A helicopter is an aircraft which is lifted and propelled by one or more horizontal rotors. Helicopters are classified as rotary-wing aircraft to distinguish them from conventional fixed-wing aircraft. The word helicopter is derived from the Greek words helix (spiral) and pteron (wing). The first single-rotor, fully-controllable helicopter to enter large full-scale production was made by Igor Sikorsky in 1942.
Compared to conventional fixed-wing aircraft, helicopters are much more complex, more expensive to buy and operate, and are more limited in speed, range, and payload. The compensating advantage is maneuverability: helicopters can hover in place, reverse, and above all take off and land vertically. Subject only to refueling facilities and load/altitude limitations, a helicopter can travel to any location, and land anywhere with enough space (approximately twice the area of the rotor disk).
Compared to other vertical lift aircraft like tiltrotors (V-22 Osprey for example) and vectored thrust airplanes (AV-8 Harrier for example), helicopters are very efficient, carrying more than twice the payload, consuming less fuel in hover and costing considerably less to buy and operate. However these other configurations have a much higher cruise speed than a helicopter (270 km/h for a helicopter, 460 km/h for a tiltrotor, 900+ km/h for a vectored thrust airplane).
Generating lift
In conventional aircraft, the wing profile (called airfoil) is designed to deflect air efficiently downward. This downward deflection causes an opposite lifting force on the wing (described by Newton's third law) and a lower pressure on the upper surface, higher pressure on the lower surface. This pressure difference integrated over the airfoil area causes a net lift. However, the more the lift of the airfoil, the more drag that is caused (induced drag by creating wingtip vortices). A helicopter makes use of the same principle, except that instead of moving the entire aircraft, only the wings themselves are moved in a circular motion. The helicopter's rotor can simply be regarded as rotating wings, from where the military name of "rotary wing aircraft" originates.
Conventional layout
There are several possible layouts for arranging a helicopter's rotors. The most common design is the Sikorsky-layout, which is used by approximately 95% of all helicopters manufactured. Turning the rotor generates lift but it also applies a reverse torque to the vehicle, which would spin the helicopter fuselage in the opposite direction to the rotor if no counter-acting force was applied. At low speeds, the most common way to counteract this torque is to have a smaller vertical propeller mounted at the rear of the aircraft called a tail rotor. This rotor creates thrust which is in the opposite direction from the torque generated by the main rotor. When the thrust from the tail rotor is sufficient to cancel out the torque from the main rotor, the helicopter will not rotate around the main rotor shaft.
The world's largest and smallest series-produced helicopters follow this Sikorsky layout. The Mil Mi-26 can lift 27 metric tons, the Robinson R22 has a crew of two and a gross weight of 1300 lb (590 kg). Almost all civilian helicopters have the main rotor and tail rotor system.
Sometimes the blades of a tail rotor are not separated by the same angle, but laid out in an X-shape, which is supposed to reduce the noise levels for military use (e.g. AH-64 Apache).
The primary reason is to make the arrangement of the pitch controls simpler. If the tail rotor is shrouded (i.e., a fan embedded in the vertical tail) it is called a fenestron. The fenestron rotor system on the model EC120 helicopter uses a shaft driven system and gearbox to turn the fan. It is less efficient but the advantages are that less noise is generated, it is safer for people that may walk near it and there is less chance of the blades being damaged by objects because it is shrouded, unlike the traditional tail rotor.
The amount of power required to prevent a helicopter from spinning is significant. A tail rotor typically uses about 5 to 6% of the engine's power, and this power does not help the helicopter produce lift or forward motion. To reduce this waste during cruise, the vertical stabilizer is often angled to produce a force which helps counter the main rotor torque. At high speeds, it is possible for the vertical stabilizer to counteract the entire torque, leaving more power available for forward flight. This is commonly known as slip-streaming and can make hovering turns difficult on windy days. Another reason for the angled vertical stabilizer is to make it possible to stage a successful high-speed, run-on landing, in case of the tail rotor failure or damage.
Many military helicopters, especially attack types, have short wings called stub wings to add lift during forward motion. They are also used as external mounts for weapons. In extreme cases, such as that of the Mil Mi-24, the wings are large enough to obstruct airflow down from the rotors, making the helicopter all but unable to hover.
Alternative layouts
There are alternatives to Sikorsky's layout, which save the weight of a tail boom and rotor. Such Coaxial rotor designs use two main rotors which turn in opposite directions, or contra-rotate, so that the torques from each rotor cancel each other out. These methods introduce even more mechanical complexity to the design and are usually relegated to specialized helicopter types.
