Helicopter, aircraft with one or more power-driven horizontal propellers or rotors that enable it to take off and land vertically, to move in any direction, or to remain stationary in the air. Other vertical-flight craft include autogiros, convertiplanes, and V/STOL aircraft of a number of configurations.
The idea of taking off vertically, making the transition to horizontal flight to the destination, and landing vertically has been for centuries the dream of inventors. It is the most logical form of flight, dispensing as it does with large landing fields located far from city centres and the inevitable intervening modes of travel—automobile, subway, bus—that flight in conventional aircraft usually requires. But vertical flight is also the most demanding challenge in flying, requiring more sophistication in structure, power, and control than conventional fixed-wing aircraft. These difficulties, solved over time by determined engineers and inventors, made the progress of vertical flight seem slow compared to that of conventional flight, for the first useful helicopters did not appear until the early 1940s.
One important characteristic of the history of vertical flight is the pervasive human interest in the subject; inventors in many countries took up the challenge over the years, achieving varying degrees of success. The history of vertical flight began at least as early as about 400 ce; there are historical references to a Chinese kite that used a rotary wing as a source of lift. Toys using the principle of the helicopter—a rotary blade turned by the pull of a string—were known during the Middle Ages. During the latter part of the 15th century, Leonardo da Vinci made drawings of a helicopter that used a spiral airscrew to obtain lift. A toy helicopter, using rotors made out of the feathers of birds, was presented to the French Academy of Science in 1784 by two artisans, Launoy and Bienvenu; this toy forecast a more successful model created in 1870 by Alphonse Pénaud in France.
The first scientific exposition of the principles that ultimately led to the successful helicopter came in 1843 from Sir George Cayley, who is also regarded by many as the father of fixed-wing flight. From that point on, a veritable gene pool of helicopter ideas was spawned by numerous inventors, almost entirely in model or sketch form. Many were technical dead ends, but others contributed a portion of the ultimate solution. In 1907 there were two significant steps forward. On September 29, the Breguet brothers, Louis and Jacques, under the guidance of the physiologist and aviation pioneer Charles Richet made a short flight in their Gyroplane No. 1, powered by a 45-horsepower engine. The Gyroplane had a spiderweb-like frame and four sets of rotors. The piloted aircraft lifted from the ground to a height of about two feet, but it was tethered and not under any control. Breguet went on to become a famous name in French aviation, and in time Louis returned to successful work in helicopters. Later, in November, their countryman Paul Cornu, who was a bicycle maker like the Wright brothers, attained a free flight of about 20 seconds’ duration, reaching a height of one foot in a twin-rotor craft powered by a 24-horsepower engine. Another man who, like the Breguets, would flirt with the helicopter, go on to make his name with fixed-wing aircraft, and then later return to the challenge of vertical flight, was Igor Sikorsky, who made some unsuccessful experiments at about the same time.
The next 25 years were characterized by two main trends in vertical flight. One was the wide spread of minor successes with helicopters; the second was the appearance and apparent success of the autogiro (also spelled autogyro).
The helicopter saw incremental success in many countries, and the following short review will highlight only those whose contributions were ultimately found in successfully developed helicopters. In 1912 the Danish inventor Jacob Ellehammer made short hops in a helicopter that featured contrarotating rotors and cyclic pitch control, the latter an important insight into the problem of control. On December 18, 1922, a complex helicopter designed by George de Bothezat for the U.S. Army Air Force lifted off the ground for slightly less than two minutes, under minimum control. In France, Argentine inventor Raúl Pateras Pescara, who designed several helicopters in the 1920s and ’30s that applied cyclic pitch control and, if the engine failed, rotor autorotation, set a straight-line distance record on April 18, 1924, of 736 metres (2,415 feet). That same year in France on May 4, Étienne Oehmichen established a distance record for helicopters by flying a circle of a kilometre’s length.
In Spain in the previous year, on January 9, 1923, Juan de la Cierva made the first successful flight of an autogiro. An autogiro operates on a different principle than a helicopter. Its rotor is not powered but obtains lift by its mechanical rotation as the autogiro moves forward through the air. It has the advantage of a relatively short takeoff and a near vertical descent, and the subsequent success of Cierva’s autogiros and those of his competitors seemed to cast a pall on the future of helicopter development. Autogiros were rapidly improved and were manufactured in several countries, seeming to fill such a useful niche that they temporarily overshadowed the helicopter. Ironically, however, the technology of the rotor head and rotor blade developed for the autogiro contributed importantly to the development of the successful helicopter, which in time made the autogiro obsolete.
