The origins and development of rotorcraft technology

Rotorcraft

The History of Rotorcraft

For ages humankind has envisioned harnessing the capability not merely to fly but to be able to lift oneself vertically from the ground and set oneself down again without forward run. However, it was not until the early twentieth century that man invented a helicopter capable of such feats. The word "Helicopter" is derived from the Greek term "helix" (spiral) and "pteron" (wing), which stamp it as a member of the "rotary-wing" family of aircraft. It is these power-driven rotor(s) that are the subject of this paper. It investigates the ancient origins of rotorcraft, how man's interest with flight has benefitted modern society and the implementation of future uses.

Origins

The ancient Chinese were the first known society to understand and recount some of the rudimentary ideas in rotary wing aircraft. Around 400 A.D. A book surfaced entitled "Bao Pu Zi" which described the concept of flight by rotary wings

. Ancient Chinese children played with bamboo dragons, a hand-spun toy that rose upward when revolved rapidly

. This toy was the first known rotorcraft, and the beginning of centuries of interest, research, tests and inventions. It consisted of a propeller on a stick, and rolling the stick in the right direction spins the propeller, causing the toy to take flight when it is let go [see image below]. This toy eventually made its way to Europe via trade and caught the attention of philosophers and artists alike. It was even depicted in a 1463 European painting

Figure 1: Ancient Chinese bamboo dragon c. 400 AD

This form of play aircraft engaged the attentions of Leonardo da Vinci as it related to his fascination with the phenomenon of flight

. In 1490 Leonardo da Vinci was the first to put down on paper a design for a man powered spiral winged device that resembled the modern helicopter and another that resembled a hang-glider. However, his designs were impractical and could not provide lift. His sketches of the "aerial-screw" or "air gyroscope" device date back to 1483 but were not published until nearly three centuries later

Figure 2: Da Vinci's design for a flying machine c. 1488

Even into the nineteenth century these toys were sold in the U.S.A. As Wooden Puddle Jumpers, and they were studied by Sir George Cayley in 1809 to play a role in the birth of modern aviation. However, It wasn't until the early 1900's that the first helicopter took flight

II. Early Developments

More than 2,000 years after Leonardo Da Vinci's sketches, the latter half of the eighteenth century marked the beginning of the industrial revolution and of the continued search for manmade flight. In 1754, Mikhail Lomonosov of Russia developed a small coaxial rotor modeled after the ancient Chinese toy but powered by a wound-up spring device

. The device flew freely and climbed to a reasonable altitude

. In 1783, the French naturalist Christian de Launoy used a coaxial version of the Chinese toy in a model consisting of a counter-rotating set of turkey feathers powered by a string wound around the rotor shaft and tensioned by a crossbow

. When the tension was released, the blades whirled to climb the device high into the air. The scientific stir created by these early successes inspired the French mathematician A.J.P. Paucton in 1786 to publish one of the first scientific papers on the dilemma of rotating wings entitled "Theorie de la vis D'Archimede."

III. The first Helicopters

Even before the Wright Brothers first airplane took flight, Igor Sikorsky was building a coaxial helicopter in Russia.

. Into the early twentieth century, there were more attempts to build helicopters than fixed wing aircrafts

. However, these early models, developed around 1907 were not able sustain flights as we know it today; they merely just hopped of the ground

. The failure of these early pioneers and their designs is due in part to a failure to truly understanding of the nature of lift and their heavy weight-to-power ratio from inadequate motor use. The development of the engine is fundamental to any form of flight. While airplanes could fly with engines of relatively lower power, the success of the helicopter's hover abilities required aircraft engine technology that could build much more powerful and lightweight engines. A look at the historical record shows that the need for engines of sufficient power-to-weight ratio was really a key enabling technology for the success of the helicopter

Prior to 1870, the steam engine was all that was available for engineers and mechanics to use in most mechanical devices. The steam engine is a relatively primitive external combustion engine. It requires a separate boiler, combustor, recirculating pump, condenser, power producing piston and cylinder and well as a fuel and an ample supply of water

. All of these heavy components weigh down the steam engine and create a power-to-weight ration that is unsuitable for aeronautical use. Nonetheless, the innovations of James Watt greatly improved on the steam engine making it relatively practical for use in early helicopters. In the 1920's, when lighter gas engines were developed, engineers were able to achieve a more balanced weight-to-power ratio necessary for sustained flight

Then the problem of torque arose to confound early pioneers until the introduction of swashplate. Torque is the effect produced by the rotor to force the fuselage to rotate in the opposite direction as the engine

