Flying: The feat we thought impossible

Jan K. Huertas de la Cruz

The most beautiful thing of physics is that no matter how complex or simple things may look, the reason of their happening can always be explained with physics. That is why the field of physics is always around us, making cars run all by themselves, explaining why when you take a shower the water goes straight down towards you instead of just floating away, why when we throw a ball it has to stop at some time, to put it in the simplest most understandable way, physics is everywhere.  But there is one thing that in older times was thought to be impossible, but in our era is one of the most important and safest ways to travel, flying.

The physics and engineering principles involved in making airplanes fly are very complex and hard to explain without using mathematics to understand it, but the idea to how airplanes fly can be explain in a nutshell. For any airplane to fly there must be four essential aerodynamic forces that must act on the plane, these are: lift, thrust, weight and drag. To better understand these forces we can draw a free body diagram to see how these act on the plane.

In this free body diagram A is the lift force, B is the thrust force, C is the weight force and last but not least D is the drag.

Now, if we want the airplane to fly straight, leveled and in a constant speed, the thrust force must be equal to the drag force and the lift force must be equal to the weight force. But everyone knows that first we need to take off the ground and then we need to land, so these equalities won’t match always. 

In the aerodynamic forces the weight force is the simplest to understand, so let’s start there. Since all objects have mass, every mass have a weight, and this weight produces a downward force. The thrust force is an aerodynamic force that the plane must create in order to overcome the drag force (which I will explain later), this thrust force will make the plane go forward, it will make the airplane accelerate, and each airplane has a different method to produce thrust, some of these methods are propellers, jet engine and even rockets. Now, the drag force is the equivalent to the normal force in one dimensional motion, it just has some different aspects. The drag force is the force which resist the motion of an object that moves in a fluid (air and water are both fluids); to better understand it we can use a simple example. When someone sticks their arm outside the window of a moving car they must have noticed that the air pushes their hand in the opposite direction the car is moving, that is drag, the more mass and weight the airplane has the more drag it produces.

Lastly we have the lift force, unfortunately this is the trickiest and most debated force of the four dynamic forces. There are different explanations to how the lift force acts on the plane, but I will use the simplest one, the “Longest Path Explanation”. This theory states that the top surface of a wing is more curved than the lower part of the wing, and when air approaches the wing the air that goes over the wing and under the wing takes the same time to cross it, and since the top surface is curved it must travel more distance and in order for both winds to cross the wing at the same time the wind at the top moves faster than the wind at the bottom. Here is a little diagram to better explain it.

Now, since the air that goes over the wing moves faster this causes the pressure to decrease according to Bernoulli's equation, and since the lower wind goes slower this causes a more normal pressure. This difference in pressures makes the wing to move up, in other words the low pressure at the top of the wing acts as a vacuum, making the wing move upward.

These are the four essential aerodynamic forces that must act on the plane in order to fly. The physicists and engineers that have put a lot of work and time in the research of this extraordinary phenomenon are the reason for this achievement of our modern era. I hope that for our future our generation can develop better ideas that can amaze everyone’s eyes. 

References:

• Giancoli, Douglas C. Physics for Scientists & Engineers with Modern Physics. 4th ed. Vol. 1. Upper Saddle River, N.J: Prentice Hall, 2009. Print.

• "Aeronautics - What Makes An Airplane Fly - Level 1." Aeronautics Learning Laboratory for Science Technology, and Research (ALLSTAR)Network. Web. 27 Apr. 2011