There are two types of heavier-than-air aircraft that achieve lift by movement through the air: (1) The airplane, which has stationary wings that create lift when propelled through the air by a thrust mechanism such as a propeller or jet engine, and (2) the rotorcraft or rotary wing aircraft in which blades rotate to describe a disc above the aircraft to create lift.
There are three types of rotorcraft that utilize a rotor blade to provide lift: (a) The helicopter, in which the rotor blade provides vertical thrust and, because the rotor disc can be tilted on a supporting and rotating vertical mast, a horizontal thrust component. (b) The autogyro, in which lift is provided by a rotary wing and forward thrust provided by a propeller. Auto rotation is achieved by tilting the rotor disc back relative to the airflow so that some air flows up between the blades and through the rotor disc rather than down through the rotor disc as in a helicopter. As the air flows up through the rotor disc, the rotor is driven much like a windmill is driven by the wind. (c) The gyroplane in which a rotor is used for vertical and slow speed flight, but at high speed cruising the rotor is completely unloaded (no lift) and the wing provides all the lift. The gyroplane holds promise as a high-speed, low disc loading rotorcraft. The gyroplane is a new type of autogyro. It uses its wings and the back propeller to fly like an airplane. It uses the rotor to slowly float to the ground for a landing.
The quest for a high-speed low disc loading rotorcraft with cruise performance equaling or exceeding airplanes has been ongoing ever since the invention of the autogyro by Juan de la Cierva in Spain in 1923. This led to successful autogyros being produced in England and by several companies in the U.S., with Pitcairn being the most notable source. In the 1930's autogyro technology was rapidly advancing and its safety and utility was being demonstrated and accepted. Mail carrying autogyros operated off of the top of the Philadelphia Post Office. Four and five place autogyros were being produced as well as smaller ones. Pitcairn alone developed and manufactured 14 models between 1930 and 1940. These aircraft had performance equaling contemporary airplanes with maximum speeds up to 150 mph.
The technology developed for the autogyro solved the rotor flapping and control problems for the helicopter and lead to its successful development starting in the late 1930's and continuing to this day. All concepts for a high-speed rotorcraft involve some rotor unloading in cruise flight. In fact, the English Frairey Rotodyne, with its tip jet autorotating rotor, used for take off and landing, set a closed course speed record for rotorcraft of 191 mph in 1959. This speed record was first broken by the Russian Kamov KA-22 in 1961 with a speed of 221 mph. In the U.S. in 1956 the McDonnell XV-1 tip jet compound autogyro reached an unofficial speed of 200 mph. Throughout the 1960's, 1970's, 1980's and still today, slowed and stopped rotor concepts were researched by Bell Helicopter, McDonnell Douglas (Hughes) and Sikorsky. The X-wing stopped rotor concept pursued by Sikorsky, DARPA and NASA received the greatest technology development of any, but was cancelled because of its extreme complexity when it was being readied to start flight development testing.
A slowed or even a stopped rotor high-speed rotorcraft need not be complex. This is attested by the fact that the Herrick 2-bladed stop-rotor autogyro made rotor stops and starts in flight in 1937. However, to achieve the highest speed flight with a gyroplane it will be necessary to unload the rotor during horizontal flight by reducing lift to as close to zero as practicable.
Gyrodynes / Compound Helicopter
Many non-conventional rotary wing concepts have been developed, including compound concepts such as the high-speed advancing blade concept (ABC) reverse velocity rotor (RVR), vectored thrust ducted propeller (VTDP), optimum speed rotor (OSR), slowed-rotor, and tandem compound; ducted fan concepts; advanced tilt-rotor and tilt-wing concepts such as the optimum speed tilt-rotor (OSTR) and Quad Tilt-Rotor (QTR); and gyrolifter and heliplane concepts.
