Aerodynamics - aerodynamic - aircraft - commercial aircraft

Commercial Aircraft Aerodynamics

Tim Takeoff
06.08.2019
1 picture
7 minutes

Humankind has wanted to capture the skies for themselves for centuries – and the dream became true. Nowadays, we are able to fly regularly from A to B at inhuman altitude, doing so practically at supersonic speed amid pleasant surroundings. However, which laws of physics enable aircraft weighing many tonnes to remain in the air almost as a matter of course? We explain the aerodynamics to you.

Give it some power and take off?

For aircraft with rigid wings, the principle of flying is almost always identical. Therefore, to keep things simple, let’s disregard all further types of flying, such as helicopters or airships, balloons and other hybrid forms and run the rule over a modern commercial aircraft: you give it some power, accelerate, and take off! It’s as simple as that. Or is it really?

Aerodynamics – Harnessing lift and gravity

In principle, the wing’s lift arises from the air being forced to flow around the foil at a specific angle (the flow angle or adjustment angle). The curved profile, i.e. the wing’s cross-section, means that the air on the upper side has to cover a slightly longer distance and thus streams more quickly than on the underside. This leads to negative pressure on the upper side (physical background: Bernoulli equation). The negative pressure ‘sucks’ the foil upwards. A positive pressure arises on the lower side.

A wing generates a different strength of lift depending on its curvature, size, inclination and speed. If this amount of lift exceeds the aircraft’s weight, the aircraft can overcome the Earth’s gravity and take off.

Optimised wings

You still need to steer, of course

Steering usually takes place using various types of rudders. These are elevators, ailerons or side rudders. The pilot can use these rudders to guide the aircraft around all three axes. It can roll around its longitudinal axis, pitch around the latitudinal axis or yaw around the vertical axis. Each rudder on a commercial jet is also equipped with a trim that can be used to reduce the arising forces. For a comprehensive article on trim, click here.

An aircraft needs to render certain forces to be able to overcome other forces: when it comes to air travel, we therefore need to pit gravity against the wing’s lift. The engines use their thrust to generate propulsion to overcome air resistance. You can find more on these four aerodynamics in this article.

Physics of flying

The requirements define an aircraft

An aircraft is initially defined based on the desired requirements and the envisaged area of use. However, engineers designing an aircraft have to make compromises due to physical and structural limits. This is still true even though they would like to transport as many passengers as possible over the greatest distance, in an aircraft that is as light as possible. This leads to small and light short-haul aircraft coming into being, as well as large and heavy wide-body models to cover inter-continental routes.

Families of aircraft

Different versions are created even within the aircraft families themselves. For example, there is not only an Airbus A340. The A340-600 was created for particularly highly-flown routes where the range is not decisive, rather the pure ‘throughput’ of loading volume. It is longer and heavier, but does not achieve the range of its smaller brother, the A340-500, by a long shot. Boeing also pursues this philosophy in its 737, 747, 757, 767, 777 and 787 aircraft models. The airlines consider exactly what they need in terms of their personal route network before they purchase aircraft.

Subsonic or supersonic?

Almost all commercial aircraft, with the special exception of Concorde, are developed to be subsonic. The established laws of physics apply. Financial constraints mean that development hardly ever extends beyond this.

Each subsonic aircraft basically flies in the same manner. The wings move leading air to stream around the foils. As air comprises different particles and different densities depending on their altitude and temperature and thus a different air resistance, each aircraft design is optimised for a specific use.

Small and deep or high and fast?

Aerodynamics - aerodynamic - glider - gliding
© Pixabay PGottschalk

For example, gliders have long, slim and scarcely pointed wings. They are optimised for deep layers of air and low speeds. Fighter jets are built with short, strongly pointed and sometimes even ‘delta’-shaped foils. They are perfectly attuned to high speeds, even supersonic speeds, and high altitude in keeping with this.

aerodynamics - aerodynamic - combat jet - military jet
© Pixabay Eageltonc

A commercial aircraft as a compromise

When it comes to a commercial aircraft, a compromise has to be struck between speed and fuel consumption. Therefore, it is beneficial to exploit the engines’ performance to the greatest extent and to assume the highest altitude with lower air resistance. However, this resistance is also required from air particles to flow around the foils. That is to say: if you fly higher, you need to fly at a speed only just below the sound barrier.

Aerodynamic problems

To cover this narrow ‘borderline area’, the wings on a commercial aircraft are pointed slightly backwards. A considerable wingspan is still required. If profiles are optimised, this ensures equalisation. In this way, an aircraft can be kept in the sky at lower altitude – for take-off and landing – with the aid of landing flaps. At high altitude, high speeds can be maintained with as little thrust as possible and low resistance.

Higher, faster, further?

New materials such as carbon fibre or glass fibre enable wing designs to be ever increasingly optimised, with increased strength and weight savings. Thanks to continuous development, maybe supersonic flights will be possible again in the near future, as was previously the case with Concorde. It was unfortunately far too ahead of its time. In our article, you can find out more about the exciting phase of aviation.

You can find a further article on the physics of flying here at WingMag.

Cover picture © Unsplash Samuels Photos

by Tim Takeoff

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