Encyclopedia
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 and
pteron . 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, relatively slow, have shorter range and restricted 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 .
Compared to other vertical lift aircraft like
tiltrotors and vectored thrust airplanes , 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 considerably more cruise speed than a helicopter .
History
Since 400 BC the
Chinese had a
bamboo flying top that was used as a children's toy. This toy eventually made its way to Europe and has been depicted in a 1463 European painting. "Pao Phu Tau" was a
4th century book in China that described some of the ideas in a rotary wing aircraft. The first semi-practical idea of a human carrying helicopter was first conceived by
Leonardo da Vinci around 1490, but it was not until after the invention of the powered airplane in the
20th century that actual helicopters were produced. Developers such as
Jan Bahyl, Oszkár Asbóth, Louis Breguet, Paul Cornu,
Emile Berliner,
Ogneslav Kostovic Stepanovic and
Igor Sikorsky pioneered this type of aircraft, with
Juan de la Cierva introducing the first practical
autogiro in 1923 that was to be the basis for the modern helicopter.
A flight of the first fully controllable helicopter was demonstrated by Raúl Pateras de Pescara 1916 in
Buenos Aires,
Argentina. In 1931, Soviet aeronautical engineers Boris Yuriev and Alexei Cheremukhin began experiments with the TsAGI
1-EA helicopter, the earliest known single lifting rotor helicopter . It reached an altitude of 605 meters on August 14, 1932 with Cheremukhin at the controls.
The German
Focke-Wulf FW-61 was the first production fully controllable helicopter and had its first flight in 1936. The FW-61 broke all worldrecords in 1937.
Nazi Germany used the helicopter in combat during
WWII in small numbers. Models such the Flettner
FL 282 Kolibri were used in the
Mediterranean Sea.
Mass production of the military version of the Sikorsky XR-4 began in May 1942 for the United States Army and was used over
Burma for . It was also used by the
Royal Air Force, the first British military unit to be equipped with helicopters being the Helicopter Training School, formed in January 1945 at
RAF Andover with nine
Sikorsky R-4B Hoverfly I helicopters.
The
Bell 47 designed by Arthur Young became the first helicopter to be licensed for certified civilian use in the
United States. Two decades later the
Bell 206 became the most successful commercial helicopter ever built with more hours and has set more industry records than any other aircraft in the world.
Reliable helicopters capable of stable hover flight were developed decades after fixed wing aircraft. This is largely due to higher engine power density requirements when compared with fixed wing aircraft. Igor Sikorsky is reported to have delayed his own helicopter research until suitable engines were commercially available. Improvements in fuels and engines during the first half of the
20th century were a critical factor in helicopter development. The availability of lightweight
turboshaft engines in the second half of the 20th century led to the development of larger, faster, and higher performance helicopters. Turboshaft engines are the preferred powerplant for all but the smallest and least expensive helicopters today.
Generating lift
In conventional aircraft, the wing profile is designed to deflect air efficiently downward. This downward deflection causes an opposite lifting force on the wing 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 . 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 . 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 . The primary reason is to make the arrangement of the pitch controls simpler. If the tail rotor is shrouded 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 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 . 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 , 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, a
South Korean Army CH-47 Chinook crashed into a bridge for that reason while being shown live on TV.
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
NO TAil
Rotor. The NOTAR eliminates the tail rotor by conducting high-velocity air through the tail boom, using the
Coanda 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 . 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 pitch or roll 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. This is also how the helicopter is manouvred, ie. pitching forward causes forward flight..
For rotation about the vertical axis the anti-torque system is used. Varying the pitch of the tail rotor alters the sideways thrust produced. Yaw controls are usually operated with
anti-torque pedals, on the floor in the same place as a fixed-wing aircraft's rudder pedals.
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.
Dual rotor helicopters follow the same principles, but differentiate in the following ways:
- Tandem rotor designs achieve yaw by applying opposite left and right cyclic to each rotor, effectively rolling both ends of the helicopter in opposite directions. To achieve pitch, opposite collective is applied to each rotor; decreasing the lift produced at one end, while increasing lift at the opposite end, effectively tilting the helicopter forward or back.
- Synchropters use a similar system to tandem rotor helicopters, but as the two rotors are side by side, they use opposite pitch for yaw, and opposite collective for roll.
- Co-axial
In the field of helicopter [i] design, there are three principal arrangements of the rotor blades, na ...
designs achieve yaw by applying opposite collective to each rotor. This increases drag, and therefore torque, in one rotor, while decreasing the drag in the other. Since the rotors spin in opposite directions, the torque difference causes the helicopter to rotate.
