Arguably the most versatile vehicles used on Earth, helicopters are used for military, civil and industrial purposes, offering unrivalled flight dynamism.
Helicopters consist of a large airfoil (a rotating blade assembly) mounted via a hinged shaft to an aircraft fuselage, engine and flight controls. Unlike fixed-wing aircraft such as planes, however, the flight principles of helicopters differ markedly, with their power emanating from the rotating motion of the airfoil instead of from the plane’s fixed wings and turbofan Jet engines. Indeed, the fact that helicopters obtain lift from this cyclical motion complicates things massively, as it is directly affected by the horizontal or vertical movement of the vehicle at all times.
For example, in a plane the flight path of a wing is fixed in relation to its forward flight, while in a helicopter the flight path advances both forward and backwards through the circulation process of the rotors, with generated thrust parallel and in the opposite direction to it at 90 degrees. Therefore, when a helicopter is hovering in a stationary position, the plane of rotor rotation is directly parallel to the ground, balancing the helicopter’s weight and drag with its generated perpendicular thrust/lift. In order to move forward, backwards or side to side, the helicopter therefore tilts the plane of rotor rotation (ie, the opposite direction to that of produced thrust) in that direction.
Complicating this process, however, are the effects of the rotor’s opposite torque reaction, which due to the high rpm speed rotates the helicopter’s fuselage in the opposite direction to that of the spinning blades. This is controlled by the addition of the helicopter’s tail rotor, which is manually controlled by the pilot with the anti-torque pedals located in the cockpit |see ‘How to fly a helicopter’). By adjusting these pedals the pilot can increase, decrease or neutralize torque dependent on the required manoeuvre.
In addition to variable rotation planes and torque reactions, helicopters are also subject to a ‘gyroscopic precession’ effect, a dissymmetry of lift caused by its forward movement. This occurs because as the rotor rotates while the helicopter is moving, the blades at the fore of any single cycle combine both their own velocity with that of the movement, while those at the rear of any cycle hold the difference between their velocity and the movement. This, in simple terms, means that the blades at the front move quicker than those at the back, producing more lift. If left unchecked, the helicopter would roll, so this is counteracted by altering the blade’s individual angle of attack (the angle between the helicopter’s lifting plane and oncoming flow of the atmosphere), decreasing that of those advancing and increasing those retreating to generate equilibrium.
How to fly a helicopter?
Think flying a plane would be hard? This is even harder…
Flying a helicopter is significantly harder than piloting a plane, due to the increased number of control inputs that need to be co-ordinated. There are four flight control inputs: the cyclic and collective controls, as well as the anti-torque pedals and throttle. The cyclic control – the joystick that sits between the pilot’s legs – alters the pitch of the helicopter’s rotor blades cyclically, allowing the pilot to change the rotor’s thrust direction and overall vehicle tilt. For example, if a pilot pushes the cyclic stick forward then the rotor disc does also, creating forward thrust. The collective control is positioned to the left-hand side of the pilot (in a handbreak-type position) and when risen changes the pitch angle of all main rotor blades collectively, independent of their position, increasing or decreasing altitude.
Anti-torque pedals are positioned at the pilot’s feet and control the direction of the helicopter’s nose when pushed. These work by adjusting the pitch of the tail rotor blades, increasing yaw (rotation around a vertical axis) either to the left or right, dependent on which pedal is pressed.
Finally, the throttle control – which is usually positioned as a twist grip on the collective control – affects the amount of power produced by the helicopter’s engine, directly affecting the rpm speed of its rotors. Professional pilots must be trained to utilize these four controls in unison.