When you throw a paper airplane, it flies for a couple of seconds before losing speed and stalling. With a glider, it has a very long wingspan, but you take off with another plane or winch tow pulling it and then it is released on its own. Shouldn't the glider lose all speed and stall?
A glider and a paper airplane operate on the same principle: Exchanging whatever potential energy (altitude) they have for the kinetic energy (airspeed) required to keep air moving over the wings so they produce lift, giving a stable, controlled descent (glide).
In both cases the airfoil will not stall unless it exceeds its critical angle of attack which is independent of the airspeed. You can continue to fly (with the wing not stalled and producing lift) as long as you still have altitude to trade for that airspeed, and if you do it right you will eventually run out of both altitude and airspeed fairly close to the ground, where you can have a soft landing at a speed where the aircraft is not damaged. In the case of a manned glider hopefully this happens on a runway where you can get another tow plane to take you up again.
Both aircraft can extend their glide time by finding other sources of lift - thermals, ridge lift, etc. - to carry the vehicle to a higher altitude, providing more potential energy. Gliders are better at this for one major reason: They have a pilot who can keep the aircraft in an area of rising air longer to gain the most altitude possible.
A paper aircraft can similarly be made to fly for a longer period of time if given an adequate source of lift (equal to or exceeding its minimum sink rate). There's a rather famous video of the "infinity paper plane" which is a paper airplane circling over an electric stovetop, but I'll spoil it for you and tell you that one is a hoax (the general consensus is the plane was on a string).
This one, however, can be replicated fairly easily: The result is more similar to a helicopter in autorotation or an autogyro in a glide than a traditional fixed-wing glider, but it's a good demonstration of the principles involved.
The primary difference between a glider and a paper airplane is that most folded paper airplanes have very bad aerodynamics.
Most folded paper airplanes have short stubby wings. This in itself makes it very draggy. The ideal wing with absolute minimum drag has a wingspan of infinity. Obviously it's not possible to build that ideal wing (we don't have infinite number of atoms in the universe). But generally, all else being equal, the longer the wingspan the less the drag.
Secondly, paper airplanes have very poor airfoils compared to real gliders. They're not even flat plates (which would actually be a fairly good airfoil at the Reynolds Number they fly). The folds typically add uneven thickness to the airfoil making it a bit wedge shaped. This adds a lot of drag.
Finally, you mention that the paper airplanes you're used to flying tend to lose speed and stall. Stalling definitely reduces glide time. Stalling wastes energy. A smooth glide will tend to stay aloft longer. A real glider is trimmed to glide smoothly and not stall.
Toy gliders need not suffer the weaknesses of paper airplanes. A good toy glider should be able to fly for at least 10 seconds per flight on average (or at least 3 seconds if you throw it level instead of up). Really well designed toy gliders can fly for more than 20 seconds and competition gliders should be able to fly for around 1 minute.
If you've never seen a balsa glider fly before you'd think that it's flying for a very long time. Here's one example (it was the video that introduced me to hand-launched gliders):. And here's a more recent example:
The world record for hand-launched gliders is almost 2 minutes:
These gliders fly longer than the typical paper airplane because they're well designed and are aerodynamically more efficient. Note that most of these balsa gliders (including the world record glider) have eyeballed, imperfect airfoils. Sometimes they're even triangular or just have the leading and trailing edges beveled. So the airfoil is not as important as the design, trim and balance (CG) of the plane.
Paper airplanes need not suck. Here's a clip of the world record (duration) flight of a paper airplane:. If you google around for the design of that plane you'll find that it has a lot of nose weight (good balance/CG) and bent up trailing edges (good trim).
So the key lessons are:
Balance your plane (adjust the CG). This is true for paper airplanes, RC planes and even real airplanes of all sizes. Bad CG is the number one reason planes stall. There are formulas and rules of thumb to calculate the correct CG for your plane but for a paper airplane I'd suggest just experimenting with adding and removing nose weight (that's what people do with balsa gliders).
Reduce drag. For paper airplanes you can try creasing the fold better so that the folded part is as thin as possible. Gliders can increase wingspan and choose low drag airfoils to reduce drag but that's not quite applicable to paper airplanes. Then again, there are paper gliders that have relatively long wingspans (compared to typical paper airplanes) that can glide very well:. But that's entering into the scissors and tape category.
The glider and a paper aircraft are designed and operated in different ways which affects their stalling characteristics.
- Both aircraft trade one form of energy (potential energy or altitude) in order to gain another (Kinetic energy or forward speed). In general, the gliders are in (a controlled) descent unless they encounter thermals or rising air. This is what causes their gliding. The same happens with any unpowered aircraft; however, the gliders have a lot of altitude to expend compared to the paper aircraft.
- The gliders are designed to be highly aerodynamically efficient, with extremely high lift to drag ratios, usual in the range of 50:1 or more. That means that they can glide for extremely long distances, left to their own devices. The paper airplanes are not like this. However, with proper design, they can be made to fly longer.
