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Autogyro History
The early history of the autogyro is basically a history of one man, Don Juan de la Cierva.
Cierva was born in Murcia, Spain, September 21, 1895. He was only eight years old when the Wright brothers first flew on Dec. 17, 1903. He was a young man on his way to becoming a civil engineer when they first demonstrated their machine to the rest of the world in France in 1908. Cierva was intrigued by this new technology and decided to build his own airplane. His first attempt was to rebuild a Sommer biplane. He fitted it with a new engine and made several modifications to the original airplane. When he completed the project in 1912, he named the airplane the BCD-1 El Cangrejo, Spanish for the BCD-1 Crab.
The plane flew well and was considered to be the first Spanish built airplane. Cierva's second attempt was the BCD-2, a small monoplane, which he built in 1913. It did not fly as well as the BCD-1 and crashed. It was rebuilt, but crashed again. The design was abandoned. Cierva's third and final airplane design was the C-3. It was for a Spanish military competition announced in September of 1918. The C-3 was entered into the bomber division of the contest. The plane was a large tri-motor biplane, and was completed in May of 1919. It flew well, but in one of the preliminary tests the pilot flew the plane too slow and it stalled. The plane was wrecked, but the pilot escaped without serious injury. This crash disappointed Cierva, and inspired him to think of a better way to fly at low speeds. After tossing a toy helicopter from his parents balcony and studying the flight, Cierva came up with the idea of an autogyro, which he called the autogiro (Notice the spelling with an "i" instead of a "y").
Cierva's first attempt at building an autogyro was the C.1. The C.1 had two counter-rotating rotors to provide lift and by counter-rotating to eliminate torque. A vertical control surface above the rotors was meant to provide lateral control, while a conventional tail rudder and elevators would provide control about the other axes. Unfortunately, this design never flew. Because of the interactions between the two rotors, the top spun faster than the bottom. This upset the balance in lift and torque, causing the machine to tilt to one side. However, when it was tested, in October of 1920, it did demonstrate successfully the principles of autorotation while taxiing on the ground.
After the C.1, Cierva started work on his next design, the C.2. The C.2, was to have only one rotor, consisting of five blades, with duralumin spars. Because of difficulty in obtaining the duralumin, and because of a shortage of funds, work on the C.2 was postponed and Cierva began work on the C.3.
The C.3 was completed in June of 1921. The C.3 had one rotor with three blades. He still had a rudder and elevator for yaw and pitch control, but for lateral control he tried collective pitch variation. This refers to changing the angle of all the different blades at the same time. However this design proved to be impractical, and the C.3 only achieved brief hops of a few inches off the ground.
Once done with the C.3, Cierva went back to the C.2. The C.2 was finally completed early in 1922. It had similar controls to the C.3. It achieved slightly better lateral control, and short hops of a few feet above the ground, but still couldn't maintain sustained flight.
One of the problems with Cierva's three designs up to this point was that the rotor was rigid. This created two problems. First was that it created a gyroscopic effect. As soon as the aircraft tried to move, this effect would cause the aircraft to tilt. The other problem came from unbalanced lift. As the rotor was spinning, one side would be moving the same way the aircraft was moving, increasing the relative wind speed, while the other side would be moving opposite the direction the aircraft was moving, decreasing the relative wind speed. The side with the higher relative wind speed would have a higher lift than the side with lower relative wind speed, causing the aircraft to tilt. Cierva came up with a solution to this problem while watching an opera. One of the props for the opera was a windmill with hinged blades. Cierva decided to use hinges in his rotor designs. This allowed the blades to rise and fall depending on what direction they were moving in. The blades moving with the aircraft rose because of the higher lift, but this also served to decrease their angle of attack. The blades traveling in the opposite direction of they autogyro would fall because of the lower lift, serving to increase their angle of attack. The combination of the rising and falling action, which came to be known as flapping, and the increase and decease this had on the angle of attack served to balance the lifts created on each side of the aircraft. The hinged blades also eliminated the gyroscopic effect caused by the rigid blades.
Cierva's next design, the C.4, incorporated these hinged rotors. For lateral control, ailerons were mounted on outriggers to the side of the aircraft. Yaw and pitch control still came from a rudder and elevators. On January 17, 1923, the C.4 flew, marking the first controlled flight of an autogyro. The C.4 also demonstrated the autogyro's safety in low speed flight. On January 20, three days after its first flight, the autogiro went into a steep nose-up attitude after an engine failure at about 25-35 ft. In an airplane, this would have almost certainly resulted in an almost unrecoverable stall. But the autogyro just descended gently to the ground without damage to the machine or injury to the pilot. This low speed safety was demonstrated even more dramatically on January 16, 1925, when another design, the C.6, lost power after take-off at about 150-200 ft. The pilot was still able to turn the autogyro around and bring it in for a safe landing, with only slight damage to the machine. This maneuver would have been much more difficult in an airplane, and quite possibly could have led to a worse accident.
