Breaking Down the 4 Principle Forces of Flight

Breaking Down the 4 Principle Forces of Flight

By Josh Page, CFI

Whenever I’m in an airplane as a passenger or a pilot, I still marvel that this heavy machine can get off the ground, soar through the sky, and land at my destination. It’s simply amazing. 

One of the first and foundational things you’ll learn in flight training are the four forces of flight. This is the science behind aviation. Understanding these four forces of flight will help you know how your airplane can actually take off and fly. We’re going to take a closer look at each one of those four forces and hopefully by the end of the article, you’ll more deeply appreciate the incredible aerodynamics of these flying machines!  


We’ll start with my personal favorite — lift. How in the world can this heavy plane get off the ground?

As the airplane is rolling down the runway, air is passing over and under the wings. That much is obvious. But what happens with that flow of air? If you look closely at an airplane’s wing, you’ll notice a curvature on the front edge (known as the leading edge) of the wing. As the air flows up and over that curve, it speeds up which causes its pressure to decrease. This results in lower pressure air above the wing and higher pressure air below the wing. If you know remember anything about high pressure vs. low pressure from physics class, you’ll remember that areas of high pressure always seek areas of low pressure. Essentially, this difference in air pressure under and over the wings is precisely what gets the airplane off the ground and keeps it in the air! 


Now let’s talk about the flip side of lift — weight. This one’s simple enough. Because of gravity, the weight of the airplane acts downward toward the center of the earth. To maintain level flight, we would need to have enough air flowing over the wings to match the amount of weight we’re lifting — the plane itself, fuel, passengers, baggage and, of course, that cup of coffee. 

Speaking of weight, you’ll learn in flight training that it’s possible to load the airplane too heavily. Among other critical problems, this can lead to the airplane not being able to take off or land with the amount of runway available. It’s crucial to make sure your aircraft is loaded and balanced properly. This is weight and balance, a very important topic in aviation.


Thrust is our forward motion. In a training aircraft, our rapidly spinning propellor powered by the engine causes the airplane to move forward. Simple as that! In a commercial airliner, the jet engines are creating the thrust required for the heavy plane to move forward. In your car, the engine under the hood is creating thrust for you to move forward, but you stay on the ground because you don’t have wings!


I don’t mean to be a drag at this point, but let’s talk about what holds the plane back. Drag! Drag opposes lift. Think of this as the force that resists the movement of the aircraft through the air. It’s helpful to understand the two types of drag: parasite and induced drag. Parasite drag is any of the forces that work to slow the aircraft’s movement: From the shape and structure of the aircraft to the individual rivets along the sides, to the wing strut and landing gear, etc. —  You’ve experienced parasite drag anytime you’ve placed your hand out the window while driving fast on the interstate. 

Induced drag, on the other hand, is the byproduct of lift. This means if you’re producing lift, you will have induced drag as a result. The more lift, the more induced drag. Remember that part about high pressure air going below the wing and low pressure air going above the wing? Well, since that high pressure air below the wing is seeking the low pressure air above the wing, it results in a vortex flow of air at the wing tips from the bottom of the wing to the top. This actually changes the airflow behind the wing, pushing it downward. Air deflected downwards is called downwash. So it makes sense that the more lift we have, the more downwash will occur. This downwash tilts our lift component backward creating induced drag. What a drag!


What’s most amazing about these four forces of flight is that they are all connected. When one force is changed, the other forces are impacted. For example, if we have more thrust on takeoff, i.e. full power, we will have more air flowing over the wings which generates more lift. But at the same time if we have more lift, we will have more induced drag opposing lift! It’s a very delicate balance. But the more you’re behind the controls flying and experiencing these forces for yourself, the more you’ll understand the beautiful science behind aviation!

By Josh Page, CFI

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