Whirlygigs

In the comments section to an earlier post I gave my initial guess about how a helicopter must work. The part about each rotor blade acting like a fixed wing in terms of the general Bernoulli lift principle was right. The guess about the tail rotor counteracting the spin that the main rotor otherwise would put on the body was right.  There are other ways to skin that cat, such as mounting two counterrotating main rotors.  Sometimes those are fore-and-aft, or side-by-side, but they can also be mounted on the same axle, one over the other.  They can even be side by side and so close that they must be carefully timed to avoid collision between the intermeshed blades.  Yikes.

But my intuition about how the main rotor could be made to tilt the body (to go forward/backwards/right/left) was completely wrong.  It turns out each rotor blade can tilt on its long axis, in two ways.  They can be tilted all in lockstep, which affects their general lifting power.  But what's really nifty, and what I never would have guessed, is that the machine will automatically cause each rotor to tilt individually just as it reaches the point in its cycle that's in the direction the pilot wants to go, and then flip back when it passes that point.  Now that's a fast adjustment!   I had vaguely in mind that the whole rotor system, including its axis, must be tiltable, but then on reflection that couldn't have been right.  Tilt each rotor blade one at a time as it reaches a particular point in its superfast rotation!  Very clever.  No wonder helicopters need such obsessive levels of maintenance.

And now for Bernoulli, and my indistinct memory of reading a quibble about the explanation for lift that we carry around in our heads:  the wing is shaped with a big curve on top and a much flatter one below, so when air passes it must go faster on top in order to traverse the same distance, resulting in higher pressure below the wing and lower pressure above, ergo lift.  But, you ask, who says the the air on top has to get to the back of the wing at the same time as the air below? And in fact it doesn't:  it gets there a lot faster; the air on the bottom never catches up.  Hmm.

 

The Wiki lift article claims that the Bernoulli equation itself just describes what will happen if you assume a speed differential above and below the wing, which can be observed experimentally.  In order to explain and predict the observed speed differential, you need a more involved treatment of conservation of momentum, mass, and energy, and Navier-Stokes equations that can't readily be solved, and simplifying assumptions about viscosity that allow them to be approximated for some conditions.  I'll just have to take their word for it:  experimentation tells us that if you mold a proper wing shape and use the right angle of attack at a good speed, you'll get an airspeed differential and lift.  In my brain, that gets filed under "magic."