13 Mayıs 2008 Salı

Comparison of Conventional and Co-axial Rotor Systems Used in Helicopters

2001 yılında üniversitede İngilizce dönem projesi olarak hazırladığım makale. Bilgisayarımın derinliklerinde buldum. Şimdi tekrar okuduğumda gözüme pek çok hata ve eksiklik çarpıyor ama yine de paylaşmak istedim.

As one of the oldest dreams of mankind, flying is the only impossible action that can be accomplished without any equipment heavier than air. Observation of flying creatures gave important clues for the first pioneers of this progress, but the lack of enough technology and knowledge was an important barrier. To pass over this barrier some centuries were needed, and through those centuries flight with fixed wings like birds took most of the attention. In the 15th century, Leonardo da Vinci came up with the first theoretical idea of vertical light and made several drawings of a manned machine that could achieve this mission ( Boyne & Lopez, 1984, p.3 ). But first appeared as a Chinese hand-spun toy, helicopter is the oldest solution. ( Gnaegi, 1997, p.1). Before the theoretical studies produced a flying helicopter, at the start of the 20th century first flight with rigid wings was made. ( Gessow & Myers, 1985, p.1 ). In the first half of this century helicopter design showed great improvement. Melting arts and science in the same pot, helicopter design is one of the most difficult tasks of modern engineering era ( Young, 1949, p.3 ). The design of the helicopter comes up with 3 basic problems: Firstly, balancing the structural loads, engine weight and the lift, secondly the negative effect of the torque and , finally controlling the helicopter during the flight. The torque and control problems were solved by different rotor configurations. ( Gessow & Myers, 1985, p.1 ). Since they are the oldest theoretical and practical solutions for flying, helicopters can contain different lift producing surfaces, or rotors, to achieve this duty, and two of them are the most popular among others: conventional and coaxial configurations.
The history of the helicopter starts at the time of the Roman Empire. A simple toy named ‘Chinese Top’ can be considered the first example of helicopters. The Chinese Top was a propeller with angled blades to produce lift. The propeller was mounted on a stick. When spun with the help of a string or hand, the propellers produced lift ( Boyne & Lopez, 1985, p.3 ). As moving forward in history, the great genius of all times is seen: Leonardo da Vinci. In the 15th century, da Vinci discussed the possibility of flying with machines which are heavier than air, and he made some drawings of possible crafts. His famous design can be described as a helix. He introduces the flight characteristics of his helix with the following sentences: “ I find that if this instrument with a screw be well made- that is to say, made of linen of which the pores be stopped up with starch- and be turned swiftly, the said screw will make its spiral in the air and it will rise high” ( Boyne & Lopez , 1984, p.3 ). His design was theoretically available, but in real life the ‘helix’ had no chance to fly. Until the end of 18th century, no important developments were made in helicopter design. But in 1796, Sir George Cayley designed and built some successful helicopter-like machines powered by strings. W. H. Philips in 1842, Enrico Foclanini in 1878 and Thomas Edison in 1880 made researches and several attempts to design, build and fly helicopters,but they were unable to succeed. Until the beginning of the 20th century, the biggest problem in the design of the helicopter was to find a light and adequate power plant . The invention of combustion engines was a great breakthrough, but many other problems followed this first one like the torque (Gnaegi, 1997, p.1 )
In 1907, the first flight of a helicopter by Paul Cornu was seen. Cornu’s helicopter reached a maximum altitude of 5 feet and kept it for about 20 seconds. From that time to the 1930’s great advances in helicopter industry were seen. Many pioneers like de Bothezat, Raoul Pescasa, Von Baumhauer and Igor Sikorsky made invaluable assistance to the progress of helicopter development. ( Gessow & Myers, 1985, p.6, 8 ). Gained momentum from these studies, helicopters showed great improvement in the second half of the 20th century.
The evolution of the helicopter was proportional to the advance in power plant and materials technology. This progress contains the experience gained from different designs such as gyroplane, autogyro, convertaplane, etc. ( Young, 1949, p.4 ). During the evolution Breguet built a coaxial helicopter, Antoine Flettner built a synchropter ( helicopter with two intermeshing rotors ) in 1937, and in the same year, Focke-Angelis built a side-by-side helicopter ( Gessow & Myers, 1985, p.9, 10). All of their designs are the predecessors of modern helicopters.
