3.3. Text helicopters General Characteristics and Structure
Helicopters are rotary-wing aircraft able to take off and land vertically, to move in any direction, or remain stationary in the air. The word helicopter comes from the Greek words meaning "helical wing" or "rotating wing". Modern helicopters can hover over one point at zero speed and attain a top speed of up to 300 knots and more, their range being up to 2000 miles with extra fuel tanks. They can climb to an altitude of 10,000 feet and carry a load amounting to 40 tons.
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Рис. 5. Principal structural units of the AH-64 Apache attack helicopter:
1 - pilot’s cabin - кабина экипажа; 2 - main rotor blades - лопасти несущего винта; 3 - main rotor hub - втулка несущего винта; 4 - tail (anti-torque) rotor - хвостовой винт; 5 - vertical fin - киль, вертикальный стабилизатор; 6 - horizontal stabilizer -горизонтальный стабилизатор; 7 - tail boom -хвостовая балка; 8 - fuselage -фюзеляж; 9 - landing gear -шасси
A helicopter consists of the airframe and propulsion system. The airframe includes a fuselage and undercarriage. The fuselage is the principal structure of the helicopter and houses the pilot's cabin for a crew of two or three, and cargo compartment designed to carry a squad of equipped infantrymen or different cargo. The tail boom is a structural element of the fuselage connecting the tail rotor to the cargo compartment. Stabilizers and tail support are also fixed to it. Undercarriage with main and nose landing gears and tail support and propulsion system - major structural elements of the helicopter - are attached to the fuselage as well.
The propulsion system consists generally of the engine, (transmission) drive system and main and tail rotors. The turboshaft engines having a high power-to-weight ratio are currently used in helicopters. An axial-flow turboshaft engine is comprised of a compressor, combustion chamber, turbines, and main gear box. The latter is of utmost importance as the safe operation of the entire system depends on it.
The rotation of the main rotor tends to cause the fuselage to rotate in the opposite direction. To prevent this, the single-rotor helicopter is provided with a tail rotor producing a counteracting thrust. Alternatively, the helicopter may have two rotors which revolve in opposite directions and thus counterbalance each other. Such helicopters can be tandem-rotor, side-by-side or co-exial.
3.4. Тext Principle of Operation and Control
The helicopter derives lift from rotating an airfoil called the rotor. Usually it is equipped with one or more power-driven rotors. When the angle of attack of rotating rotor blades attains a certain value, the lift overcomes the weight of the aircraft and then it takes off vertically.
To achieve horizontal flight, the pilot tilts the rotor forward at a certain angle. This is done by what is known as cyclic pitch change, i.e. changing the pitch of each blade once per revolution. More particularly, the angle of attack of each blade is increased every time it sweeps over the tail of the machine, thereby temporarily developing a greater amount of thrust than the other blades.
Additionally each blade can swivel about its longitudinal axis and its pitch is changed cyclically, through a linkage system, by a so called swash-plate, which performs a sort of wobbling rotary motion around the shaft and swivels the blades to and fro as they rotate. The tilt of the swash-plate can be varied by the pilot, and the tilt of the rotor follows the tilt of the plate.
So, by changing the pitch angle of the main rotor blades, by manipulating the engine speed and by tilting the main rotor, the pilot controls the helicopter's flight in any direction.
Hovering over one point is done by selecting the correct speed and setting the rotor blades so that their vertical lift is exactly equal to the weight of the helicopter.