How Airplanes Fly?

How do Airplanes Fly?

Every aviation enthusiast’s first question probably was how airplanes fly? So in this blog I’m writing about the physics and science behind how airplanes fly.

First, let’s see forces that can make the airplane stay in air.

1.      We need a force that will pull the aircraft so it moves forward. Let’s call this force ‘Thrust’

2.      We need a force that will balance the weight of aircraft so that it takes off from ground, stays in air and don’t fall back to earth surface. Let’s call this force ‘Lift’

Thrust, as we know is provided by the engines, but how is lift generated?

Yes, the answer is WINGS. But how do wings generate lift?

Fig 1: A simple wing of airplane

If we look at the wing as shown in Fig 1, when looked from a side the profile looks as the figure 2. This is a special geometry called ‘airfoil’

Fig 2: Cross section of wing: Airfoil

So, what’s so special about the airfoil?

The airfoil as seen in Fig 2 has the length at lower portion lesser than that of upper portion. What it does? The air entering the starting of airfoil must leave the airfoil at the same time because of conservation of mass. So, as the lower length of airfoil is small the air travels slower in lower portion and faster in higher portion so as to meet at the same time at the end of the airfoil.

Now we know that the air velocity is high above the airfoil and low below the airfoil. So according to Bernoulli’s equation given below:

P+¹/₂ ρv²+ρgh=Constant

Where,Pressure P is inversely proportional to Velocity V. ρ is density, g is acceleration due to gravity and h is height.

So, as we know the velocity is low below the airfoil, the pressure is high below airfoil.

If we take for the whole wing, the pressure below wing is higher than the pressure above, the difference in pressure will cause a force that will lift the airplane above and make it stay in the air.

The pressure from Bernoulli equation is given by:

P=¹/₂ ρv²

The lift force is Pressure X Area:

F= ¹/₂ ρv2s

Where s is surface area of wing. (Planform area)

If C_L is a non dimensional quantity that relates to the lift generated by wing lift can be given as:

L= ¹/₂ ρv²S_wC_L

Hence, we seen that for airplane to fly, the thrust generated by the aircraft pulls the aircraft front, and the force generated by the wings due to pressure difference keeps the airplane airborne.