The main 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 hovering, 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 advancing 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. See Wave drag.
Because the advancing blade has higher airspeed than the retreating blade and generates a 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. 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 airspeed of a helicopter is given a design rating called VNE, Velocity, Never Exceed.[35] In addition, at extremely high speeds, it is possible for the helicopter to travel faster than the retreating blade which would inevitably stall the blade, regardless of the angle of attack.
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.
Hazards
As with any moving vehicle, unsafe operation could result in loss of control, structural damage, or fatality. The following is a list of some of the potential hazards for helicopters:
- Settling with power, also known as a vortex ring state, is when the aircraft is unable to arrest its descent due to the rotor's downwash interfering with the aerodynamics of the rotor.
- Retreating blade stall is experienced during high speed flight and is the most common limiting factor of a helicopter's forward speed.
- Ground resonance affects helicopters with fully articulated rotor systems having a natural lead-lag frequency less than the blade rotation frequency.
- Low-G condition affects helicopters with two-bladed main rotors, particularly lightweight helicopters.
- Dynamic rollover in which the helicopter pivots around one of the skids and 'pulls' itself onto its side.
- Powertrain failures, especially those that occur within the shaded area of the height-velocity diagram.
- Tail rotor failures which occur from either a mechanical malfunction of the tail rotor control system or a loss of tail rotor thrust authority, called Loss of Tail-rotor Effectiveness (LTE).
- Brownout in dusty conditions or whiteout in snowy conditions.
- Low Rotor RPM, or rotor droop, in which the engine cannot drive the blades at sufficient RPM to maintain flight.
- Wire and tree strikes due to low altitude operations and take-offs and landings in remote locations.[36]
Deadliest helicopter crashes
- Khankala attack - Mi-26 shot down over Chechnya in 2002; 127 killed.
- 1997 Israeli helicopter disaster - MH-53 crash in Israel in 1997; 73 killed.
- 1977 Israeli CH-53 crash - CH-53 crash near Yitav in the Jordan Valley on 10 May 1977; 54 killed.
- 1986 Sumburgh disaster - a British International Helicopters Boeing 234LR Commercial Chinook, Shetland Islands; 45 killed.
- 26 January 2005 - a CH-53E Super Stallion from HMH-361 crashed near Ar Rutbah, Iraq killing all 31 service members onboard.
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