Spinning Into Flight
Imagine the rotors are made of simple flat plates of metal. That’s not the case, of course, because as we’ve
seen, the rotor blades have a special airfoil shape. But to understand what forces are starting to take effect in the rotor disc as the blades spin faster and the helicopter prepares for flight, it helps to imagine the rotor blades as flat.
By the Book
The collective pitch control is
a lever located on the left ride of the pilotT seat It resembles a parking brake hand lever, but is more beefed up. Moving it up and down changes the amount of twist, or "pitch,* of the rotor blades.
When the helicopter is on the ground and the pilot doesn’t want it to take off yet, he positions the collective pitch control so that the blades are perfectly horizontal. It’s as if you flattened the blades of a room fan so that the fan created no breeze.
Now let’s say the rotors are spinning at full speed. Of course, like our room fan with the flattened blades, the rotor blades won’t be creating much of a breeze. Imagine placing that fan on a slippery block of ice. It’s easy to visualize that with flattened blades, even on the most slippery surface, the fan won’t move.
Now imagine you can somehow cause the spinning fan blades to regain their twist a few degrees. Immediately, the fan will begin producing a breeze. Not only that, on the slippery ice it will gradually begin to slide in the opposite direction that the breeze is blowing. The action of the fan blades regaining their twist is a room-fan version of collective pitch control, and the fan beginning to slide is the helicopter beginning to rise.
The force we’ve been talking about is lift, the simple reaction that happens, for example, when the wind blows a barn door shut. To paraphrase Sir Isaac Newton, every action produces a reaction, and in the case of a helicopter, the action of deflecting air downward produces a reaction, which is to drive the rotor blade upward, and the helicopter along with it.
When you calculate the weight of the air that is being deflected by a helicopter‘s spinning rotors at full speed, and you apply some aerodynamics formulas filled with Greek letters and plenty of algebra, you discover that a rotor produces more than enough force to enable the helicopter to fly.