Helicopters can be considered as aircraft with rotating and lift producing surfaces known as rotors instead of fixed wings ( Young , 1949, p.3 ). The most typical physical property of a helicopter is its rotor and absence of wings. Unlike aircraft, helicopters do not need long runways and they can increase or decrease their altitudes vertically. The other typical manoeuvre of helicopters is hanging in the air, or ‘hovering’. To achieve all these manoeuvres , helicopters must develop thrust. Developing thrust and making these manoeuvres are described by means of momentum theory, blade-element theory and vortex theory. On the other hand, vertical flight is based on the rules of momentum theory. ( Gessow & Myers , 1985, p.46 )
To understand the basis of vertical flight, a little basic knowledge of the momentum theory will be necessary. This theory comes from Newton’s second law of motion, which indicates that the force acting on an object is the product of the mass and the acceleration of that object. The momentum theory is derived from the fact that the initial velocity a fluid passing through an airscrew is higher than its velocity passed by the airscrew. Resulting from the thrust, the difference between these two velocities takes the name of ‘ induced’ or ‘downwash’ velocity. So, according to Newton’s second law of motion, the thrust which is produced by the airscrew is equal to the multiplication of the amount of air advanced through the airscrew and the difference of velocities passed through the disk ( Gessow & Myers, 1985, p.46, 47 ). In helicopters, the name of this airscrew is rotor, and rotors can be considered as disks in flight, because while turning to produce lift, they act like disks. The momentum theory develops three conclusions: Firstly, rotors can be thought as ‘actuator disks’ because they can be thought as the composition of infinite number of blades. This ‘actuator disk’ pass the air homogeneously and there is no decrease of thrust at the edges of the blades, secondly, “The power required to produce the thrust is represented only by the axial kinetic energy imparted to the air composing the slipstream. A frictionless fluid is assumed so that there is no blade friction or profile-drag losses. Rotational energy imparted to the slipstream is ignored”, and finally, the thickness of the disk can be ignored, so there is no discontinuity of velocity between the upper and the lower sides of the disk. (Gessow & Myers, 1985, p.47, 48).
To achieve vertical flight, the rotor system is needed, as mentioned. But the requirement for rotating surfaces makes rotors very complex and difficult to design. They must provide freedom of movement in 6 axes and develop lifting force at the same time. The other problem is known as ‘torque’. When the main rotor turns in one direction, torque forces the helicopter to turn in the opposite direction ( Gnaegi, 1997, p.3). There are several solutions for eliminating the torque effect by applying different rotor configurations. The most common arrangements are conventional and coaxial rotor systems.
The most common rotor type used in helicopters today is conventional, or single rotor with tail rotor design. This configuration is quite simple. It consists of a main lift producing rotor, a transmission system and an anti-torque rotor ( Gessow & Myers , 1985, p.16). The first conventional rotor helicopter was designed and built by Van Baumhauer, who was a Dutch scientist. The helicopter was unable to raise above two feet from the ground, and after an accident in 1929, the project was cancelled. Later, in 1941, Igor Sikorsky designed VS-300 helicopter. The first version of this helicopter operated a main rotor and three auxiliary rotors, but later the design efforts came to a conclusion of a conventional helicopter. After the success of VS-300, Sikorsky designed XR-4 and YR-4 military helicopters. In the last years of WW-II, the United States Army bought and operated lots of these helicopters (Gessow & Myers , 1985, p.10, 12 ).
Conventional rotor comes up with blades which are fifteen to twenty times longer than they are wide ( Gessow & Myers, 1985, p.32 ). This type of rotor configuration is used for helicopters below 6000 pounds. The blades of the rotor can be made of wood, plywood, metal or fabric covered ( Gessow & Myers, 1985, p.34 ). Nowadays composite materials are also being used. The rotor contains a rotor hub which is above the mast, connecting the rotor blades to the control system ( Bloom, 1998, p.4 ). The swash plate which transforms control movements is the backbone of the control system ( Bloom, 1998, p.4). And the transmission system provides a gear ratio between rotor and the engine of 10:1. In conventional helicopters, the anti-torque rotor at the tail is connected directly to the main rotor. This means that if the power plant fails to operate, the movement of the main rotor makes the tail rotor turn ( Gessow & Myers , 1985, p.36). The aerodynamic shapes of the rotor blades are similar to those of aircraft wings. They usually involve airfoils like NACA 0012, 0015, 23012. And their ratio of thickness varies from 9 % to 20 %.
The conventional rotor configuration contains some advantages like low requirement for torque correction, involvement of a non-complex control system, reduced number of transmission problems, high performance and manoeuvrability, and finally low cost.
