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The Battle to the Skies

Author: Raja H R Bobbili, MIT

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It has been a century long struggle. The battle to fly and to move in mid-air has been of great interest to the human-race. In this magnificent battle to the sky, many accidents have occurred, many lives have been sacrificed. What causes a plane to lift itself up and fly up to the heavenly skies? Have you ever been on a plane? If so, have you ever been on a 747-400? It is arguably one of the most magnificent inventions of the man-kind. This machine, dear readers, weighs 847,000 kilograms on take off and carries with it 215,000 liters of fuel. Its engines are powerful enough and its fuel is sufficient enough to exert a thrust of 57,000 pounds each. This giant body of metallic mass can travel a maximum of 7200 kilometers without even showing signs of exhaustion. Man-kind will never be forgotten for its remarkable scientific invention of one of the most beautiful machines that a common-man has ever been exposed to: the airplane.

A few important, but not too accurate theories had been suggested by scientists regarding the mechanisms behind lift creation. Even though these theories were incorrect, they nevertheless gave us a basic understanding of lift creation. One of these not-too-accurate explanations was contributed by Isaac Newton. He derived his theory of aerodynamics from his third law of motion. Newton s third law of motion simply states that for every force, there is an opposite and equal reaction force. Hence, if the wing of a plane in mid-air is bombarded with randomly-moving air particles which exert forces upward on it, the wing will exert the same force on the particles. When this happens, it will push itself up. Thus, Newton concluded that his theory would make an aerodynamically-shaped object life itself up into the sky.

Newton s theory is not entirely wrong. Newton s explanation, that air molecules will hit the wing and push it upward, holds true only for hypersonic flights (those which travel at more than 5 times the speed of sound). It does not, however, apply to our passenger flights. The major flaw here is that air does not travel towards the wing and collide with the wing. About a century after this theory was brought forward, Leonhard Euler proved that mid-air particles approaching an object do not actually hit the body; they deflect away. Therefore, Newton s aerodynamics theory was disproved. Even if Euler s explanation was not used to disprove this theory, it can be simply stated that this theory is fallacious because it only considers the lower wing; it does not consider the upper wing. If mid-air particles exert a force on the lower wing and cause the plane to raise, then an equal number of particles above the wing should push the plane down, isn t it? Newton s aerodynamic theory was unanimously voted against.

Another explanation propped up very soon. The prerequisite here is that the wing should be shaped in such a way that the top of the wing should be curved outward (like a spoon), while the bottom of the wing should be flat. This air-wing design creates a scenario where the top of the wing is longer than the bottom of the wing. Let us assume a situation where two particles approach the wing with equal velocity. What will happen to the two particles as they approach the vertical cross section of the wing? There can be many scenarios, but let us assume that one will go over the wing and the other underneath the wing. They will both travel along the wing in the same time period and rejoin at the other end of the wing. What can we conclude from this assumption? One important point to note is that the particle traveling above the wing (along the curved surface) has to travel a greater distance than the one which travels underneath the wing (the flat plane). But they both travel the two different distances in the same time. Therefore, the one flowing above the wing should be traveling faster than the one traveling below. Bernoulli s equation, a very basic equation in fluid dynamics, suggests that when velocity of fluid flow increases, the pressure decreases. Hence, the particle traveling above the wing should exert less pressure than the one traveling below the wing. When pressure above is less than the pressure below, the pressure below will overcome the pressure above and lift the object.

This explanation, just like Newton s explanation, is not entirely incorrect. There are two aspects of this explanation which do apply to aerodynamics. The first one is that the air above the wing does actually flow faster than the air below the wing. The second correct aspect is that the air pressure below the wing is greater than the pressure above the wing, resulting into an aerodynamic lift. However, this explanation has its own flaws. One major flaw is that this explanation assumes that the two particles traveling along the wing rejoin. But this is fallacious because the two particles have no idea where the other one is located. This explanation also assumes that the wing is curved at the top and flat at the bottom. Many flights, however, have symmetrical wings. This explanation also ensures that the flights can not travel upside down (with the curved surface underneath), but it is well known that many flights do actually travel upside down.

Even though these theories might not be very correct, they gave scientists some idea of aerodynamics and these theories formed the aerodynamic theory. How exactly is lift created? Numerous aspects should be considered when talking about the lift. When air particles come toward the wing, some of them deflect upward to travel above the wing while others deflect downward to travel below the wing. The air that travels above the wing undergoes very fast changes in its motion. As soon as it reaches the upper surface of the wing, the curve on the other side of the wing ensures that this air is sucked downward. Therefore, there is very little time when air actually stays above the wing. On the other hand, the air which deflects downward to travel below the wing is made to move very slowly and is compressed. Therefore, the pressure exerted by it is very high. As it approaches the end of the wing, it regains its initial speed, rejoins the air coming from the top, and moves off. The pressure exerted upward (by the air flowing below the wing) is far greater than the pressure exerted downward (by the air flowing above the wing).

I hope that this qualitative treatment of aerodynamics has given you an understanding of the mechanisms of lift creation. Read next week for more.

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