Helicopters
Model helicopterflying - The pinnacle of modelling
The hobby of modelling can be broadly divided into static and working models, and within the latter category there are many different branches. From the ocean-going yacht to the off-road buggy, from the glider to the aerobatic biplane, modelling is as multi-faceted as society itself. Arguably the high point of the whole hobby is the model helicopter. Discipline, accuracy and a rigorous approach are required from the moment you start building the machine and installing the working systems, but the most demanding aspect of model helicopters comes later: the art of controlling this flying machine is a challenge to anyone. Like a juggler, keeping four balls in the air with his head, hands and feet, the helicopter pilot has to control all three primary axes of his model separately and simultaneously.
Hovering may look like child's play, but mastering a steady hover is the beginner's first major obstacle and he can only progress to 'real' flying in easy stages once that hurdle is overcome. As confidence and skill increase, many helicopter pilots find that their ambitions are widening too. You may want to indulge in aerobatics, or commit many hours to building a full-fuselage scale model, but one thing is for sure: you will never run out of challenges, because the fascinating combination of physics, aerodynamics and model making which constitutes the model helicopter represents an infinite breadth of variety.
How a model helicopter flies
If you wish to fly a model helicopter, there is really no alternative to learning the fundamentals of how the machine stays in the air. The spinning rotor supplies the upthrust required to defeat gravity, but at the same time it assumes several control functions. The helicopter has four primary controls: collective pitch (rise and descend), roll (right and left), pitch-axis (forward and back) and tail rotor (rotation right and left). The next section explains briefly how these functions work, and how they are superimposed on each other.
Roll-axis, pitch-axis
Control of the helicopter in the forward/aft direction and to both sides rests with the swashplate, which causes the pitch angle of the rotor blades to alter constantly as they rotate (cyclic pitch control). The result is that the amount of upthrust generated by the rotor blades varies according to their momentary position on the rotor disc. If more upthrust is generated on one side of the disc, that side of the helicopter rises, and the model moves in the corresponding direction. However, when the helicopter changes direction, the total amount of upthrust also changes, and the machine will tend to rise or fall in consequence. This effect can be corrected by altering the pitch of all the blades – this is the task of the collective pitch function.
Tail rotor
The power produced by the motor is transmitted to the rotor head, where it causes a rotational movement, or torque. The fuselage reacts by rotating in the opposite direction to the main rotor (torque reaction). It is the task of the tail rotor to compensate for this unwanted movement, and this it does by producing a sideways thrust to counteract main rotor torque. At the same time the tail rotor blades pitch can be altered directly from the transmitter; this increases or reduces tail rotor thrust and produces a rotational (yaw) movement of the helicopter. It is important to remember that you are controlling the model's nose, rather than it's tail.
Main rotor collective pitch
In modern model helicopters the rotational speed of the rotor head is maintained more or less constant and the amount of upthrust produced by the rotor disc is controlled by altering the pitch angle of the rotor blades simultaneously (collective pitch). If you increase the pitch of the rotor blades, the total upthrust increases and the helicopter climbs vertically. When upthrust is exactly the same as the weight of the model, the helicopter hovers, i.e.remains motionless in the air. Reduce the blade pitch angle and you reduce upthrust; the helicopter then descends vertically.
Gyro
Every time you change the rotational speed of the motor, or adjust the pitch angle of the main rotor blades, there is a corresponding change in main rotor torque and this would cause the tail to swing round if not corrected. The constant need to correct the tail rotor setting places a great strain on the model helicopter pilot and this stress can be significantly relieved by installing a gyro in the machine. When a change in main rotor torque occurs, the gyro automatically detects the alteration and adjusts the pitch angle of the tail rotor blades to correct any incipient change in heading. Early gyros were mechanical in nature, but an important recent development in this area is the piezo or positional sensor gyro. These units are entirely electronic in operation and the technology has allowed the development of much smaller, lighter gyros. A further development is the 'heading hold' gyro, which maintains the helicopter's nominal direction, or heading, as commanded by the transmitter stick; this it does regardless of the momentary flight situation. This makes many flight manoeuvres easier to carry out.
