Gravity vs Lift

A David and Goliath Story

By the staff at MySkyForce

Gravity did its best to hold us down for a long time. The powerful force confined our travel to the surface of the Earth for centuries. Things that went into the air always came right back to the ground (usually broken). It’s easy to see why people in the early days were not standing around looking up into the sky and thinking “now there is the future of travel.”

Gravity was still the big boss when the Vikings took to the water with oarsmen and long ships to raid distant settlements in the 700’s. And hundreds of years later when the pioneers hauled their Conestogas through streams and up and down the mountains, travel across the country still took months.

Flapping was not the Secret

The birds that filled the skies offered some hope that we could get off the ground someday, but a lot of people who jumped off high things with their arms flapping could not find the answer. An assortment of unpowered gliding devices showed promise in the late 1800’s, but the flights were short and often really exciting.

Everyone knows all of that changed in 1903 when the Wright Brothers built a flying machine combining wings and an engine to power through the air. So, what kept their plane in the air?

The short story is that airplanes can fly because the wings create a force called lift. That simple explanation will make you sound well-informed when you are making conversation at the bar. So, if you are headed to the bar you have what you need.

Pilots need more 

But if you are going to become a pilot there is a lot more to understand about lift. You will need to understand the physics of lift and how you can manage it as a pilot. You will need to understand how lift keeps the plane in the air, and how you can use it to control the movements of the plane.

The Federal Aviation Administration will expect you to thoroughly understand the concept of lift before you can be certified as a private pilot. They will test you on it on both your written and practical tests.

Planes Fly by Creating Lift 

An airfoil is any device that generates an aerodynamic force known as lift by moving through a body of air. The wings, horizontal stabilizer and propellers are all examples of airfoils. When people think of lift they usually think of a force acting upward, but lift can be applied in any direction. The wings on an airplane tend to generate lift upward, but the wings of race cars create lift in a downward direction. Race car designers refer to this as down force, but it is the very same force that allows an airplane to fly.

How does an Airfoil Create Lift? 

The specific shape of the wing causes the air to accelerate as it moves over the top of the wing. The accelerating air leads to a reduction of pressure above the wing relative to the pressure below the wing. In very basic terms the high-pressure air below the wing attempts to push the wing surface toward the lower pressure air above the wing. Lifting.

But wait. How is the wing causing the air to accelerate? And why is the faster moving air creating a low-pressure area? There is a lot happening to create the lift required for your flight. And it can seem a little confusing if we don’t start with a couple of the principles of physics. Don’t break into a sweat. We will make it easy.

To really get this, we have to go back and look at the work of three early scientists. These men were not giving thought to flying when they did their experimentation, but the principles they discovered were the concepts later applied by engineers to create airfoils.

Isaac Newton

Newton’s laws from the 1600’s are considered the cornerstone of modern physics. Pilots need to become familiar with Newton’s three laws of motion, because they relate directly to lift and control of the airplane. For the purpose of our overview on lift, we need to know that his third law states: “For every action, there is an equal and opposite reaction.”

Daniel Bernoulli

Bernoulli was studying the movement of fluids in the 1700’s when he observed the Venturi Effect. The principle states that as the velocity of a moving fluid increases, the pressure in the fluid decreases. (Gasses, like air, are also considered a fluid. So as air accelerates, its pressure decreases.)

Heinrich Gustav Magnus

Magnus, in the 1800’s, experimented with the movement of moving cylinders in fluids.  He discovered that a spinning cylinder split the flow of the fluid and caused the fluid flowing over the top of the cylinder to accelerate. The wing of an airplane performs like a spinning cylinder in a body of fluid. The wing splits the airflow and causes the flow over the top of the wing to accelerate.

And putting all that together …

So back to the airfoil. Magnus shows us the wing splits the airflow and causes the air over the top of the wing to accelerate. Bernoulli tells us the accelerated airflow over the top of the wing decreases the pressure, causing the higher-pressure air below the wing to push the wing surface up into the lower pressure area.

The final component of lift production is described by Newton’s third law of motion. As the airflow moves rapidly down and back across the top of the wing (called downwash), it collides behind the trailing edge of the wing with the airflow from the lower side of the wing. In accordance with Newton’s third law, the downwash action creates an equal and opposite upward force that drives the wing up and forward.

Whenever a wing (applying these three physical laws) generates a lifting force greater than the weight of the airplane, flight can occur.

Okay. This is an overview. But there is so much more a good pilot will need to know on this topic, so we will be talking about lift a lot as we blog on.

Superman defied gravity

But he had to wear that weird blue suit

You can defy gravity in your regular duds, if you just learn to control lift.

In an upcoming Vertical Speed, you will learn about the three most important words in aviation. The words are the key to allowing the pilot to fly an airplane by changing the relationship between the airfoil and the airflow — to control lift.

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