Putting a Whole New Twist on It

Now that the rotor is turning at full speed, it’s creating a force called torque. Torque is the measure of how much a force acting on an object causes it to rotate. At least that’s its scientific definition. In practice, it means that when the rotor turns counterclockwise, the rest of the helicopter will tend to rotate clockwise.

To counteract the torque effect, helicopter engineers created the antitorque rotor. That’s the small rotor spinning in the tail of the helicopter, and it’s controlled in the cockpit by two floor pedals called, logically enough, antitorque pedals. (Helicopter pilots are a literal bunch.)

By the Book The throttle in a helicopter perform? the same function that it does in a car or motorcycle. It connects to the engine and con­trols the amount of fuel being burned and the power output of the engine. In fact, a helicopter throttle is located on the collec­tive pitch control and is a rotat­ing handgrip exactly like those used on motorcycles.

Turbulence There’s a natural rivalry between airplane pilots and designers and helicopter pilots and designers. One of the ways this rivalry manifests itself is in terminology. You’ll notice plenty of examples where helicopter pilots use dif­ferent names for objects similar to those in an airplane. Helicop­ter pilots even fly from a differ­ent side of their craft than airplane pilots do: Helicopter pilots fly from the right side, air­plane pilots from the left

If the antitorque rotor was not operating, here’s what would happen. The pilot would use the throttle to spin the rotors at the proper number of revolutions per minute, or rpms, for flight. With his left hand, he would pull up on the collective pitch control, which would increase the pitch of the blades, as we’ve just seen. The lift created by the rotor disc would slowly lift the helicopter off the ground. And then all hell would break loose.

Because the rotor is spinning in a counterclockwise direction, as viewed from the top, a helicopter without an antitorque rotor would begin spinning in the opposite direction as soon as it broke free of friction on the ground. (It would spin in the opposite direction because of old Ike Newton, remember, who said every reaction is opposite of every action. Since the rotor is spinning in one direction, the body of the helicopter would spin in the other.) The antitorque rotor, which spins about six times faster than the main rotor because its smaller size means it must work harder for the same effect, is used to stop that spinning.

The helicopter designer, in effect, took a smaller version of the main rotor, tipped it on its left side, and mounted it on the tail, which has a tendency to swing in a clockwise direction. The blades are angled in such a way that when the antitorque rotor is spinning, its force tends to push the tail in a counterclockwise direction, which counteracts the main rotor’s torque.

In essence, then, the antitorque pedals perform the same role for the tail rotor as the collective pitch control performs for the main rotor. They alter the angle of the blades to adjust for changes in main-rotor torque when the pilot changes power.

Because the natural torque effect on the helicopter is in a clockwise, or “right-turn,” direction, when a pilot

presses down on the right antitorque pedal, the tail rotor blades flatten to produce less force. That lets the natural torque effect take over and rotate the helicopter to the right. On the other hand, when the pilot applies left-foot pressure, the tail-rotor blades go to a higher pitch, causing the helicopter to rotate toward the left.

Going Places

So far, we’ve seen how the helicopter lifts off the ground by increasing the pitch, and thereby the lift, of the main rotor. We’ve also seen how the antitorque rotor prevents the helicopter from spinning in circles. Now, we’ll look at how the helicopter moves forward, backward, and side to side.

To begin, visualize the main rotor disc as a solid object. The lift it creates can be thought of as a force arrow sticking out of the disc and pointing exactly perpendicular to it, or in the example we’ve been looking at so far, straight upward.

If the pilot could tilt that disc a few degrees forward, backward, left, or right, that arrow—or “lift vector,” to give it its impressively technical-sounding name—would then tilt with it. Whichever direction that lift vector points is where the helicopter’s going to go. Helicopter pilots control the direction of the lift vector using the cyclic pitch control.

When the pilot moves the cyclic pitch control, a whole lot of complex things begin to happen in the hub of the main rotor, where all the mechanisms that control the rotor are located. The simple thing to say would be that the cyclic pitch control tilts the rotor one way or another, which in turn deflects the lift vector and changes the path of the helicopter. But that would gloss over a universe of complicated machinery that people like Igor Sikorsky spent decades of energy to perfect.

So let’s take a few paragraphs to understand the complex inner workings of the swash plate assembly (the cluster of mechanical components and links located near the rotor hub), which transfers the pilot’s control movements in the cockpit into pitch changes in the spinning rotor disc.

Plane Talk Of course, no technology pioneer, including 5ikorsky, succeeds independently of others. There is always a trail of innovation that can be traced back for decades, centuries, even millennia. In the case of helicopters, the Chinese deserve credit for the first written description of a rotary wing. Their version involved rotor blades made of wood carved from the inner part of a jujube tree and straps of ox leather to set the whole thing in motion. Other inventors conceived of the various components that eventually came together in the modern helicopter, and it only awaited energetic and clever thinkers to 1 put the pieces together into a flyable helicopter. —————————————————————————- ——— – —

The Swash Plate Assembly By the Book The cyclic pitch control is a metal stick that projects from the helicopter floor between the pilot’s knees. With her right hand, the pilot moves the stick front to back to control the for­ward or backward speed, and left and right to move sideways.

To visualize the swash plate assembly, let’s construct an analogy of it from familiar objects. We’ll start with two hockey pucks, laid flat and stacked one on the other. While you’re at it, imagine there’s a layer of tiny ball bearings between the two hockey pucks so the top one can spin easily while the lower one remains stationary.

Now imagine standing four pencils on end and placing the stacked hockey puck and ball bearing unit on top of them. One pencil is toward the nose of the imaginary helicopter, another toward the tail, and the others are on the left and right sides. (Sure, right about now the whole thing comes tumbling down. But cut us some slack and imagine the whole assembly is being held firmly upright and nothing’s falling apart.)

Next, place another four pencils on the upper puck. In a helicopter, those pencils represent the “pitch links” that control the pitch of the individual rotors, either increasing or flattening their pitch.

Finally, drill a hole through the hockey pucks to allow a main rotor shaft to pass through. In a helicopter, the bottom of the shaft would be attached to the engine and transmission and the top would serve as an anchor for the pivoting rotor blades. The main rotor would be linked to the upper swash plate, or top puck in our

On Course Torque is created only when the helicopter’s engine is delivering power to the rotor. Reducing power reduces torque. So if a tail rotor or the tail-rotor controls fail during flight, the pilot will cut power to the engine to stop the tendency for the helicopter to rotate toward the right. With­out power, the pilot will have to begin planning a forced landing, which we’ll discuss shortly.

On Course j MD Helicopters of Mesa, Arizona, has a unique alternative to the traditional tail rotor. It devised a system called NOTAR, for "no tail rotor," which uses a jet of air to produce the thrust normally created by the spinning blades of the rotor. And because the tail rotor is responsible for much of the noise a helicopter makes, • NOTAR helicopters are much і quieter. I

In its barest terms, the hockey – puck – and – pencil model you’ve imagined represents the swash plate assembly, and it contains the essence of the cyclic pitch control system. (Of course, the technically minded can research the system in far more detail in books listed in Appendix C, “Recommended Reading,” and on Web sites listed in Appendix D, “World Wide Web Resources.”)