Sky is the Limit: How the Forces of Lift, Drag, Thrust and Gravity Work Together to Keep an Aircraft Aloft

Author: Arnav Nagpal

Among the greatest achievements of modern science and engineering, airplanes have made traveling much easier, carrying millions of passengers to different destinations around the world. But have you ever stopped to wonder what keeps these giant metal tubes airborne? The answer lies in invisible teamwork: four main forces acting together on the aircraft - lift pushes it upward, gravity pulls it downward, thrust moves it forward, and drag pushes it backward. This short article will take a dive into how these forces help us fly. To understand this dynamic better, let us first look at some basic parts of an airplane.

Parts of an airplane. Source: airheadatpl.com

The first force to explore is one that is perhaps most well-known for making apples fall from trees - gravity. It pulls objects toward one another and thus, is not exactly helpful for a massive aircraft trying to take to the skies. An airplane needs the help of another force to fly, one that can compete with gravity - namely, lift.

Lift is almost like gravity's opposite partner in flight. It is the upward force that helps an aircraft rise in the air and remain there. Lift is generated mainly by the wings, also called airfoils. They are shaped with a gentle curve on top and a flatter surface underneath. The wings push air downward, and by Newton's Third Law, the air pushes the wings upward, contributing to lift. As the aircraft moves forward, air flows faster over the curved upper surface and slower beneath. This difference in airspeed creates lower pressure above the wing and higher pressure below it, producing an upward force strong enough to counter gravity. The faster the air moves over the wings, the greater the lift, and when it becomes higher than the weight, the airplane climbs!

Now, let us turn to the more commonly known force, weight. As you might have guessed, it is a force that is always directed toward the center of the earth, due to the earth's gravity. The weight of an aircraft can be written as W = mg, where m is the mass of the aircraft and g is the acceleration due to gravity. Its amount depends on the mass of all the airplane parts, plus the amount of fuel, plus any payload on board. This immense weight is actually distributed throughout, but we can think of it as acting through a single point on the airplane called the center of gravity. In flight, the airplane rotates about the center of gravity, but the direction of the weight force always remains toward the center of the earth. The airplane's weight constantly changes as it consumes fuel. Thus, although weight does not directly support an aircraft's flight, it ultimately defines the amount of lift the wings must generate to overcome gravity.

Another force that comes into play is drag! Drag is the aerodynamic force that resists forward motion. In layman's terms, it is the force exerted by fluids on an object moving through them. Drag comes in two main forms:

1. Parasitic drag: Includes form drag, skin friction, and interference drag.

2. Induced drag: A byproduct of generating lift.

Taking inspiration from nature, scientists and engineers saw that birds' streamlined body shape was perfect for overcoming drag. So, they used similar shapes for designing aircrafts. Pilots and engineers constantly work to reduce drag using smooth surfaces and features like winglets. During landing, however, drag can become a helpful force because it slows the aircraft down. Spoilers and air brakes are designed to increase drag when needed, making descent and landing safer and more controlled.

Forms of drag. Source: Arbaz Rizvi (LinkedIn).

Next comes thrust. Thrust is the forward force needed to overcome drag during the passage of an aircraft through air. To maintain level flight at constant speed, constant thrust is required; to climb or descend the aircraft whilst maintaining constant speed, the thrust must be increased or decreased; to increase or reduce the speed of the aircraft whilst maintaining level flight, the thrust must also be increased or decreased.

How thrust works.

The next time you see an airplane fly by, remember that it is the dream team of four invisible forces that has made the fantasy of flying a reality. Lift, weight, thrust, and drag work together in a careful balance to keep an aircraft in the sky. Inspiration from nature, years of scientific study, and many brilliant minds made this vision possible.

References

1. European Union Aviation Safety Agency (EASA): https://www.easa.europa.eu/en/light/topics/how-do-planes-fly

2. NASA:

   https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/forces.html

3. BAA Training: https://baatraining.com/blog/fundamentals-of-aerodynamics-for-pilots/

4. SKYbrary: https://skybrary.aero/articles/thrust

5. AeroToolbox: https://aerotoolbox.com/lift-weight-thrust-drag-explanined/

6. California Aeronautical University: https://calaero.edu/aeronautics/principles-of-flight/newtons-third-law-in-aviation/