How Do Airplanes Fly ?

Modern airplanes are truly engineering marvels. They overcome high turbulence and unpredictable currents in the air and undertaking many complex maneuvers to complete their flights. Have you ever thought of how the pilots are able to do this? Or what happens to the aircraft when the pilot operates plane controls? In this article, we will explore how an airplane flies, and how pilots are able to control an airplane in a logical yet simple way. 

First, let's have a closer look at modern aircraft's wings and tails. One amazing thing you will notice is that they are not made as a single body. The wings and tails of the aircraft contains many movable parts. The most fascinating thing about the whole wing, and the different parts of it, is that they form a very special shape in fluid mechanics. That is the airfoil shape. Just by understanding the physics behind this simple shape will allow you to completely understand airplane physics. 

Let's learn more about airfoils. An airfoil produces a lift force when moved relative to the air. This lift force makes an airplane fly. How is this lift produced? The airfoil produces a downwash (flow deflected down) as shown. This causes a pressure difference at the top and bottom of the airfoil, and hence produces lift. Generally, the higher the angle of attack, the greater will be the downwash, and therefore the lift force. A increase in airspeed also increases the lift force significantly. Interestingly, in human history's first successful flight, the Wright Brothers also made use of this same airfoil principle. Even though their airfoils was sufficient to produce a good downwash with a simple curved shape. More specifically, their airplane had two such airfoils.

Let's apply this airfoil knowledge to the airplane. If we activate the flaps and slats, it increases the downwash and increases the lift. The ailerons can move up and down, and for that reason, the lift force can decrease and increase respectively. At the tail of the aircraft, you can see two attachments rudder and the elevators. By moving the elevators, you can control the vertical force on the tail. By moving the rudder, you can control the horizontal force.

 

Now, let's get into the most interesting part of the article, controlling the aircraft using these simple wing attachments. Let's start with the takeoff part of the flight. To get the airplane to take off from the ground, what you have to do is increase the lift force using various techniques and make sure that this force is more than the gravitational pull. Pilots apply all of the three lift increase techniques together for a successful takeoff. First, the speed of the airplane is increased by increasing the thrust of the engines. When the airplane's speed is high enough, lift is increased due to the pilots activates the flaps and slats. When the airplane is ready for takeoff, they activate the elevators upwards. The tail force tilts the aircraft, and the angle of the airfoil will be increased. The lift is suddenly increased due to this, and the airplane takes-off. Usually, an angle of 15 degrees is maintained for the takeoff. 

In all these discussions, we are talking about the airplane takeoff, but, how is the engine able to produce thrust? Modern airplanes use special engines known as turbo fan engines for this purpose. 

 

In this, the fans reaction and the reaction force of the exhaust give the necessary thrust force. By burning more fuel, the pilot can achieve more thrust. The fuel is stored inside the wings of an airplane. After the takeoff, next comes the climb phase of the aircraft. As long as the engine's thrust is more than the drag, the speed of the airplane will keep on increasing. The greater the speed, the higher will be the lift force. This will cause the airplane to go up. When the airplane reaches level flight, there won't be any acceleration or change in altitude. You can see that with this condition, the thrust should be exactly equal to the drag, and the lift should be exactly equal to the weight of the airplane. 

 

Now, let's discuss the most critical part, how does an airplane changes its direction? You might think that just by moving the rudder, you would be able to do this. The rudder produces a horizontal force, and can turn the airplane with this force. However, such a instant change in direction will cause discomfort to passengers, and it's not a practical method. To make a turn as shown, we need is a centrifugal force. 

Let's see how pilots achieve this centrifugal force. Pilots need to make one aileron go up and the other aileron go down. The difference in the lift force will make the aircraft turn. In this condition, there is no vertical lift. The horizontal component of the lift can provide the necessary centrifugal force to bank the aircraft. This way, the pilot can make a turn of any radius depending upon the angle of turn and the speed of the aircraft. However, this banking technique has some drawbacks. When you keep one aileron up and the other aileron down, the drag forces induced on the wings are not the same. This will cause the airplane to yaw. This phenomenon is known as adverse yaw. The rudder has to be operated simultaneously to prevent the adverse yaw. To descend the airplane, what pilots do is decrease the engine's thrust and keep the nose of the airplane down. As the airplane loses speed, it gets ready for landing. At this stage, the flaps and slats are activated again. These devices also increase the drag. To increase the drag further, a wing attachment called a spoiler is also activated. The pilots use one more trick here to reduce the stopping distance, which is reverse thrust. 

Here, the engine covers open wide, and the air which was supposed to go backwards is forcefully directed forwards. This will obviously generate reverse thrust, and will make the stopping of the airplane easier.