It’s fascinating to think that it was only a little over a century ago — on December 17, 1903, to be exact — that Wilbur and Orville Wright first flew their heavier-than-air powered airplane at Kitty Hawk, North Carolina. Within a few short years, they patented their flying machine — U.S. patent 821,393 — and claimed their rightful place in aviation history.
Back then, it would have been next to impossible for the Wright brothers to fully conceive the impact their invention would have. In terms of the sheer amount of today’s commercial air traffic, the numbers are staggering — the National Air Traffic Controllers Association estimates that it tracks over 87,000 flights in American airspace each day.
It might seem counterintuitive that giant tubes of metal are capable of soaring through the air for hours at a time. What’s so important to understand about airplane flight, however, is that all aircraft — from an Airbus A380-800 to a Cessna 172 — obey the same laws of physics. And once you examine the combined forces that give passenger planes, fighter jets and even helicopters the ability to fly, you can comprehend how something heavier than air can take to the skies.
THE SCIENCE OF FLIGHT — THE FOUR FORCES
What are the principles that enable an airplane to fly? To start, here’s a simple look at the physics of airplane flight with what are known as the four forces of flight — lift, weight, thrust and drag. Each of these forces plays a role individually, but it’s the way they work together that helps an airplane fly.
Lift is generated by an aircraft’s wings. If you look at the cross-section of a wing, you might notice that it’s a unique shape. While the bottom of the wing is relatively flat, the top is curved. The amount of curvature, or the camber, along the top and bottom of the wing can vary depending on what the airplane is designed to do, such as an aerobatic airplane or a fighter jet that can also fly inverted. However, many aircraft utilize a noticeably larger curve along the top of the wing than along the bottom. This shape is known as an airfoil. In addition to the wing, the propeller, the aircraft’s control surfaces and even the airplane’s fuselage are airfoils — though the fuselage is not good at generating lift.
While the intricacies of lift are quite complex, there are two scientific principles that help to explain the overall concept.
As the wing moves through the air, the air flowing over the top of the wing moves faster than the air flowing below the wing. This creates an area of lower pressure on the top of the wing. Higher pressure remains below the wing and is able to lift the wing into the air. This principle is known as Bernoulli’s principle, named after Swiss mathematician Daniel Bernoulli.
NEWTON’S THIRD LAW OF MOTION
The second concept that applies to the creation of lift is Newton’s third law of motion. The law states that for every action, there is an equal and opposite reaction.
As the air flows around the wing, it gets deflected downward. The equal and opposite reaction then is that the wind pushes the wing upward.
It’s the combination of both the lower pressure air flowing above the wing and this equal and opposite reaction that enables a wing to generate lift.
Weight is a force that pulls the airplane downward because of gravity, and it refers to the combined load of the airplane itself as well as other components such as passengers, crew and fuel. If lift is not sufficient enough to overcome the effect of weight, then the airplane cannot fly.
A plane’s size and shape are responsible for displacing air as it moves. This force of resistance is known as drag. It essentially refers to air resistance, pushing back against the plane as it flies.
Imagine walking through waist-high water with your hands at your sides — you can feel drag as your body displaces water molecules while you move forward. If you cup your hands and close your fingers, you can even feel how drag increases.
The design of an airplane can have a significant effect on the way drag influences its movement. To return to the analogy of moving your hand through water, imagine the difference that could be made by how your hand was shaped and turned. A cupped hand facing into the direction of movement would cause much more drag than a flat hand angled parallel to it.
In the same way, the structure of a plane has a marked effect on how much drag it produces. There are two main types of drag: induced drag and parasite drag. Induced drag is the result of an airfoil producing lift. Parasite drag refers to the other components — such as skin friction and airplane parts like antennas that affect the smoothness of the airflow around the plane — that attempt to slow an aircraft’s forward movement.
The last of the four forces is thrust, which is the forward-moving force — or momentum — produced by a plane’s engine. It’s usually accomplished via either a propeller or a jet engine. While these are two vastly different forms of propulsion, they both push air backward and away from the plane. For an aircraft to be able to move, thrust must be able to overcome the effects of drag.
Newton’s third law applies here as well. As the propeller pushes the air backward, and a jet engine creates a blast of backward moving air, the aircraft moves in the opposite direction — forward.
Note that thrust is a major reason that most unpowered gliders must first be towed by powered planes — so that upon release, they have sufficient forward speed. After they are released, a glider’s design allows them to descend very slowly. To stay aloft, gliders rely on areas of rising air.
HOW IS AN AIRPLANE CONTROLLED?
When you drive a car down the road, you only have to worry about two dimensions. If you make a turn, you know the car is simply going to move to the left or the right. An airplane, however, operates in three dimensions, and it has three different axes around which it can rotate.
Yaw refers to a plane’s position relative to the vertical axis. When a plane rotates about this axis, it acts similarly to a car turning on the road, remaining parallel to the ground as it moves.
Where cars use their wheels to make turns, planes use a rudder, located on the vertical stabilizer. This works very similarly to a rudder on a ship. Just as a ship uses a rudder to redirect the flow of water, a plane uses the rudder to redirect the flow of air. This redirection causes the plane’s nose to move from side to side.
Pitch refers to a plane’s position relative to the latitudinal axis. This axis stretches through the wings from wingtip to wingtip. When a plane rotates about this axis, the nose tilts up in a climb and down in a descent.
Planes use either an elevator or stabilator to adjust pitch. The elevator is located on the horizontal stabilizer. A stabilator combines the elevator and horizontal stabilizer into one control surface.
