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Before we see how these amazing principles of physics are applied, we have to understand what they are first.

Aircraft fly based on the principles of a branch of physics called aerodynamics.  

All things have forces acting on them. Whether you are moving or just standing still, there are always forces acting on you. It is no different with aircraft. In flight, there are altogether FOUR forces acting on the aircraft. Up, down forward and backward. Only they are not known as that. They are known as:

1. Lift (UP)
2. Weight (DOWN)
3. Drag (BACKWARD)
4. Thrust (FORWARD)

LIFT

Lift is basically an upward force created when air flows over the wings of an aircraft. The greater the lift, the higher the aircraft climbs. Simple? Simple. Please be informed that this is a very, very simplified view of lift in aerodynamics.

Bernoulli's Equation

You might ask: How is lift created? It has to do with  the Bernoulli principle, which states that as the velocity of a fluid increases, its internal pressure decreases. Surprisingly, air is considered to be a fluid here.

Thus, a flow that is travelling faster will have a smaller pressure, and airplane wings are shaped to make use of that.The curved part on top of the wing accelerates the air flowing over it, resulting in lower pressure on top than underneath. Thus, the relatively higher pressure below will cause an upward force we call lift.

However, not all lift is created in this way. A small fraction of the lift is created by Newton's famous Third Law of Motion, which states that "every action has an equal and opposing reaction". Thus, the air that is deflected downwards by the wing produces an upward reaction that lifts the airplane.

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Weight is simply the force of gravity that is acting on the airplane.  The weight of an airplane changes with the amount of load it carries, even fuel.  For the airplane to lift off from the ground, the lift created by the wings has to be larger than its weight. 

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Drag is the force that limits aircraft speed.  It consists of four parts:

Form Drag
Skin Friction Drag
Inteference Drag
Induced Drag

Form Drag is created by the plane's structure.  It increases when more of the plane is blocking the wind that is rushing past it.  That's why planes have to have streamlined bodies to 'block' as little air as possible.

Skin friction Drag is due to the surface of the airplane body.   The smoother the surface, the less drag.

Inteference Drag is caused when the air currents across the airplane body meet and interact, causing some resistance.

Induced Drag is the by-product of lift, and increases with more lift.

The total drag that results from all these forms of drag form the total drag that holds the airplane back.

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Thrust is the force that is needed to overcome drag.  It is produced by the engines of the plane, and it is needed for the plane to move and fly.   The more drag on the plane, the more thrust required.

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Now that you have understood the basic concepts that make an airplane fly, take some time to understand how these planes fly and serve their purpose.

B-2 Spirit Bomber
F-15 Eagle

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