Why pilot extends full flaps during take-off and landing?

Landing and take-off are the two flight phases where almost everyone is mesmerized by the outside view and cruising flight may bring some astonishing views. Well the full flight phase is amazing but if you travel at night or in the morning then the views are too beautiful. But wait a second did you ever notice the whirring sound of the plane before landing and it may happen that you noticed or felt that something is trying to stop the plane in mid air which is exactly the same as braking the car.

 

 

Let’s answer the question. Let me tell you that the ‘whir’ sound that you notice is of flaps deploying at the time of landing. But why pilot deploys flaps if plane have wings? If you are a avid flyer then you must know that these flaps are also deployed during take-off but at that time no one notices the sound because the noise from runway and engine absorbs this sound. Again the question is why? The answer is going a little bit technical.

 

An airplane faces various types of limitations during the entire flight time and one of the problem is that they have to maintain speed according to their geopotential altitude and this speed has a range too i.e. there is a maximum and minimum speed in which flight has to be operated. At this point I would like to clarify that if you go technically you will find that speed of the aircraft doesn’t depend on the geopotential altitude but instead of it, it depends of density of air which in turn varies according to altitude. I am not using term air density because it will become more technical. Here we are for simple language. I hope you understand the purpose to use geopotential altitude. 

 

Deployed Flaps

 

Now back to the answer. An airplane normally encounters its lowest flight velocity at the moment of take-off and landing and these two periods are most critical for the aircraft safety. At this stage I would like to introduce stalling speed. It is the slowest speed at which an airplane can fly in straight and level flight and it is always endeavor of an aeronautical engineer to make this stalling speed as small as possible. If you go for the formula for stalling velocity you will find that it is directly proportional to weight of the aircraft and inversely proportional to air density, wingspan and coefficient of lift. From this point you could conclude that only way to minimize stalling velocity is maximize coefficient of lift because all the parameter are constant in the equation, hence coefficient of lift is to be maximized. But by the nature coefficient of lift is fixed i.e. lift properties of an airfoil shape including maximum lift depends on the physics of the flow over the airfoil. As the lift properties are limited by the nature we need some extra mechanism to assist nature and this assistance is provided by the artificial high lift devices called flaps.

 

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