People have long found the idea of flight to be fascinating. What could be more majestic than soaring through the air like a bird with no cares in the world? Humanity’s bird-like flight capabilities are limited to activities such as paragliding and hang gliding. But we humans are smart creatures. We figured out that there are more than just those two ways to take to the skies. There are also airplanes. Although flying in an airplane is a somewhat less majestic experience than flying like a bird might be (and airplane passengers seem to have significantly more cares than birds do), planes are still a nice way to get from here to there. But how do they work?
When a plane is in flight, there are four forces acting on it: weight, lift, thrust, and drag.
Weight is the downward force on the plane and is created by the gravitational attraction between the earth and the plane. Unlike lift and drag, which are aerodynamic forces, weight is a field force. The weight of the plane creates two challenges: controlling the plane and overcoming the weight. Do these two things right, and the plane flies. Because the plane uses up its fuel during flight and things on the plane (like people) move around during flight, the distribution of weight and the plane’s center of gravity change continuously, so the pilot must compensate during the flight in order to keep the plane steady in the air.
Lift is the upward force that opposes the plane’s weight. It is a mechanical aerodynamic force. The force is generated when an object deflects or turns a flow of gas. Because of Newton’s Third Law of Motion, the lift is in the opposite direction of the deflected gas flow. Basically, the wings push the air down. There is an equal force of air pushing back up on the wings. That is the lift. There are three factors associated with lift: object, motion, and air. For an airplane, the “object” factor is primarily the size and shape of the plane’s wings. The motion depends on the velocity of the air and the airplane. The “air” factor is the mass, viscosity, and compressibility of the air. Wings are designed to curve slightly downwards, so the air going over the top of the wings moves faster than the air doing under the wings. Because a quickly moving fluid (in this case, air) exerts less pressure than a slowly moving fluid, the pressure on the top of the wings is less than the pressure on the underside of the wings.
Thrust is the forward force on the plane and is a mechanical force. Engines generate thrust using one of four systems of propulsion: propeller, jet turbine, ramjet, and rocket. Using Newton’s Third Law of Motion, thrust is the reactive force that occurs when the airplane’s engines accelerate a mass of gas. The factors that decide the magnitude of thrust are the amount of gas that is accelerated and the difference of the velocities of the engine exhaust and the outside air.
Drag is the force that acts in the opposite direction of the plane’s motion and is a mechanical force created by the difference in velocity between the airplane and the air. Drag has two main sources: skin friction between the air and the airplane and form drag. Skin friction depends on the viscosity of the air and the smoothness of the outside of the plane. Form drag is resistance to the plane’s motion through the air and is caused by the airflow over the aircraft. The shape of the aircraft separates the air, creating areas of high and low pressure. The varying distribution of pressure creates a force on the plane, and that force is form drag. Streamlining the shape of the plane can reduce form drag.
While the plane is in the air, as long as the lift and thrust are greater than or equal to the weight and drag, the plane flies. Next time you see a plane overhead, don’t wonder if it’s a bird, a plane, or Superman. Instead, think about science.