. Due to dissymmetry of lift the early single-rotor helicopters tended to flip over when changing between hovering to forward movement

. The swashplate provides a means of varying the pitch of the blades in a cyclic fashion as they rotate around the central shaft to create the freedom of independent blade motions. Cyclic pitch control equalized the lift on each side of the shaft and eliminated the tendency to tip over sideways. Gaetano A. Crocco was one of the first designers to propose cyclic pitch for rotor control in 1906 Italy

. Crocco, who pioneered the idea of hydrofoil boats, recognized that proper forward flight in a helicopter required a means of changing the pitch on the blades to account for dissymmetry of the lift

The coaxial design's basic simplicity further reduced torque issues. By placing two rotors on top of each other with counter-rotation, the power train remains short and the airframe can take many shapes. Counter-rotation eliminated the need to force torque into the airframe. In hovering flight the lift force is only vertical without a tail rotor producing a side force that requires constant pilot intervention to avoid tip-over. This design was originally produced by the Russian Kamov helicopter design bureau

. Well-known names in coaxial designs include Cornu, Asboth, Pescara, De Bothezat, Berliner, Bendix, Hiller and others.

On November 13, 1907, the French pioneer Paul Cornu lifted a coaxial rotor helicopter into the air for a few seconds

. The airframe was very simple, with a rotor at each end and two large blades set at the periphery of a spoked wheel. Power was supplied to the rotors by a gasoline motor and belt transmission. Also in the early 1900's, Henry Berliner created the first powered coaxial rotorcraft that successfully made a controlled flight. Berliner's helicopter only traveled about 100 yards at an altitude of about 15 feet, but the flight was successfully controlled by a pilot

The VS-300, the first practical helicopter was designed by Sikorsky in 1939

. The VS-300 was a single rotor (as opposed to the coaxial rotor discussed above) with an open cockpit welded together by steel tubing frame. Although the VS-300 could ascend vertically and could also fly sideways and to the rear, it could not fly safely forward

. Sikorsky continued to redesign the safety quotient of the aircraft, on May 6, 1941 he set a new world helicopter endurance record by flying the VS-300 for 1 hour, 32 minutes and 26 seconds

. From the VS-300 was developed the R-4, more than 400 of which served in China, Burma, India, the Pacific, Europe and Alaska in the second World War and afterwards

IV. The Autogiro

During the roaring twenties, Juan de la Cierva developed an aeronautical design independent of the contemporary engineering paradigms

. This design led to the Autogiro, the first successful rotating -- wing aircraft to demonstrate a useful role in aviation

. However, it wasn't until hinged rotor blade and the swashplate, discussed above, that design was laid out for the helicopter as a practical form of air-transport.

Cierva's Autogiro differed from the later helicopter in that the Autogiro rotor was not run by a motor (which removes the power-to weight ration problem that many developers struggled with approximately fifteen year later) such that the rotor movement came from a flow of air directed upwards through the rotor disk

. However, only a very small flow is necessary to turn the rotor, so as long as the machine continues to move through the air, the rotor will continue to turn

. Therefore, the early autogiro was mechanically simpler that the helicopter, but in no way was it technologically any less.

By the early thirties the autogiro had a relatively stable control system that used the rotating blades for control of vertical flight at very low speeds

. By the end of the decade, World War II was under way and the United States Military released a request for bids from aircraft manufacturers to design and build a helicopter. Contracts were given to Sikorsky Aircraft in Bridgeport and Platt-LePage in the Philadelphia, and from this the American helicopters were developed.

V. American Helicopters

There are four general helicopter designs which have been developed and produced in the United States: the most common being the tail rotor design, more popularly known as the single rotor design, which has one main rotor and a small tail rotor. Other rotor designs produced in America include the tandem rotor design, the syncropter design and the coaxial design

. The single main rotor and a small anti-torque rotor dominate modern helicopter design today.

By 1940, just after the start of WWII Sikorsky had developed a helicopter that could fly but it was impractical for military use because the system was too complicated for trainee pilots

. Within a few years of this early design, Sikorsky gained access to the Autogiro Company's patents and designs. Once the autogiro rotor system was added to Sikorsky's earlier designs he was able to deliver a simple and flyable helicopter to the United States Military.

Gyrodyne Company of America, Inc. began to occupy the field of design, development, testing, and production of coaxial helicopters starting in the late 1940s once Sikorsky's WWII contract with the military ended

. From the mid-fifties to the early seventies, Gyrodyne manufactured and sold approximately 800 coaxial helicopters. Gyrodyne ceased development on coaxial designs in the mid 1970s due to financial difficulties but it was at the forefront of numerous technological milestones that paved the path for the contemporary helicopter.

