Childhood Wonders

Today the question is simple but it’s something that I used to wonder a lot as a child.

How Do Airplanes Fly?

What I mean is they are so heavy and some even carry up to a hundred people (along with their luggage!), so how is it that it’s able to fly?

When an airplane is in motion, there are 4 forces that act on it:

• Thrust - Produced by the engine/propeller and makes the plane move forward
• Drag - The force of resistance of air that prevents the plane from moving forward
• Gravity - The force of attraction towards the Earth
• Lift - The special force that lifts up the airplane against the force of gravity

We don’t need to worry about the first three forces as they don’t significantly (if at all) contribute towards taking an airplane upwards or keeping it afloat. What we are interested in is the last force, the force of lift.

Lift is produced by the difference in air pressure above and below the wings of a plane. Most airplane wings have a curved upper surface and a flat lower surface, making a cross-sectional shape called an aerofoil or airfoil (for the Americans, P.S. Improve your language people). A lot of literature explains lift in the following way:
When air rushes over the curved upper surface of the wing, it has to travel further than the air that passes underneath, so it has to go faster (to cover more distance at the same time). According to Bernoulli’s law, fast-moving air is at a lower pressure than slow moving air, so the pressure above is lower than the pressure below, this creates a force of lift.

Although we arrive at the right answer, the explanation is nonsense. Why does air travelling on the upper part have to keep up with air travelling below the wing? Come to think of it, if that was the case, the lift would always be towards the curved side of the wings. Thus, no plane would be able to do acrobatics (turn upside down) without crashing (Thank God that’s not the case!). So what is the right explanation?

As the wing proceeds forward in the sky (thanks to the thrust from the engine), it splits the air at front into two different paths. The air below the wing smoothly flows downwards, while the air above it has a natural inclination to go in a straight line (in a slightly upwards direction). But the curve of the upper part of the wing pulls it around and back down. This has a stretching effect on the air - the same number of air molecules now occupy a larger space above the wing - this leads to a low pressure area being created on top of the wing. The difference in air pressure now creates a difference in air speed above and below the wing (not the other way round). As you can see in the figure, at the end of the wing, the air accelerates down at high speed.
In accordance with Newton’s third law of motion, the air pushes the plane upwards (both due to air being directed downwards and the pressure difference). Voila! we fly to the clouds.