In 1936 Germany stepped to the forefront of helicopter development with the Focke Achgelis Fa 61, which had two three-bladed rotors mounted on outriggers and powered by a 160-horsepower radial engine. The Fa 61 had controllable cyclic pitch and set numerous records, including, in 1938, an altitude flight of 11,243 feet and a cross-country flight of 143 miles. In 1938 the German aviator Hanna Reitsch became the world’s first female helicopter pilot by flying the Fa 61 inside the Deutschland-Halle in Berlin. It was both a technical and a propaganda triumph. Germany continued its helicopter development during World War II and was the first to place a helicopter, the Flettner Kolibri, into mass production.
In the United States, after many successes with commercial flying boats, Igor Sikorsky turned his attention to helicopters once again, and after a long period of development he made a successful series of test flights of his VS-300 in 1939–41. Essentially a test aircraft designed for easy and rapid modification, the VS-300 was small (weighing 1,092 pounds) and was powered by a 65-horsepower Lycoming engine. Yet it possessed the features that characterize most modern helicopters: a single main three-bladed rotor, with collective pitch, and a tail rotor. As successful as the VS-300 was, however, it also clearly showed the difficulties that all subsequent helicopters would experience in the development process. For many years, compared with conventional aircraft, helicopters were underpowered, difficult to control, and subject to much higher dynamic stresses that caused material and equipment failures. Yet the VS-300 led to a long line of Sikorsky helicopters, and it influenced their development in a number of countries, including France, England, Germany, and Japan.
After World War II the commercial use of helicopters developed rapidly in many roles, including fire fighting, police work, agricultural crop spraying, mosquito control, medical evacuation, and carrying mail and passengers.
The expanding market brought additional competitors into the field, each with different approaches to the problem of vertical flight. The Bell Aircraft Corporation, under the leadership of Arthur Young, began its long, distinguished history of vertical-flight aircraft with a series of prototypes that led to the Bell Model 47, one of the most significant helicopters of all time, incorporating an articulated, gyro-stabilized, two-blade rotor. Frank Piasecki created the Piasecki Helicopter Corporation; its designs featured a tandem-rotor concept. The use of twin tandem rotors enabled helicopters to grow to almost twice their previous size without the difficulty of creating very large rotor blades. In addition, the placement of the twin rotors provided a large centre of gravity range. The competition was international, with rapid progress made in the Soviet Union, the United Kingdom, France, Italy, and elsewhere.
To an even greater extent than fixed-wing aircraft, the development of the helicopter had been limited by engine power. Reciprocating engines were heavy, noisy, and less efficient at high altitude. The first application of jet-engine technology to the helicopter was accomplished in 1951 by the Kaman Aircraft Corporation’s HTK-1, which had Kaman’s patented aerodynamic servo-controlled rotors in the “synchropter” configuration (i.e., side-by-side rotors with intermeshing paths of blade travel).
In conventional aircraft the power of the jet engine was used primarily for increased speed. In the helicopter the thrust of the jet turbine had to be captured by a gearbox that would turn the rotor. The jet engine had many advantages for the helicopter—it was smaller, weighed less than a piston engine of comparable power, had far less vibration, and used less expensive fuel. The French SNCA-S.E. 3130 Alouette II made its first flight on March 12, 1955, powered by a Turbomeca Artouste II turbine engine. It rapidly became one of the most influential helicopters in the world and started a trend toward jet-powered helicopters everywhere.
There are now a vast number of helicopter types available on the market, ranging from small two-person private helicopters through large passenger-carrying types to work vehicles capable of carrying huge loads to remote places. All of them respond to the basic principles of flight, but, because of the unique nature of the helicopter’s rotor and control systems, the techniques for flying them differ. There are other types of vertical-lift aircraft, whose controls and techniques are often a blend of the conventional aircraft and the helicopter. They form a small part of the total picture of flight but are of growing importance.
