How do planes Fly?

 

As stated it Sir Cayley to maintain a plane in flight, three elements must be assured: the lift of the plane, its propulsion and finally its stability.
Two of these forces are generated by the relative movement of the air compared to the plane. The first one is the lift. This force is directed upwards and is acting perpendicular to the displacement of the plane. It is thanks to this force that the plane is maintained in the air. The second is the drag. It is exerted in the direction opposed to the displacement of the plane. It is due to the breaking action of the air on the plane and is opposed to the advance of the plane. The lift and the drag are called aerodynamic forces because they are resulting from the action of the air due to the displacement of the airplane.
The force due to gravity, the weight of the plane, is opposed to the  
lift. The balance of the lift and the weight leads to the fact that the plane is maintained at constant altitude. To ensure that the plane continues to move forward, it is necessary to provide a force that compensates for the force called drag. This force is called the thrust. The thrust is generated by the system of propulsion of the planes, the engines. In the case of the flight at cruising speed, the role of the engine is thus to compensate for the force of drag, but not to make the plane climb. On the other hand, at the time of takeoff, the engine power will be used to bring the plane to the altitude of flight.
And what about sailplanes which do not have any engines? The lift of the sailplane is generally initiated by a plane which tows it until a certain altitude. Then, the sailplane is released and starts its autonomous flight. As it does not have any engine, it must follow a downward trajectory relative to the air to keep its speed. To avoid approaching the ground too quickly, it must find zones where the air follows an ascending direction. That enables him to continue its flight.

Weight and lift

We all know that gravity is a force that pulls everything towards the Earth’s surface. This pull is called the weight force.
Planes and birds have to be able to provide enough lift force to oppose the weight force. Lift is a force that acts upwards against weight and is caused by the air moving over and under the wings.

Thrust and drag

The power source of a bird or plane provides the thrust. Thrust is the force that moves the object forward. Thrust is provided by:

• muscles – for birds and other flying animals
• engines – for flying machines
• gravity – for gliders that actually fly by always diving at a very shallow angle (birds do this too when they glide).

The force working against thrust is called drag. It is caused by air resistance and acts in the opposite direction to the motion. The amount of drag depends on the shape of the object, the density of the air and the speed of the object. Thrust can overcome or counteract the force of drag.

How it works

An object in flight is constantly engaging in a tug of war between the opposing forces of lift, weight (gravity), thrust and drag. Flight depends on these forces – whether the lift force is greater than the weight force and whether thrust is greater than drag (friction) forces.
Lift and drag are considered aerodynamic forces because they exist due to the movement of an object (such as a plane) through the air. The weight pulls down on the plane opposing the lift created by air flowing over the wing. Thrust is generated by the propeller (engine) and opposes drag caused by air resistance. During take-off, thrust must counteract drag and lift must counteract the weight before the plane can become airborne.
If a plane or bird flies straight at a constant speed:

• lift force upwards = weight force downwards (so the plane/bird stays at a constant height)
• thrust force forwards = opposing force of drag (so the plane/bird stays at a constant speed).

If the forces are not equal or balanced, the object will speed up, slow down or change direction towards the greatest force.
For example, if a plane’s engine produces more thrust, it will accelerate. The acceleration increases air speed past the wing, which increases lift so the plane gains altitude. Then, because the plane is moving faster, drag (air resistance) is increased, which slows the plane from speeding up as quickly until thrust and drag are equal again. The plane can now remain at a constant but greater height.
A plane can lose altitude by reducing thrust. Drag becomes greater than thrust and the plane slows down. This reduces lift and the plane descends.