Jet Engines and How Airplanes Fly

Since the mid Twentieth Century, the jet engine, which operates according to Newton’s Third Law, has dominated aviation technology. Last year we asked Guy Ellis, author of Britain’s Jet Age From the Meteor to the Sea Vixen (2016) and Britain’s Jet Age From the Javelin to the VC10 (2016), to help us understand the theory behind how jet engines work.

Now we are turning to Simone Paternostro, Space Engineer and University of Nottingham fellow, to explain the principles of flight, and how they relate to modern jet aircraft. This is the second part of our interview with Simone, see the first part here.

Four forces of flight

The design of a jet aircraft is complex, with variations depending on the manufacturer, economic constraints, and numerous other factors. But all jet engines’ flight depends on the four forces of flight. This article will explore each force in turn, explaining how airplanes fly with reference to lift, drag, propulsion, and mass.

Lift

Lift, also known as aerodynamic force, allows the aircraft to take off. Air flowing past the surface of the jet exerts force, lifting the aircraft off the runway. Lift is generated by the pressure of air on the wings, which have a slight tilt in relation to the horizon. Higher pressure in the lower part of the wings, and lower pressure in the higher parts, creates an upward force. The force of this lift also depends on the velocity of the aircraft; how fast it moves and in what direction.

Aerodynamics: The aerodynamics of an aircraft – in other words, the qualities of the jet that affect how easily it is able to move through the air – are crucial, because they harness or mitigate lift and drag. The aerodynamic design of the jet includes factors like the shape and dimension of the wings, and the fuselage.

Drag

Drag is the aerodynamic countering of an aircraft's movement through the air. It is a mechanical force, generated by every part of the airplane. How? Well, the interaction and contact of a solid body with a fluid or gas (in this case, the air) generates drag. How much depends on the shape of the aircraft, e.g., the fuselage and wings, and also the speed and the density of the air. Drag works against the direction of motion, slowing down the aircraft unless another force compensates for it.

Mass

The third feature is the mass of the aircraft. Naturally, the aircraft should be as light as possible in order to minimise the amount of fuel used. The mass, or the weight, is the last force that acts on the aircraft. The lift generated needs to at least counteract the weight of the aircraft, before it can take off.

Propulsion

The fourth most important feature of how a jet engine works is propulsion. The word propulsion comes from the Latin “pro” meaning before, and “pellere”, meaning to drive; propulsion means to push forward an object. The engine itself defines the force that acts on the aircraft. In jets, thrust is generated through the application of Newton's third law, which posits that for every action there is an equal and opposing reaction. Thrust, which is the force that pulls the airplane forward and accelerates it, increases at the same time with speed and lift. This is why aircraft need runways to take off. They must reach a certain velocity before lift enables the plane to take off.

Propulsion and the jet engine: All jet engines create propulsion in the same way, whether powering a 747 Jumbo Jet or a light private aircraft. Air is pulled into the engine with a fan. A compressor (made of blades attached to a shaft) raises the pressure of the air. The shaft blades spin fast, compressing the air. This compressed air is sprayed with fuel. This compressed, fuel-infused air is ignited with a spark. The now-burning gases expand, blasting out through the nozzle at the back of the engine. As these jets of gas shoot backward, the aircraft is propelled forward.

This process is described by Newton's third law, which states that for every action there is an equal and opposing reaction. The exact same principle would come into action if you stood on a cart with wheels, throwing bowling balls off the back. As you threw the balls from the back of the cart, it would move forwards.

The overall performance of a jet depends on these four forces, which define its capability. However, they are not the only features that are necessary for an aircraft. While there are four forces, there are many other critical design features, such as the controls, the stability, and the structure of the aircraft.

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Timeline: the birth of the jet plane:

• 1st Century AD - The aeolipile – a precursor of the jet engine – is invented, most likely in Greece or Alexandria. This device pushes steam through two nozzles, causing a sphere to spin rapidly. As engineers do not recognise its practical applications, it is exhibited as a curiosity.
• 1232 - The Chinese begin to use gunpowder-powered rockets as weapons.
• 1633 - Lagâri Hasan Çelebi builds a 7-winged rocket fuelled by gunpowder, and ascends from the coast below Topkapı Palace in Istanbul. He lands in the sea, winning silver and a military position from the sultan, Murad IV.
• 1687 - Sir Isaac Newton presents his three laws of motion, laying the foundation for modern propulsion theory. Newton theorizes that a rearward-channeled explosion could propel a machine forward at high speed.
• 1791 - The gas turbine – which the jet engine will later utilise – is patented by John Barber in England.
• 1913 - French engineer René Lorin patents a design for the world's first ramjet. However, Lorin cannot develop a working prototype because an aircraft with sufficient speed to operate the jet has not yet been invented.
• 1930 - Frank Whittle, an engineer and officer in the British Royal Air Force designs the first turbo jet engine. In 1932, Whittle is granted a patent for a two-stage axial compressor feeding a single-sided centrifugal compressor.

• 1936 - German physicist Hans von Ohian, working with Max Hahn, develops and patents his own design for the jet engine.
• 1939 - The first successful jet aircraft, Heinkel He 178, utilising von Ohian’s jet engine, has a successful maiden flight.
• 1941 - Frank Whittle designs the first successful turbojet, the Gloster Meteor.
• 1942 - Dr. Franz Anselm develops the axial-flow turbojet, Junkers Jumo 004, used in the Messerschmitt Me 262, the world’s first operational jet fighter.
• 1948 - The USA’s turbojet, Bell X-1, breaks the sound barrier for the first time.
• 1952 - The first purpose-built jet airliner, the British de Havilland Comet, enters service on the British Overseas Airways Corporation’s London to Johannesburg flight.
• 1955 - Reheat is first used to increase the turbojet’s thrust.
• 1960s - Relentless improvements in the jet engine push the piston engine out of mainstream use. The jet has achieves near-universal use, in less than twenty years since its invention.