Another problem with early autogyros was rotor spin-up. To achieve autorotation, the rotor had to be spun to a minimum speed before the aircraft began moving. In early autogyros, this was achieved mainly in four ways, spinning the rotor by hand if the rotor was small enough, or if it was bigger, with a team of horses, a team of people, or by connecting it to a car engine through a drive shaft. This need for an external source to spin the rotor kept the autogyro from being a self sufficient machine capable of working anywhere. Cierva's solution for this problem was to design the tail in such a way that it would deflect the slipstream from the propeller up into the rotor. This deflected wind would cause the rotor to spin up. It was achieved simply by adding flaps to the tail that angled upwards. This design was known as a scorpion tail. He tested this idea on a C.19 in 1929. The problem was that the tail did not deflect enough wind. The rotor did spun some, but not to the minimum speed needed for takeoff. A short takeoff run was still needed to spin the rotor the rest of the way. Another method used to solve the problem of rotor spin-up was, instead of connecting a drive shaft to a car, just connect it right to the autogyro's engine. The first attempt to do this was in a modified C.11 early in 1930. A drive shaft was connected to the engine through a clutch. Unfortunately, this mechanism weighed about 165 lb. and was too heavy for the autogyro to fly. An improved clutch and drive shaft system was introduced late in 1930. It was first used in April 1931 on the PCA-2, an autogyro developed by Harold Pitcairn, who had the licensing rights for Cierva's invention in the United States. The drive shaft and clutch system wasn't used on a Cierva design until almost a year later in March of 1932 on the C.19. This new design worked well on both the PCA-2 and the C.19, and became the dominant style of spin-up in all later models of autogyros where the rotor was too large to be spun by hand.
The next major advance in autogyros came on August 5, 1931. This was the first flight of the Wilford WRK. This new autogyro replaced the hinged rotors with a rigid rotor with cyclic pitch variation. Cyclic pitch variation is a method where the pitch of the blades is changed as they spin. The pitch is lowered when they are moving in the direction of the aircraft, and raised when they are moving in the opposite direction. This does the same thing as flapping to balance the lift created by the blades. The WRK was the first autogyro to successfully fly with a rigid rotor.
Earlier in this paper, it was stated that autogyros have the potential for vertical take off and landing. Although all the autogyros discussed so far have been capable of vertical landings at least in an emergency, they have also all needed some minimum takeoff run. But, if the rotor was powered before take off to make it spin at the minimum speed for autorotation, why not just continue to power it to a higher speed and take off from the lift created that way. That is exactly what happened. In August of 1933, experiments were begun on a C.30 in this new method of takeoff, which came to be known as a jump takeoff. These first experiments were promising, but not satisfactory. Spinning the rotor on the ground caused too much vibration, and the aircraft was only capable of making low jumps. By October 28, 1934, after over a year of experimenting and refining, the C.30 finally made a successful jump takeoff. Many later autogyros were also designed for jump takeoffs, most using the same method as the C.30. A few later autogyros had tip driven motors where either a jet or a rocket was put at the end of each rotor blade to spin the rotor that way.
The C.30, besides being the first autogyro to make a successful jump takeoff, was notable for another aspect as well. It was the first autogyro to use direct control. Direct control was a method where the pilot tilted the rotor instead of a rudder and ailerons. This greatly simplified the control of the aircraft, as well as the design. A pilot now had one control for yaw, pitch, and roll, and designers only needed to design that one control. In the C.30 and later autogyros of comparable size, this consisted of a bar connected directly to the rotor hub that extended into the cockpit. For larger machines, the controls of the pilot were mechanically linked to the rotor hub. The C.30 also proved to be the most popular production autogyro ever designed, with more than 180 of them being built.
On June 26, 1935, the Breguet-Dorand 314 was the first successful helicopter to fly. It incorporated many of the features developed for autogyros, such as collective and cyclic pitch control. On December 8, 1941, Igor Sikorsky's V.S.300 flew, another of the first successful helicopters. The V.S.300 was only a test aircraft, but led to the VS-316, a more refined helicopter using the same principles. The U.S. Army ordered the VS-316, and 400 of these aircraft were produced along with the R-5 and R-6, two other Sikorsky helicopters of similar design.
