Of Airplanes and Stuff – Basic Physical Principles.

Hello there,

Airplanes have been an object of fascination from childhood for me, and since I’m extremely bored at the moment with quite some time to kill, I decided to write a little introduction to how airplanes work and examine the physics involved.

The behaviour of air is treated as part of fluid dynamics in Physics, and the explanations that I will write in this post can be visualized with air as a fluid. The dynamics of airplane flight can be explained in the context of four different forces or factors.

To achieve flight, airplanes must defeat gravity and their weight, and they do this through lift, to move forward and generate lift in the process, they need to overcome air resistance (called drag) and they do this through thrust.

Forces acting on a plane.

I will now go on to describe how lift and thrust are produced.

Aerodynamic Lift.

Lift is generated by the way aircraft wings interact with air. Before moving on, please do remember that aircraft wings can generate lift only when moving forward (i.e, there is air flowing over and under the wing at high velocity). Aircraft wings, which we shall call airfoils from now, have a flat surface on the bottom and a curved surface on the top. The way these surfaces interact with the air of the atmosphere is governed by Bernoulli’s principle, air flowing over the wing does so faster, and does so with increased kinetic energy, which in turn creates a low pressure area above the wing, enabling the relatively higher-pressure air under the wing to lift it into the low pressure area above.

Lift and Bernoulli's principle

The reason that the speed of air flowing over the curved surface is greater is the increased distance air has to flow over to cross the wing.

So that does it for lift.

Thrust

Thrust is required to propel airplanes forward such that their wings are able to generate the required lift to keep them airborne, thrust is produced either through propellers or through jet engines, the former relying on manipulating airflow and pressure differentials and the latter relying on Newton’s third law of motion.

Propellers and airflow.

The blades of a propeller are like tiny wings, but they are oriented at angles that make forward movement, as opposed to upwards movement, possible.

A schematic diagram of a generalized jet engine.

Of Pitch, Roll and Yaw.

Now lift and thrust per se are not sufficient to account for everything that airplanes need to do in flight, including moving from side to side, up and down, and left to right. Since the flight of airplanes is three dimensional, we need three terms to describe these movements.

Pitch is when the airplane is pointed up or down, roll is when it rolls to the left or right (obviously) and yaw is when it is re-oriented to the left or right. See the image below if you have trouble grasping it.

Pitch, Roll and Yaw

These movements are executed by utilizing airflow and Bernoulli’s theorem again, through the use of distinctive control surfaces that are found on airplane wings, stabilizers and tails. The surfaces that control rolling are called ailerons and are found on the wings in most conventional aircraft designs. The surfaces that control pitch are called elevators and are found on the tailplane or in the form of canards in front of the wing.
The surface(s) that control yaw is/are called the rudder and these too are found on the tailplane.

I will blog about the nuances and the intricacies of aircraft parts and systems and control surfaces and aerodynamics and so on in the future, but this is it for this post. Thanks for reading.

– Ankur.

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