The term "rotary wing aircraft" implies that such aircraft have rotating wings used to generate lift unlike other "fixed wing aircraft" with stationary lift devices. Individually known as rotor blades, and collectively as a rotor, these rotary wings give helicopters versatility - their ability to take off and land vertically, to hover and to fly sideways and rearwards. This versatility comes at a price, however, that being that the rotor system is an extremely complex dynamic system of rotating blades with fundamental and higher order vibratory response, complex unsteady aerodynamics verging upon aeroelastic instabilities and blade stall phenomena.
Helicopters differ from fixed wing aircraft in that both lift and propulsion are provided by a single item: the rotor. Each main rotor blade acts as slender wing with the airflow producing a high reduction in pressure above the front of the blades, thereby producing lift. Although of high aspect ratio, the blades are proportionately thicker than those of fixed wing aircraft and are often of symmetric profile. The principle of helicopter airfoil operation is illustrated below:
At the heart of a helicopter lie the engines, transmissions, drivetrain and rotors. These components are made up of a multitude of smaller constituents which are highly loaded in both magnitude of the applied loads and number of cycles. Failure of any one of these critical components, as they are referred to, can often be catastrophic as distinct from fixed wing aircraft where the number of critical components is less and structural redundancy is easier to build in.
In relation to loads, the two most important distinctions between fixed and rotary wing aircraft are as follows:
Considering the case of steady flight (a condition in which nearly all aircraft spend most of their time), the loads on a fixed wing aircraft are essentially static in nature as opposed to dynamic and highly vibratory complex loading observed on helicopters.
Whereas on a fixed wing aircraft one manoeuvre goes with one incremental load, a manoeuvre of a helicopter results in a number of incremental load cycles. This time dependence is the result of the cyclical loading produced by a rotor blade as it rotates during flight.
There are four main conventional configurations used in helicopter design: single main and tail rotor, twin tandem rotors, twin coaxial rotors and twin intermeshing rotors. Other types such as the NOTAR (No Tail Rotor), tilt rotor or X-wing are still in development and as such are not considered conventional. The most commonly used type is the single main and tail rotor which is largely employed in civilian applications. The torque produced by the main rotor is counterbalanced by the lateral force of the tail rotor thus preventing the aircraft from spinning out of control in one direction. This was often the case with early design of rotary wing aircraft. Another popular configuration is the twin tandem type in which two intermeshing rotors spin in different directions to counteract each other. This type of configuration is mainly used on large aircraft used for transport.
In all design, lift force is transmitted through the blade roots via the rotor hub into the main drive shaft, so the aircraft effectively 'hangs' of this shaft.
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| Bell 206B Kiowa - Single Main and Tail Rotor [http://www.ipas.com.au/pages/Page_Commissions.htm] |
Kamov Ka-25 Hormone - Twin Coaxial Rotor [http://www.aircraftinformation.info/gallery_military_helicopters_utility.htm] |
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| Kaman HH-43B Huskie - Twin Intermeshing Rotor (Synchropter) [http://marvellouswings.com/Museum/22 Castle Air Museum/22 Castle Air Museum.html] |
Boeing CH-47C Chinook - Twin Tandem Rotor [http://www.flickr.com/photos/87791108@N00/2630144972/] |
Helicopters require a higher installed power per unit of weight than fixed wing aircraft. A large proportion of the power is needed simply to overcome the force of gravity, and overall specific fuel consumption is high. Most helicopters are powered either by a single piston engine or by one, two or three gas turbine turboshaft engines. A typical gas turbine model of 1343 kW (1800 hp) comprises centrifugal and axial compressor stages and two stage 'free power' turbine.
Despite their unique abilities, helicopters are much slower than their fixed wing counterparts, produce a significant amount of noise pollution and are subject to complex and unique fatigue problems. As such their main applications are limited to those where fixed wing aircraft are inferior, i.e. surveillance, rescue, operations in environments inaccessible to fixed wing aircraft (e.g. mountain summits, urban communities, offshore locations, etc) and others.