy design. With this design I finally had a very successful flight: My first actually flying glider(see picture above) got all the way down the hill used for sledding in the winter.

The airfoil design was a lot more repeatable, but I still wanted to get a flat wing to fly because I had read that airfoil is not necessary for flight. So I went back to my thin cardboard and glued a cardboard strip on top. I also built this design in different sizes. The medium-sized wing(blue wing) is ~30cm long and 8cm wide. For the longer wings(double in length) and the wider wings(double in width) I reenforced it with sticks. For the smaller sized wing(half the length), during the gluing of the front cardboard strip the cardboard warped, making it into a camber wing(a "hollow wing", so they have the top bump but a bottom which is curved inward. So a wing that is not flat at the bottom, but rather has a cavity in the same shape as the top). Camber wings are supposed to be more stable for small wings, but for me this was not the case. Probably because despite now being a lot less floppy this wing design both the long, the wide short and the small version where still more floppy than the airfoil design.
Then I tried thick cardboard box cardboard with sticks in the inside slots that this cardboard has. For small and medium-sized wings this design is ok but airfoil wings are better. However, for bigger wings this wing design is surprisingly good because it is quit sturdy. The advantage of this design is that the wing size can easily be extended by putting sticks in the inside. This allows to make big wings that are relatively light.

I eventually went back to my airfoil design and made them in many different shapes and sizes.

   -small wing(16cm long and 8.5 cm wide) was relatively stable in the air(flying straight) and relatively repeatable, but did not go super far because it behaves like an arrow but with a little bit of gliding.

   -medium wing(35cm long and 9cm wide) flew well.

   -long thin wing(high aspect ratio)(44cm long and 9cm wide) was not much different from medium wing.

   -wide short wing(low aspect ratio)(35cm long and 19.5cm wide) flew well. Slightly more left-right unstable, but otherwise flew like medium wing. Also, it needs to be balanced a bit more in the middle of the wing.

   -angled up wing. Upward angled wings are supposed to make the plane stay straight more easily because if it falls to the left, the right wing will have more lift and if it falls to the right, the left wing will have more lift bringing it back straight. However, it is harder to make a stable wing that is angled upward. My angled up wing was at first floppy because I wanted the angle to be adjustable. But when I made it more sturdy by removing that option the angle was to high so it did not provide enough lift. Then I made another wing the size of the medium wing but slightly angled upward. This wing however did not fly any better nor worth then the medium wing so probably the angle was not big enough.

   -adjustable horizontal angle wing was floppy because I wanted the angle to be adjustable.

   -super long wing(~3times longer then medium wing)(this one however was not rectangular shaped, but rather a tappered wing(straight in the front and triangular in back)) flew very well. With a big enough horizontal stabilizer it got all the way down the hill and was more repeatable than all my previously built planes. This wing was also slightly bent/angled upwards but only slightly.

   -another supper long wing(~same size) but reversed relative to the other super long wing(straight in back and triangular in front). It is also more angled up and has wing tip fences on the ends so cant directly compare it to the other superlong wing. It flew really well like the other superlong wing.

I also tried other designs without airfoil:

  -X shaped wing was relatively stable, flying straight. But not very far.

  -triangular wing also flies, but needs to be balanced differently since wing size is not the same in front and back. If triangle points forward then the balance point needs to be more in back to have more wing area in back. If triangle points backwards then it does not fly properly for some reason.

  -90 degrees wing(2 cardboard strips glued at 90 degrees horizontally) only flew a little when pointing forward and back heavy. If pointing backward it did not fly.

  -Many small wings made of thick cardboard strips with a stick inside for stability. I thought this would fly well since wings push air down to make lift so many small wings pushing air down should create more lift. This design however did not fly well, probably because too many parts so more floppy.

I also tried changing the angle of attack of the wings by sliding thick cardboard strips under the front of the wing. I thought this would improve lift because with a steeper angle the air pushed down by the wing will have a force that is more downward oriented creating more(upward force) lift. I tested this with both flat thick cardboard medium-sized wing and airfoil medium-sized wing and in both cases the more cardboard the more unstable. My theory is that the steeper angle made more drag which outweighed the benefits.

So in short, airfoil is not required but does greatly improve flight and repeatability. Long thin airfoil wings fly more repeatedly than other wings I tested if attached to the body such that they dont fall left or right. Smaller wings can also fly well, but they seem to be a bit more sensitive about balance.

Other Discoveries

Airfoil does help create lift. I put one of the super long airfoil wings upside down and it did not fly, but when I changed nothing but put it right side up it flew well. Also, putting the super long wing wrong way round such that the airfoil bump is in the back of the wing does not make it fly well either. This is because wings push air down which hits the air thats already there which because of inertia does not want to move out of the way and so instead pushes back and this force lifts the wing(Its action reaction). That's why airfoil wings fly more reliably then flat wings because flat wings need the plane to be slightly tilted up to push air down. When the airfoil is upside down it pushes air up instead which makes the plane go down. If the airfoil is upright but pointing the wrong way then instead of the air that made it over the bump of the wing being guided down gradually the air is now guided up gradually and then suddenly guided down. It still provides lift just not as much since the air is not guided down nicely.

I also discovered that if my cardboard gliders(no matter what wings and tails used) were just a little bit off balance they had the tendency of flying straight for a short stretch and then abruptly falling sideways. Usually to the left. I thought it fell to the left because I throw with the right hand but when I tried throwing with the left hand then sometimes the plane fell right but othertimes it still fell left. So there is some other cause, maybe the wind.

Another important thing to keep in mind is that wind plays a big role. If the wind is opposite to the planes flying direction then more air is pushed down by the wings and more lift is created. So in stronger head on wind the plane needs to be more nose heavy to be balanced properly.

Side Note on Frisbees

Frisbees are like planes. Instead of a tail they get most of their stability from turning rapidly. Like planes, they also need wing area to fly. Lids from ice cream or yogurt containers make excellent frisbees. They are light and the rim makes them stable. If you cut of the rim it will have a tendency of going "crazy" and of flipping sideways which makes it lose its wing area. If you use only the rim it does not fly because there is no wing area and the rim is straight so it does not push air down to make lift. If you throw a ring(from ring toss) that is bulky it will have the same problem while if you throw a ring(from ring toss) that is flatter and angled inwards down then it will fly like a frisbee.
Frisbees can also fly upside down but they are more unstable. The aerodynamic side will push air down in the front and the otherside will make a lot of drag.
Usually, frisbees are like a wing with an airfoil, so the airfoil pushes air down creating lift. Ice cream or yogurt containers lids dont have this airfoil, so they produce a bit more drag, but because they are lighter they can still fly almost as well as real frisbees.

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