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Flight mechanics in the birds

Through an evolutionary process that has taken millions of years, birds have developed internal and external anatomical and physiological adaptations that give them absolute flexibility and freedom in the air. Even after the incredible progress aviation has experienced in the last century, bird flight continues to exceed artificial flight in many ways. Despite being slower, birds still have more versatile, silent and efficient flight. Below, we explore some of the lessons we have learned from birds.

From the earliest emergence of aeronautics, bird flight has always been a source of inspiration for aircraft design.

Many of the revolutions in commercial aviation were based on the study of birds. For example, the Wright brothers’ great contribution was the aircraft control system, which allowed rotation to be controlled along three axes: roll, pitch and yaw. They discovered that they could control the roll of an aircraft using wing torsion, a technique used by birds.

Both aircraft and birds must generate lift in order to be able to fly, and both use wings: in the case of aircraft they are fixed, whereas in birds they are flapping wings. Interestingly, the similarities between both types of wings are remarkable, and the shape of their cross-section is very similar. However, the superiority of bird wings is considerable.

One of the main advantages birds have derives from their feathers, which unite the qualities of strength, flexibility and lightness to the highest degree. Feathers allow birds to change the shape of their wings instantly, and with more flexibility than the mobile surfaces of aircraft wings. Bird feathers are made up of a shaft called a rachis, which divides the feather into two asymmetrical halves called vanes. These in turn are formed of branches or barbs, made up of barbules that overlap each other and thus trap air. There are different types of feathers, according to their function and location on the wing:

  • Alulae: This group of feathers is located on the anterior leading edge, in the position corresponding to what would be the thumb in humans. They allow birds to increase wing lift at low speeds, reducing turbulence.
  • Remiges: In charge of generating most of the lift during flapping.
  • Coverts: Smaller feathers that give shape to the wing, favouring lift by creating the wing’s aerodynamic profile.
  • Scapulars: Feathers that cover the bird’s humeral region.

The layout of feathers on the wing is also very important, affecting its shape and function. Primary remiges are in charge of propulsion, and secondary remiges are in charge of lift.

Aircraft flight control surfaces were inspired by bird wings and the movement of their feathers. For example, in modern aircraft, the front flaps have a similar function to alula feathers.

However, in terms of thrust, the differences are even greater, as aircraft use engines, whereas birds flap. There are two distinct movements involved in flapping. When the wing comes down feathers tend to rise and come into contact with their neighbours. Thus, the air is trapped, and the bird achieves a huge thrust force. At the same time, the wing goes back slightly in a rowing motion, which increases the thrust. When the wing goes up, the feathers bend down and separate, causing air resistance to be about ten times lower. In this way, birds achieve huge energy efficiency. This is greatly influenced by the asymmetry of the two vanes.

Just as there are different types of feathers, there are also different categories of wings, for both birds and aircraft. Each type of wing is optimised for different types of flight. In birds, there are three large groups of wing shapes. Some have a short wingspan in relation to their chord. This feature is called a low aspect ratio. They have an oval shape and end in a tip. This type of wing is optimised to achieve rapid take-off and make sudden changes of direction, but is not suited to quick and sustained flight.

Another type of wing with a higher aspect ratio, characteristic of vultures and large eagles, is specialised in gliding. These have greater separation between the primary remiges, so that each behaves like a small wing with a high aspect ratio, allowing slow flight. This separation is easily modifiable by the bird – they can vary the flow of air at will, achieving great versatility of flight and manoeuvrability.

The third type of wing, typical of swallows and hawks, has a very high aspect ratio, which becomes extreme in seabirds like the albatross. They are very long and narrow and end in a tip. Vortex formation is minimal with this type of wing, which maximises energy efficiency.

This is a very simplified classification, as there is a wide spectrum of wing types according to each species’ habitat and way of life. For example, birds that live in shrubland have totally different flight needs compared to steppe birds.

In order to further improve the mechanics of flying, we should continue to study birds. We can learn so much from them.

 

 

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05/11/2018

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