Time to get out of the water

Water isn’t the only fluid that’s in everyday experience. Air is as well – the word can apply to either a liquid or a gas. And things moving in air can behave pretty oddly too. I think the best example is an aerofoil – that relies the fact that a fluid’s pressure reduces as it moves more quickly (Bernoulli’s equation). The top surface of the aerofoil is curved outwards, so the air has further to go, and speeds up a bit as it travels – that means less pressure on the top of the wing and the aircraft rises.

Of course, areofoils were around a long time before the Wright brothers got their hands on them; anyone who has sailed a yacht will tell you that. The sail does the same thing. Being able to sail upwind (or about 45 degrees to it) is a concept that I find rather perplexing, despite having a pretty decent physics training and having sailed dinghys a fair bit. Knowledge of neither the theory nor the practice is quite good enough to get over the impression that travelling upwind just doesn’t make sense. Taking the wind’s energy, and turning it around to move against the wind, it’s just utterly un-natural.

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