AUTOGYRO OTHERWISE KNOWN AS GYROCOPTER

An autogyro is a type of rotorcraft that uses an unpowered rotor in free autorotation to develop lift. Forward thrust is provided independently, by an enginedriven propeller. While similar to a helicopter rotor in appearance, the autogyro’s rotor must have air flowing across the rotor disc to generate rotation and the air flows upwards through the rotor disc rather than down.

The autogyro was invented by Spanish engineer Juan de la Cierva in an attempt to create an aircraft that could fly safely at low speeds. He first flew one on 9 January 1923, at Cuatro Vientos Airfield in Madrid. The aircraft resembled the fixed-wing aircraft of the day, with a front-mounted engine and propeller. Cierva’s autogyro is considered the predecessor of the modern helicopter.

The success of the autogyro garnered the interest of industrialists and under license from Cierva in the 1920s and 1930s, the Pitcairn & Kellett companies made further innovations.Late-model autogyros patterned after Etienne Dormoy’s Buhl A-1 Autogyro and Igor Bensen’s designs feature a rear-mounted engine and propeller in a pusher configuration.

The term Autogiro was a trademark of the Cierva Autogiro Company, whilst the term ‘Gyrocopter’ (derived from helicopter) was used by E. Burke Wilford who developed the Reiseler Kreiser feathering rotor equipped gyroplane in the first half of the twentieth century. The latter term was later adopted as a trademark by Bensen Aircraft.

Principle of operation

An autogyro is characterised by a free-spinning rotor that turns because of the passage of air through the rotor from below. The downward component of the total aerodynamic reaction of the rotor gives lift to the vehicle, sustaining it in the air. A separate propeller provides forward thrust and can be placed in a puller configuration, with the engine and propeller

at the front of the fuselage, or in a pusher configuration, with the engine and propeller at the rear of the fuselage. Whereas a helicopter works by forcing the rotor blades through the air, drawing air from above, the autogyro rotor blade generates lift in the same way as a glider’s wing, by changing the angle of the air as the air 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 force and the thrust force in balance. Because the craft must be moving forward with respect to the surrounding air in order to force air through the overhead rotor, autogyros are generally not capable of vertical take-off.

Pitch control is achieved by tilting the rotor fore and aft, roll control by tilting the rotor laterally. The tilt of the rotor can be affected by means of a tilting hub (Cierva), a swashplate (Air & Space 18A), or servo-flaps. A rudder provides yaw control. On pusher configuration autogyros, the rudder is typically placed in the propeller slipstream to maximise yaw control at low airspeed.

Flight controls

There are three primary flight controls: control stick, rudder pedals and throttle. Typically, the control stick is termed the cyclic and tilts the rotor in the desired direction to provide pitch and roll control (some autogyros do not tilt the rotor relative to the airframe, or only do so in one dimension and have conventional control surfaces to vary the remaining degrees of freedom). The rudder pedals provide yaw control, whilst 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 take-off and collective pitch to reduce blade pitch before driving the rotor.

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’. Its main advantages are the simplicity and lightness of its construction and the unobstructed visibility. It was developed by Igor Bensen in the decades following World War II, who also founded the popular rotorcraft association (PRA) to help it become more widespread.

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 autogyro. Nonetheless, the tractor configuration has some advantages compared to a pusher, namely greater yaw stability (as the center of mass is farther away from the rudder) and greater ease in aligning the center of thrust with the center of mass to prevent ‘bunting’ (engine thrust overwhelming the pitch control).