The Fundamental Forces of Flight

When designing an airplane, engineers focus on four primary forces: weight, thrust, drag, and lift. These forces inform the aerodynamic shape of the aircraft, the dimensions of the wings, and the passenger capacity of the airplane.

For instance, during takeoff, the thrust must exceed drag, and lift must surpass weight. Observing an airplane's takeoff reveals the adjustment of wing flaps, which enhance lift but also increase drag, necessitating powerful engines to generate sufficient thrust. Once the aircraft reaches cruising altitude, lift must balance weight, and thrust must equal drag, allowing the pilot to retract the flaps and reduce engine power.

At its core, a force is defined by Newton’s Second Law, which states that force equals mass multiplied by acceleration (F = ma).

Understanding Gravity and Thrust

Gravity is a force that we encounter daily, grounding us and influencing our weight. When you step on a scale, it measures the gravitational force your body exerts. In flight, gravity pulls the airplane downward, representing its weight. Meanwhile, the engines generate thrust, propelling the airplane forward by drawing in air and expelling it rapidly, thus adhering to the principle of mass multiplied by acceleration.

According to Newton’s Third Law, every action has an equal and opposite reaction. Thus, as air exits the engines, it creates a reaction that propels the airplane forward. As the airplane navigates through the atmosphere, its shape displaces air, resulting in drag. You can experience a similar effect while swimming: paddling provides thrust, but halting your movements results in slowing down due to the drag from the water.

The Complexity of Lift

Lift, unlike weight, thrust, and drag, involves more intricate dynamics. Generated by the aircraft's wings, lift depends heavily on the wing's shape, known as an airfoil, which has curved surfaces on both the top and bottom. This design leads to differential air pressure: lower pressure on top compared to the bottom, resulting in lift.

Understanding the reasons behind this pressure difference is crucial for optimizing lift. Advancements in our comprehension of lift can lead to the design of more fuel-efficient airplanes, enhancing passenger comfort.

The Ongoing Mystery

Despite extensive research, the reasons for varying airspeeds around an airfoil remain a mystery, prompting ongoing investigation. Aerospace engineers have conducted wind tunnel tests and in-flight pressure measurements to model wing performance. Although we can manipulate wing shapes for desired lift characteristics, the fundamental physics of lift continues to elude complete understanding.

Mathematical models, particularly those involving circulation, enable engineers to predict airflow patterns around wings. Even without full comprehension of the underlying processes, we can create safe and efficient aircraft. If we can ultimately decipher why air moves differently across the wing, it may lead to the development of airplanes that are more fuel-efficient and environmentally friendly.

This article is derived from The Conversation, an independent nonprofit news platform committed to providing context for global events. Learn more about them or subscribe to their newsletter.

Craig Merrett receives funding from the Office of Naval Research and L3Harris. He is a licensed professional engineer in Ontario, Canada, and an associate professor at Clarkson University's Department of Mechanical and Aerospace Engineering.