"The cyclic," I said, "is the large vertical stick that you hold in your right hand. It controls the attitude of the rotor disc. If you imagine that the turning rotors are like a large plate, or disc, balanced on top of the rotor mast, moving the cyclic in any direction causes the rotor disc to tilt in the same direction."
"Because the main rotor gives a helicopter both lift and directional thrust, tilting the rotor disc vectors the rotor thrust and makes the helicopter move in the same direction the disc is tilted. Thanks to the main rotor's circular shape, the pilot can tilt it in any direction. And, as far as the rotor is concerned, it's just as happy going backwards or sideways relative to the helicopter's fuselage as it is going forwards."
"The next major control," I said, "is the collective pitch lever. We usually just call it the collective. It increases the pitch on all the blades at the same time (or collectively), when you raise the collective. If you lower the collective, the reverse happens, and you reduce the pitch to all the blades. This is a direct mechanical connection and is easy to see. But," I added, "when we get flying, you'll find out that there are some secondary effects to collective movement that require quite a bit of coordination with all the other controls."
Lastly," I said, "we have the pedals. They are easy to understand, unless you happen to be an airplane pilot, which I remember you are. In that case, we have some unlearning to do."
"The pedals," I explained, "are always an interesting problem for fixed-wing pilots who are learning to fly helicopters. On an airplane, the pedals are connected to the rudder. The rudder's job is to counter the effects of adverse yaw during a turn, and to correct the yaw caused by P-factor, torque, and the propeller's spiraling slipstream, especially at high angles of attack." It's this latter use that comes closest to the way a pilot uses the tail rotor pedals in a helicopter.
"The tail rotor pedals," I said, "are used to counter the effects of engine torque. In an airplane you've become used to using pedal in conjunction with the ailerons when entering a turn. This isn't necessary with a helicopter because there are no ailerons and, therefore, no adverse yaw." The tail rotor pedals increase and reduce the pitch on the tail rotor blades, varying the amount of tail rotor thrust. In effect they are the tail rotor's collective pitch control. Left pedal increases the amount of tail rotor thrust and causes the helicopter to yaw to the left. Right pedal reduces the amount of tail rotor thrust and causes the helicopter to yaw to the right.
"In a hover you'll use the pedals to alter the helicopter's heading. That's straightforward in concept, but in reality, the degree of control is nowhere so positive as, say, turning a taxiing airplane," I explained. A helicopter requires differing amounts of pedal when making a hover turn because crosswinds constantly affect the fuselage and the airflow through the tail rotor. Crosswinds both vary the amount of thrust the tail rotor produces and make the fuselage want to weathercock into the wind.
In forward flight, it's different - the pilot uses the pedals to counter changes in torque only. "If you raise the collective, you'll have to put in left pedal, and if you lower the collective, you'll have to put in right pedal. Where it gets interesting - and really confusing for airplane pilots - is the fact that if you increase collective to climb and then turn to the right, you will be turning right with left pedal. If, on the other hand, you reduce power to descend and then enter a left turn, you will be turning left with right pedal applied."
Like most airplane students who transition to a helicopter, my student raised his eyebrows at this. "Don't worry," I said, "it all becomes easier after the first thousand or so iterations. Okay, the only other control you need to think about is the throttle. It is a twist grip control mounted on the end of the collective. Oh, you don't ride motorcycles do you?" My student said he didn't. "That's good," I said. "Some motorcycle riders have problems with the throttle because to increase throttle, you have to twist it away from you, not towards you, which is what a motorcycle rider must do."
Our next step is to visit the aircraft sitting on the sunlit ramp to see how this all fits together, and then to go for our introductory effects-of-controls flight. "Okay, reach in and move the controls, and let's see what's happening to the three main-rotor blades," I suggested.
He waggled the cyclic stick. "Hey," he said, "when I move the cyclic to the side, the rotor blade to the right side there doesn't move. That's one of the checks on my airplane, when I move the yoke to the right, I expect to see the aileron tilt up on that side. Why doesn't the rotor blade move? I don't see how the helicopter turns if the rotor blade doesn't change its pitch."
Impressed with his observation, I said, "You have discovered the next part of the lesson for yourself." If you move the cyclic to the side, the rotor blades to the sides don't move. But look at the blade to the front."