- The gliders are usually flown by (skilled) pilots, who know how to find and make use of the aforementioned thermals, upward air currents etc, which helps in increasing altitude and keeping the aircraft in air longer.
- Most of the gliders are extremely stable compared to the paper aircraft (which usually has only a wing and no stabilizer), which prevents them from stalling when left alone. In fact, in some gliders, the best way to recover control is to let go of the controls. It basically means that the glider returns to level flight when no control inputs are given.
In order to keep an aircraft flying, you need to keep adding energy. In an aircraft with an engine, the energy is primarily in the form of fuel.
In a glider, the additional energy comes from the lifting action of rising air. Glider pilots will find areas of rising air (they have sensitive instruments to assist with this), and circle in that area to increase altitude. Once they have enough altitude, they will fly in a more or less straight line toward their destination, stopping along the way to circle again and gain more altitude.
Rising air may come from "thermals" (warm air bubbling up through the atmosphere), "ridge lift" (air rising up a slope), "wave lift" (produced by mountains), and others. You can read more about lift at Lift (soaring). There are different flying techniques required to best take advantage of each kind of lift.
You compare 2 different things, the paper plane and the glider. Gliders take weather into advantage to increase their altitude and sustain flight. Paper planes do not experience the same weather phenomena as gliders do. And of course they lack pilot and control surfaces.
Wikipedia mentions 4 ways to gain energy:
- Thermals (where air rises due to heat),
- Ridge lift, where air is forced upwards by a slope,
- Wave lift, where a mountain produces a standing wave,
- Convergence, where two air masses meet
From the moment the tow or winch is released, a glider starts descending. If the atmosphere were completely still, apart from trading speed for height, it will soon have to land.
Gliders are designed with very high lift/drag ratios. 40:1, 50:1 even 60:1 or higher. They stay aloft for long periods because of this and their ability to take advantage of, and the pilots skill in finding, thermals and upward air currents caused by turbulence off ridges, mountains etc.
Paper aircraft have a completely different design (you could not build a glider like the one you show from paper) and can fly for a lot longer than a couple of seconds.
Many people mentioned the difference in the shape of the wings and airfoils. But even if you just simply downscale a real glider to a 5 cm wingspan it won't fly as well as the big one.
The Reynolds number for the small scale plane is very small and the viscous drag becomes large. Turbulence will not be strong enough to prevent flow separation at angles of attack which are still OK for the real plane, because the laminar boundary layer is more susceptible to separation. The separation strongly reduces the generated lift and may lead to stall.
It doesn't mean your paper plane will always stall, but you must be careful with the angle of attack and play with the centre of gravity.
Small models have to use special low Reynolds number airfoils, but will never be as efficient as larger gliders, because the skin friction drag is so important. Even typical radio controlled glider models are usually much larger than typical paper planes.
One way or another, a flying object has to push down on the air to keep flying. Given that air is fluid, we have to make the change in momentum of the air at least as big as the weight of the aircraft. Now:
- force = difference in (mass * velocity)
But the change in energy required to do this alone goes as
- change in energy = change in (mass * velocity squared)
So for efficient flight, we want to push down on a large mass of air with a small change in vertical velocity, rather than a small mass of air with a large velocity.
Gliders, with their long skinny wings, are optimized to do this: they fly relatively slowly intercepting a relatively large mass of air and pushing down on it gently.
At the other extreme, vertical take-off jets, like the Harrier, have to take the opposite approach. They need to fly fast, so in hover they push down a relatively small mass of air very quickly. This is horribly inefficient so a Harrier can not hover for very long before its fuel is exhausted.
In addition to this, a glider is relatively light as it does not have to carry fuel or an engine. It also is made a smooth an slippery as possible to reduce air resistance.
Other people have already mentioned the use of weather to keep the glider afloat longer.
I think you are asking, "How do wings work?"
Aircraft wings are modeled after what we have seen in nature. The basic shape is the wing is flat on the bottom and curved on the top. This curve is similar to a curve found on part of an egg with the bulk of the width located in front of the wing. (You can google up more optimized shapes.)
Forward motion will compress the air above the leading portion of the wing but will also create some drag in the process. The following portion of the wing slowly thins to the trailing edge. This portion of the wing is the area that has the negative air space (lift). It is this negative space that all aircraft rely on. Helicopter blades and engine propellers also make use of this basic shape.
A gliders initial propulsion is a tow, but after that they rely on gravity for propulsion and the lift created by the wing shape, in addition to weather assists.
Comparatively, [without using specialized measurement tools or going into a higher math explanation, the flatness of] a paper airplane has [only negligible] lift [when comparing to an optimized man made wing or wing found in nature] and relies solely on you for a one time propulsion [unless you are lucky enough to find an exhaust grate or the like to fly over].