How Do Airplanes Create Lift?

Aerospace Education

Introduction

In this post, we will look at how an airplane flies. We will cover the basic science of it, without getting too mathematical, and we will also address a popular misconception.

Forces

An airplane in flight is under four basic forces. They are thrust, drag, lift, and weight (an aircraft’s mass times gravitational acceleration). Thrust and drag act in opposite directions to each other. Lift and weight do the same, but on a different axis to thrust and drag.

When thrust equals drag, the aircraft is either stationary, or moving at a constant velocity. When thrust is greater than drag the aircraft accelerates. If drag is greater, the aircraft has a negative acceleration (deceleration) and slows down. For the purposes of this post, we are not going to cover thrust and drag, as an aircraft that doesn’t produce any thrust can still fly.

Figure A-1-1: Four forces of flight diagram

Our main focus will be on lift and weight. Lift is a force that acts perpendicular to the relative wind and weight always points towards the center of the earth. When lift is greater than weight, the aircraft ascends. When lift is less than weight the aircraft descends.

Important Principles

Continuity Equation: States that in a closed system, where mass flow rate is constant A₁V₁ = A₂V₂. What this means is that if the flow hits a place where the area is smaller, in order to maintain the mass flow rate, the fluid will have to increase its velocity

Figure A-1-2: Continuity Equation showing on a river with an obstruction

Bernoulli’s Principle: States that with a steady flow of a fluid, when the relative velocity of that fluid increases, there is a subsequent drop in pressure. When the relative velocity of a fluid decreases, there is an increase in pressure.

Figure A-1-3: Derived Bernoulli equation depicted on a river with an obstruction

Newton’s Third Law of Motion: States that for every action (force) there is a force that is equal in magnitude and opposite in direction.

Basic Definitions

Airfoil: An airfoil is the 2 dimensional shape of an airplane wing. If you were to take a slice of the wing from the leading edge to the trailing edge and then look at the profile, that would be an airfoil. An airplane wing is a 3 dimensional object that may consist of multiple airfoils arranged at different angles along the wing span.

Cambered Airfoil: Most airfoils used in aircraft wings are cambered. This means that if you draw a straight line from the front of the airfoil to the trailing edge of the airfoil, the section that is above the line will be thicker than the section below the line.

Figure A-1-4: Basic shape of an airfoil with chord and camber lines shown.

Requirements For Lift: In order to have lift, you need to have a fluid to move through. Air is a gas and gases are treated like fluids. Bernoulli studied flowing water in the 1700’s, but his principles still applied to aircraft two centuries later because gases and liquids (water) behave similarly.

Another requirement for lift is a relative velocity. The wing needs to be moving through the fluid to create lift. If the wing is moving at a speed and direction, but the wind is also moving at the same speed and same direction as the wing, the speed relative to each other is zero and no lift is produced. This is one reason why airplanes take off into the wind and never with a tail wind, because a tail wind reduces the relative velocity and therefore the lift.

Liftoff!

Bernoulli’s Explanation

When an airplane flies through the air, the wing is causing the air molecules to move out of the way. Some of those molecules go over the wing and some go under the wing. When the wing goes through a particular column of air, the wing causes the air above it to compress a little more than the air below it. This causes the air flowing over the top of the wing to accelerate more than the air flowing over the bottom.

Figure A-1-5: The airflow over an airfoil from Bernoulli’s equation

According to Bernoulli, when the air flowing over the wing accelerates, that causes the air pressure to drop. Meanwhile, the air flowing under the wing also accelerates, but not as much. Therefore the pressure above the wing is lower relative to the pressure below the wing.

$Pressure = Force * Area$

When there is a pressure difference between the top and bottom of the wing, the high pressure air wants to go to the low pressure side of the wing. That creates a force on the wing. That force is lift!

What Does Newton Say About Lift?

Newton’s Third Law of Motion explains lift in a different way. We all know his second law of motion F = ma. Since air has mass, a force has to be applied in order to move it, or make it change directions. As an wing cuts through the air, it forces the air over its surface and down at the trailing edge. That makes the air deflect downward at the back of the wing, more commonly known as downwash.

According to Newton’s Third Law of Motion, when the air is forced downward, there is another force pointing upward that is equal in magnitude and opposite in direction. That is the force the air exerts on the wing.

You’ll notice in the diagram below that this force does not point straight up. It points a little towards the rear. Forces are vector quantities, so they can be broken down into components. The vertical component of the force vector is lift. It acts perpendicular to the relative wind. The horizontal component is called Lift-induced drag (D_i). The act of making lift, also creates drag.

Figure A-1-6: Lift as explained by Newton’s Laws of Motion

Why Two Explanations of Lift? Who is Correct?

The answer is that both Bernoulli and Newton are correct. It turns out that the force acting on the aircraft’s wing can be explained with both methods. Pressure is a force times an area, and a difference in pressure creates a net force. A force is required to turn mass of air to create the downwash, so an equal force has to be applied in the opposite direction. Airplanes came long after Bernoulli and Newton existed, so they actually never studied the airflow over a wing. Other scientist used their principles to explain and calculate the lift of a wing.

Equal Transit Theory

The equal transit theory assumes that two molecules of air are suddenly split apart from each other by a fast moving airplane wing. The molecule that goes over the top of the wing has a longer journey than the one going under the wing. That makes the air molecule going over the top of the wing race to meet his long lost partner at the trailing edge of the wing at the same time.

This is a cute love story between two air molecules. The problem is that air molecules don’t have feelings and are not in love. So the act of separating them does not make them suddenly accelerate to meet their presumed partner at the back edge of the wing.

If, for arguments sake, air could fall in love, that would also make them super geniuses! Because, in the instant that the two molecules are suddenly split apart, they would have to do some amazing computational fluid dynamics to analyze the wing that split them and compute how fast they would need to go to meet each other on the other side.

I will admit that the equal transit theory is an explanation (though false) that can get people thinking about the air flow over a wing, but it is incorrect in its assumption that two air molecules that are split apart must be reunited in the same place at the same time. Sadly, this is still taught in schools and textbooks.

How do Helicopters and Rockets Fly?

Most people see helicopters as wingless wonder machines. They can be very confusing, especially since they can fly without moving through the air, which is a requirement to make lift. What people don’t realize is that helicopters do have wings. They all have at least two, but some have three or four, or as many as eight! The difference between the airplane and the helicopter is that in an airplane, the wind is created by moving the whole airplane through the air. While in a helicopter the wind is created by spinning the wings very fast. Yes, the rotor blades on a helicopter are thin, long wings that rotate. Pilots that fly helicopters don’t have a helicopter license, they have a rotor-wing endorsement.

Figure A-1-7: CH-53 with six rotor blades (wings)

Rockets Don’t Have Wings or Rotors.

Correct. Rockets have neither wings (aside from the former Space Shuttle), nor rotor blades. But they do have lots of power. If you strap a big enough engine (or multiple engines) to anything and point it in the right direction, you can get it to fly. Rockets use their thrust to oppose their weight and drag. While airplanes use thrust to move from point A to point B, rockets only want to go up, so they put their engines on the bottom and that forces them straight up. Unlike airplanes, a rocket’s weight and drag forces act in the same direction, which is how thrust can overcome both.

This system is very direct, but not without its drawbacks. The main drawback is that they are opposing two forces, gravity (weight) and drag with one force (thrust). This means that up to 90% of the weight of the rocket must be fuel to produce enough thrust to overcome these forces long enough to get to space. That’s a very simplified explanation, but it sums it up.

Figure A-1-8: SpaceX rocket at launch.

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