9 Dec 2016

Bernoulli Is Not Enough

By Simon Shapiro

Probably all of us science geeks think we know how aeroplanes fly. It’s thanks to the Bernoulli Principle, which says that faster flowing air exerts less pressure than slower air. Aeroplane wings are designed with flat bottoms and rounded tops. Air has to flow more quickly around the longer top surface than the shorter bottom surface. That gives us higher pressure below the wing and lower pressure above the wing.

This diagram is from my book, Faster, Higher, Smarter: Bright Ideas that Transformed Sports. (More information here; available from Amazon, Indigo, etc.)

So, the upward Bernoulli force overcomes the downward force of gravity, and the aeroplane can fly.

But there's a problem:

Think about a plane doing aerobatics. A pilot will happily flip her plane upside down and can fly that way indefinitely (at least until the plane runs out of fuel). But in the upside down configuration the Bernoulli force is directed downwards, just like gravity. So with both forces acting downward, how come the aeroplane doesn’t fall like a stone? Worse than that, like a stone with an extra downward force?

The answer is that there’s another factor, usually overlooked. And that’s the Angle of Attack. Aeroplanes don’t usually fly with the bottom of the wing parallel to the ground. They usually fly with the leading edge tilted up. That tilt angle is called the Angle of Attack.

In this configuration the wing pushes against the air and (thank you, Sir Isaac) experiences a normal (perpendicular to the surface) reaction  of wind resistance. That’s shown as the red arrow in the diagram above. The force shown by the red arrow is the same as two forces – a vertical one (shown by the green arrow) and a horizontal one (shown by the tiny blue arrow). That vertical “green arrow” force counteracts gravity, and is what keeps an upside-down aeroplane in the air.

In fact the Bernoulli effect alone isn’t strong enough to keep a heavy modern jet plane up. The Angle of Attack is critical even when the plane is flying rightside up.

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