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Helicopter From Wikipedia, the free encyclopedia Jump to: navigation, search For other uses, see Helicopter (disambiguation). The Bell 206 of Canadian Helicopters Robinson Helicopter Company (USA) R44, a four seat development of the R22A helicopter is an aircraft which is lifted and propelled by one or more horizontal rotors consisting of two or more rotor blades. Helicopters are classified as rotorcraft to distinguish them from fixed-wing aircraft because the helicopter derives its source of lift from the rotor blades rotating around a mast. In fact, the word 'helicopter' originates from the Greek words elikoeioas (helical or spiral) and pteron (wing or feather).[1]

The primary advantages of the helicopter are due to its rotor, which provides lift in a vertical direction, giving it the ability to take off and land vertically and to maintain a steady hover in the air over a single point on the ground. This allows the helicopter to land and take off from pinnacles and confined areas that airplanes are not able to take off from, including heliports in the middle of busy cities and rugged terrain in remote areas. The helicopter is used for rescue, medical evacuation and as an observation platform. Other operations that involve the use of helicopters are fire fighting, tours, as an aerial crane, logging, personnel transport, electronic news gathering, law enforcement, military and for pleasure.

Although helicopters were developed and built during the first half century of flight, some even reaching limited production, it wasn't until 1942 that a helicopter designed by Igor Sikorsky became the first helicopter to enter full-scale production,[2] totalling over 400 copies. Even though most previous designs utilized more than one main rotor, it was the single main rotor with antitorque tail rotor configuration of this design that would come to be recognized worldwide as the helicopter.

Contents [hide] 1 History 2 Generating lift 2.1 Conventional layout 2.2 Alternative layouts 3 Controlling flight 4 Stability 5 Limitations 6 Landing on a ship 7 Hazards of helicopter flight 8 Helicopter models and identification 9 Use 10 References 11 External links 12 See also

[edit] History Paul Cornu's helicopter built in 1907, this helicopter was the first flying machine to have risen from the ground using rotor blades instead of wings.Since 400 BC, the Chinese had a bamboo flying top that was used as a children's toy. Eventually, this flying top toy made it to Europe,[citation needed] and is depicted in a 1463 European painting.[citation needed] Pao Phu Tau (抱朴子) was a 4th-century book in China describing some of the ideas inherent to rotary wing aircraft.[citation needed] Around 1490, Leonardo da Vinci first conceived the semi-practical, manned helicopter.[citation needed]

The word "helicopter" (hélicoptère) was coined in 1861 by Gustave de Ponton d'Amécourt,[1] a French inventor who demonstrated a small steam-powered model. In 1907, the French inventor Paul Cornu made a helicopter that used two 20-foot (6-meter) counter-rotating rotors driven by a 24-hp (18-kW) Antoinette engine. It lifted its inventor to about five feet (1.5 meters) and remained aloft one minute.

In the early 1920s, Raúl Pateras de Pescara, an Argentinian working in Europe, demonstrated one of the first successful applications of cyclic pitch.[3] His coaxial, contra-rotating, biplane rotors were able to be warped to cyclically increase and decrease the lift they produced; and the rotor hub could also tilt, both allowing the aircraft to move laterally without a separate propeller to push or pull it. Pescara is also credited with demonstrating the principle of autorotation, the method by which helicopters land safely after engine failure. By January 1924, Pescara's helicopter No. 3 was capable of flights up to 10 minutes. One of Pescara's contemporaries, Frenchman Etienne Oemichen, set the first helicopter world record recognized by the Fédération Aéronautique Internationale on 14 April 1924, flying his helicopter 360 meters (1,181 feet). On 18 April 1924, Pescara beat Oemichen's record, flying for a distance of 736m (nearly a half mile) in 4 minutes and 11 seconds (about 8 mph, 13 km/h) maintaining a height of six feet.[4] Not to be outdone, Oemichen reclaimed the world record on 4 May when he flew his No. 2 machine again for a 14-minute flight covering 5,550 feet (1.05 mi, 1.692 km) while climbing to a height of 50 feet (15 meters).[4] Oemichen also set the 1-km closed-circuit record at 7 minutes 40 seconds.[5]