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 . In practice, a single 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 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
servo-feedback loop 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, causing 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.
- 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.
- 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.
- Any low rotor RPM flight condition accompanied by increasing collective pitch application will cause aerodynamic stall.
- Unique to helicopters is the vortex ring state 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. .
Stability
Fixed wing aircraft are usually inherently stable. If a gust of wind or a nudge to one of the controls causes a fixed wing aircraft to pitch, roll, or yaw, the aerodynamic design of the aircraft will tend to correct the motion, and the aircraft will return to its original attitude. Many small, fixed wing aircraft are stable enough that a pilot can let go of the controls while looking at a map or dealing with a radio, and the plane will generally stay on course.
In contrast, helicopters are very unstable. Simply hovering requires continuous, active corrections from the pilot. When a hovering helicopter is nudged in one direction by a gust of wind, it will tend to continue in that direction, and the pilot must adjust the cyclic to correct the motion. Hovering a helicopter has been compared to balancing yourself while standing on a large beach ball.
Adjusting one flight control on a helicopter almost always has an effect that requires an adjustment of the other controls. Moving the cyclic forward causes the helicopter to move forward, but will also cause a reduction in lift, which will require extra collective for more lift. Increasing collective will reduce rotor RPM, requiring an increase in throttle to maintain constant rotor RPM. Changing collective will also cause a change in torque, which will require the pilot to adjust the foot pedals.
Small helicopters can be so unstable that it may be impossible for the pilot to ever let go of the cyclic while in flight. While fixed-wing aircraft are generally designed so pilots sit on the left side of the aircraft, freeing up their right hand for dealing with radios, engine controls, and the like, helicopters are generally designed so pilots sit on the right side of the aircraft so they can keep their right hand on the cyclic at all times, leaving the radios and engine controls for their left hand .
Limitations
The single most obvious limitation of the helicopter is its slow speed. There are several reasons why a helicopter cannot fly as fast as a fixed wing aircraft.
- When the helicopter is at rest, the outer tips of the rotor travel at a speed determined by the length of the blade and the RPM. In a moving helicopter, however, the speed of the blades relative to the air depends on the speed of the helicopter as well as on their rotational velocity. The airspeed of the forward-going rotor blade is much higher than that of the helicopter itself. It is possible for this blade to exceed the speed of sound, and thus produce vastly increased drag and vibration. It is theoretically possible to have spiralling rotors, similar in principle to variable-pitch swept wings, which could exceed the speed of sound, but no presently known materials are light enough, strong enough, and flexible enough to construct them.
- Most rotors are not rigid. Because the advancing blade has higher airspeed than the retreating blade, a perfectly rigid blade would generate more lift on that side and tip the aircraft over. In consequence, rotor blades are designed to "flap" - lift and twist in such a way that the advancing blade flaps up and develops a smaller angle of attack, thus producing less lift than a rigid blade would. Conversely, the retreating blade flaps down, develops a higher angle of attack, and generates more lift. At high speeds, the force on the rotors is such that they "flap" excessively and the retreating blade can reach too high an angle and stall. In some designs the hub is rigid. The blades are made from composites which can bend without breaking. Fully rigid rotors exist and create very responsive helicopters. In most such designs, the lift is varied cyclically and according to the speed of the helicopter. The adjustment is either by adjusting the angle of attack of the blades, or by engine-powered vacuum devices that suck air into the blades, adjusting the lift.
- Rotorhead design is a limiting factor on many helicopters. Low or negative-G situations encountered in a semi-rigid system will result in blade flapping down until it hits the tail boom or other airframe structure, followed by rotor separation, causing a crash.
- Helicopters are susceptible to potentially disastrous vortex ring effects. In these, the downward wind from the rotor causes a circular vortex to form around the rotor. If this ring is augmented by terrain, wind, rain, or sea spray, the helicopter can lose enough lift to experience settling with power and hit the ground.
During the closing years of the
20th century designers began working on
helicopter noise reduction. Urban communities have often expressed great dislike of noisy aircraft, and police and passenger helicopters can be unpopular. The redesigns followed the closure of some city heliports and government action to constrain flight paths in
national parks and other places of natural beauty.
