Helicopters come in many sizes and shapes, but most share the same major components. These components include a cabin where the payload (passengers, baggage, and cargo) and crew are carried; an airframe, which houses the various components, or where components are attached; a power-plant or engine; and a transmission which takes the power from the engine and transmits it to the main rotor, which generates the aerodynamic forces that make the helicopter fly. To keep the helicopter from spinning due to main rotor torque, there must be some type of “anti-torque” system. Finally there is the landing gear, which could be skids, wheels, skis, or floats. See figure i-1.
There are four basic controls used during flight:
Collective – Controls main rotor thrust by changing pitch of all blades simultaneously or collectively.
Throttle – Controls engine power by changing engine RPM
Cyclic – Controls aircraft direction by “tilting” the main rotor thrust through individual blade pitch adjustment during rotation cycle
Anti-torque Pedals – Counters main rotor torque by controlling tail rotor thrust through tail rotor blade pitch
There are three different types of main rotor systems:
- Rigid – Blades, hub, and mast are rigid with respect to one another. Blades can “feather” (change pitch) but cannot “flap” (move up or down during rotation) or “drag” (move fore and aft in rotation plane)
- Semi-rigid – Main rotor hub can “tilt” with respect to the main rotor shaft on a “teetering” hinge, allowing blades to flap as a unit.
- Fully Articulated - Each blade is attached to the rotor hub via a “flapping” and “lag” hinge, allowing each blade to move independently of the others.
Most helicopters with a single, main rotor system require a separate rotor to overcome torque. This is accomplished through a variable pitch, anti-torque rotor or tail rotor. See figure i-2. The thrust of the anti-torque system must be varied in order to maintain directional control whenever the torque of the main rotor changes, or to make heading changes while hovering.
Helicopters are powered by one of two power-plant types:
- Reciprocating – Typically used in small and light helicopters. A series of pistons drive a crankshaft which connects to the main rotor transmission. The engine can be mounted horizontally or vertically with the transmission supplying the power to the vertical main rotor shaft. See figure i-3.
- Turbine – Typically used in most medium to heavy lift helicopters due to its large horsepower output. Comprised of three axial stages, compression, combustion, and turbine. Output shaft drives a gearbox connected to main rotor transmission and tail rotor.
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