During this time, Juan de la Cierva was developing and introducing the first practical autogyro. In 1923, the rotorcraft that became the basis for the modern helicopter began to take shape, in the form of an autogyro.[6] Cierva discovered aerodynamic and structural deficiencies in his early designs that could cause his autogyros to flip over after takeoff. The flapping hinges Cierva designed allowed the rotor to develop lift equally on the left and right halves of the rotor disk. A crash in 1927 led to the development of the drag hinge.[6] These two developments allowed for a stable rotor system, not only in a hover, but in forward flight.

In 1922, Albert Gillis von Baumhauer, a Dutch aeronautical engineer, started studying VTOL rotor craft. His first prototype 'flew' ('hopped' and hovered really) on September 24, 1925, with Dutch Army-Air arm Captain Floris Albert van Heijst at the controls. The controls that Captain van Heijst used were Von Baumhauer's inventions, the cyclic and collective. Patents were granted Von Baumhauer by the British ministry of aviation on January 31, 1927, under number 265,272.

Soviet aeronautical engineers Boris Yuriev and Alexei Cheremukhin began experiments with the TsAGI 1-EA helicopter in 1931. A single rotor helicopter, with forward and aft anti-torque rotors, it reached an altitude of 605 meters (1,984 ft) on August 14, 1932 with Cheremukhin at the controls.[7]

The German Focke-Wulf FW 61 was the first viable helicopter first flying in 1936. The FW-61 broke all of the helicopter world records in 1937. Nazi Germany used helicopters in small numbers during World War II. Models such as the Flettner FL 282 Kolibri were used in the Mediterranean Sea, while the Focke-Achgelis Fa 223 Drache was used in Europe.

Mass production of the military version of the Sikorsky XR-4 began in May 1942 for the United States Army; this was used over Burma for rescue duties.[8] 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 (in March 1946) 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 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 than fixed-wing aircraft. 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.

[edit] Generating lift The eight-bladed fenestron of the Eurocopter EC120BIn 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.

[edit] Conventional layout The major components of a Bell 204/UH-1B-type helicopterThere are several possible layouts for a helicopter's rotors. The most common design is the Sikorsky-layout, which is used by approximately 95% of all helicopters manufactured.[citation needed] Turning the rotor generates lift but it also applies a reverse torque, 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 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. Respectively, 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 four-bladed tail rotor are not separated at right angles, but laid out in an X-shape, primarily to make the arrangement of the pitch controls simpler, but with the added benefit of reduced noise levels for military use (e.g. AH-64 Apache).[9] 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 Eurocopter EC120 helicopter uses a shaft driven system and gearbox to turn the fan. It is more efficient (because the shroud prevents lift loss at the tips), with the advantages that less noise is generated, it is safer for nearby people and there is less chance of the blades being damaged by objects because it is shrouded, unlike the traditional tail rotor.[9]

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[citation needed] and can make hovering turns difficult on windy days.[citation needed] 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.[citation needed]

Some helicopter designs, especially military attack types, incorporate short stub wings. These are primarily used for additional fuel capacity and for a place to mount weapons. While the wings will generate some lift which can be used by designers to reduce overall rotor disk area, wings have some significant disadvantages as well. They produce an unwanted down-force while the helicopter is in a hover, and the increase the aerodynamic drag, resulting in the need for an increase in both collective pitch (to overcome inflow) and cyclic pitch to keep the disk trimmed. In most applications, these increases in pitch, and the consequent increase in retreating blade angle-of-attack, result in an overall penalty to the disk efficiency greater than the benefit of the wing's lift.[9]

[edit] Alternative layouts There are alternatives to Sikorsky's layout, which save the weight of a tail boom and rotor.[citation needed] Such designs use two main rotors that turn in opposite directions, or contra-rotate, so the torque from each rotor cancels out. These methods introduce even more mechanical complexity to the design and are usually relegated to specialized helicopter types.[citation needed]

The co-axial design, where rotors are mounted on top of each other at the top of the fuselage and share a common rotor mast, 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, NATO codename "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.

