Chapter 4: Fluid Mechanics

Critical Questions:

• How does an airplane stay up in the air?

You might think that today, more than a century after the Wright brothers flew that first airplane of theirs, most of the people who build and fly airplanes would agree on an explanation for how they stay up there. But just try walking into a room full of pilots and airplane engineers and asking them to explain this to you. The heated arguments and vicious character attacks that will result may well convince you never to step foot on an airplane again.

The reason why it’s so hard to pin down one easy explanation for the force that keeps an airplane up in the air – the ‘lift’, as it is called – is that it’s actually pretty complicated. Most simplified explanations of lift leave out some crucial details, or else they add in some incorrect ones.

The most common incorrect explanation of lift involves Bernoulli’s principle, which is interesting enough that we can spend some time getting to know about it before coming back to the problem of airplanes.

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Critical Questions:

• Why do things float?
• What is density?

I think it’s illegal to write anything about buoyancy without telling the story of Archimedes. So here goes.

Archimedes was a scholar who lived in Greece in the 200s BC. As the story goes, the king of Syracuse gave a local smith some gold and requested that it be turned into a crown. When the crown was finished, the king suspected that the smith had kept some of the gold for himself, replacing the stolen quantity with silver, which was much less valuable. Luckily for the king, there was a person in the kingdom who is recognized today as one of the greatest scientists, mathematicians, and inventors of all time. The king went to Archimedes and asked him to sort out the whole crown situation.

The crown was found to have the correct weight, but the smith could have put in just enough silver to match the weight of the missing gold. However, gold was known to be almost twice as dense as silver – in other words, one piece of gold would weigh about twice as much as an equal-sized piece of silver. So if the smith had indeed cheated, the crown would be bigger than expected – specifically, it would have a greater volume.

But nobody could figure out how to determine the volume of a shape as strange as an intricately-wrought crown. They could melt it and re-shape it into a nice cube (and just multiply length x width x height), but then the crown would be ruined.

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In 1903, at Kitty Hawk, North Carolina, Orville and Wilbur Wright flew their homemade airplane for 59 seconds, travelling in the air a distance of 260 meters. Their aircraft is generally recognized as the first heavier-than-air, powered vehicle, meaning that anything that flew before then was either a glider, a balloon, a blimp, or a bird.[1. Other possibilities include a bee, a bat, and a pterodactyl.]

Although rocket technology is a whole different field, it’s nonetheless interesting to note that it was only 66 years later – less than a human lifetime – that the first human beings walked on the surface of the moon. Somebody could have witnessed, as a child, the first airplane to ever take to the sky, and then grown up and gotten on a commercial airliner to Florida to watch Neil Armstrong and Buzz Aldrin take off in a Saturn V rocket.

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