Roll refers to a plane’s position relative to the longitudinal axis. This axis stretches from the nose of the airplane all the way through to the tail. When a plane moves about the longitudinal axis, it rolls from side to side.
To adjust roll, planes use ailerons, which are located on the trailing edge of the wings closest to the wingtips.
WHAT DO A PLANE’S ENGINES DO?
Two types of motion are happening when an airplane flies — movement upward and movement forward. The latter is caused by the engines, which propel the plane forward through the air. This propulsion is what enables planes to travel such great distances at such high speeds.
Here are a couple of ways airplane engines can generate that forward movement:
On single-engine aircraft, the type of engine most commonly used is a spark ignition, four-stroke engine. These engines are connected to a propeller, and the power they generate will make the propeller spin. As engines go, this method isn’t complicated — it’s essentially the same process that generates the spin of something like a lawn mower blade or the operation of a car engine. The difference is in function. The spin of the propellers pushes air backward, moving the plane forward.
Larger planes with multiple engines usually have turbine engines. These engines have many different types, but most of them function in the same basic way. As the plane moves forward, air rushes in through the front of the turbines. The turbines then compress the air, mix it with fuel and ignite the resulting mixture. The burning gases blast out the back of the turbines, propelling the plane forward. So while propellers and turbines function differently, they achieve the same result — pushing air backward to move the plane forward.
WHAT OTHER FACTORS AFFECT FLIGHT?
While the four forces of flight are the main factors driving aircraft’s ability to stay in the air, they aren’t the only ones that contribute. It’s not as though planes fly in a vacuum. Flight is performed by vehicles that are susceptible to a variety of conditions, and it takes place in a sky with no shortage of natural atmospheric activity. Here are some of the main additional factors driving the way planes fly.
WEIGHT AND BALANCE
Different planes are built in different ways. Part of the reason for this fact goes back to the four forces of flight. A plane has to be built so that it can achieve enough lift to overcome its weight, but that all depends on what its weight is. Larger planes will have to generate more thrust and lift than smaller ones, for instance. If a plane does not have sufficient thrust and lift to overcome weight and drag, it won’t be able to get off the ground.
It also matters how weight is distributed throughout the plane — an airplane must be loaded according to its center of gravity envelope. The center of gravity is the point at which if an aircraft were suspended, it would be balanced at that location.
The load of the aircraft affects the center of gravity’s exact position, and consequently, how an airplane flies. Having too much weight toward the nose, for instance, would move the center of gravity too far forward and make it difficult, if not impossible, for the plane to take off — you would not be able to lift the nose off the ground, and the airplane would not be able to climb. Any given aircraft must be within its center of gravity limits to fly the way it’s intended to.
It may not seem like it, but air takes up space and exerts weight. At sea level, a square inch column of air exerts 14.7 pounds of pressure. As you go higher in the atmosphere, the density of the air decreases. For airplanes such as airliners that can fly at higher altitudes, this decreased air density means less resistance and helps the plane move faster and more efficiently. For other airplanes, however, such as single-engine piston-powered aircraft, a decrease in air density results in a less efficient engine.
Humidity refers to the amount of moisture in the air. More specifically, relative humidity measures what percentage of moisture is present relative to how much a given sample of air can contain. This means that if a given sample of air has a relative humidity of 40%, it contains 40% of the maximum amount of moisture it’s capable of containing. Once the relative humidity reaches 100%, it means that the air is saturated — it can’t hold any additional water.
The higher the humidity, the more water is present in the air. Because aircraft rely on air to operate, higher humidity can cause their engines to function less efficiently. Additionally, once the air becomes saturated, the water it holds can begin to condense into fog and clouds, which, of course, reduces visibility both on the ground and in flight.
Much like humidity, weather can be a factor in how and where an airplane is able to fly. One of the biggest weather drivers is the uneven heating of the earth’s surface. As air is warmed, it begins to rise, and cooler air moves in underneath to take its place. This movement of the air causes what are known as convection currents. Convection helps to create winds as well as other weather phenomena such as thunderstorms. It can also be a cause of turbulence, which affects how bumpy a flight is.
Even when there’s no storm going on, winds can have a significant impact on a plane’s movement through the sky. In addition to convection currents, several other factors can generate wind, such as:
High and low pressure systems: If you pay attention to weather forecasts, you’ve likely heard the terms high and low pressure. Air always flows from high to low in an attempt to equalize the pressure, and this air movement creates wind.
The jet stream: The jet stream is a powerful wind current that travels at high speeds throughout the atmosphere. Its position changes due to high and low pressure systems, the time of year and the temperature of the air.
Winds are especially important to be aware of when taking off and landing, where all of a plane’s movements must be as controlled and precise as possible. Headwinds and tailwinds also affect the speed an airplane travels over the ground.
With the jet stream, for instance, a plane traveling in the same direction as the jet stream’s winds will experience a tailwind. The airplane essentially rides the stream of air, like a ship might ride an ocean current, and can reach its destination faster. One passenger jet recently broke the transatlantic subsonic speed record for an airline by doing just that.
HOW SAFE IS FLYING IN PLANES?
Despite common fears about air travel, often generated by the rare sensationalistic news story, plane crashes are just that — rare. The National Safety Council, which collects data on the likelihood of different causes of death, reports that the odds of dying in a plane crash are too low for them to even calculate. To put that in perspective, they reported the odds of dying in a car crash at one in 106. In other words, you’re significantly safer in a plane than in a car — no matter how frequently you fly.