Helicopter

A helicopter is an aircraft which is lifted and propelled by one or more horizontal rotors (propellers). Helicopters are classified as rotary-wing aircraft to distinguish them from conventional fixed-wing aircraft. The word helicopter is derived from the Greek words helix (spiral) and pteron (wing). The engine-driven helicopter was invented by the Slovak inventor Jan Bahyl. The first stable, fully-controllable helicopter placed in production was invented by Igor Sikorsky.

Alternative layouts

There are alternatives to Sikorsky's layout, which save the weight of a tail boom and rotor. Such designs use two large horizontal rotors which turn in opposite directions, or contra-rotate. All of these systems are designed for the same purpose: to produce a net rotational speed of zero. These methods introduce even more mechanical complexity to the design and are usually relegated to specialized helicopter types.

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The co-axial design, where rotors are mounted on top of each other at the top of the fusalage and share a common main axle complex, was first built by Theodore von Karman and Asbóth Oszkár in 1918 and later became the hallmark of soviet Kamov design bureau. Co-axial helicopters in flight are highly resistant to side-winds, which makes them suitable for shipboard use, even without a rope-pulley landing system. The complexity of a contra-rotating mechanism introduces extra costs and is hard to justify for civilian use (although the Ka-26 was a successful crop duster aircraft).

Related Topics:
Theodore von Karman - Kamov - Ka-26 - Crop duster

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The Kaman-system, which was developed in Nazi Germany for a small anti-submarine warfare helicopter, features two interlapping main rotors on separate, obliquely mounted axles. The contra-rotating rotors are located on top of the fuselage, close to each other. During the Cold War the American Kaman company started to produce similar helicopters for USAF firefighting purposes. Kamans have high stability and powerful lifting capability, thus the latest Kaman V-Max model is a dedicated sky crane design, used for construction works.

Related Topics:
Germany - Anti-submarine warfare - Cold War - Kaman - USAF - Kaman V-Max

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In the flying-waggon system (sometimes called "banana" for the peculiar shape of early US examples), the two main rotors are located at the front and rear extremity of a long, boxy fuselage that resembles a railway wagon. A prime example is the Boeing CH-47 Chinook, that can carry 14 tons of payload. Waggon helicopters are practical for military logistical purposes, because entry and unloading is easily facilitated via the unobstructed front and rear ramps. The rotors and turbines are located very high on top of the fuselage, making them less sensitive to damage and dirt. The main drawback of a waggon is limited agility in air and the need for a highly trained crew, as the large main rotors have long outreach beyond the fuselage and may easily hit nearby obstacles (in 2001, a South Korean army CH-47 Chinook crashed onto a bridge for the same reason while being shown live on TV).

Related Topics:
Railway wagon - Boeing - CH-47 Chinook - Payload - Waggon - Logistical

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A helicopter built by Cierva had three main rotors. These were placed at the corners of an equilateral triangle and all turned the same direction.

Related Topics:
Cierva - Equilateral triangle

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In the cross system, the rotary wing aircraft resembles a traditional fixed-wing airplane, with the two main rotors mounted at the extremities of its wings. Such helicopters are rare, because structural integrity of the wings is difficult to maintain against the amplified resonance of far off-board rotor-turbine units. The 1930's German FW-61 helicopter was built to such design. The world's largest ever helicopter, the Soviet Mil-V-12 prototype was a cross of two Mil Mi-6 turbine-rotor units built onto a modified Antonov cargo plane. The US V-22 Osprey tilting rotorcraft is similar, although its nacelles can be rotated, and shares some of the inherent technical problems of a cross system.

Related Topics:
Mil Mi-6 - V-22 Osprey - Nacelle

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The most unusual design is the roto-rocket principle, where the single main rotor draws power not from the shaft, but from its own wingtip jet nozzles, which are either pressurized from a fuselage-mounted gas turbine or have their own pulsejet combustion chambers. Although this method is simple and eliminates precession, development of such helicopters ceased soon, because their extreme noise levels preclude both military and civilian use.

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