The slightly different system of intermeshing rotors, also called a synchropter,[citation needed] 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 contra-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[verification needed] or tandem rotors system (sometimes called "flying banana" for the peculiar shape of early U.S. examples), the 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, able to carry a 14 ton payload. Wagon helicopters[verification needed] are practical for military logistical purposes, because entry and unloading is easy via unobstructed front and rear ramps. The rotors and turbines are located very high on the fuselage, making them less susceptible 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 stretch beyond the fuselage and may easily hit nearby obstacles.[citation needed] In 2001, a South Korean Army CH-47 Chinook crashed into a bridge on live TV in just such an incident.

A helicopter named Air Horse built after WW2 by the Cierva Autogiro Company had three main rotors. These were placed at the corners of an equilateral triangle and all turned in the same direction. Torque correction was achieved by rigging each rotor with a tilt to provide a corrective thrust moment.

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.

MD 600N (Helicopters of America)A 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. It uses considerably less horsepower than a tail rotor.[citation needed]

The most unusual design is the roto-rocket principle[citation needed], 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 torque, the prototypes built lack the efficiency of conventional helicopters.

[edit] Controlling flight Controls of an Alouette IIIUseful 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.

Enstrom (USA) 280FX Shark, an aerodynamically restyled F28 for the corporate market.For pitch (tilting forward and back) or roll (tilting sideways) the angle of attack of the main rotor blades is altered - cycled - during the rotation creating different amount of lifts at different points in the cycle. This is also how the helicopter is propelled: pitching forward causes forward flight.

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. Yaw controls are usually operated with anti-torque pedals corresponding to 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 all together, i.e. equally throughout the 360 degree rotation of the rotor. 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 the helicopter to rise.

Dual rotor helicopters follow the same principles, but differ 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. Kamov Ka-50 helicopter with contra-rotating co-axial rotors.Co-axial 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. Sikorsky S-92The throttle controls the power produced by the engine, which is connected to the rotor by a transmission. The throttle control is a motorcycle-style twist grip mounted 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 reduced centrifugal forces are then no longer able to keep the rotor blades straight: 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 can result. In general, RPM must be maintained within a tight tolerance, usually a few percent, and the RPM indicator dial is marked accordingly. 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 loops in their engine controls to maintain rotor RPM and relieve the pilot of routine responsibility for that task.

The cyclic (pitch control lever) changes the pitch of the blades cyclically, causing the lift to vary across the plane of the rotor disc. This variation in lift causes the rotor disc 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 are referred to the advancing blades. As the blades swing to the other side of the helicopter, they move at rotor tip speed minus aircraft speed and are referred to as the retreating blades. To compensate for the added lift on the advancing blades and the decreased lift on the retreating blades, the angle of attack of the blades is regulated as they spin round. 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, or rotor blade, is too high, the airflow above it separates and this causes an 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 speeds increase, airflow over the advancing blades approach the speed of sound and generate shock waves that disrupt the airflow over them, 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 dissymmetry 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 (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. Ex-military Westland Scout AH.1 (XV134), now on the UK Civil Register.Helicopters are powered aircraft but to a limited extent they can glide without power by using the momentum in the rotors and using downward motion to force air through them. 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).

[edit] 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.

Bell 407In 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 (usually the strong hand) on the cyclic at all times, leaving the radios and engine controls for their left hand (usually the weaker hand).

[edit] 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. To counter this dissymmetry of lift, 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. For this reason, the maximum safe forward speed of a helicopter is given a design rating called VNE, Velocity, Never Exceed. 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. The Bristol Type 192 Belvedere (then taken on by Westland) twin rotor helicopter had a large cargo door and external hoist, and was used as personnel/paratroop transport, casualty evacuation, and for lifting large loads. The Belvedere had a production run of only 26 and went into RAF service in 1961.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.

[edit] Landing on a ship