Avro Type 671 Rota
By 1934, the autogyro form of aircraft, developed by Señor Juan de la Cierva, a Spaniard, was sufficiently advanced for the Royal Air Force to order a small number of C-30As for evaluation on army co-operation duties. However, by the outbreak of the Second World War they had been allocated for use by a highly secret unit engaged on ground radar calibration duties.
The C-30A was the most widely produced Cierva autogyro design. Avro built the type under licence, as the Avro Type 671 Rota, both for the civil and military market.
Evaluation of the type took place at the School of Army Co-operation at RAF Old Sarum. Neither the C-30A, nor the later C-40, was adopted by the Royal Air Force as an observation or communication aircraft, but the development of ground radar, and in particular the need for a slow-flying aircraft for its calibration, meant the Rota had a valuable wartime role.
Early British radar operated on a wavelength of 11 meters, making the spar of each C-30 rotorblade a perfect ½ wave dipole reflector.
Range measurement with early radar was simple and straightforward, being based on fundamental physical quantities but azimuth determination was a serious challenge.
An antenna aperture of more than 10 wavelengths is required for producing narrow beams, making steerable directional antennas impractical.
The 11-meter radar, rather than using steerable antenna arrays for azimuth determination, used widely separated receiving antennas and the phase difference of the returning echo from each antenna was compared. This utilized a device called a goniometer, a variable inductor, not very repeatable from one to another.
Each site therefore had to be individually calibrated in azimuth and the C-30 was ideal for this purpose; being able to loiter over a fixed location and producing a huge echo.
A revolutionary advancement in machines capable of vertical flight came in 1923 with the Spanish civil engineer Juan de la Cierva’s C- 4 autogyro. The autogyro (gryoplane) appeared as a hybrid between a helicopter and an airplane, with a conventional propeller, airframe, and tail, but with the rotors mounted above the fuselage. De la Cierva’s design consisted of a converted fixed-wing aircraft with a coaxial rotor system that he hoped would solve the problem of asymmetrical lift. Unfortunately, the rotors failed to control the rolling motion of the aircraft, and his first autogyro crashed.
“Gryoplane” is an official term designated by the Federal Aviation Administration (FAA) describing an aircraft that derives lift from a freely spinning rotary wing (rotor blades), and that receives its thrust from an engine-driven propeller. Historically this type of aircraft was known as the autogyro and the gyrocopter. The early names and variants were filed as trademarks. Early autogyros were powered by engines in a tractor (pulling) configuration and were relatively heavy. Modern gyroplanes, with a rear-mounted engine and pusher propeller, are lighter and more maneuverable.
Gyroplanes exhibit qualities of both helicopters and airplanes, thus are a type of hybrid. Gyroplanes can fly at slower airspeeds than airplanes and are less likely to stall, but they cannot hover. Helicopters draw air down through engine-powered rotor systems in order to hover. Since the rotor system on a gyroplane is powered only by the air rushing over the rotor blades, much like a windmill, no antitorque rotor is required. Air velocity turning the rotor blades is known as autorotation, and it occurs in helicopters as well. Both gyroplanes and helicopters utilize this flight characteristic to descend to the ground in the event of an engine failure.
Controversy evolved over the actual date of the first flight of de la Cierva’s autogyro, but on either January 9 or January 17, 1923, the first controlled gyroplane flight in history occurred at Getafe Airdrome, near Madrid, Spain. After his initial failure de la Cierva conducted wind tunnel tests on several small models and solved most of his aerodynamic problems. His fourth machine incorporated mechanical “flapping” hinges at the blade root, which somewhat equalized the lift on each side of the rotor disk. The hinges allowed the blades to flap up or down, reacting to the varying pressures exerted on the spinning blades. Although Charles Renard had suggested the principle of flapping blades in 1904, and Louis Breguet patented the idea in 1908, de la Cierva received credit for the first successful practical application to a rotating-wing aircraft.
Notwithstanding de la Cierva’s successful flight, his rudimentary designs required improvements on successive models. On his first models the engine drove the propeller only. A ground crew started the rotors turning by pulling a rope wound around the rotor shaft. Otherwise the pilot taxied around on the ground until the rotors, or “windmills” as de la Cierva called them, began spinning, then the pilot opened the throttle to gain sufficient airspeed to lift off into flight. To synchronize the hinged blades, the inventor linked the rotors together with small cables and turnbuckles near the midpoint of each blade. De la Cierva also installed a clutch on later models that allowed the pilot to power the rotors with the engine, and to disengage them for autorotation.
In later models of his autogyros, de la Cierva incorporated even more sophisticated controls to compensate for asymmetrical lift. He added a lag hinge, which allowed the blades to move freely fore and aft, further alleviating the aerodynamic forces exerted on the blades. His modifications eventually evolved into a fully articulated rotor hub. A control stick allowed the pilot to tilt the entire rotor disk to fly the aircraft, eliminating the need for ailerons, but not the elevator and rudder. In 1935 de la Cierva introduced an advanced rotorblade pitch-change mechanism in versions of the C-30 model autogyro. De la Cierva referred to his integrated control system as “orientable direct rotor control.”
Although de la Cierva’s autogyros exhibited some helicopter capabilities, they could not hover. His machines, nonetheless, required minimal forward airspeed to maintain flight. An inestimable number of flights proved that his autogyros were very safe and essentially stall-proof. The autogyros could land in small, confined areas because they handled well at low speeds. Takeoffs, however, required a short runway to attain flying speed. De la Cierva’s autogyro was not a true helicopter, but his contribution to rotorcraft control systems was nonetheless significant. Some aviation scholars consider the first flight of the autogyros almost as momentous as the Wright brothers’ success at Kitty Hawk.
Variants of Avro 671 Rota
Powered by a 78-kW (105-hp) Armstrong Siddeley Genet Major I radial piston engine.
Improved model, powered by a 104-Kw (140-HP) Armstrong Siddeley Genet Major IA radial piston engine.
Main production model, powered by a 104-kW (140-hp) Armstrong Siddeley Genet Major IA radial piston engine.
Rota Mk I
RAF designation of the Cierva C.30A.
Crew: one, pilot
Length: 19 ft 8 in (6 m)
Rotor diameter: 37 ft (11.28 m)
Height: 11 ft 1 in (3.38 m)
Empty weight: 1,220 lb (554.5 kg)
Loaded weight: 1,800 lb (818 kg)
Powerplant: 1× Armstrong Siddeley Genet Major IA 7-cylinder air-cooled radial engine, 140 hp (104 kW)
Maximum speed: 110 mph (177 km/h)
Cruise speed: 95 mph (153 km/h)
Range: 285 mi (458 km)
Rate of climb: 700 ft/min (213.4 m/min)