Why Autogyros Weren't Accepted
At this point we can ask the question of why autogyros were never widely accepted. Just about every aviation historian has their own answers to this question, but here is this author's opinion. Early autogyros, although they had a higher speed envelope than airplanes, had a higher drag and so were not as efficient at higher speeds, and absolutely could not attain the maximum speeds of the faster airplanes. Also, the early autogyros did not have the vertical takeoff and landing capabilities that would have made them more attractive to potential buyers. When the C.30 finally demonstrated a successful jump takeoff in 1934, it was less than a year until the first successful helicopter flew, and only a few more years until the very successful Sikorsky V.S.300 and VS-316. Although helicopters had a smaller speed envelope than autogyros, they were capable of hovering, and their envelope could fill the role that airplanes couldn't. In other words, anything an autogyro could do could be done by another aircraft. Also, Cierva, who was doing most of the development of autogyros, was funding much of the development on his own. When the army ordered the VS-316, that money went in to Sikorsky's company. This gave Sikorsky the funding for development that Cierva was running out of. Without the money, Cierva just couldn't fund the research. And then, on December 9, 1936, Cierva was killed in a plane crash (a DC-2 operated by KLM). He was only 41 years old. There were other people developing autogyros, but Cierva had been one of the main driving forces behind the movement. Much was lost when he was killed.
Another factor that kept the autogyro from being accepted was purely psychological. Even though helicopters weren't successful until 1935, they had been under development for as long as airplanes. The general public knew about helicopters, and understood the principle of a powered rotor. Autogyros had an unpowered rotor that spun due to aerodynamic forces. Most people did not understand how it worked and so did not trust it. Although it is actually safer than either helicopters or airplanes, people did not realize this. They wanted something powered.
Autogyros After Helicopters
After helicopters flew successfully and the companies that designed them got military grants for further research, the autogyro was pretty much abandoned. Except for a few concepts and only a handful of attempts at civil designs, autogyros were kept alive only as home built aircraft, and that mostly as ultralights. Recently, there have been two companies to resurrect the idea of the autogyro, Groen Brothers and CarterCopter.
The Groen Brothers design is the more conventional of the two. The unique innovation of the Groen Brothers machine is that it is using ram jets at the tips of its rotor blades to power them for spin up. Ram jets are not very efficient engines, but they will only need to be run for ten to fifteen seconds, so efficiency is not much of an issue. The ram jets will spin the rotor fast enough to enable the machine to take off vertically. Their autogyro will also use a collective pitch control to help reduce drag. It is being marketed to corporations that currently use helicopters, or can't quite afford helicopters, to travel from rooftops in cities. These corporations really have no need for hover, they just need to be able to take off and land vertically. The autogyro can do this faster and for less money than a helicopter. They are also trying to market it to organizations that only need low speed flight and not complete hover. In fact, most observation of stationary objects done by a helicopter is done by flying above it in slow circles, not just hovering.
The CarterCopter is more of a hybrid between an airplane and an autogyro. The rotor will be spun up with a conventional drive shaft and clutch system, but the rotors will have 60 pound weights at their tips. This will cause the rotor to function as a flywheel for the jump takeoff. It also allows the rotor to be slowed down in cruise and still maintain enough rigidity from centrifugal force to be stable. Once flying, the pitch on the rotor blades will be lowered as much as possible to reduce the drag created by them to as little as possible. The lift will come from conventional airplane wings mounted on the sides of the aircraft. The company is predicting that the CarterCopter could surpass the performance of all aircraft except jet propelled airplanes and spacecraft. This includes helicopters, all previous autogyros, piston powered prop planes, and turboprop airplanes. With a turboprop engine, the craft should be able to fly 400 mph at 45,000 feet. A modified propeller version could be able to fly at 70,000 ft., while a jet powered version could fly 500 mph or more. A specially modified version of the aircraft should be able to fly 25,500 miles on one tank of gas, which would allow it to equal the Voyager's round the world flight of 1986, but with a vertical takeoff and landing. With its performance, the CarterCopter would not only be able to fill the roles the Groen Brothers would like to fill, but also those currently belonging to propeller driven aircraft. (In the interest of full disclosure, it should be stated that the author is an employee of Carter Aviation Technologies - although the CarterCopter was included in the original version of this paper while the author was still a student.)
Concluding Remarks
Autogyros were the first successful rotary wing aircraft to fly. They marked a departure from conventional fixed wing aircraft and an attempt to fill a role that airplanes couldn't. They can fly slowly due to a phenomenon known as autorotation, where the rotor is unpowered and is made to spin by aerodynamic forces. Autorotation allowed the wings to move faster than the aircraft. Although autogyros were never widely accepted by the public, the military, nor aircraft companies, they were very important in the development of the helicopter. Many technologies essential for practical helicopters were first developed for the autogyro. If Cierva had not pursued the autogyro, it almost certainly would have delayed the development of the helicopter, maybe even for decades. After the introduction of the first successful helicopters, autogyros were largely forgotten except as kit aircraft and ultralights. Recently, two companies, Groen Brothers and CarterCopter, have brought back the autogyro using modern technologies. Neither plans to replace the helicopter entirely, only in places where low speed flight or vertical take off and landing are all that are needed. Perhaps, if these companies have their way, the future will be more kind to the autogyro than has the past.
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