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One common, but slightly incorrect explanation for flight involves Bernoulli's Law. (Source: Getty)

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How on Earth do planes fly?

Tuesday 17th November 2015 11:51 am

Seeing a plane get off the ground is an amazing sight! Dr Karl investigates the science of flying planes.

I always get amazed when I see that giant double-decker plane, the Airbus-380. I wonder to myself — how on Earth do they get that off the ground. It weighs nearly 600 tonnes, as much as a small ship — and yet it flies.

How does it fly? Well, the answer is kind of “complicated”, but it’s related to Newton’s famous Third Law of Motion — “For every action, there is an opposite but equal reaction”.

An important fact to realise is that air has weight. This was first discovered in 1640, by the physicist, Evangelista Torricelli. That’s right, air has weight, even if it is invisible. Down at sea-level, a cubic metre of air weighs about 1.25 kilograms.

This applies to our 600-tonne passenger plane cruising at 900 kilometres per hour at some 40,000 feet.

First, the plane is moving forwards, because it’s pushing air backwards from its four jet engines. These forces are in balance, because it’s cruising at a constant velocity of 900 kph.

Second, the plane is staying up at a constant 40,000 feet, because it’s pushing air down.

Think of a tiny plane, such as the single-engined, four-seater Cessna 172. It weighs just over a tonne. When it’s flying at 220 kilometres per hour, its wings are pushing vertically downwards some five tonnes of air each second.

That brings us back to Newton’s Third Law, “For every action, there is an equal but opposite reaction”.

One common (but slightly incorrect) explanation for flight involves Bernoulli’s Law. This explanation claims that the air is split when it is hit by the front of the wing, and then meets up again at the back of the wing. It also says that since the wing is kind of flattish on its bottom surface, and curved on its upper surface, the distance that the air has to travel is slightly greater on the top surface. Following on from this, because the air on top has to travel further to meet up again at the back of the wing, it has to travel faster. Sounds reasonable.

And then Bernoulli’s Law tells us that if a packet or streamline of moving air travels faster, then the pressure drops. So because we have lower pressure on the top surface of the wing, and higher pressure underneath, the wing rises upward, carrying the plane with it.

There are a few problems with this explanation.

First, how does the air “know” that it has to meet up again at the back of the wing?

Second, tests in a wind tunnel show us that it doesn’t meet up. In fact, the air on top of the wing travels much faster, and gets to the back of the wing first.

Third, this explanation totally ignores the viscosity (or resistance to flow) of the air. But you can get around this by applying some fancy physics called the Kutta-Joukowski condition.

And fourth, and this one is a doozy, if Bernoulli is the only explanation, how does a plane fly upside down?

It turns out that Bernoulli is a minor part of the explanation of flight, but the major part is angle of attack.

Imagine you’re in a swimming pool, and you’re moving a table-tennis paddle horizontally through the water — edge first. The paddle moves horizontally. Now tilt the paddle upwards a little at the front. As you move it through the water, the paddle tries to rise towards the surface. Yup, Newton’s Third Law again.

So on large long-haul passenger jets, the wings are stuck onto the long body (or fuselage) at an angle of about 5-10 degrees, tilted upwards at the front. This means that while the wings are angled upwards, the body is flying straight through the air, like a horizontal pencil.

The big advantage of the horizontal body is that the plane gets maximum fuel economy, because the wind resistance is less. And another advantage is that when the flight attendants push those heavy food trolleys along the aisles, they don’t have to push them uphill …


This blog first appeared on Dr Karl's Great Moments in Science

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