How does a jet engine work?
What goes on inside a turbofan engine?
What is reverse thrust during landing?
How does a super-heavy jet get off the ground?
What do you need to know in order to explain the phenomenon of "flying"?

How does a jet engine work?

Aviation owes a lot to the jet engine. This revolutionary invention made it possible to exceed the speed of the fastest propeller plane, or 700 km/h. Modern commercial aircraft fly with a jet engine at speeds of 850 km/h and more.

Jet engines even made it possible to fly faster than the speed of sound. Here we explain exactly how a jet engine works.

The start of a new propulsion principle

As early as the beginning of the 20th century, a number of scientists and engineers began playing around with the idea of using the recoil principle. The plan was to utilise this principle to generate thrust for propulsion. Success came in the second half of the 1930s. Frank Whittle in England and Hans von Ohain in Germany each developed a new drive of this type independently. This led to the first plane that used a jet engine instead of a propeller drive.

The recoil principle

Today there are different types of jet engines, and they are used for different purposes. Two such types are the turbojet engine (typically used in fighter jets) and the turbofan engine (typically used for commercial aircraft). Despite rapid advances in technology, the principle itself--the recoil--has remained the same. Let's take a closer look at a turbofan engine. The Americans describe the principle of engine operation like this: "suck - squeeze - bang - blow!"

What goes on inside a turbofan engine?

Jet engines operate on the basis of the turbofan principle. The fan of a Boeing 747-400 uses its 38 propeller blades to draw in air from the front and thus generate 80 percent of the total thrust. The smaller portion of the air volume is compressed by low- and high-pressure compressors and passed on to the combustion chamber. This is where the fuel is injected and the resulting air/fuel mixture is burned continuously. The gas expands enormously as it is heated and then emerges from the combustion chamber with a great deal of energy. It flows through the high- and low-pressure turbines, causing them to rotate, and thus delivers the power needed to drive the compressor and fan.

There are two ways to increase thrust. By increasing either the air mass flow or the air discharge rate. When the flying speed and air mass flow are the same, thrust is zero. Driving the engine at maximum power produces a 30-fold increase in pressure. Compressing air has a heating effect, resulting in air temperatures of up to 580 degrees Celsius. Some of the air removed from the compressor is used for aircraft systems such as the three air conditioners that regulate the temperature, or for de-icing the wings. The compressed air enters the combustion chamber, where the inflowing air is mixed with fuel. The fuel is injected into the combustion chamber from the tanks via pumps, valves, filters and fuel injectors. Spark plugs ignite the flame with the help of the inflowing air. This is the "bang". During combustion, the temperature rises above 2,000 degrees Celsius. This heating effect causes the air to expand so that it flows backward toward the turbine.

The high- and low-pressure turbines convert the gases flowing at high pressure out of the combustion chamber into useful work: driving the compressor and fan via a shaft. The kerosene burns and releases gases that contain H2O (water), among other things. They form the long white condensation trails (contrails) that you can see in the sky on a clear day. Contrails are made of steam that has frozen to form ice crystals and is a product of kerosene combustion.

What is reverse thrust during landing?

Passenger planes can fly as long as 19 hours non-stop to reach their destinations as long as they have sufficient fuel, i.e. kerosene. On landing, the aircraft needs to brake, which is done with a process known as reverse thrust. Pilots use reverse thrust to save wear and tear on the wheel brakes when the plane touches down on the runway. Once the plane has slowed to a safe taxiing speed, the pilot turns the reverse thrusters off again. The aircraft can then taxi to its parking position. A few seconds after the plane stops, and before the engines stop humming, the aircraft is surrounded by vehicles and personnel.

Aerodynamics

How does a super-heavy jet get off the ground?

Large commercial aircraft like the Boeing 747 often weight hundreds of tonnes, and yet they seem to fly effortlessly through the air. We pay little attention to the way a plane flies, except during take-off and landing. But how does it all work?

What do you need to know in order to explain the phenomenon of "flying"? This is where aerodynamics comes in. To test the flying behaviour of planes, for example, tests are carried out in enormous wind tunnels.

The four forces

Four different forces always act upon a plane as it flies. They are: 1. lift, 2. the weight of the aircraft (or gravity), 3. thrust, and 4. drag.

Drag is the friction produced between the air and the plane. As you can see, thrust and drag work against each other. The amount of drag depends on the speed and shape of the object moving through the air.

If you have ever held your hand out of the window of a moving car and observed how your hand's position changes, you are familiar with this phenomenon. You clearly feel the different forces acting upon the surfaces of your hand. If you hold your hand at a slight angle, you will even notice that it wants to move upward. This is lift.

Lift therefore depends to a large extent on thrust or speed: In order for lift to occur in the first place, the plane must be moving through the air. Without air or movement, the plane cannot experience lift. Lift is the key. The plane can remain in the air only if lift and weight are equal to each other.

The fact that a plane can fly, of course, also has to do with its wings and the air flowing past it. To explain the lift acting upon a plane in greater detail, we therefore need to take a closer look at the wings. All wings are constructed more or less like this one.

They are slightly curved upward and a bit thicker in front than at the rear. The way air behaves as it flows past the wings is very important. The different pressure ratios above and below the wings are what allow the aircraft to lift off the ground and remain in the air. The Swiss physicist Daniel Bernoulli (1700 -1782) was the first person to describe the relationship between flow velocity and pressure.

He discovered that airflow accelerates when air must flow around an obstacle. When a plane moves, air flows against its wings from the front. The airflow thus splits in two. The bottom of the wing is relatively flat, which allows the air to flow past it relatively unhindered. The air is displaced on the top of the wing, which has a more pronounced curve. Here the air must move out of the way, and its speed increases. As a result, the air pressure is lower. The air pressure below the wings is higher than it is on the top of the wing. This produces a suction effect above the wing, and the plane is pressed up from below by its wings.

You can experience this effect yourself by conducting a simple experiment. Just hold a strip of paper against your lower lip and blow hard on it. You will notice that the paper is drawn upward. The fast-flowing air produces a low pressure, and the paper is drawn upward by suction.

Our explanation would not be complete if we failed to mention something else that happens on the wing: The air flowing over the wing generates a vortex at the rear edge of the wing. This results in a counter-vortex, which, in turn, brakes the air flowing below the wing so that the pressure there increases even further and intensifies the lift.