Rotary Aircraft Operations
Helicopters have directly powered main rotors, usually a jet engine with a transmission linkage to the rotor, but some helicopters have internal combustion engines as their power supply. The flight characteristics of the helicopter permit it to hover or move in any direction. Helicopters, in principle, use either a single main rotor combined with tail rotor that counteracts the torque of the main rotor, or two main rotors that rotate in opposite directions, thus canceling out the torque.
Helicopters are subject to an aerodynamic speed limit. This is because as a helicopter flies forward, one blade is also moving forward relative to the airflow and meets the air at a much higher velocity and provides lift. The opposite blade, known as the retreating blade, however, is moving much slower than the helicopter, and as its speed drops relative to the air, its angle of attack, or the angle in which the rotor blade is going through the air relative to the flight path, must be increased in order to provide the same amount of lift as the faster moving, advancing blade. However a limit is reached when a helicopter is flying at the same speed as the rotors are turning. This effectively creates a zero air speed of the retreating blade. It provides no lift, and the retreating blade then stalls. The retreating blade stall actually occurs below this velocity and retreating blade stall causes the helicopter to fall off towards the side of the loss of lift.
In theory, it would be possible to increase the speed of the rotor blades, but there is a point at which the blades would become supersonic and the incredible racket of several thousand sonic booms per minute striking the pilots has been demonstrated in experimental (XF-94) aircraft to be completely intolerable, not to mention structurally very demanding on the rotor blades and the vehicle. Thus, for all practical purposes, the top speed limit for helicopters is in the order of 250 knots, and most helicopters cruise in the 100 to 150 knot range. Autogyros are similarly limited.
Operations of rotorcraft are essentially identical to fixed wing aircraft for private, commercial and air transport operations. A separate pilot's license for rotorcraft is required even if the pilot already holds a pilot's license for other kinds of aircraft. The same medical certification for the operation of rotorcraft applies for conventional aircraft with a Third Class for private, Second Class for commercial, and First Class for airline transport operations. Because of their utility, helicopters are extremely versatile, and are not only used for transporting passengers, but are seen in law-enforcement, news reporting, cargo operations, agriculture, and rescue and medical operations.
The nature of the rotary blade system causes considerable low-frequency vibration and higher noise levels. Typically, helicopter pilots are required to wear helmets with high quality hearing protection, and the seats in many helicopters are designed to absorb more of the low frequency vibration exposure. Overall, the cabin environment is more similar to a heavy, over-the-road truck than other forms of aircraft. In longitudinal studies, noise-induced sensory-neural hearing loss is the most common problem encountered with helicopter crew members.
As with any other aircraft, spatial disorientation can be a problem, but helicopter operations provide two additional problems. One is known as a flicker vertigo. In this situation, a light source, usually above the rotor blades, such as an overhead sun or moon, will be observed blinking off and on, and at certain frequencies of flicker, angular vection illusions occur. This may result in motion sickness or become a source of irritation and distraction. However, one should be aware that photic stimulation and frequencies in the 8 to 14 hertz range is that of the encephalographic alpha rhythm, and can produce seizures in rare individuals who are susceptible to flicker-induced vertigo. The prevalence of this condition is very low (less 1 in 20,000), and the number of pilots affected are very few, although some helicopter crashes are felt to have been caused by this.
Because helicopters often operated near the ground, crash worthiness and crash survival are important features to helicopter design and operations. The controls of a helicopter are mounted somewhat differently from a fixed wing aircraft. The pilot typically flies with the right hand on the control stick (which tilts the rotor mast) and the left hand on a "collective". The collective is a combined throttle and pitch control. By twisting the throttle, engine power is controlled, and by pulling back and forth on the collective, the pitch angle of attack for the rotor blades is changed. Due to the fact that the helicopter is unstable in flight and must be flown constantly, the pilots cannot take their hands off the control stick. If the pilot needs to manipulate controls on the center console, it must be done with the left hand. Therefore, unlike conventional aircraft, a helicopter pilot will be seated in the right seat, whereas in conventional aircraft, the pilot in command is seated in the left seat.
Additional modifications of a helicopter are required for crash worthiness. Because the helicopter's descent is usually vertical rather than in a longitudinal axis, the stress loading for crashing tends to be in the vertical axis, with energy absorbing seats and crushable material in the belly of the aircraft.
Also, unlike fixed wing aircraft, ejection/bailout is not an option. The rotor also creates an additional problem because of the blade's inertia upon crashing may cause them to bounce and shatter and enter the cockpit. Special rotors with frangible tips have been built into modern helicopters, and the transmission has been moved from out of the cockpit and tends now to be mounted above the crew compartment. Fuel lines are usually changed to quick disconnects and new experimental helicopters, using engine thrust, instead of an anti-torque tail rotor are being evaluated to reduce the potential problems of striking objects in flight with the tail rotor, or more importantly, on the ground. All too frequently, ground personnel have accidentally walked into a tail rotor with fatal results.
Post accident evacuation of a helicopter is extremely difficult, due to its tendency to roll over, both from the dynamic roll over effect, as well as torque transfer from the rotating blades. Therefore, part of aircrew training for helicopters requires specialized training for emergency evacuation, on land and water, including "worst-case" scenario is inverted underwater.
Increasing demands on helicopter pilots involve night and all-weather operations. Although this has become routine with military personnel, the use of night vision devices, for light amplification, and infrared detectors are now entering civilian, police and law-enforcement departments. These create new problems with limited field of vision and decreased acuity at night. Both require extensive training and safety procedures to avoid accidents.
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