Physics + Engineering = Flight

How does a plane fly? I thought I knew, so it was very embarrassing when this TechStuff podcast on that very question started by saying they’d first describe the wrong answer to the question:

1. The shape of the wing causes the wind above the wing to have to travel a longer distance than the wind flowing below the wing.
2. Ergo, the wind above has to flow faster than the wind below.
3. By Bernoulli’s principle, if the wind velocity is higher above the wing, the pressure must be lower above the wing.
4. In turn, the higher pressure below the wing would produce a lift force, thereby offsetting the downward pull of good old gravity.

All very logical, right? Which is why it is taught world-over. It is also what I believed. Unfortunately, it’s the wrong answer!

The podcast deconstructs why it is the wrong answer, not by physics but by simple logical thinking:

1. If the shape of the wing is the key, why does a paper plane fly?
2. If the shape of the wing is the key, how can planes fly upside down? Surely, an upside down plane would find all the forces now pulling it down to earth, right?

This was proving to be very humbling. The right reason as to why a plane flies involves both physics and engineering:

1. The shape of the wing blocks a part of the air that would have otherwise flown above the wing.
2. Now that there is less air (and hence less pressure) above the wing, the “more air” (and thus higher pressure) below the wing exerts a lift force on the wing (and the plane).

Engineering adds to this by smart design choice e.g. making the wind that flows off the tip of the wing to slant downwards thereby generating a secondary “equal and opposite” Newtonian upward force on the plane (That, by the way, is how a helicopter works). But more importantly, the wings are not parallel to the plane’s body. Instead, they are slightly slanted, aka the “angle of attack” plays a role too.

All of this explains why it took so long to make a heavier-than-air device fly. The physics was known long before, getting the engineering right took a lot of trial and error. Plus, there was the matter of how to make a plane turn. Just making it fly when you can’t steer it is, of course, pointless.

The answer to the steering question was to tilt the plane sideways, aka “banking”. But why does tilting make a plane turn? Because the air now hits the wings at different angles, thus generating different lifts on the two wings, which in turn make the plane turn. But this created a new problem: turning would cause a plane to lose speed, which in turn meant lesser lift force. And so other mechanisms had to be found to fix the problem of speed loss during turning.

Now you see why it took as long as it took before the Wright brothers got it all solved. As an interesting aside, the podcast says that the Wright brothers’ plane had the engine not at its center but slightly to one side. So the pilot had to lie a bit to the other side to compensate. But the engine weighed around 71 kg, and the pilot was only 65 kg. The difference in weights meant the plane would always tilt in one direction. And remember how tilting causes turning?

The Wright brothers solved this problem by applying one of the principles described above. They made one of the wings slightly longer than the other one. How did that solve the problem? Well, the (slightly) longer wing generated slightly more lift compensating for the tilt problem created by the difference in weights between the engine and the pilot!