All About Flight

The Essential Ingredient of Flight

To be able to fly, you need lift. This is an easy concept because we have all lifted things at one time or another. When you lift a book off a desk, you are supplying a force through your muscles that is enough to overcome the force of gravity, which pulls down on the book.

Lift gets a flying machine—an airplane, a helicopter, a blimp, a hot air balloon, or a rocket—off the ground.

Each of these flying machines has a different way to achieve lift.

How Do Airplanes Fly?

What is the most important feature of an airplane, the one that really makes it possible to fly? Is the most important thing the propeller or jet engine, which gives it speed? The jet fuel? Or how about the wings?

If you guessed the wings, you are right. But there is more to it than that. It is the shape of the wings that makes it possible for a plane to fly. We're not talking about the length of the wings, although that is important. If you sawed straight through the wing and looked at it, you would see a shape like this:

This shape is very important—it makes it possible for a plane to lift off the ground. This is the shape of an "airfoil." Notice the features of the airfoil: a curved top surface and a flatter bottom one. This is the secret of heavier-than-air flight.

What is it about an airfoil that makes it so special? Air flows over the top, curved surface of the airfoil faster than under the bottom, flat surface. This creates a difference in air pressure between the top and bottom of the airfoil-shaped wing. The air above the wing is at a lower pressure than the air under it, and the higher pressure pushing up on the bottom of the wing overcomes the lower pressure, pushing it down. When an airplane is moving fast enough down the runway so that the upward pressure is greater than the force of gravity, the airplane lifts off.

Of course, the plane could not reach high speed without a jet engine (or a propeller) and fuel, so these are important components too. And if the weight of a plane is too great it might never go fast enough to leave the ground. But without the key shape of the wing—the airfoil—none of these things would matter.

Here is a simulation of how an airfoil works.

How Do Helicopters Fly?

A helicopter has a rotor—pretty much a rotating wing—on top and another rotor in its tail. The rotors are made in the shape of—you guessed it—airfoils. When the helicopter's engine turns the main rotors at high speed counterclockwise (looking from above), the pressure on the bottom side of the rotor is greater than the pressure on the top, making the helicopter lift off straight up into the air.

However, even though they share the same basic way of getting off the ground, helicopters are much more complicated than airplanes. Planes can go forward, up and down, and left and right. Helicopters can do the same, while adding the ability to go straight up, backwards, or just hover in midair, going nowhere.

The key to these extra abilities is that the helicopter pilot can change the angle of the rotor blades. The steeper the angle of the blades, the greater the amount of lift, and the faster the helicopter rises.

To go forward or backward, right or left, the pilot tilts the blades in the direction he wants to move.

It is interesting to note that the same engine that spins the main rotors counterclockwise also tries to spin the helicopter body clockwise—every action has an equal and opposite reaction, just as Isaac Newton said. The tail rotor produces a force to counter the tendency of the helicopter body to turn, thus stabilizing it.

How Do Blimps Fly?

Blimps are filled with helium, a gas that is lighter than air. You may have seen helium balloons at a parade or a party. What happens when you let go of the string of a helium balloon? It rises into the sky. Similarly, helium provides the lift in a blimp.

Once in the air, small engines turn propellers that control the speed and direction of the blimp.

How Do Hot-air Balloons Fly?

Just like a blimp, a hot-air balloon uses a gas that is lighter than air to achieve lift. In this case, the lighter-than-air gas is simply hot air. The pilot of a hot-air balloon controls a flame located at the bottom opening of the balloon. By turning on the flame and heating the air inside the balloon, he can make it rise. By turning off the flame and letting the air inside cool, he can make it descend. Unfortunately, a pilot has little control over the direction his balloon moves across the sky, because there are no motors or propellers on a hot-air balloon. To move in a certain direction, the pilot has to move the balloon up or down to catch a wind current moving in the desired direction.

How Do Rockets Fly?

A rocket relies on high-pressure exhaust to propel it into the sky. It is a case of one of Isaac Newton's laws again—for every action there is an equal and opposite reaction. High-pressure exhaust gases blasting downward cause the rocket to blast upward—a blastoff!

The burning of fuel (called combustion) makes the exhaust gases. Many different types of fuels are used in rockets. The most common fuels used by NASA are liquid oxygen and liquid hydrogen. These fuels are held in separate tanks located on the booster rocket. When mixed together, hydrogen and oxygen burn to produce enormous amounts of energy, as you have seen if you ever watched a launch from the Kennedy Space Center. The tremendous flames that shoot out the bottom of a rocket when the controller says "ignition" provide the energy to "blast off."

Sometimes solid-rocket fuels are used instead of liquid ones, but the principle is the same—high-energy combustion of fuels launches a rocket into space.

How Does a Spacecraft Break Free of Earth's Gravitational Pull?

If you toss a baseball straight up into the air, it keeps rising until Earth's gravitational pull overcomes the force you put into the throw, and it falls back to the ground. However, if you could toss it hard enough to reach a speed of 7 miles per second, or 25,000 miles per hour, the ball will keep going into outer space. This is known as the "escape velocity." At that speed, the force of gravity can never be stronger than the force causing the ball to rise, so it will escape the gravitational bonds of Earth. Of course, the best baseball pitchers can only throw at a velocity of 95-100 miles per hour, which is why you don't see baseballs in orbit. Rockets, however, can reach escape velocity, so we can send spacecraft beyond Earth's grasp to explore the other planets in our solar system.