Principles of Flight - Why can airplanes fly?

This is perhaps the most fundamental question that any aviation enthusiast needs to understand. 

When you try to ask a 3-year-old why an airplane can fly, he/she may probably said "Because it has wingsssss!!!" Well...that's correct...partially. If anything has wings can fly, why not chicken? why not ostrich? why not penguin?  

In fact, a better answer could be "An airplane can fly because it has wings that generate sufficient LIFT (升力)." So what is lift? What creates lift on a wing? There are two ways to explain it.

(1)

Mathematical-wise, we have the Lift Equation (below) which lists out all the factors contributing to lift creation.

p = air pressure

V = air velocity

S = wing surface area

CL = coefficient of lift (includes factors like angle of attack, wing shape, etc)

From the above equation, we know the relationship between Lift and other factors :

• The higher the air pressure, the greater the lift
• The faster the air velocity, the greater the lift
• The larger the wing surface area, the greater the lift
• The greater the number of the lift coefficient, the greater the lift

Here I don't want to go into details (maybe later). Otherwise, it will become an academic paper and bore everyone. Yet it's definitely vital as a pilot to remember this equation.

(2)

Theoretical-wise, since wing is the major part that creates lift, we can take a look at the cross section of it to see what happen when air is passing through it.

When I first learned about this, I used to imagine the air particles as two runners :p They split apart at the leading edge of the wing, one travels above the wing while another one travels beneath it. These two "little runners" have to arrive at the "finishing line" (here refers to the trailing edge of the wing) at the same time. 

As a result, the air particle above the wing, which has a longer distance to travel, must "run" faster (i.e. faster flow) than the air particle below. 

As air flows faster at the top, it exerts a lower pressure. While as air flows slower on the bottom, it exerts a higher pressure. The difference in pressure above and below the wing create a force called Lift pushing the airplane upward. This concept is known as the Bernoulli's Principle.

On the other hand, when air particles hit the bottom part of the wing, they are deflected downward. According to Newton's Third Law of Motion: To every action there is an equal and opposite reaction. The downward defected air particles exert an action force which leads to an equal an opposite upward force as the reaction. This force also pushes the airplane up.