"That's interesting," he said. "When I move the cyclic to the right side, the rotor blade to the front is changing pitch nose up, but the blade on the right isn't changing pitch at all. Why not?"
"The big difference between airplane and helicopter control, is that airplane controls act directly, but because of gyroscopic precession, helicopter controls have a built-in lag," I explained. "If you move the cyclic to the right, the blade over the helicopter's nose pitches nose up and the rotor blade over the helicopter's tail pitches nose down.
"Remember," I told him, "gyroscopic precession causes the force of control inputs to be delayed by 90 degrees in the direction of rotation before they take effect. Helicopter manufacturers overcome gyroscopic precession by rigging the controls so as to put control movements into the rotor blades 90 degrees early. When seen from above, our rotor turns counterclockwise, so putting a nose down control input to the blade over the tail and a nose up input to the blade over the front, will cause the entire rotor disc to tilt right. But enough theory," I said, "let's go flying."
We cleared the airport, climbed to altitude, and then I began the first effects-of-controls lesson. "As I briefed you, if you want to go faster, you push the cyclic forward. The rotor disc tilts down in front, and the fuselage also pitches down. We go faster, but note we are also losing altitude because I didn't make a power adjustment to compensate."
"Just like my airplane," said my student.
"That's right," I agreed, "Note that when we pull the cyclic back, the primary effect is to reduce speed, and as a secondary effect, we're also climbing." I showed him how moving the cyclic to the side turned the helicopter in the appropriate direction, but he found it hard to stop himself from putting in pedal at the same time, despite my reminders.
"Okay," I said, "I'm going to change the collective pitch setting without moving any of the other controls, except the throttle. I'll have to adjust the throttle to stop the engine from under- and over-speeding when I make the collective adjustments."
With him following me on the controls to make sure I moved the collective and throttle only, I lowered the collective. Immediately, the nose of the helicopter dropped, and the aircraft yawed left and began to roll to the left. My student was a little surprised, to say the least. "Now I'm going to raise the collective," I said. "Let's see what happens." Without moving the other controls I raised the collective, and the nose of the helicopter pitched up, the aircraft began yawing to the right and rolling to the right.
"Of course we don't fly like that," I said, "but I just wanted you to see the secondary effects of collective movement. Raising and lowering the collective had the desired effect of causing the helicopter to climb and descend, but we have to counter the secondary effects that caused the pitching, rolling, and yawing. Now let me show you how we do that."
This time, as I raised the collective and increased the throttle to maintain the engine and rotor rpm, I moved the cyclic stick forward enough to prevent the pitch up that had occurred the first time. At the same time, I applied enough left pedal to keep the nose from yawing from the heading. The rolling didn't occur, because it had been caused by the first two effects. When I lowered the collective, I reduced the throttle, moved the cyclic stick back to stop the nose from pitching down, and applied right pedal.
"That's a lot of coordination just to make a simple movement," my student said. "Why is all that necessary?"
"Most of it has to do with the airflow around the horizontal stabilizer that's mounted on the right hand side of the tail," I said. "But there are some other coupling effects that we need to discuss in the classroom. Basically, when we descend, the airflow from below hits the stabilizer and pushes the tail up. Because the stabilizer is mounted on the right, the upward airflow also causes a roll to the left. The left yaw occurs because lowering the collective and reducing the throttle greatly reduces the amount of rotor torque, so we need less left pedal. When we increase power to climb, the airflow from above pushes the tail down and causes the aircraft to pitch up and roll right. The helicopter yaws to the right because the increased torque at the main rotor causes an opposite reaction in the fuselage"
"Huh?" my student said. "I'm supposed to remember all that?"
"Oh, don't worry about it too much just now," I said, "It will soon become second nature. Anyway, let's just make some small turns in each direction to get the feel of it, and then we'll do some power changes."
My thoughts came back to the present. I watched as my student's control touch became less tense and he began to enjoy himself. At least, the next time he said "I have the controls," he would have a clearer idea exactly what that statement implied. Looking at the grin on his face as he waggled the cyclic to make small turns in each direction, it was clear to me that theory was taking a distinct back seat to the practical (and infinitely more fun) aspect of flying a helicopter.