The autogiro was for many years the most reasonable alternative to the helicopter as a means of vertical flight. Because the rotor is not powered, the autogiro does not have to contend with torque (the tendency of the aircraft to turn in the opposite direction of the rotor) and thus avoided many of the control problems that impeded the development of the helicopter. The autogiro’s rotor is designed so that a blade set at a low positive angle of pitch will rotate automatically as long as an airstream is kept flowing through the rotor (autorotation). As the autogiro is propelled forward through the air, with a stream of air flowing upward through its rotor, lift is generated. Control is effected in part through a universal joint at the rotor head, which tilts the blades creating a force that pulls the autogiro in the direction of the tilt. An elevator and rudder are maintained within the propeller slipstream for additional control. While prospects for commercial development of the autogiro evaporated with the success of the helicopter, sport autogiros known as “gyroplanes” became very popular.
Powered-lift aircraft can change the direction of their propulsion system’s thrust in flight. They characteristically have the airframe and propulsion system closely integrated so that the propulsion system exhaust flow influences the aerodynamics of the airframe. They encompass a number of types; among the most successful are the vectored jet, the externally blown wing, and the externally blown flap.
The most successful of all the alternatives to the helicopter is one of the most technically complex, the vectored jet, best exemplified by the Harrier, developed initially by Hawker Aircraft and brought to maturity by British Aerospace and McDonnell Douglas. In the vectored jet, nozzles are designed to rotate so that the thrust can be applied vertically for takeoff and then moved to a horizontal position for conventional flight.
In an externally blown wing system, the exhaust from the jet engines is directed over the upper surface of the wing (and in some cases over the outer surface of the flap area. Exhaust from the jet engines in the externally blown flap vehicle is directed against a large flap extension surface.
Other types of vertical-takeoff aircraft include convertiplanes. There are two types of V/STOL (vertical- or short-takeoff-and-landing) aircraft that may alternate between vertical takeoff and conventional horizontal flight. These are convertible rotorcraft and convertible airplanes.
The first group consists of two types, the most important of which is the tilt-rotor aircraft, such as the Bell/Boeing V-22, in which a helicopter rotor is tilted vertically for vertical lift and horizontally for ordinary flight. The V-22 stemmed from more than three decades of development, which began with the Bell XV-3 in the early 1950s. It represents a configuration offering the greatest promise for intercity air transportation, combining the utility of the helicopter with speeds approaching that of turboprop transports. The second type is the less frequently found compound helicopter, which has driven rotors and uses both an additional power source and an additional means of generating aerodynamic lift.
The second group, convertible airplanes with propellers, has four basic configurations. The first of these are the deflected thrust type, in which large propellers exert thrust against a wing deflected into a broad arc. The second type is the tilt wing. In these aircraft, the wing is rotated to point the propellers vertically for takeoff and landing, then adjusted for horizontal flight by bringing the wing to a normal angle of attack. The third is the tilt duct, in which propellers shrouded in ducts are rotated from one flight mode to the other. The fourth is the tilt propeller, perhaps the least successful of the group. The Curtiss-Wright Corporation built the X-100 test-bed, which was successful enough to allow the building of the more advanced but ill-fated X-l9 prototype that crashed during testing.
A number of attempts have been made to use the power of jet engines to lift an aircraft vertically from the ground and then shift to forward flight, but in every case the difficulties involved in recovery have inhibited the program. An early example, the Ryan X-13 Vertijet, was launched from a trailer bed that was erected vertically prior to takeoff. The aircraft flew successfully in vertical and horizontal modes, including takeoff and “tail-sitter” landings, but the operational limitations in terms of speed, range, and payload were too great for further development. The Ryan XV-5A Vertifan used a jet engine to drive horizontally mounted fans in the nose and wing; it was nominally successful. Another type of fixed jet used separate batteries of jet engines, some dedicated to vertical flight and some to horizontal flight, but this expensive technology was ultimately rejected.
Over time there have been a host of miscellaneous attempts at vertical flight. These include propeller-driven tail-sitters, dusted disc platforms, ground-effect aircraft (Hovercraft [trademark]), and deflected jet thrust. In most cases, the advantages sought were offset by the difficulties encountered, and the tilt rotor, the vectored jet, and especially the helicopter have remained the most successful means to vertical flight.
Principles of flight and operation
Unlike fixed-wing aircraft, the helicopter’s main airfoil is the rotating blade assembly (rotor) mounted atop its fuselage on a hinged shaft (mast) connected with the vehicle’s engine and flight controls. In comparison to airplanes, the tail of a helicopter is somewhat elongated and the rudder smaller; the tail is fitted with a small antitorque rotor (tail rotor). The landing gear sometimes consists of a pair of skids rather than wheel assemblies.
