Just before my student lifted off I watched the tilt of his head and eyes to see where he was looking. Instructors can gain a lot of cues to correct student faults by noting where the student is looking during a maneuver. My student looked away from the helicopter into the middle distance. Here, using cyclic and pedals, he could better see and correct divergences from the proper fuselage attitude and heading before the errors became too large. Student helicopter pilots can make the mistake of fixing their gaze too close to the aircraft. By looking too close, they don't become aware of changes in the helicopter attitude until the changes have become too large for comfort.
The helicopter lifted. I could tell by the rate at which it rose that my student was a little taken aback by the extra performance gained by the off-loading of my 160 pounds in the cockpit, even though I had briefed him to expect it. After a brief wobble, he stabilized the aircraft at a five foot hover. As it rose, I watched the rotor blades, which had been horizontal while the helicopter was on the ground, rise as they pivoted on the inboard flapping hinges, until, seen from the side, they resembled a shallow cone (Figure 1).
This coning angle is where the centrifugal force acting outwards on the rotor blades and the lift acting vertically on the blades comes into balance. As my student paused before beginning the transition into forward flight, I saw a small piece of paper, probably a candy wrapper, get caught in the rotor wash. At first the candy wrapper blew away from the helicopter, then it rose into the air, perhaps 60 or 70 feet above the helicopter, before plunging down, with increasing speed, through the rotor and whirling away to begin the rotation again. This was a most graphical demonstration of the induced flow that a rotor system causes.
I almost wished my soloing student was beside me to see this demonstration. When discussing the phenomenon of induced flow a week or so earlier, this student had found it difficult to visualize the relative wind vector caused by the dual effects of rotational velocity and induced flow upon the rotor. In frustration he said, "I can't see those arrows!"
I watched, amused, as the little candy wrapper arrowed straight down through the rotor for a second time while my student, all concentration, prepared for take off, oblivious of the object lesson that he had no time to observe (Figure 2).
His voice came strongly over my handheld radio, "Helicopter Six-Zero-Zero-Two-X-ray, taking off on runway Two-Seven." He moved the cyclic forward to begin the transition into forward flight. The helicopter moved forward, and then, just as it reached the area where the forward limits of the rotor downwash disturbed the grass, the helicopter rolled slightly to the right and began to pitch nose up. Two separate effects occurring more or less simultaneously caused the pitch and roll.
The pitch-up was caused by an effect known as dissymmetry of lift. The rotor blade on the right side of the helicopter, moving toward the nose, had a greater relative airspeed than the rotor blade on the left of the helicopter that was moving toward the tail. The helicopter's forward movement added to the blades' rotational velocity and resulted in more lift on the right side of the rotor disc than on the left.
The extra lift caused the blades on the right side of the helicopter to begin flapping up about the flapping hinge, while those on the left side flapped down. Instead of causing a roll to the left, gyroscopic precession moved the effect 90 degrees in the direction of rotation, which caused the helicopter to pitch nose up (Figure 3).
The roll to the right was caused by the front of the rotor disc entering undisturbed air before the back of the rotor disc as the helicopter moved forward. For a brief moment, the front of the rotor disc had a greater angle of attack than the rear of the disc because the front had entered undisturbed air and the rear was still in the influence of the induced flow.
A higher angle of attack and more lift at the front of the rotor causes the blades to begin flapping up. Gyroscopic precession again rotates the effect 90 degrees in the direction of rotation and causes a roll to the right or advancing side. This difference in lift is known as transverse flow effect (Figure 4).
I could imagine, but not feel, the slight shudder that went through the helicopter's fuselage at that point of my student's solo flight, as the rotor system reacted to the sudden gain in efficiency known as effective translational lift (ETL). This is where the aircraft comes alive and wants to climb. Instead of the contrary and willful machine that it seems to be in the hover, the helicopter transforms into a responsive and seemingly much more powerful aircraft.
