An autogyro is a rotary-wing aerodyne whose rotor is turned throughout its flight by air forces resulting from the motion of the craft through the air. Autogyros are not commonly seen and are distinguished from helicopters, in that while they do have a rotary wing, unlike a helicopter, the autogyro's rotary wing is unpowered. It swings freely, and its motion is due to the forward motion of the aircraft. The autogyro power source is usually a pusher propeller.
Juan de la Cierva's invention and development of the autogyro began in 1919. The word Autogiro was a Cierva Company trademark, to be spelled with a capital A and with the "i". The generic term is autogyro, with a lower case "a" and a "y". Autogiro is Spanish for autogyro. He progressed from failure (with his Model C-1 in 1920) to production (with his Model C.19) in ten years. His aircraft and his engineering, marketing and business skills attracted Harold Pitcairn, the Kellett brothers and others to the autogyro field. Together, these pioneers laid the foundation to today's rotorcraft industry.
The autogyro was thought to have great promise. At first glance, it looked like a helicopter, with a huge multi- bladed rotor situated above the fuselage. Unlike the helicopter, the autogyro had stubby wings and used a nose-mounted engine with a conventional propeller for forward momentum. In moving ahead, the main rotor turned, so that its long thin airfoil blades provided lift, with some assistance from the shortened wings. The autogyro could not take off or land vertically, nor could it hover, but its abbreviated landing and takeoff runs were dramatic, and proponents claimed that the aircraft minimized dangerous stalls. Some writers of the era envisioned the autogyro as a replacement for the family sedan.
While there are few autogyros today, they were very popular in the 1930s and the design features of helicopters, save for the anti-torque tail rotor, were all pioneered with the autogyro. Successful autogyros were produced in England and by several companies in the U. S., with Pitcairn being the most notable manufacturer. In the 1930's autgyro technologywas rapidly advancing and its safety and utility were being demonstrated and accepted. Mail carrying autogyros operated from the top of the Philadelphia Post Office. Four- and five-passenger 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. However, the advent of the helicopter in the mid-1940s quickly relegated the autogyro to a curiosity, because the autogyro is capable of short field takeoffs and landings, but cannot perform vertical takeoffs and landings, and more importantly, cannot hover.
The basic aerodynamic technology of the autogyro and its many derivatives (aka compound helicopters, convertiplanes, airplane with a rotor, etc.) is captured with a few simple equations. These equations show that the drag of the rotor blades alone is of minor importance to the successfully performing convertiplane. A much more serious concern is the drag of hubs and their pylon support to say nothing about fuselages and un-retracted landing gear. Controlling rotor speed at high advance ratio is a very difficult task for a pilot and some form of autopilot appears mandatory. The conventional rotor experiences, around unity advance ratio, a reversal in the change of thrust with collective pitch. Stability of the flapping motion is a serious question when the advance ratio approaches 1.5. Instabilities in both sub harmonic and higher harmonic blade motions are quite real high speed limitations. Fortunately, today's comprehensive rotorcraft theory and wind tunnel model technology are capable of discovering these rotor system instabilities before detailed design is completed.
The autogyro era began with Juan de la Cierva's development of his C-1 (from the latter part of 1919 to unsuccessful flight during October 1920). Cierva developed the flapping hinge for rotor blades before his early C-4 Autogiro was really flyable. Later, he introduced the lead-lag hinge. These two articulated joints insured that blade loads would be minimal in flight. These and other improvements were incorporated into his C.30 Autogiro, his most successful production configuration.
The 25 year era ended, for all practical purposes, by 1943 after the U.S. Army Air Corp selected the underpowered Sikorsky R-4 helicopter instead of the competing Kellett XO/YO-60 autogyro or the less satisfactory Pitcairn XO-61 autogyro. The choice was made primarily upon the fact that the helicopter could hover and the autogyro could not. A configuration comparison (A-14 & A-19) shows the XO/YO-60 bested the R-4 in every performance category. The rotor and control systems were functionally identical in that both aircraft had 3-blades and fully articulated hubs; both used collective and cyclic control. Laying cost aside, the discriminator was simply a short takeoff and landing (STOL) autogyro versus a vertical takeoff and landing (VTOL) helicopter.
The autogyro industry, while it existed, developed some 46 aircraft types and delivered about 450 rotorcraft. The aircraft's safety record was easily 5 times better than general aviation experience over the 25 year period. The cost per pound of weight empty varied from $3.50 for the Cierva C.30 (of which 180 were produced) up to about $8.00 for the Pitcairn PCA-2 (of which 25 were produced), these costs being in "back then dollars." The industry reduced the civil autogyro's initial weight empty to gross weight fraction from 0.81 to 0.58 by the end of the era. From a business point of view, our pioneers (a) created a flying machine other than an airplane, (b) acquainted the public with the aircraft and (c) pursued a vigorous product improvement program.
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