The fact that the helicopter obtains its lifting power by means of a rotating airfoil (the rotor) greatly complicates the factors affecting its flight, for not only does the rotor turn but it also moves up and down in a flapping motion and is affected by the horizontal or vertical movement of the helicopter itself. Unlike the usual aircraft airfoils, helicopter rotor airfoils are usually symmetrical. The chord line of a rotor, like the chord line of a wing, is an imaginary line drawn from the leading edge to the trailing edge of the airfoil.
The relative wind is the direction of the wind in relation to the airfoil. In an airplane, the flight path of the wing is fixed in relation to its forward flight; in a helicopter, the flight path of the rotor advances forward (to the helicopter’s nose) and then rearward (to the helicopter’s tail) in the process of its circular movement. Relative wind is always considered to be in parallel and opposite direction to the flight path. In considering helicopter flight, the relative wind can be affected by the rotation of the blades, the horizontal movement of the helicopter, the flapping of the rotor blades, and wind speed and direction. In flight, the relative wind is a combination of the rotation of the rotor blade and the movement of the helicopter.
Like a propeller, the rotor has a pitch angle, which is the angle between the horizontal plane of rotation of the rotor disc and the chord line of the airfoil. The pilot uses the collective and cyclic pitch control (see below) to vary this pitch angle. In a fixed-wing aircraft, the angle of attack (the angle of the wing in relation to the relative wind) is important in determining lift. The same is true in a helicopter, where the angle of attack is the angle at which the relative wind meets the chord line of the rotor blade.
Angle of attack and pitch angle are two distinct conditions. Varying the pitch angle of a rotor blade changes its angle of attack and hence its lift. A higher pitch angle (up to the point of stall) will increase lift; a lower pitch angle will decrease it. Individual blades of a rotor have their pitch angles adjusted individually.
Rotor speed also controls lift—the higher the revolutions per minute (rpm), the higher the lift. However, the pilot will generally attempt to maintain a constant rotor rpm and will change the lift force by varying the angle of attack.
As with fixed-wing aircraft, air density (the result of air temperature, humidity, and pressure) affects helicopter performance. The higher the density, the more lift will be generated; the lower the density, the less lift will be generated. Just as in fixed-wing aircraft, a change in lift also results in a change in drag. When lift is increased by enlarging the angle of pitch and thus the angle of attack, drag will increase and slow down the rotor rpm. Additional power will then be required to sustain a desired rpm. Thus, while a helicopter is affected like a conventional aircraft by the forces of lift, thrust, weight, and drag, its mode of flight induces additional effects.
In a helicopter, the total lift and thrust forces generated by the rotor are exerted perpendicular to its plane of rotation. When a helicopter hovers in a windless condition, the plane of rotation of the rotor (the tip-path plane) is parallel to the ground, and the sum of the weight and drag forces are exactly balanced by the sum of the thrust and lift forces. In vertical flight, the components of weight and drag are combined in a single vector that is directed straight down; the components of lift and thrust are combined in a single vector that is directed straight up. To achieve forward flight in a helicopter, the plane of rotation of the rotor is tipped forward. (It should be understood that the helicopter’s rotor mast does not tip but rather the individual rotor blades within the plane of rotation have their pitch angle varied.) For sideward flight, the plane of the rotation of the rotor is tilted in the direction desired. For rearward flight, the plane of the rotation of the rotor is tilted rearward.
Because the rotor is powered, there is an equal and opposite torque reaction, which tends to rotate the fuselage in a direction opposite to the rotor. This torque is offset by the tail rotor (antitorque rotor) located at the end of the fuselage. The pilot controls the thrust of the tail rotor by means of foot pedals, neutralizing torque as required.
There are other forces acting upon a helicopter not found in a conventional aircraft. These include the gyroscopic precession effect of the rotor—that is, the dissymmetry of lift created by the forward movement of the helicopter, resulting in the advancing blade having more lift and the retreating blade less. This occurs because the advancing blade has a combined speed of the blade velocity and the speed of the helicopter in forward flight, while the retreating blade has the difference between the blade velocity and the speed of the helicopter. This difference in speed causes a difference in lift—the advancing blade is moving faster and hence is generating more lift. If uncontrolled, this would result in the helicopter rolling. However, the difference in lift is compensated for by the blade flapping and by cyclic feathering (changing the angle of pitch). Because the blades are attached to a rotor hub by horizontal flapping hinges, which permit their movement in a vertical plane, the advancing blade flaps up, decreasing its angle of attack, while the retreating blade flaps down, increasing its angle of attack. This combination of effects equalizes the lift. (Blades also are attached to the hub by a vertical hinge, which permits each blade to move back and forth in the plane of rotation. The vertical hinge dampens out vibration and absorbs the effect of acceleration or deceleration.) In addition, in forward flight, the position of the cyclic pitch control causes a similar effect, contributing to the equalization of lift.
