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Why don’t aeroplanes fall to the ground because of gravity?

Why don’t aeroplanes fall to the ground because of gravity?
Issy from Greater London and Nitisha Muddaloor from Hertfordshire (age 5-14)


This is due to the shape of the wings. As the plane moves forward air flows over and under the wings. These are designed to let the air pass more easily over the upper surface than under the underneath. Hence a build up of excess air occurs underneath the wing compared to above it. An excess pressure under the wing results which causes a resultant upward force (= excess pressure X wing area) and when this reaches the value of the weight of the aircraft (acting down), (i.e. at the take off speed) the plane takes off and flies. The shape of the wings can be altered by opening and closing flaps and so the height and speed of the plane can be adjusted so that it can climb, descend or fly at constant height. Some people are led to wonder, if it is as simple as that, how come aircraft can fly upside down?

John Kilcoyne, Scientist behind Brainiac LIVE


5 Responses

  1. An aircraft flies because of the reduction in pressure above the wing, not the increase below it. The curved upper surface of the wing section causes a reduction in pressure, as the air has further to go in the same time as air passing underneath. In effect, it is half a Venturi. An aircraft can fly inverted by adjusting the angle of attack of the wing until lift occurs. This is very inefficient, requiring a lot more thrust to overcome the resultant drag.
    (Flight Engineer with 10,000hrs flying experience)

  2. Malcolm Granville’s answer is correct. As the air accelerates over the curved upper surface of the wing, its kinetic energy increases, kinetic energy is proportional to mass times the square of the speed and the speed has increased. The total energy of the air doesn’t change, the thermal energy of the air reduces a little which is why aeroplane wings can ice up in cold weather, but the big drop in energy equivalent to the rise in kinetic energy is in pressure energy. The reduction in air pressure times the area of the wing is the lift generated.

  3. The simple answer to your questions is that the wings of the aircraft produce LIFT. While gravity is trying to the pull the aircraft down the lift is an equal and opposite force trying to push it up.

    When an aircraft is flying straight and level the lift force equals gravity. To climb, the lift force must be greater than gravity.

    That’s the simple answer. To know how the lift is produced read the answers given by Malcolm Granville and David Maw Cornish.

  4. 1.The ‘Bernoulli makes airplanes fly’ explanation was taught to me as an undergraduate, and I’m sorry to say that I only questioned it when I hit 30. According to Mr Bernoulli, air travels faster over the top of the wing that it does over the bottom. This creates a pressure difference and lift. However, this depends on assuming that air travels over the top and bottom of the wing in the same time, and this is not true. There is simply no reason at all why the air should do this, and in any event, the wing would either need to look like a humpbacked whale, or be moving very fast indeed. Aircraft wings produce lift by creating a down wash of air. When fluids flow over any surface, they stick to it more that we might expect (the Coanda effect) so that the air flow has to travel over the wing surface. This allows wing designers to shape wings such that air flowing over the top of the wing sticks ti it and leaves the back of the wing in a net downward direction. This plus a bit of tilt (‘attack’) to generate lift from the bottom of the wing (and, OK, some Bernouli effect too) gives an upward force on the aircraft- lift.
    2.For a very good explanation of all this stuff, try ‘How Airplanes Fly’, Scott Eberhardt, Sport Aviation, February 1999. And don’t look in any physics/engineering books cause they’re still blaming Bernoulli!

  5. As you can tell from the replies so far there is some dispute as to how aeroplanes fly. Many people still favour something called the Bernoulli effect named after Daniel Bernoulli who did experiments about air pressure. However I favour something called theCoanda Effect named after Henri Coanda. You might have noticed the Coanda Effect when you pour something from a rounded jug and the liquid follows the shape if the jug and drips on the floor! Fluids such as water or air follow the shape of a surface when they flow close to the surface so when the air flows over the curved surface of a wing it follows that shape. The useful thing is that the air carries on curving after it leaves the wing and moves down. This is a downward action and the reaction is that the wing lifts. As Isaac Newton explained in one of his laws of motion; to every action there is an equal and opposite reaction. You might be interested to know that Henri Coanda designed and flew a type of jet propelled aeroplane in 1910. He called his engine the Air Reactive Engine. It was when he was testing his ideas that the Coanda Effect was noticed.
    Peter Halford. Science and Technology Teacher. Imperial War Museum. Duxford.

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