But there are also some handicaps of conventional rotor systems, some resulting from the vertical anti-torque rotor. First of all, the length of the tail carrying the anti-torque rotor produces a problem of area. Secondly, foreign objects like trees or wires can give critical damage to the tail rotor. Tail rotor does not produce lift, so the power spent for it is useless. Finally, strong control over the centre of gravity is required during the flight. ( Young, 1949, p.5)
The other most common rotor design is coaxial arrangement. This type consists of two counter-rotating and superimposed rotors. The first coaxial system was designed by Emile and Henry Berliner in 1909. The craft had two engines ( Gessow & Myers, 1985, p.5 ). Also Raoul Pascara from Spain designed and built a coaxial helicopter with biplane rotors in 1925. In Pascara’s helicopter, each rotor had ten biplane surfaces fixed to the shaft ( Gessow, 1985, p.6 ). D’Asconio in 1930 and Rene Breguet in 1936 also designed successful coaxial helicopters ( Gessow & Myers, 1985, p.6, p.9 ).
The coaxial rotor system solves the torque problem with two rotors mounted one above the other. They are superimposed and they rotate in different directions. This means that the torque of one rotor’s is balanced with the other one’s torque. The rotors do not have to be in the same size or they do not need to rotate with equal speeds ( Gessow & Myers, 1985, p.18). While explaining the design considerations of helicopters, Young wrote:
When the helicopter employs coaxial, counter-rotating rotors to produce a torque-free lifting system, the problem involved in the interference between the rotors can be solved in a manner similar to that of counter-rotating propellers. The lower rotor operates in the wake of the vortices of the upper rotor, and the behaviour of the upper is affected by the interaction of the induced velocities between the two (Young, 1949, p.61).
According to Watson, coaxial helicopters are ideal for mass production. He states that coaxial helicopters are advantageous for their small sizes and they occupy the smallest space when not operating. He also tells that, because of their small sizes, coaxial helicopters need the smallest area for landing and take-off operations ( Watson, 1948, p.2). There is no requirement for the synchronisation of the rotors in the coaxial arrangement, this is one of the most characteristic advantage of this system. Manoeuvrability of coaxial helicopters is very high, and ground clearance performances are good.
One of the most known examples of coaxial helicopters is Kamov Ka-50 attack helicopter. According to the studies over this helicopter, it appeared that coaxial rotors’ torque is 12% better than conventional design, and the power spent to rotate the tail rotor is saved (Teknik Ucak, 1999, p.67).
On the other hand, there are a number of disadvantages of coaxial design. Firstly, the control mechanism of the rotor system is very complex, and this complex structure causes more drag than other rotor configurations. Secondly, the height of the helicopter is increased because of the second rotor. Certain stability problems were observed during experiments, and finally the overall weight of the system is too high. (Young, 1949, p.6)
During the development of helicopters, other types of rotor configurations also appeared. Some of them are tandem, intermeshing (synchrocopter), side by side and jet reaction rotors. In fact they all do the same job: produce lift, provide basic flight manoeuvres and carry a useful load. They all have some advantages and disadvantages. In tandem rotor configuration, two main rotors are placed (one in front of the other). The characteristic field of use of this design is carrying heavy loads. Side by side rotors are used for helicopters over 5000 pounds of weight, but they have great overall weight and parasite drag. Intermeshing rotors come up with an interesting solution. Two main rotors are synchronised in this type of design. But this is also the main drawback of the design. And finally, jet reaction rotors use jet propulsion to rotate the main rotor. No transmission or tail rotor are used in this arrangement, but it’s biggest handicap is high fuel consumption ( Young, 1949, p.5, p.6, p.7).
In conclusion, as the oldest solution for flying, helicopters involve a wide variety of rotor solutions for lift producing to do their job; two of these solutions are the most popular: conventional and coaxial arrangements. Both of them have some similarities with other rotor configurations, but they both have some advantages and disadvantages. Conventional rotors are ideal because of their simplicity and good manoeuvrability. On the other hand, coaxial arrangement deserves attention for it’s high lift producing capability and requirement for small areas for landing. Of course they are not for every kind of helicopters, designers must show utmost care during the selection of the rotor system. Requiring both artistic and scientific background, helicopter and rotor system design seems to be one of the most difficult engineering tasks of modern engineering ea. It is clear that this task cannot be accomplished without the knowledge of design materials.

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