Helicopters vibrate. An unadjusted helicopter can easily vibrate so much that it will shake itself apart. To reduce vibration, all helicopters have rotor adjustments for height and pitch. Most also have vibration dampers for height and pitch. Some also use mechanical feedback systems to sense and counter vibration. Usually the feedback system uses a mass as a "stable reference" and a linkage from the mass operates a flap to adjust the rotor's
angle of attack to counter the vibration. Adjustment is difficult in part because measurement of the vibration is hard. The most common adjustment measurement system is to use a stroboscopic flash lamp, and observe painted markings or coloured reflectors on the underside of the rotor blades. The traditional low-tech system is to mount coloured chalk on the rotor tips, and see how they mark a linen sheet.
Landing
On a ship
A
helicopter deck is a helicopter pad on the deck of a ship, usually located on the
stern and always clear of obstacles that would prove hazardous to a helicopter landing. In the
U.S. Navy it is commonly and properly referred to as the
flight deck. In the
Royal Navy,
landing on is usually achieved by lining up slightly astern and on the port quarter, as the ship steams into the wind and the aircraft captain slides across and over the deck.
Shipboard landing for some helicopters is assisted though use of a haul-down device that involves attachment of a cable to a probe on the bottom of the aircraft prior to landing. Tension is maintained on the cable as the helicopter descends, assisting the pilot with accurate positioning of the aircraft on the deck; once on deck locking beams close on the probe, locking the aircraft to the flight deck. This device was pioneered by the
Royal Canadian Navy and was called "Beartrap". The U.S. Navy implementation of this device, based on Beartrap, is called the "RAST" system and is an integral part of the LAMPS MK III weapons system.
A secondary purpose of the haul-down device is to equalize electrostatic potential between the helicopter and ship. The whirling rotor blades of a helicopter can cause large charges to build up on the airframe, large enough to cause injury to shipboard personnel should they touch any part of the helicopter as it approaches the deck.
Coaxial rotor helicopters in flight are highly resistant to side-winds, which makes them suitable for shipboard use, even without a rope-pulley landing system.
Hazards of helicopter flight
As with any moving vehicle, operation outside of safe regimes could result in loss of control, structural damage, or fatality. For helicopters the hazards are particularly acute since they are flying at relatively low altitude, with little time to react to a sudden event. The following is a list of some of the potential hazards for "conventional" helicopters:
Reduction & Elimination of Common Helicopter Flight HazardsThe U.S. Department of Transportation has published a “Basic Helicopter Handbook”. One of the chapters in it is titled, “Some Hazards of Helicopter Flight'. Ten items of hazards have been listed to indicate that a typical single rotor helicopter has to deal with. The unique
Coaxial rotor design either reduces or completely eliminates these hazards. The following list indicates which:
1. Settling with power - Reduced
2. Retreating blade stall - Eliminated
3. Ground resonance - Eliminated
4. Low-frequency vibrations - None
5. Medium frequency vibrations - None
6. High frequency vibrations - None
7. Transition from powered flight to autorotation - Eliminated
8. Anti torque system failure in forward flight - Eliminated
9. Anti torque system failure while hovering - Eliminated
10. Height-Velocity Curve - Eliminated
The reduction and elimination of these hazards are the strong points for the
Coaxial rotor safety design.
SEE * + *
Helicopter models and identification
In identifying helicopters during flight it is helpful to know that when viewed from below, the rotor of a French, Russian, or Soviet designed helicopter rotates counter-clockwise, whilst that of a helicopter built in
Italy, the
UK or the
USA rotates clockwise.
Some companies, notably
Schweizer Aircraft Corporation in the USA, are developing remotely-controlled variants of light helicopters for use in future battlefields. is currently selling a line of small rotorcraft
UAVs, including an all electric helicopter.
Hybrid types that combine features of helicopters and fixed wing designs include the experimental
Fairey Rotodyne of the
1950s and the Bell Boeing
Osprey, which is on order by the U.S.
Marine Corps and will be the first mass produced
tilt-rotor aircraft to enter service.
A helicopter should not be mistaken for an
autogyro, which is a predecessor of the helicopter, that gains lift from an unpowered rotor.
Some common nicknames for helicopters are "copter", "chopper", "whirlybird", "windmill", "helo" or "paraffin Budgie" .
Helicopters are useful for landing in tight spaces.
Many companies have helicopters for transport.
See also
In the field of helicopter [i] design, there are three principal arrangements of the rotor blades, na ...
References
- Thicknesse P, Jones A et al, Military Rotorcraft, 2nd edition, 2000, Brassey's World Military Technology series, Shirvenham UK, xvi + 160pp, ISBN 1857533259
- Wragg D, Helicopters at War: A pictorial history, 1983, Robert Hale Ltd, London UK, 283pp, ISBN 0709008589
External links
: "Aircraft, especially aircraft of the direct lift amphibian type and means of construction and operating the same"
