Paper Airplanes: Basic Theory

Force Balancing Unlike conventional airplanes, paper airplanes are essentially gliders because they do not have engines to propel them forward in a horizontal flight.

Paper airplane depends on its weight, thanks to gravity, to keep it moving in a downward sloping path.

All together, there are three forces, weight, lift and drag, acting on a paper airplane in flight.

Once a paper airplane is launched with a forward thrust from your fingers, it moves forward, and gravity exerts it is influence to pull the airplane earthward at the same time.

The resulting air current, moving over it is wings, generates an upward lifting force called lift, which is perpendicular to the flight path, to counteract the opposite weight component of the airplane.

Lift helps to keep the airplane aloft.

The same air current, moving over the airplane, also generates drag, which acts in the opposite direction of the flight path, due to air resistance.

Drag slows the airplane down; however, during the gliding phase of the flight, the weight component, parallel to the downward sloping path, counteracts drag to keep the airplane trades altitudes for speed as it moves along the downward sloping path until it hits the ground, or something else in it is path.

The angle of the downward sloping path is equal to the lift/drag ratio, hence the more lift and less drag an airplane has, the smaller will the angle of the downward sloping path be, and the further will the airplane glide for a given amount of altitude loss.

For the initial phase of flight, immediately after launching, the airplane seems more as if it is flying instead of gliding.

In fact, it can actually maintain level flight for a certain distance or even gain altitude.

Why is this the case if paper airplanes are supposed to behave like gliders? The answer is, at the point of launch, you impart a certain amount of energy to the airplane.

The harder the throw, the more energy is imparted.

The airplane leaves your fingers at a higher speed than it would normally require to glide on it is own.

The higher speed and energy level allow the airplane to generate extra lift to maintain level flight or to climb.

This energy will eventually be used up to counteract drag, resulting from air resistance, and to attain a higher altitude if the airplane climbs.

The airplane will slow down to a point where it begins to glide, and start trading altitude for speed.

Where does lift come from? Paper airplane wings, although more layered towards the leading edge than the trailing edge, are still relatively flat when compared to the airfoil-shaped wings on conventional airplanes.

Nevertheless, the same aerodynamic principles still apply where lift generation is concerned.

As air approaches the wing is leading edge, at a small angle of attack, it divides at a point called the stagnation poing, which is a little below the front of the wing is leading edge.

The air going over the top of the wing then progresses forward around the leading edge, where it separates from the wing because it is unable to adhere to the surface around the sharp edge.

However, it is turned backward by the upper surface a short distance from the leading edge.

The overall non-symmetrical flow pattern around the wing causes the air to get sucked down, and to accelerate over the top surface of the wing, so that it exits the trailing edge in a streamlined manner.

The end result is that the air above the wing travels at a faster speed than the air below the wing.

The faster moving air, above the wing, produces a lower air below.

The difference in air pressure across the wing then produces a net upward force called lift, which keeps the airplane aloft.

The greater the angle of attack, the greater is the amount of lift and drag being generated.

However, there is a limit, which is not very much, and typically less than 10 degrees, where linear lift generation is possible.

Beyond this limit, the air will suddenly separate from the upper wing surface completely and cause the wing to stall.

When the wing stalls, total lift will be lost and the airplane crashes.

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