physics of flying

The physics of flying – a brief outline

Esther Nestle
22.02.2019
1 picture
4 minutes

Why do aircraft fly? The solution to this problem revolves around four opposing forces whose perfect interaction allows aircraft which weigh many tonnes to take to the air, “just like that”.

Simply lifting off from the ground just like a bird and moving weightlessly through the air – knowledge of how to accomplish this ancient human desire did not fall from the skies “just like that”. We owe flying now being almost as natural a part of our lives as, say, cycling or singing or cleaning vegetables to a long line of flight visionaries who gradually became wise to how the aforementioned four forces interact. Some of these brilliant brains will soon have the honour of being the subject of future articles.

So today’s question is:

Why do aircraft fly?

A large passenger or freight aircraft easily weighs 100 or 200 tonnes. TWO HUNDRED THOUSAND kilograms! As the largest and heaviest passenger aircraft in the world, the Airbus 380 can bid farewell to the ground with an unbelievable 560 tonnes starting weight (The fact that this lump of an aircraft will nevertheless soon no longer leave the ground can be found in another article). So here, a box weighing many tonnes rolls along the runway and lifts off at some point like a feather and is lifted higher and higher by an invisible hand.

A portion of our readership may know why. Give us your expertise! You are very welcome to contribute to WingMag! Another portion possibly feels a little less sure and wonders “How does that work?”

The physics of flying – four aerodynamic forces

Still on land or have you taken off yet? Whether there is ground traction or the craft takes to the skies is decided by the interaction of these four forces:

Gravity pulls the aircraft downwards and initially keeps in on the ground. This is really not surprising … By contrast, lift “pulls” the weight upwards (it’s all in the name) and keeps the aircraft in the air. The aircraft only lifts off when the lift is greater than gravity. Thrust? This drives the machine forwards. In turn, the opposing drag force slows the machine down and has to be overcome. Highly complex knowledge of how these four forces interact forms the basics of aircraft construction.

How does lift manage to overcome gravity?

The wings, whose curved shape is clearly visible in profile, are primarily responsible for dynamic lift. The shape ensures that the air flows over the curved upper surface of the wing much quicker than over the flat lower surface. The quicker the air flows around the wings, the more lift is created. This means that there is a strong upward pull on the wings. The faster the aircraft travels, the greater the pull, or rather, the lift. Beyond a certain point, the pull (i.e. the lift) is stronger than gravity – the aircraft lifts off. The engines which are used for thrust provide the power required for high speeds so that the air passes over the wings quickly enough and the lift can overcome gravity.

Many readers will be aware that the scientifically correct explanation as to how lift is created in detail is highly mathematical and brings a number of further elements into play: circulation, rotation, gradient, vortices (demonstrated using vector fields), starting vortices, streamlines. But even if the above explanation is old hat for a portion of our estimated readership, the other portion is perhaps now a little more in the know thanks to this simplified explanation on the main features of the secrets of flying.

by Esther Nestle

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