VII. Future Rotorcraft Capability

Many engineers and aeromechanics believe, due to vague government projections for improvement of rotorcraft capability, that any advances in helicopter-type crafts will be made in the civilian sector

. The building social trend toward miniatures has extended into the designs of future rotorcrafts as many attempts have been made to reduce the rotorcrafts size and to realize fully autonomous ?ight. A multi-year, 35-million-U.S.-dollar launched by DARPA in 1997, directly aims to develop micro-air- vehicles such as the Fancopter UAV system, whose largest rotor span is no more than 15 cm. Unfortunately, results have exhibited that a 15 cm UAV is too small to be useful or workable

. However, significant strides have been made in the way of the Epson Tokyo micro-flying robot [see image below]. The mini rotorcraft has potential for providing unique combat support for battles in con-ned areas such as streets

Figure 5: The 2004 Epson Tokyo Micro Flying Robot

Attention has recently shifted to the development of larger miniature unmanned aerial rotorcraft vehicles with practical applications. One successful application of the rotorcraft unmanned aerial vehicles (UAV) is in the civil domain is the pesticide spraying in agriculture and forestry. As a result of the Japanese Ministry of Agriculture's 1983 request for crop dusting helicopters from Yamaha, the most representative examples of UAV's for this purpose are the Yamaha R50 and Rmax helicopters

. These UAV's reduced the labor and costs in the rice farming industry. The R50 and Rmax series have now emerged as the dominating force in the pesticide spraying service

The unmanned aerial vehicle also provides a platform for surveillance and reconnaissance in some crucial tasks such as range safety monitoring, arms transfer monitoring, and mine detecting

. As far as arts and entertainment are concerned, the miniature UAV'S are capable of providing a birds-eye view of stadiums or capturing aerial footage for special effects in movies

. Further applications include assisting workers and engineers to solve engineering and construction problems, which are dif-cult to resolve through conventional means. For instance, the Automatic Lab of Chiba University has successfully developed a rotorcraft UAV, called Sky Surveyor2 equipped with a vibration-free PTZ camera to help Chugoku Electric Power conduct inspection on power transmission lines between each conjoined high-voltage tower

The civilian sector however is not solely engaged in miniature aircraft, but is also focusing on improving the traditional manned helicopter. The current tilt-wing MV-22 Osprey, can hit speeds upward of 240 knots, but it's a hybrid machines, thus to reach heightened speeds it needs to fly like a fixed wing airplane. However, the X2 technology, currently in development by Sikorsky will allow a coaxial helicopter to reach speeds almost twice as fast as today's helicopters

. Sikorsky showed off its X2 Technology Demonstrator suite as Heli-Expo 2008

in which it suggested that an X2 aircraft could match the current Osprey MV-22 in speed while having far superior vertical take-off due to vertical take-off jets and in hovering

. The X2's coaxial setup would eliminate the torque, allowing the tail rotor to propel the helicopter forward much like the propeller on a ship. Internally, an X2-based aircraft would have fly-by-wire (that is, electronic) flight controls, as opposed to traditional mechanical or hydraulic controls. Sikorsky says the eventual aircraft would serve as a "flying wind tunnel" to measure the performance of the rigid main rotors and their relationship to the rest of the aircraft's aerodynamics.

Even though the civilian sector is busy developing faster, smaller, and unmanned helicopters, the government is also making progress in the field of aeronautics. NASA is currently doing research into rotor blades made with piezoelectric actuators that can change and adapt the rotor blade while in motion. Initial test data has shown that the new rotor technology can halve the amount of noise it puts out within the controlled environment of the wind tunnel

According to William Warmbrodt, chief of NASA'S aeronautics branch, "today's limitations preclude us from having [large rotorcraft that make short hops between cities such as New York and Washington, carrying as many as 100 passengers at a time in comfort and safety] so NASA is reaching beyond today's technology for the future.

" This new rotor technology will substantially improve the performance of the rotor which will allow helicopters to fly farther and engage in more tactical operations that require a quiet, fuel efficient aircraft.

V. Conclusion

From ancient times, humankind has been obsessed not only with flight but with the ability to hover and as a result have invested countless effort, trials, prototypes, money and even human life to satisfy this desire to hover in the air. Due to complexity, difficulty of learning to pilot and cost and regulation, the helicopter has never reached a mass consumer market, but has instead remained a military device capable of many government applications and minimal civilian use such as in traffic reporting and minor media applications. However, though the development of safer tilt-rotor aerial vehicles and unmanned miniatures, the helicopter will soon become a mass-market product.