Unveiling the Secrets of Flight: How Airplanes Take to the Skies
Airplane flight is one of the most significant technological achievements of the 20th century. At any given moment, roughly 5,000 airplanes crisscross the skies above the United States alone, amounting to an estimated 64 million commercial and private takeoffs every year. It is easy to take the physics of flight for granted, as well as the ways in which we exploit them to achieve flight. Understanding precisely why airplanes fly is an ongoing challenge for aerospace engineers, who study and design airplanes, rockets, satellites, helicopters and space capsules. Their job is to make sure that flying through the air or in space is safe and reliable, by using tools and ideas from science and mathematics, like computer simulations and experiments. Because of that work, flying in an airplane is the safest way to travel - safer than cars, buses, trains or boats. So, what makes an airplane fly, then?
The Four Forces of Flight
At the core of aerodynamics are the four forces that act on an aircraft: lift, weight, thrust, and drag. These aren’t just technical terms from a textbook. They’re the choreographers behind every smooth takeoff, steep climb, and graceful landing. Engineers use these forces to help design the shape of the airplane, the size of the wings, and figure out how many passengers the airplane can carry. For example, when an airplane takes off, the thrust must be greater than the drag, and the lift must be greater than the weight. When the airplane is high enough and is cruising to your destination, lift needs to balance the weight, and the thrust needs to balance the drag.
Lift: Defying Gravity
Lift is the upward force generated by an aircraft’s wings. It acts perpendicular to the relative wind and opposes weight. Everything that flies must have lift. For an aircraft to move upward, it must have more lift than weight. The shape of an airplane’s wings is what makes it possible for the airplane to fly. Airplanes’ wings are curved on top and flatter on the bottom. That shape makes air flow over the top faster than under the bottom. As a result, less air pressure is on top of the wing. This lower pressure makes the wing, and the airplane it’s attached to, move up. Using curves to affect air pressure is a trick used on many aircraft. Helicopter rotor blades use this curved shape. Lift for kites also comes from a curved shape. Even sailboats use this curved shape. A boat’s sail is like a wing.
Air may be invisible, but it’s made up of molecules that have mass. That mass means air has weight, and because of that, it applies force. Enter: Bernoulli’s Principle. This scientific concept states that faster-moving air has lower pressure, and slower-moving air has higher pressure. The curved upper surface of the wing causes air to speed up as it flows over the top, creating lower pressure above the wing than below it. Pilots also control lift by adjusting the angle of attack (AoA) and airspeed. More angle, more lift-up to a point.
However, the standard explanation of lift is problematic for another important reason as well: the air shooting over the wing doesn't have to stay in step with the air going underneath it, and nothing says it has to travel a bigger distance in the same time. Imagine two air molecules arriving at the front of the wing and separating, so one shoots up over the top and the other whistles straight under the bottom. There's no reason why those two molecules have to arrive at exactly the same time at the back end of the wing: they could meet up with other air molecules instead.
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As a curved airfoil wing flies through the sky, it deflects air and alters the air pressure above and below it. As a plane flies forward, the curving airflow over the wing lowers the air pressure directly above it, while the curving airflow under the wing increases the air pressure directly beneath it. The angled wings push down both the accelerated airflow (from up above them) and the slower moving airflow (from beneath them), and this produces lift. The curved shape of a wing creates an area of low pressure up above it (red), which generates lift.
Weight: The Pull of Gravity
Weight is the force of gravity pulling the aircraft toward the Earth. A force that everyone encounters every day is the force of gravity, which keeps us on the ground. While an airplane is flying, gravity is pulling the airplane down. That force is the weight of the airplane. Weight is also the downward force that an aircraft must overcome to fly. Weight is the amount of gravity multiplied by the mass of an object. It acts through the center of gravity (CG)-a crucial point that affects balance and stability. Managing weight is equally about how much you’re carrying and how it’s distributed. For instance, the position of the CG changes as fuel is consumed.
Thrust: Powering Forward
Thrust is the forward movement of the plane. Thrust, whether caused by a propeller or a jet engine, is the aerodynamic force that pushes or pulls the airplane forward through space. Thrust is what propels the airplane forward and is produced by the engines. The engines pull in air, which has mass, and quickly push that air out of the back of the engine - so there’s a mass multiplied by an acceleration. For an aircraft to keep moving forward, it must have more thrust than drag.
There are different types of engines, but most commonly, jet engines and propellers generate thrust in aircraft. Propeller-powered planes are widely used in initial pilot flight training. Jet engines produce thrust by rapidly expelling exhaust gases. They’re efficient at higher speeds and altitudes but come with their own quirks-for instance, a slight delay when pilots adjust throttle settings.
Drag: Resisting Motion
As the airplane flies through the air, the shape of the airplane pushes air out of the way. Again, by Newton’s Third Law, this air pushes back, which leads to drag. You can experience something similar to drag when swimming. Stop paddling, and you will keep moving forward because you have mass, but you will slow down. The reason that you slow down is that the water is pushing back on you - that’s drag. Drag is the aerodynamic force that resists forward motion. Drag provides resistance, making it hard to move. For example, it is more difficult to walk or run through water than through air. Water causes more drag than air. The shape of an object also affects the amount of drag. Round surfaces usually have less drag than flat ones. Narrow surfaces usually have less drag than wide ones.
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Pilots and engineers constantly work to reduce drag through aerodynamic design-smooth surfaces, streamlined shapes, and features like winglets. But drag isn’t always the enemy. During landing, it’s a helpful force that slows the aircraft down.
Controlling the Aircraft
The tail of the airplane has two types of small wings, called the horizontal and vertical stabilizers. A pilot uses these surfaces to control the direction of the plane. On the horizontal tail wing, these flaps are called elevators as they enable the plane to go up and down through the air. Meanwhile, the vertical tail wing features a flap known as a rudder. Finally, we come to the ailerons, horizontal flaps located near the end of an airplane's wings. These flaps allow one wing to generate more lift than the other, resulting in a rolling motion that allows the plane to bank left or right. Ailerons usually work in opposition. As the right aileron deflects upward, the left deflects downward, and vice versa. By manipulating these varied wing flaps, a pilot maneuvers the aircraft through the sky.
Stalling
Should lift decrease and drag increase suddenly, such as when an aircraft's angle of attack surpasses that for maximum lift, a stall occurs. At a certain angle (generally round about 15°, though it varies), the air no longer flows smoothly around the wing. There's a big increase in drag, a big reduction in lift, and the plane is said to have stalled. The airframe shakes and the plane falls, at least for a few feet. In most cases the pilot merely corrects for the stall by lowering the plane's angle of attack.
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