The co-axial design, where rotors are mounted on top of each other at the top of the fuselage and share a common main axle complex, was first built by Theodore von Karman and Asbóth Oszkár in 1918 and later became the hallmark of soviet Kamov design bureau (see for example the Kamov Ka-50 "Hokum"). Co-axial helicopters in flight are highly resistant to side-winds, which makes them suitable for shipboard use, even without a rope-pulley landing system. Another example is the Kamov Ka-26, a successful crop duster aircraft. See Coaxial rotor.
The slightly different system of intermeshing rotors, also called a synchropter, which was developed in Nazi Germany for a small anti-submarine warfare helicopter, the Flettner Fl 282 Kolibri, features two main rotors on separate, obliquely mounted axles. The counter-rotating rotors are on top of the fuselage, close to each other. During the Cold War the American Kaman company started to produce similar helicopters for USAF firefighting purposes. Kamans have high stability and powerful lifting capability. The latest Kaman K-Max model is a dedicated sky crane design, used for construction works.
In the flying-wagon or tandem rotor system (sometimes called "flying banana" for the peculiar shape of early U.S. examples), the two main rotors are located at the front and rear extremity of a long, boxy fuselage that resembles a railway wagon. A prime example is the Boeing CH-47 Chinook, that can carry 14 tons of payload. Wagon helicopters are practical for military logistical purposes, because entry and unloading is easy via the unobstructed front and rear ramps. The rotors and turbines are located very high on top of the fuselage, making them less sensitive to damage and dirt. The main drawback of a tandem rotor is limited agility in air and the need for a highly trained crew, as the large main rotors have long outreach beyond the fuselage and may easily hit nearby obstacles. In 2001, while on live TV, a South Korean Army CH-47 Chinook crashed into a bridge for this reason.
A helicopter built by Juan de la Cierva had three main rotors. These were placed at the corners of an equilateral triangle and all turned the same direction. In the cross system, the rotary wing aircraft resembles a traditional fixed-wing airplane, with the two main rotors mounted at the extremities of its wings. Such helicopters are rare, because structural integrity of the wings is difficult to maintain against the amplified resonance of far off-board rotor-turbine units. The 1930s German FW-61 helicopter was built to such design. The world's largest ever helicopter, the Soviet Mil-V-12 prototype, was a cross of two Mil Mi-6 turbine-rotor units built onto a modified Antonov cargo plane. The U.S. V-22 Osprey tilting rotorcraft is similar, although its nacelles can be rotated, and shares some of the inherent technical problems of a cross system.
A recent development in helicopter technology is the NOTAR system, which stands for N'O' TAil Rotor. The NOTAR eliminates the tail rotor by conducting high-velocity air through the tail boom, using the Coandă effect to produce forces to counter the torque. NOTARs adjust thrust by opening and closing a sliding circular cover near the end of the tail boom. The NOTAR system was developed in the United States and is used exclusively by McDonnell Douglas Helicopters.
The most unusual design is the roto-rocket principle, where the single main rotor draws power not from the shaft, but from its own wingtip jet nozzles, which are either pressurized from a fuselage-mounted gas turbine or have their own pulsejet combustion chambers. Although this method is simple and eliminates precession, development of such helicopters ceased because their extreme noise levels preclude both military and civilian use.
Controlling flight
Useful flight requires that an aircraft be controlled in all three dimensions (see flight dynamics). In a fixed-wing aircraft, this is easy: small movable surfaces are adjusted to change the aircraft's shape so that the air rushing past pushes it in the desired direction.
In a helicopter, however, there is often not enough speed for this method to be practical. For rotation about the vertical axis (yaw) the anti-torque system is used. Varying the pitch of the tail rotor alters the sideways thrust produced. Dual-rotor helicopters have a differential between the two rotor transmissions that can be adjusted by an electric or hydraulic motor to transmit differential torque and thus turn the helicopter. Yaw controls are usually operated with anti-torque pedals, on the floor in the same place as a fixed-wing aircraft's rudder pedals.
For pitch (tilting forward and back) or roll (tilting sideways) the angle of attack of the main rotor blades is altered or cycled during the rotation creating a differential of lift at different points of the rotary wing. More lift at the rear of the rotary wing will cause the aircraft to pitch forward, an increase on the left will cause a roll to the right and so on.
