Why Paper Airplanes Fly?

Why Paper Airplanes Fly? by Ken Blackburn
Despite my degree in aerospace engineering and many years work in the field, explaining why a paper airplane stays in the air is not easy.

I’ll begin by saying that paper airplane fly using the game general principles as real airplanes, both are controlled by four basic forces: weight, lift, thrust, and drag.

Think of it as two teams each playing tug-of-war with the airplane attached to a rope in the middle.

Weight vs. lift

The first team, weight and lift, play a vertical game of tug-of-war. Because gravity is constantly trying to pull the airplane to the ground, WEIGHT is obviously on the downward side of the rope.

LIFT exerts an upward pull.
As an airplane flies, it is wings are angled with the front edges higher than the back edges.
This causes the air going over the top of the wings to speed up slightly as it gets sucked downward across the wing.
The air travelling under the wing slows down a bit a it gets shoved by the bottom of the wing.
The speeding up and slowing down of the air is what creates lift.

What makes a paper airplane fly? Air — the stuff that’s all around you. Hold your hand in front of your body with your palm facing sideways so that your thumb is on top and your pinkie is facing the floor. Swing your hand back and forth. Do you feel the air? Now turn your palm so it is parallel to the ground and swing it back and forth again, like you’re slicing it through the air. You can still feel the air, but your hand is able to move through it more smoothly than when your hand was turned up at a right angle. How easily an airplane moves through the air, or its aerodynamics, is the first consideration in making an airplane fly for a long distance.
Drag & Gravity
Planes that push a lot of air, like your hand did when it was facing the side, are said to have a lot of “drag,” or resistance, to moving through the air. If you want your plane to fly as far as possible, you want a plane with as little drag as possible. A second force that planes need to overcome is “gravity.” You need to keep your plane’s weight to a minimum to help fight against gravity’s pull to the ground.

Thrust & Lift
“Thrust” and “lift” are two other forces that help your plane make a long flight. Thrust is the forward movement of the plane. The initial thrust comes from the muscles of the “pilot” as the paper airplane is launched. After this, paper airplanes are really gliders, converting altitude to forward motion.

Lift comes when the air below the airplane wing is pushing up harder than the air above it is pushing down. It is this difference in pressure that enables the plane to fly. Pressure can be reduced on a wing’s surface by making the air move over it more quickly. The wings of a plane are curved so that the air moves more quickly over the top of the wing, resulting in an upward push, or lift, on the wing.

The Four Forces in Balance
Long flights come when these four forces — drag, gravity, thrust, and lift — are balanced. Some planes (like darts) are meant to be thrown with a lot of force. Because darts don’t have a lot of drag and lift, they depend on extra thrust to overcome gravity. Long distance fliers are often built with this same design. Planes that are built to spend a long time in the air usually have a lot of lift but little thrust. These planes fly a slow and gentle flight.

As eighteenth-century Swiss scientist Daniel Bernoulli discovered, when air speed up it is pressure is reduced, and when the air slows down it is pressure is increased.

Therefore the air that speeds up over the top of the wing creates a slight suction which pulls upward on the wing. At the same time, the air below the wing creates extra pressure which pushes upward on the wing.

Lift is the combination of these two forces. During straight level flight both lift and weight are pulling equally.
If lift pulls harder than weight, the plane begins to accelerate up, if weight pulls harder, the plane accelerates down.

Drag vs. Thrust
While lift and weight are tugging up and down on our airplane, thrust and drag are pulling forward and aft.
Let is look at DRAG first. When you fly a paper airplane level across a room, drag is what pulls back on your airplane and slows it down. Most of drag comes from air resistance. Thin and insubstantial as air may seem, it does have mass and in some ways is like runny maple syrup. As a plane flies, the air is viscosity makes it stick to the plane, creating resistance to motion. Another source of drag comes from lift never pulls directly up, but rather tugs up and a little back, and that backward pull contributes to brag.

THRUST is on the opposite end of our imaginary rope forward. Real airplanes get their thrust from a propeller or a jet engine. Paper airplanes get it from your arm, then from gravity. A throw gives them their initial speed, and then they fly a title downward, letting gravity pull them along (like a bicycle coasting down a hill). When you throw a paper airplane from a building or hill, the plane descends at an angle that lets gravity balance the pull of drag so the plane won’t slow down. A typical paper airplane is drag is about one fifth it is weight. This requires the paper airplane to fly at an angle approximately 11 degrees below level in order for gravity (weight) to pull forward enough to counteract drag.

Originally posted 2012-03-24 19:27:48. Republished by Blog Post Promoter

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