Other forces acting upon helicopters include coning, the upward bending effect on blades caused by centrifugal force; Coriolis effect, the acceleration or deceleration of the blades caused by the flapping movement bringing them closer to (acceleration) or farther away from (deceleration) the axis of rotation; and drift, the tendency of the tail rotor thrust to move the helicopter in hover.
A helicopter has four controls: collective pitch control, throttle control, antitorque control, and cyclic pitch control.
The collective pitch control is usually found at the pilot’s left hand; it is a lever that moves up and down to change the pitch angle of the main rotor blades. Raising or lowering the pitch control increases or decreases the pitch angle on all blades by the same amount. An increase in the pitch angle will increase the angle of attack, causing both lift and drag to increase and causing the rpm of the rotor and the engine to decrease. The reverse happens with a decrease in pitch angle.
Because it is necessary to keep rotor rpm as constant as possible, the collective pitch control is linked to the throttle to automatically increase power when the pitch lever is raised and decrease it when the pitch lever is lowered. The collective pitch control thus acts as the primary control both for altitude and for power.
The throttle control is used in conjunction with the collective pitch control and is an integral part of its assembly. The throttle control is twisted outboard to increase rotor rpm and inboard to decrease rpm.
The antitorque controls are pedals linked to operate a pitch change mechanism in the tail rotor gearbox. A change in pedal position changes the pitch angle of the tail rotor to offset torque. As torque varies with every change of flight condition, the pilot is required to change pedal position accordingly. The antitorque control does not control the direction of flight.
It was stated above that the lift/thrust force is always perpendicular to the plane of rotation of the rotor. The cyclic pitch control, a stick-type control found to the pilot’s right, controls the direction of flight by tipping the plane of rotation in the desired direction. The term cyclic derives from the sequential way each blade’s pitch is changed so that it takes the flight path necessary to effect the change in direction.
Differences in helicopter and airplane design and construction
The most immediate and obvious difference in the construction of a fixed-wing aircraft and a helicopter is of course the latter’s use of a rotor instead of a wing. There are many other critical additions, however, including the use of a tail rotor to offset torque. (Some helicopters use a “no tail rotor” system, in which low-pressure air is circulated through a tail boom to control the torque of the spinning main rotor.) Less obvious are such additions as the transmission system, which is used to transfer power from the engine to the rotor, tail rotor, and other accessories; the clutch, used to engage the engine and transmission with the rotor; and the mechanics of the rotor system itself.
The first helicopters were quite primitive, with skids instead of wheeled landing gear, open cockpits, and unaired fuselage sections. Helicopters are now as fully equipped as airplanes, with retractable landing gear and full instrumentation and navigation equipment, and are provided with whatever accoutrements may be necessary to accomplish the specific task at hand. For example, some helicopters are flying ambulances, especially equipped with a complete set of intensive-care accessories. Others function as electronic news gatherers, with appropriate sensors and telecommunications equipment.
The design and operation of helicopters have derived the same advances from computers and composites as have other aircraft, especially in the design and construction of the rotor blades. One of the more important improvements is in the simplification of flight-control systems, where a simple side stick controller, with the assistance of computers, performs the functions of the collective, cyclic, and throttle controls.
Helicopter designs have included a number of optional rotor configurations, such as rotors that stop to serve as a fixed wing for forward flight; rotors that fold in a streamwise direction to blend in with, or be stowed within, the fuselage contours, lift being provided by a stub wing; and X-shaped rotors that rotate for takeoff and landing but are fixed for lift in flight.
In sum, the additional forces imposed upon a helicopter by its very concept delayed its development, made it relatively more difficult to control than fixed-wing aircraft, and, in general, impeded its use. While generally considered more expensive to operate than conventional fixed-wing aircraft, a true comparison of costs cannot be made without assessing the additional advantages conferred by the vertical flight capability. The popularity of the helicopter indicates that users willingly pay any additional costs involved to obtain that capability. In some applications—medical evacuation, supplying of oil drilling rigs, spreading of certain agricultural agents, to name but a few—it is irreplaceable.Walter James Boyne