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What are the forces that make an airplane fly?

What are the forces that make an airplane fly? - By Ahmed Haouaria

21 Avr 2020
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‘’To invent the airplane is nothing. To build one is something. But to fly is everything’’.Otto Lilienthal, German aviation pioneer.1848-1896.

From bird to plane: Inspiration and Design.

I am sure you have already asked yourself these questions:

How does a plane hold up?

How do you get planes to fly? 

To build an airplane as it is done today, the man had to work his imagination and was inspired by the bird to design it.Like birds, planes have a pair of wings, but unlike birds, these wings are stationary.Leonardo da Vinci, one of the main innovators and great visionary of the 15th century, was one of the first to draw inspiration from birds to produce architectural plans for a flying machine named ORNITHO PTERE "(from the Greek:" ornithos " which means "bird", and "pteron" for "wings").The bird and the plane, therefore, have a similar internal and external configuration, which reinforces the observation that man, in the design of the plane, is strongly inspired by birds.But The birth of the first plane did not take place until 400 years later, on October 9, 1890, where Clément Ader succeeded in flying for the first time.The French engineer took off from the ground aboard his aircraft capable of rising and circulating in the air: the Eole aircraft, the primitive model of what became the plane.A few years later, he created another aircraft, which became universal in the world of aviation, the Plane, meaning according to Ader: Flying Device Imitating the Natural Bird.According to Ader, the term  ‘’avion’’ "plane" was also declined from the Latin « avis» meaning "bird". Subsequently, many new planes were created, as in 1908 when the "Wright Flyer" was built by the Wright brothers, but also, the "LE BLERIOT manufactured by Louis Blériot who was the first plane to cross the Channel, on July 25, 1909.

 

The 4 forces acting on an airplane in flight

Now let's come to the question of how a plane can hold up?

The Lift

Every object has a mass that, along with Earth's gravity influences its weight.Let's talk about the lift. This vertical force which allows the plane to hold in the airThe wings of an airplane are essential because although airplanes do not flap their wings, they play a major role in flight.Picture a plane in mid-flight. The penetrated air flows horizontally along the plane and therefore its wings.But the upper surface of the wings, will, of course, disrupt this flow of air, creating a strong depression which itself will create a vertical force: lift.And it is this force which will compensate for the significant weight of the plane and always keep it up.But lift alone cannot explain why planes stay in the air, a vertical force is not enough, you also need a horizontal one.So, contrary to popular belief, it is not the engine that makes an airplane fly, but its wings, which generate lift.In aerodynamics, the lift is directed perpendicular to the axis of any flying device and is balanced with the weight of the plane, which allows it to stay in the air.Once we have explained the lift we are going to focus on the 3 other forces acting on the plane.

Lift: the relative wind created by the engine and the headwind passes on the upper surface of the wing. This results in a depression on the upper surface and overpressure on the lower surface by virtue of the Bernoulli principle.The addition of the overpressure and the depression then gives the lift which sucks the wing and therefore the plane upwards.

Bernoulli's principle summary :

Daniel Bernoulli is a Swiss doctor, physicist, and mathematician,Bernoulli's principle relates the pressure of a fluid to its elevation and its speed. Bernoulli's equation can be used to approximate these parameters in water, air or any fluid that has very low viscosity.

A fluid: Anybody that follows the shape of its container (liquids, gases) opposite to solid. It is characterized by:

  1. Instability
  2. Movement
  3. FluctuationAir is a compressible fluid, that is to say, that its density varies. It is able to flow and change shape.

 

The Weight

The Earth attracts to it all the objects which surround it, therefore, all the objects which are dropped above the ground, are attracted by the center of the Earth. It exerts weight on them, a force distributed at a distance from the object. It is also called the force of gravity. The weight of an object is, therefore, the force exerted by the Earth on this object. It acts in a downward directiontoward the center of the Earth.

Newton has shown that "all bodies attract each other, directly because of their mass and inversely because of the square of their distances".

Weight characteristics:

  • direction: vertical (direction of a plumb line)
  • direction: down
  • point of application: this is the center of gravity of the object
  • intensity: you can measure it with a dynamometer and calculate it using a formula

Like any force, we will represent the weight with a vector. On the other hand, weight is a force and as g depends on the altitude, the intensity of the weight depends on the place! The product of its mass by the acceleration of gravity represents a force due to the Earth's attraction. The greater the weight, the greater the lift. It is, therefore, necessary to deflect larger air masses downwards, which is possible with larger wings. Given the incidence of the wings, they inevitably produce greater drag so that ultimately the pulling force must be more intense. More powerful reactors are then necessary for consuming more fuel.

The Thrust

Any object equipped with a motor or other means of propulsion or traction/thrust, is "propelled" in a certain direction.

So, in the same way that the lift and the weight that we have just seen are two forces which balance each other, the traction or the thrust of the engine balances another force: the drag which is directed the opposite side. The thrust is exerted by the reactors which push the air backward. Thus, by the law of action and reaction, the air pushes the plane forward. Therefore, the thrust provided by aircraft engines is used to propel it into the air. The push is opposed to the drag; the two forces balance during a stabilized flight. At the same altitude, if the thrust increases, it momentarily exceeds the drag and the aircraft picks up speed. But the drag also increases in this case and catches up with the thrust after a certain time. The plane then stops accelerating and resumes a stabilized flight, at a higher but constant relative speed.

Thrust is also the most important factor in determining the upward capabilities of an aircraft. In fact, the maximum climb speed is not linked to the lift generated by the wings, but to the amount of energy supplied in addition to that necessary to maintain level flight.

The Drag

Any body in motion must overcome the resistance of the air. The larger or faster it is, the greater the force opposing it. The total drag of an aircraft is actually the addition of 3 drag of different origins: The drag can be a so-called “friction” or “parasitic” drag, caused by the friction of the air on the entire aircraft, its structure: landing gear, masts, antennas, etc. It increases by the square of the speed of the plane. If the speed doubles, the parasitic drag quadruples. The induced drag because it is a direct consequence of the Prandtl vortex (from the name of the German aerodynamicist who first described that the asymmetry of the airflow around the wing is equivalent to a whirlwind and that it is, therefore "induced", curiously, by the lift. Indeed, each time there is a lift, there is necessarily a whirlwind around the profile.

key points to remember

In the end, we, obtain a diagram of four forces which compensate each other Two to Two.The lift compensates for the weight and allows the plane to fly. It is created by the relative wind, which is to say the flow of air on the surface of the wing. The wing profile is therefore very important. The weight results from universal gravitation. It is represented by a vector applying to the center of the inertia of the plane, directed towards the center of the earth along the vertical of the place and whose value varies according to the mass of the plane.

Thrust is provided by the powertrain (engine and propeller). Its intensity varies according to the power delivered by the engine. It is represented by a vector directed in the direction of movement. Drag is an aerodynamic component parallel to the air streams of the relative wind. It is the force opposed to the movement of the plane and which results from its resistance to air.

The Physics of an airplane in flight can be summarized by the balance of the 4 opposable forces. The lift opposes the weight and the thrust opposes the drag. It is assumed that the four forces in flight are exerted at a single point called the center of gravity.

 

 

 

 

 

 

 

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