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 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 in the U.S. 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 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 the successful development of the helicopter 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 Fairey 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 starts (but not stops) 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. An identification of this goal is disclosed in the patent issued to The Ryan Aeronautical Company, U.S. patent No. 3,155,341, November 3, 1964. Yet, the potential of the gyroplane remains largely unrealized, and its development stunted by a shift in the interest of the aircraft industry to other goals.

More Info:

For a slightly more detailed account of autogyro history, visit:
Autogyro History and Theory on jefflewis.net

or for a much more detailed account, read the book:
From Autogiro to Gyroplane : The Amazing Survival of an Aviation Technology by Bruce H. Charnov