Configuration
An autogyro is characterised by a free-spinning rotor that turns due to passage of air upwards through the rotor. The vertical component of the total aerodynamic reaction of the rotor gives lift for the vehicle, and sustains the autogyro in the air. A separate propeller provides forward thrust and can be placed in a tractor configuration, with the engine and propeller at the front of the fuselage (e.g., Cierva), or pusher configuration with the engine and propeller at the rear of the fuselage (e.g., Bensen).
Whereas a helicopter works by forcing the rotor blades through the air, pushing air downwards, the autogyro rotor blade generates lift in the same way as a glider's wing by changing the angle of the air as it moves upwards and backwards relative to the rotor blade. The free-spinning blades turn by autorotation; the rotor blades are angled so that they not only give lift, but the angle of the blades causes the lift to accelerate the blades' rotation rate, until the rotor turns at a stable speed with the drag and thrust forces in balance.
Pitch control of the autogyro is by tilting the rotor fore and aft; roll control is by tilting the rotor laterally (side to side). Three designs to affect the tilt of the rotor are a tilting hub (Cierva), swashplate (Air & Space 18A), or servo-flaps (Kaman SAVER). A rudder provides yaw control. On pusher configuration autogyros, the rudder is typically placed in the propeller slipstream to maximize yaw control at low airspeed (cf. McCulloch J-2).
Flight controls
There are three primary flight controls: control stick, rudder pedals, and throttle. The control stick is termed cyclic and tilts the rotor in the desired direction to provide pitch and roll control. The rudder pedals provide yaw control, and the throttle controls engine power.
Secondary flight controls include the rotor transmission clutch, also known as a pre-rotator, which when engaged drives the rotor to start it spinning before takeoff, and collective pitch to reduce blade pitch before driving the rotor. Collective pitch controls are not usually fitted to autogyros, but can be found on the Air & Space 18A and McCulloch J-2 and are capable of near VTOL performance. Unlike a helicopter, autogyros without collective pitch need a runway to takeoff; however they are capable of landing with a very short, or zero ground roll.[2]
Pusher vs tractor configuration
Modern autogyros typically follow one of two basic configurations.
The most common design is the pusher configuration, where the engine and propeller are located behind the pilot and rotor mast, such as in the Bensen "Gyrocopter". It was developed by Igor Bensen in the decades following World War II, and came into widespread use shortly afterward.
Less common today is the tractor configuration. In this version the engine and propeller are located at the front of the aircraft, ahead of the pilot and rotor mast. This was the primary configuration in early autogyros, but became less common after the advent of the helicopter. It has enjoyed a revival since the mid 1970s however, in the "Little Wing" autogyro concept.
History
Juan de la Cierva was a Spanish engineer and aeronautical enthusiast. In 1921, he participated in a design competition to develop a bomber for the Spanish military. Cierva designed a three-engined aircraft, but during an early test flight, the bomber stalled and crashed. Cierva was troubled by the stall phenomenon and vowed to develop an aircraft that could fly safely at low airspeeds. The result was the first successful rotorcraft, which he named Autogiro in 1923. Cierva's autogyro used an airplane fuselage with a forward-mounted propeller and engine, a rotor mounted on a mast, and a horizontal and vertical stabilizer.
Early development
Cierva's first three designs (C.1, C.2, and C.3) were unstable due to aerodynamic and structural deficiencies in their rotors. His fourth design, the C.4, made the first successful flight of an autogyro on 9 January 1923, piloted by Alejandro Gomez Spencer at Cuatro Vientos airfield in Madrid, Spain. Cierva had fitted the rotor of the C.4 with flapping hinges to attach each rotor blade to the hub. The flapping hinges allowed each rotor blade to flap, or move up and down, to compensate for dissymmetry of lift, the difference in lift produced between the right and left sides of the rotor as the autogyro moves forward. Three days later, the engine failed shortly after takeoff and the aircraft descended slowly and steeply to a safe landing, validating Cierva's efforts to produce an aircraft that could be flown safely at low airspeeds.
Cierva developed his C.6 model with the assistance of Spain's Military Aviation establishment, having expended all his funds on development and construction of the first five prototypes. The C.6 first flew in February 1925, including a flight of 10.5 km (7 miles) from Cuatro Vientos airfield to Getafe airfield in about 8 minutes, a significant accomplishment for any rotorcraft of the time. Shortly after Cierva's success with the C.6, Cierva accepted an offer from Scottish industrialist James G. Weir to establish the Cierva Autogiro Company in England, following a demonstration of the C.6 before the British Air Ministry at RAE Farnborough, on 20 October 1925. Britain had become the world center of autogyro development.
A crash in February 1927, due to blade root failure, led to an improvement in rotor hub design. A drag hinge was added in conjunction with the flapping hinge to allow each blade to move fore and aft and relieve in-plane stresses, generated as a byproduct of the flapping motion. This development led to the Cierva C.8, which, on 18 September 1928, made the first rotorcraft crossing of the English Channel followed by a tour of Europe.
The U.S. industrialist Harold Frederick Pitcairn, upon learning of the successful flights of the autogyro, had previously visited Cierva in Spain; in 1928, he visited Cierva again, in England, after taking a C.8 L.IV test flight piloted by Arthur H.C.A. Rawson. Being particularly impressed with the autogyro's safe vertical descent capability, Pitcairn purchased a C.8 L.IV with a Wright Whirlwind engine. Arriving in the United States on 11 December 1928 accompanied by Rawson, this autogyro was redesignated C.8W. Subsequently, production of autogyros was licensed to a number of manufacturers, including the Pitcairn Autogiro Company in the U.S. and Focke-Wulf of Germany.
Development of the autogyro continued in search for a means to accelerate the rotor prior to takeoff (called prerotating). Rotor drives initially took the form of a rope wrapped around the rotor axle and then pulled by a team of men to accelerate the rotor - this was followed by a long taxi to bring the rotor up to speed sufficient for takeoff. The next innovation was flaps on the tail to redirect the propeller slipstream into the rotor while on the ground. This design was first tested on a C.19 in 1929. Efforts in 1930 had shown that development of a light and efficient mechanical transmission was not a trivial undertaking, but the Pitcairn-Cierva Autogiro Company, of Willow Grove, Pennsylvania, finally solved the problem with a transmission driven by the engine in 1932.
Cierva's early autogyros were fitted with fixed rotor hubs, small fixed wings, and control surfaces like those of a fixed wing aircraft. At low airspeeds, the control surfaces became ineffective and could readily lead to loss of control, particularly during landing. In response, Cierva developed a direct control rotor hub, which could be tilted in any direction by the pilot. Cierva's direct control was first developed on the Cierva C.19 Mk. V and saw production on the Cierva C.30 series of 1934.
When improvements in helicopters made them practical, autogyros became largely neglected. They were, however, used in the 1930s by major newspapers, and by the US Postal Service for mail service between the Camden, NJ airport (USA) and the top of the post office building in downtown Philadelphia, Pennsylvania (USA). [3]
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