Sergat: Specialist Communications Agency for Tourism

foto plane

Why does an airplane fly, even if it weighs 500 tons?

Today, Insights interviews Marc, Sergat's senior expert in communication for the aviation industries, an aviation geek and passionate about everything that flies and in his free time a private pilot with a JAR (European Joint Aviation Rules) license.

Insights: We want to know why the “giants of the air”, such as the A380s or B777s, weighing hundreds of tons, can rise from the ground and soar through the skies.

Marc: These “giants of the air” fly for exactly the same reason as a small plane or even an ultra-light aircraft, although it is true that every time we observe a wide-body takeoff it seems a miracle. So heavy and yet it flies. Even as a child, when I first saw a B747 Jumbo Jet I was fascinated by this phenomenon, although what seems like a miracle is not really so, since it is pure physics and has to do with the speed and density of the air over the wings.

Insights: Sounds complicated. Do you have to be an expert in physics to understand this or is it an easier concept?

Marc: I'll try to explain it in an understandable way. The plane, when it starts the take off roll, is powered by its engine or engines and picks up more and more speed. The plane is dashing on the runway, which increases the speed of the air above its wings while decreasing the density of this airflow above the wing because this airflow above the wing is less dense than the airflow below the wing. The wing is curved at the upper edge, the path for the air particles above the wing is longer than the one under the wing, so as the speed of this airflow increases the density decreases. And there comes a moment when the speed reached by the aircraft -what in aviation we call rotation speed and which is naturally different for each type of aircraft- allows the aircraft to overcome the force of gravity with the lift that is created above the wing and rises. In this context it is also important to point out that there are basically four forces acting on the aircraft in two groups: thrust/advancement vs. resistance, lift vs. gravity.

Insights: Is it supposed that if there is a wind that pushes the plane from behind this helps to take off?

Marc: Not at all, quite the contrary. If the wind comes from behind, it decreases the air speed over the wing. And this is the reason why -with few exceptions- you want to take off into the wind because the headwind helps you to increase the speed over the wing which favors the shortening of the take off roll and you get to the rotation speed earlier. On the contrary, on approach and landing the headwind helps to slow down the plane. But not only the wind influences the take off, but also other factors like for example the outside temperature or the elevation of the airport. You don't need the same runway distance with a plane with the same weight taking off in August in Madrid with a field elevation of about 600 meters as in January in Barcelona with a field elevation almost at sea level. The reason, again, is the density of the air.

Insights: One last question that always intrigues us: why at an airport with only one or a couple of runways can they be called “Runway 36”, for example?

Marc: The numbering of runways (RWY) has nothing to do with the number of runways at a given airport, but refers to the direction of the runway as indicated by a 360-degree compass and according to the magnetic north. In the numbering of  RWY the 0 is removed from the end, so in the example you mention the direction in which the RWY points would be the north.

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