Rotary Aircraft Operating Principles
The principle of the helicopter can be understood contrasting it with the airplane and the autogyro. In an airplane, the lift or the sustention is provided by the fixed wings; in the autogyro, a free rotor which is turned by the air-flow set up by the forward motion of the aircraft serve the same function. The forward motion or propulsion in both the airplane and the autogyro is obtained from the thrust of an ordinary propeller which absorbs the whole power of the engine. But in the helicopter both the sustention and the propulsion are provided by the engine-driven rotors. Thus basically the helicopter is an ordinary airplane with its fixed wings stripped off and its propeller shifted overhead in the form of longer rotor blades turning about a vertical axis which can be tilted forward to achieve propulsion.
Helicopters achieve lift and forward propulsion from the rotor system. Forward (and sideward and rearward) flight is achieved by tilting the rotor system in the desired direction of flight. The engines do not provide any forward propulsion. Helicopters are most efficient in forward flight and in a hover near the ground. They require the most power to achieve out-of-ground-effect hover. As the helicopter increases forward speed, the rotor blades change their pitch throughout the 360° rotation. That is, while moving through the advancing (right) side, the blade takes a lower bite (pitch) and a larger one on the retreating side. Consequently, the helicopter's top speed is limited by the ability of the rotor blade to pitch any higher during one phase of the rotation. In the event of an engine failure the helicopter descends in controlled flight through a maneuver called "autorotation." It is a maneuver requiring great pilot skills. Frequently the aircraft is landed without damage and many pilots would prefer to have an engine failure in a helicopter versus an airplane. Autorotations are less common as the fleet has evolved to twin engine helicopters.
Helicopters are designed to carry a maximum given load (maximum gross weight). This maximum is reduced in hot temperatures and high altitude conditions. The difference between the maximum gross weight and the empty weight is referred to a "usable load." In some older aircraft, the usable load is insufficient. Consequently designers are particularly weight conscious. Since the early 1970's military helicopters have also been designed with "crashworthy" characteristics (e.g., impact bearing landing gear, stroking seats, and a cabin frame that remains intact). The latest helicopter (Comanche) is designed to minimize the radar cross section. The most significant design feature of the past decade has been the incorporation of more systems into existing aircraft.
Given a suitable shape, any aerodynamic body will create lift as the air flows around it. It makes absolutely no difference if the shape is a wing, a propeller, or a rotor blade. The faster the speed of the air, the more lift generated. The forces, however, do not increase uniformly. An airplane which accelerates from 100 to 300 miles per hour (mph) does not triple the amount of lift from the wings. The increase is nine-fold, for lift is created by the " square" of the velocity of the air. (100 X 100 versus 300 X 300). A small change in speed, obviously, creates a disproportionate difference. In a fixed-wing aircraft, with both wings firmly attached to the airplane and moving through the air at the same speed, this is no problem. There is no difficulty with a helicopter either as long as the machine is in a hover in calm air. In such a case, the rotor blades are passing through the air at the same speed at all points around the aircraft. But when a helicopter begins to move forward, the conditions change rapidly. Now as the rotor blade begins to sweep forward to the front of the aircraft, the forward speed of the helicopter is added to the velocity of the air. Conversely, as the blade retreats from the front, the velocity is subtracted. The amount of lift generated on opposite sides of the helicopter is drastically out of balance. This disparity of lift was a major stumbling block to the design of helicopters.
Several solutions were proposed. The most common was to install two rotors which turned in opposite directions. In forward flight portions of each were always spinning into the wind, and equal portions turning away from the wind. There was a balance of lift, but two rotors usually turned out to be a complicated and expensive solution.
There were other methods. Igor Sikorsky's rightful claim to be the inventor of the first successful helicopter in the western hemisphere is based on his development of a method for equalizing lift on both sides of the aircraft using a single lifting rotor. As his rotor blades moved around the helicopter, they automatically changed pitch, flexed, twisted, and even adjusted speed so that no matter where they were in relation to the wind, they produced the same amount of lift. The result is termed a "fully articulated" rotor head. Modifications to Sikorsky's basic invention have provided the basis for rotors by most other manufacturers.
