Moulton Lava

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Location: New England, United States

Thursday, November 03, 2005

The Amusing Helicopter

Once upon a time there was an amusement park where you could go for a ride in a helicopter.

Oh, it was a wonderful ride, because they let you fly the helicopter yourself.

Or I should say yourselves.

You see, these were specially designed helicopters, custom-made for the amusement park.

Some were two-seaters, and some were three-seaters. There were no one-seater helicopters in this amusement park. You'll soon understand why.

It was easy to spot the two-seater helicopters, because they had two rotor blades, 180° apart. And the three-seater helicopters had three rotor blades, spaced 120° apart, just as one would expect.

What made these amusement park helicopters special was how the controls worked. The pilot sitting in a given seat only controlled the pitch angle of one of the blades. Each pilot controlled the pitch angle of a different blade. That's why the two-seater helicopters had two blades and the three-seater models had three blades.

Now if you know anything about helicopters (or airplane or ship propellers), you know that the pitch or bite angle of the blade determines how big a slice of air it grabs as it whirls around. If you 'feather' the blade, you set the pitch angle to zero, and it has no 'bite' at all. If you set the pitch angle to 90°, the blades would just churn up the air without generating any lift or propulsion. The pitch angle has to be set to some ideal value for the propeller to do its job.

In the amusement park, most of the visitors who go on the helicopter ride don't know very much about flying a chopper. They have no idea what pitch angle to set their blade to.

But what makes the ride fun is that each pilot is independently controlling just one of the two or three blades. So, unless they are coordinated, they typically have the various blades set to different pitch angles.

And as you might expect, this unbalances the helicopter and prevents it from flying very well, if it flies at all.

Mostly, the chopper just sits on the ground, bouncing spastically, shaking and shimmying, and going nowhere.

The operator of the helicopter ride never explains to the visitors how to coordinate their piloting to uniformly set all the rotor blades to the same well-chosen pitch angle. Nope nope nope. That would spoil the show.

Now usually there is one pilot who knows better than the others which pitch angle is best if you really want to get the chopper off the ground. But the other pilots don't have a clue. They set the pitch angle of the blade under their control to some arbitrary and capriciously chosen value which, in all likelihood, is unsuitable for flight.

And so the question is, what should the more knowledgeable pilot do?

Should he set his own blade to the optimal pitch angle and just keep it there, waiting for the other pilot to catch on?

Should he match his own blade to the setting of his moronic co-pilot, just to keep the chopper from flipping over on the ground? Moulton's Purple Helix

Or should he set it to something partway between those two values in an effort to coax his partner to discover the two fundamental laws of smooth ascent — that the blades have to all be balanced against each other and they all have to be set to a functionally efficient absolute pitch to achieve liftoff?

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