Helicopters maneuver with three flight controls besides the pedals. The collective pitch control lever controls the collective pitch, or angle of attack, of the helicopter blades altogether, that is, equally throughout the 360 degree plane-of-rotation of the main rotor system. When the angle of attack is increased, the blade produces more lift. The collective control is usually a lever at the pilot's left side. Simultaneously increasing the collective and adding power with the throttle causes a helicopter to rise.
The throttle controls the absolute power produced by the engine that is connected to the rotor by a transmission. The throttle control is a twist grip on the collective control. RPM control is critical to proper operation for several reasons. Helicopter rotors are designed to operate at a specific RPM. However, for each weight and speed there would be an ideal RPM (design-rpm). In practice, a single (higher) RPM is used in order to minimize resonance design requirements and add a safety margin to rotor stall RPM. Usually only in autorotation are different RPMs used to increase rotor efficiency, which can be crucial in the case of an emergency without engine power.
If the RPM becomes too low, the rotor blades stall. This suddenly increases drag and slows the rotor down further. The centrifugal forces are then not able to straighten the rotor blades any more, excessive coning ("tuliping") develops and a catastrophic accident is certain.
If the RPM is too high, damage to the main rotor hub, power transmission and engine from excessive forces could result. In general, RPM must be maintained within a tight tolerance usually a few percent. In many piston-powered helicopters, the pilot must manage the engine and rotor RPM. The pilot manipulates the throttle to maintain rotor RPM and therefore regulates the effect of drag on the rotor system. Turbine engined helicopters, and some piston helicopters, use a servo-feedback loop, otherwise known as a governor, in their engine controls to maintain rotor RPM and relieves the pilot of routine responsibility for that task.
The cyclic changes the pitch of the blades cyclically, that is, during the rotation of the blades around each complete circle (2 pi radians). This causes the lift to vary across the plane of the rotor disk. This variation in lift causes the rotor disk to tilt and the helicopter to move during hover flight or change attitude in forward flight. The cyclic is similar to a joystick and is usually positioned in front of the pilot. The cyclic controls the angle of the stationary section of the swashplate, which in turn controls the angle of the rotating section of the swashplate. The rotating section rotates with the rotor and is connected to blade pitch horns through pitch links, one link for each blade. When the swashplate is not tilted, the blades are all at the collective angle. When it is tilted, the links give a pitch-up at some azimuthal angle and a pitch-down at the opposite angle, hence creating a sinusoidal variation in blade angle of attack. This causes the helicopter to tilt in the same direction as the cyclic. If the pilot pushes the cyclic forward, then the rotor disc tilts forward, and the rotor produces a thrust in the forward direction.
As a helicopter moves forward, the rotor blades on one side move at rotor tip speed plus the aircraft speed and is called the advancing blade. As the blade swings to the other side of the helicopter, it moves at rotor tip speed minus aircraft speed and is called the retreating blade. To compensate for the added lift on the advancing blade and the decreased lift on the retreating blade, the angle of attack of the blades is regulated as the blade spins around the helicopter. The angle of attack is increased on the retreating blade to produce more lift, compensating for the slower airspeed over the blade. And the angle of attack is decreased on the advancing blade to produce less lift, compensating for the faster airspeed over the blade.
If the angle of attack of any wing, including rotor blades, is too high, the airflow above the wing separates causing instant loss of lift and increase in drag. This condition is called aerodynamic stall. On a helicopter, this can happen in any of four ways.
1. As helicopter speed increases, airflow over the advancing blades approaches the speed of sound and generates shock waves that disrupt the airflow over the blade causing loss of lift.
2. As helicopter speeds increase, the retreating blade experiences lower relative airspeeds and the controls compensate with higher angle of attack. With a low enough relative airspeed and a high enough angle of attack, aerodynamic stall is inevitable. This is called retreating blade stall. See dissymetry of lift for a fuller treatment of cases 1 and 2 together in a single analysis.
3. Any low rotor RPM flight condition accompanied by increasing collective pitch application will cause aerodynamic stall.
4. Unique to helicopters is the vortex ring state (also known as settling with power) which is when a helicopter in a hover or descent comes into contact with its own down wash causing immense turbulence and loss of lift.
Helicopters are powered aircraft but they can still fly without power by using the momentum in the rotors and using downward motion to force air through the rotors. The main rotor acts like a "windmill" and turns. This technique is known as autorotation. A transmission connects the main rotor to the tail rotor so that all flight controls are available after engine failure. Autorotation can allow a pilot to make an emergency landing if the engine failure occurs while the helicopter is traveling high enough or fast enough.