In any event, my student controlled the pitch-up by steadily moving the cyclic stick to keep the tip-path plane, or the blur of the rotor tips as seen from the cockpit, on the horizon. He controlled the roll to the right by a cyclic input to the left. He did very well until a distinct "yee hah!" burst from my handheld radio. Oh, well, perhaps it wasn't my student, I thought as I turned, coughing, to hide a grin. In my heart of hearts, I knew it was, and made a note to chide him for his burst of misplaced enthusiasm over the Unicom frequency.
From where I stood his airspeed during the climb-out was difficult to judge, but it looked to be nicely situated in the Height/Velocity chart - ahead of the leading edge of the "avoid" section. This chart is really an energy management chart, and it gives the altitude and airspeed combinations from which a pilot can make a safe autorotative landing following an engine failure.
Naturally, my student and I had practiced a number of autorotations to full touchdown before I sent him solo, but the possibility, however unlikely, of an emergency is always a real concern for an instructor during a student's early solo flights.
My student's crosswind and downwind legs looked normal, but I detected the slight wobble on downwind when he gripped the cyclic between his knees so he could turn on the carburetor heat with his right hand. One of the problems with flying a helicopter is that it occupies both hands and feet all of the time. Helicopter pilots need to get inventive in order to free up a hand to perform other tasks.
His radio calls were now standard, with no extra rebel yells. Perhaps I'll go easy, I thought, if he doesn't do it again.
He didn't. His turn from base to final appeared a little on the high side, leaving him with a steeper-than-normal final approach to the hover spot. I watched anxiously, ready to radio him to go around if he let the situation get out of control. As it was, he did fine, making an almost constant-angle approach all the way down, as viewed from my position to the side of the runway where I sighted the descending helicopter against the background of a cloud.
An approach in a helicopter is a different affair from what it is in an airplane. With an airplane, the technique is to get the approach stabilized, with no changes, other than minor corrections, all the way down to the landing flare. It's different in a helicopter. The speed decreases all the way from the start of the approach to the hover point, and the power, controlled by the collective-mounted throttle, decreases during the first half of the approach, and then increases during the latter half - as the helicopter nears the hover spot and loses translational lift. Because the speed is decreasing all the way down the approach, the helicopter's pitch attitude changes, too. It's this constant change in pitch attitude that makes it so difficult to fly, or even to explain how to fly, a constant angle of approach in a helicopter.
Throttle control is critical during the approach. The constant collective adjustments during a helicopter approach require corresponding throttle movements to keep the rotor rpm in the green. Students commonly allow the rotor rpm to increase too much in the early stages of the approach as they lower the collective, and then let the rotor rpm drop below the green band as they raise the collective to achieve a hover.
Letting the rotor rpm drop is dangerous because, at a certain point, the drag on the rotor system becomes so large that the rpm cannot be recovered. As the rotor rpm drops, you also lose tail rotor rpm, which causes you to lose directional control. I listened carefully to the sound of the engine as the helicopter approached.
Early in his training this student had a control technique that more resembled alligator wrestling than helicopter flying, with gross over-controlling and correspondingly large throttle movements. Then he suddenly overcame the problem, and if I could only pinpoint exactly what I had said or done to bring about the improvement, I could sell it - and make a fortune. As it was, the improvement appeared seemingly out of the blue, and my reward (and his) was the solo flight that was taking place on this lovely summer evening.
He brought the helicopter to a hover, with the engine note remaining constant, and with only a slight fishtail as he applied the last of the hover power. Like many students early in their training, this one had regularly been slow to apply left pedal to correct the rapid increase in torque and had consistently allowed the aircraft to yaw to the right. We cured that, as I usually do with students, by getting him to focus his gaze farther ahead down the runway, and to apply the corrective pedal a little earlier instead of waiting for the aircraft to begin its swing to the right. In other words, learn to anticipate.
After another two traffic patterns at the sleepy little airport, my student set down reluctantly alongside the runway so I could climb back in and shake his sweaty hand.
During our flight back to our busy home base airport north of Chicago, my normally articulate student seemed to have lost the power of coherent speech. When he stopped grinning, about the only words I could get out of him were "Wow! That was a blast!"
I could only agree.
Related Articles