The two basic elements of flying are lifting up in the air and moving around, once up in the air. To achieve these two basic steps, power must also be provided by or to such hardware. Two major generic types of such hardware have evolved during the past century, on a large scale: airplanes and helicopters. During the second half of this century, attempts were made to combine the two basic features of these two main types of aircrafts: VTOL (Vertical Take Off & Landing) airplanes. The basic attempts involved such approaches as: (1) rotating the propulsive means upward, (2) deflecting the propulsion means airstream downward, and (3) providing the craft with two separate sources for lifting power and forward thrust, one being best adapted to and for each one of these two types of operational modes.
Each of the rotary wing aircraft and the fixed wing aircraft has certain advantages and disadvantages. The rotary wing aircraft has the advantage of vertical takeoff and landing and hovering maneuvers due to the vertical thrust from a rotating rotor. Compared with a fixed wing aircraft, the rotary wing aircraft has the disadvantage of inefficient cross country travel, the inability for high speed horizontal flight, and lower operational altitudes.
Compared with the rotary wing aircraft, the fixed wing aircraft has the advantage of being able to travel more efficiently at high speeds and fly at high altitudes. The fixed wing aircraft has the disadvantage of substantial takeoff and landing speed thus requiring a substantial runway space for takeoff and landings. In addition, the fixed wing aircraft is incapable of flight below operating airspeeds or hovering maneuvers due to the loss of lift over the fixed wing of the aircraft due to insufficient speed.
Those attempts which have tried to create a heliplane, or some similar conversion of a helicopter into an airplane by attaching a wing to the vehicle, have been quite unsuccessful. The spinning rotor creates enormous drag at high forward speeds, and one still has all the problems with the forward and rearward sweeping rotor blades and the problem that the center of mass remains inherently different from an airplane.
None of these attempts has been very successful in the case of aircraft powered by propellers. However, each basic type of craft, airplanes and helicopters, has done very well on its own and is widely used for its best suited applications. Therefore, it seems natural to attempt to combine the favorable attributes of each type and to eliminate their disadvantages in an effort to provide lifting capability and speed in the same aircraft. Such a new type of aircraft is needed and considerable efforts are now being made worldwide to that effect.
Prior gyrodynes or compound helicopters include the Lockhead AH-56 and the McDonald XV-1. Both of these gyrodynes had small fixed wings and single rotors. A gyrodyne is defined in the National Aeronautics and Space Administration Aeronautical Dictionary, 1959, as: "A rotating-wing aircraft whose rotor or rotors provide lift only, the system customarily being powered for take-off, hovering, landing, and for forward flight throughout part of its speed range, but usually autorotating at the higher flight speeds, forward propulsion being provided by a propeller or jet." The NASA Aeronautical Dictionary states that the term "compound helicopter" is a synonym for gyrodyne but is used rarely. The term compound helicopter, according to NATO, is a helicopter with an auxiliary propulsion system which provides thrust in excess of that which the rotor(s) alone could produce, thereby permitting increased forward speeds; wings may or may not be provided to reduce the lift required from the rotor system.
As the auxiliary thrust system begins to provide the majority of propulsive force at airspeeds above 180 knots, if no power was supplied to the rotor, the disk would assume an aft tilting tip path plane and rotor drag would increase significantly. This situation is avoided completely in actual compound helicopters in high-speed flight by providing a relatively small amount of power to stabilize the rotor, while keeping the tip path plane at a near zero angle of attack.
Gyrodyne, a generic term as in the FARs, is used to describe an aircraft with dual propulsion units for vertical and horizontal flight [it is also used in an unrelated sense as a gyroscopic stabilizer used on the Space Shuttle]. There is also a company called the Gyrodyne Company of America, an aerospace contractor, which had a trademark, which used the name gyrodyne. The trademark has lapsed.
A gyrodyne can be manufactured in a number of different varieties: with or without wings in addition to its rotor; with different mechanical arrangements for lift equalization over the rotor; with or without separate power plants for the rotor and forward-propulsion system; etc.
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