A helicopter in a substantial forward speed (e.g., 100-200 mph) experiences problems of control, vibration and limitations in performance resulting from the asymmetry in the speeds of the advancing and retreating blades. When traveling in a forward direction 8, the advancing blade has a speed equal the rotational speed of the blade plus the forward speed of the helicopter, whereas the retreating blade has a speed equal the rotational speed of the blade minus the forward speed of the helicopter. As a result, the advancing blade has more lift than the retreating blade. To avoid helicopter roll over due the airspeed asymmetry, the lift on the retreating blade has to be increased while the speed on the advancing blade has to be decreased. Because, lift is inversely proportional to the velocity (i.e., speed) of the blade squared (V.sup.2) a substantial increase in the coefficient of lift (C.sub.L) of the retreating blade is required. Consequently, the asymmetry in speeds between the advancing and retreating blades has to be limited thereby limiting the forward speed of the helicopter.
Increasing the RPM of the rotor reduces the relative asymmetry of the airspeed distribution, thus reducing the effects of forward speed on roll control limits. But such RPM increase is constrained by the maximum allowable rotor tip speed. The maximum allowable tip speed is typically lower than the speed of sound (i.e., Mach 1) so as to avoid the substantial increases in drag, vibration and noise encountered when the tip speed approaches Mach 1.
Current helicopter rotors turn at a constant RPM throughout the flight because of the complex and severe rotor dynamics problems. Generally, helicopter designers are content if they succeed in the development of a single speed rotor, which can go from zero to design RPM when not loaded on the ground during start and stop without encountering vibration loads which overstress the helicopter and rotor structure. When the blades of a conventional rotor are producing lift, a significant change of the rotor blade RPM from the design RPM may yield catastrophic results.
Conventional helicopter rotors are designed to achieve blade flap, lag and torsional natural oscillation frequencies, at the operating RPM, which are adequately separated from the rotor excitation frequencies occurring at the rates of 1 per revolution, 2 per revolution, 3 per revolution and so forth. For example, for a rotor operating at 360 RPM, the frequency corresponding to the occurrence of a rotor excitation frequency of 1 per revolution is 6 Hz (360 RPM is 6 cycles per second), 2 per revolution is 12 Hz, and so forth. As the rotor RPM is changed so are the excitation frequencies.
One of the greatest difficulties in the design of the helicopter is the method of counteracting what is known as torque. It is the tendency of the machine to twist itself counter to its lifting rotor. Various configurations have been used to counteract this torque. One of the most common methods is to use an engine-driven small auxiliary rotor revolving along a horizontal axis and mounted at the tail of the aircraft. This tail rotor pushes the aircraft opposite to the normal direction of the torque. Their effects can be exactly balanced and a straight course is maintained by the machine. Such well known aircraft as the Sikorsky and Bell helicopters have used this configuration.
Rotors disposed on either side of the fuselage and rotating in opposite directions have also been employed. Two partly overlapping rotor discs on either side of the fuselage have been employed to cut down the total width of the helicopter and the air drag of the supporting trusses. The arrangement of two rotors in tandem possesses the advantages of providing a large central space for the passengers. Bendix and Hiller have used two coaxil rotors one over the other and turning in opposite direction. This arrangement has three advantages of eliminating (i) the loss of power, (ii) the complexities of a long transmission (iii) the danger to the public of the tail rotor.
A fully articulated rotor system, however, has one serious drawback. It results in an aircraft that is completely unstable. The difference in stability between a helicopter and a fixed-wing aircraft is often compared to a child's swing which is hung by steel rods. If it is pushed from its normal motionless position and then left alone, the swing will sooner or later of its own accord stop exactly where it was originally. The stability of a fixed wing is similar. A helicopter, however, is like the same swing, only this time balanced upside down. If disturbed it will fall away from where it was with ever increasing speed and will never attempt to return to its original position. To an outside observer a helicopter's instability seems impossible. The whirling rotor blades very much appear to resemble a giant gyroscope - one of the most stable devices known. What is seen as a smooth blur, though, is each individual blade moving, twisting, and changing speed to adjust constantly for the differences of lift created by the wind. To demonstrate this phenomenon, cameras have been mounted on a rotor blade and after carefully counterbalancing the others, the helicopter flown. The resulting movie indicates, not the rigid structure of a gyroscope, but what most observers describe as a "writhing wet noodle. "
It is somewhat as if an airline pilot were flying a jet liner that had wings made of rubber which constantly changed shape without his knowledge. Sikorsky's solution to the difference in the amount of lift generated on opposite sides of a helicopter is the ultimate source of its instability and vibration.
