Helicopter

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.
[edit]



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. (see

Height-velocity diagram).