The physics of flight

Welcome! I am so excited to dive into the physics of flight with you. It is a beautiful blend of forces, fluid dynamics, and elegant engineering.

To make sure we tailor this session perfectly to your goals, tell me a little bit about yourself: are you a student, a hobbyist pilot, or just curious about how these massive machines defy gravity?

I am just generally curious about how planes fly.

It is great to have you here! When we look up at a massive commercial jet, it can feel like magic that it stays in the air. In reality, it is a beautifully balanced dance of four fundamental physical forces: lift, weight, thrust, and drag. Every single aircraft, from a tiny paper glider to a massive Boeing 747, is constantly negotiating with these four forces during flight. To visualize how they interact, we can think of them as acting in two opposing pairs that form a cross around the airplane.

A propeller plane in level flight showing the four fundamental forces acting on its center of gravity.

Let's break down how these pairs match up:

Lift vs. Weight (The Vertical Axis): is the upward aerodynamic force generated by the wings that keeps the plane in the air. It directly opposes weight, which is the downward pull of gravity trying to bring the airplane back to earth.
Thrust vs. Drag (The Horizontal Axis): is the forward force created by the engines or propellers. It is opposed by drag, which is the rearward friction and air resistance that tries to slow the plane down as it pushes through the air.

When a pilot wants to climb, speed up, or descend, they have to manipulate these pairs. But what happens when all four of these forces are perfectly balanced?

Continue to see what happens when the forces are in equilibrium

When all four forces of flight—lift, weight, thrust, and drag—are perfectly balanced, the airplane enters a state of aerodynamic . Contrary to what you might expect, this doesn't mean the plane stops in mid-air; rather, it behaves exactly according to Newton's First Law of Motion.

Straight and Level Flight

In physics, when the net force on an object is zero, it preserves its current state of motion. For an airplane, this translates to maintaining a constant altitude and a constant forward speed. Pilots refer to this highly stable condition as straight and level flight.

\begin{aligned} \text{Lift} &= \text{Weight} \\ \text{Thrust} &= \text{Drag} \end{aligned}

Think of it like driving a car down a highway at a steady 60 miles per hour. Your engine is actively pushing the car forward, but it is doing so just enough to cancel out the wind resistance and tire friction pushing back. You aren't accelerating, but you are still traveling fast. On a plane, the same thing happens, just in three dimensions.

To see how a pilot changes this state to climb or speed up, we have to look at what happens when we intentionally break this equilibrium.