Designers, engineers, and manufacturers devised a number of systems to compensate for the lack of stability. Most utilized a combination of sensors, electronics, and hydraulic controls. By the late 1960s considerable progress had been made and further refinements were being incorporated into new helicopters. The stability problems that confronted helicopter designers brought out the very best technology as tough engineering problems always seem to do. By the 1970s the basic trim system in some modern helicopters actually amounted to an autopilot, providing stability and control that first line helicopters demonstrate through a wide airspeed envelope. In spite of the improvements in handling characteristics brought about by the sophisticated systems, helicopters are still basically no different than the first machines. They remain unstable. With all the stability systems turned off, in most machines the smallest movement will induce an ever increasing swing away from the conditions which prevailed before. If the nose of the aircraft deviates ever so slightly from the intended direction of flight, only the most delicate and precise reaction from the pilot will prevent it from moving even further askew. Even with clear skies and an unencumbered view of the ground, a helicopter without stability systems challenges the very best of pilots. At night or on instruments such flight is seemingly impossible.
Another unique characteristic of a helicopter is termed ground effect. A helicopter rapidly loses efficiency as the air becomes thinner, whether due to an increase in altitude or temperature. The reverse is true also. Under certain circumstances, the rotor can create an artificially dense cushion of air and its lifting ability is dramatically increased. This occurs as the aircraft is close to the ground. The effect is first noticeable when the rotors are at the same altitude as their diameter and continues to intensify until the helicopter lands. The down wash from the rotor literally packs the air under the helicopter and as the aircraft flies in this mass of "thick" air the blades greatly increase their efficiency. A pilot, therefore, finds that it takes less power from the engines to fly at 10 feet than at 100.
Ground effect, however, is present only under specific conditions. The helicopter must be in a hover or moving very slowly. Otherwise it will slide right off the top of the cushion and derive no benefit. The effect is present only when there is a steady wind. If it is gusty from any direction, particularly from the side, it will blow parts of the ground cushion out from under the aircraft. The surface under the helicopter must be relatively smooth. Otherwise the rotor wash breaks up into a chaos of turbulence. Unless the landing zone is level and the wind steady, the pilot finds ground effect building up momentarily on one side of the aircraft, only to disappear and be created somewhere else for an instant. It makes a smooth landing impossible. The result is much like a sportsman trying to bring his fishing skiff to a perfect docking while bobbing in a fierce storm.
One more phenomenon associated with helicopters is translational lift. As the aircraft is picking up forward speed and passes through approximately 15-20 knots, there is a sudden decrease in the amount of power required to fly. On landing just the reverse occurs and once the helicopter slows below the critical speed, additional power must be added to maintain flight. The aerodynamic forces which create this paradox are exceedingly complex, but basically involve the relative direction of the wind over the rotor blades. Helicopter pilots quickly learn to take advantage of both ground effect and translational lift whenever they can. If takeoff is to be made from an open field and the load is heavy, the pilot will raise the helicopter into a very low hover taking full benefit from the dense air in the rotor wash. By starting forward very slowly and keeping the cushion under the aircraft he can accelerate until translational lift is reached and then begin to climb. Likewise on landing, sufficient speed is maintained to keep translational lift until the helicopter is low enough to enter ground effect.
In either case the helicopter can lift extra heavy loads. If neither condition is present, the ability is greatly reduced. This was the cause of some serious misunderstandings. For troops unaware of these characteristics, it was difficult to believe that a helicopter pilot could lift a large load from an open field where both translational lift and ground effect were present and yet could not hover 100 feet in the air to deliver the cargo to a small, rocky mountain top landing zone.
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