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Phase Lag
In the aerodynamics of rotorcraft like helicopters, phase lag refers to the angular difference between the point at which a control input to a rotor blade occurs and the point of maximum displacement of the blade in response to that control input. This displacement occurs in the direction of rotor rotation. Phase lag may vary depending on rotor tilt rate, ratio of aerodynamic damping to blade inertial forces (Lock number), offset of flapping hinge from axis of rotation (e/R ratio), and coupling of blade flap, drag, and feather motions, and often results in ''cross-coupling'' between the aircraft control axes. Phase lag is a property of all rotating systems acted upon by a periodic force. Because of phase-lag, rolling a rotorcraft to the left or right would theoretically require a forward or backward cyclic if there were no mechanical correction. The rotor control system is angularly shifted as much as necessary to compensate for phase-lag and provide helicopter response that match ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Aerodynamics
Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dynamics and its subfield of gas dynamics. The term ''aerodynamics'' is often used synonymously with gas dynamics, the difference being that "gas dynamics" applies to the study of the motion of all gases, and is not limited to air. The formal study of aerodynamics began in the modern sense in the eighteenth century, although observations of fundamental concepts such as aerodynamic drag were recorded much earlier. Most of the early efforts in aerodynamics were directed toward achieving Aircraft#Heavier than air – aerodynes, heavier-than-air flight, which was first demonstrated by Otto Lilienthal in 1891. Since then, the use of aerodynamics through mathematical analysis, empirical approximations, wind tunnel experimentation, and computer simu ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Rotorcraft
A rotorcraft or rotary-wing aircraft is a heavier-than-air aircraft with rotary wings or rotor blades, which generate lift by rotating around a vertical mast. Several rotor blades mounted on a single mast are referred to as a rotor. The International Civil Aviation Organization (ICAO) defines a rotorcraft as "supported in flight by the reactions of the air on one or more rotors". Rotorcraft generally include aircraft where one or more rotors provide lift throughout the entire flight, such as helicopters, autogyros, and gyrodynes. Compound rotorcraft augment the rotor with additional thrust engines, propellers, or static lifting surfaces. Some types, such as helicopters are capable of vertical takeoff and landing. An aircraft which uses rotor lift for vertical flight but changes to solely fixed-wing lift in horizontal flight is not a rotorcraft but a convertiplane. Classes of rotorcraft Helicopter A helicopter is a rotorcraft whose rotors are driven by the engine(s) throughout ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes allow helicopters to be used in congested or isolated areas where fixed-wing aircraft and many forms of STOL (Short TakeOff and Landing) or STOVL (Short TakeOff and Vertical Landing) aircraft cannot perform without a runway. In 1942, the Sikorsky R-4 became the first helicopter to reach full-scale production.Munson 1968.Hirschberg, Michael J. and David K. Dailey"Sikorsky". ''US and Russian Helicopter Development in the 20th Century'', American Helicopter Society, International. 7 July 2000. Although most earlier designs used more than one main rotor, the configuration of a single main rotor accompanied by a vertical anti-torque tail rotor (i.e. unicopter, not to be confused with the single-blade monocopter) has become the most comm ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Lock Number
In helicopter aerodynamics, the Lock number is the ratio of aerodynamic forces, which act to lift the rotor blades, to inertial forces, which act to maintain the blades in the plane of rotation. It is named after C. N. H. Lock, a British aerodynamicist who studied autogyros in the 1920s. Typical rotorcraft blades have a Lock number between 3 and 12, usually approximately 8. The Lock number is typically 8 to 10 for articulated rotors and 5 to 7 for hingeless rotors. High-stiffness blades may have a Lock number up to 14. Larger blades have a higher mass and more inertia, so tend to have a lower Lock number. Helicopter rotors with more than two blades can have lighter blades, so tend to have a higher Lock number. A low Lock number gives good autorotation characteristics due to higher inertia, however this comes with a mass penalty. Ray Prouty writes, "The previously discussed numbers: Mach, Reynolds and Froude are used in many fields of fluid dynamic studies. The Lock number is our ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Rotorhead
In helicopters the rotorhead is the part of the rotor assembly that joins the blades to the shaft, cyclic and collective mechanisms. It is sometimes referred to as the rotor "hub". The rotorhead is where the lift force from the rotor blades act. The rotorhead is connected to the main drive shaft via the jesus bolt, and houses several other components such as the swash plate, flight control linkages and fly-bars. The rotor hub is also where the centre of gravity In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force ma ... acts on the helicopter. Types Rotorheads can be classified into 3 main types: * Articulated * Semi-Rigid * Rigid Articulated Rotorhead An articulated rotorhead system is one where the individual blades are free to flap, lag and change pitch. This is done by mounting t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gyroscopic Precession
Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In other words, if the axis of rotation of a body is itself rotating about a second axis, that body is said to be precessing about the second axis. A motion in which the second Euler angle changes is called ''nutation''. In physics, there are two types of precession: torque-free and torque-induced. In astronomy, ''precession'' refers to any of several slow changes in an astronomical body's rotational or orbital parameters. An important example is the steady change in the orientation of the axis of rotation of the Earth, known as the precession of the equinoxes. Torque-free Torque-free precession implies that no external moment (torque) is applied to the body. In torque-free precession, the angular momentum is a constant, but the angular velocity ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Transverse Flow Effect
Transverse flow effect is an aerodynamic effect encountered when a helicopter moves horizontally (typically forward) through the air, which causes the rotor disc to roll to the side. It is also known as transverse roll or inflow roll. Transverse flow effect is not experienced when hovering, because the air above the rotor disc is being pulled down from above (known as induced flow or downwash), and is equally distributed around the rotor disc. The air is descending from above, which has the effect of reducing angle of attack. However, when the helicopter starts moving into undisturbed air, a portion of the disc is in clean, unaccelerated air, while the remaining portion of the rotor disc is still working on descending air. The part of the disc working on clean air therefore sees a higher angle of attack than the portion of the disc which is working on descending air. The result is that the portion in clean air develops more lift. The disc rolls to the side, rather than pitching ba ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Transverse Flow Effect
Transverse flow effect is an aerodynamic effect encountered when a helicopter moves horizontally (typically forward) through the air, which causes the rotor disc to roll to the side. It is also known as transverse roll or inflow roll. Transverse flow effect is not experienced when hovering, because the air above the rotor disc is being pulled down from above (known as induced flow or downwash), and is equally distributed around the rotor disc. The air is descending from above, which has the effect of reducing angle of attack. However, when the helicopter starts moving into undisturbed air, a portion of the disc is in clean, unaccelerated air, while the remaining portion of the rotor disc is still working on descending air. The part of the disc working on clean air therefore sees a higher angle of attack than the portion of the disc which is working on descending air. The result is that the portion in clean air develops more lift. The disc rolls to the side, rather than pitching ba ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Helicopter Rotor
A helicopter main rotor or rotor system is the combination of several rotary wings (rotor blades) with a control system, that generates the aerodynamic lift force that supports the weight of the helicopter, and the thrust that counteracts aerodynamic drag in forward flight. Each main rotor is mounted on a vertical mast over the top of the helicopter, as opposed to a helicopter tail rotor, which connects through a combination of drive shaft(s) and gearboxes along the tail boom. The blade pitch is typically controlled by the pilot using the helicopter flight controls. Helicopters are one example of rotary-wing aircraft (rotorcraft). The name is derived from the Greek words ''helix'', helik-, meaning spiral; and ''pteron'' meaning wing. Design principles Overview The helicopter rotor is powered by the engine, through the transmission, to the rotating mast. The mast is a cylindrical metal shaft that extends upward from—and is driven by—the transmission. At the top of the mast i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Helicopter Flight Controls
A helicopter pilot manipulates the helicopter flight controls to achieve and maintain controlled aerodynamic flight. Changes to the aircraft flight control system transmit mechanically to the rotor, producing aerodynamic effects on the rotor blades that make the helicopter move in a deliberate way. To tilt forward and back (pitch) or sideways (roll) requires that the controls alter the angle of attack of the main rotor blades ''cyclically'' during rotation, creating differing amounts of lift (force) at different points in the cycle. To increase or decrease overall lift requires that the controls alter the angle of attack for all blades ''collectively'' by equal amounts at the same time, resulting in ascent, descent, acceleration and deceleration. A typical helicopter has three flight control inputs—the cyclic stick, the collective lever, and the anti-torque pedals. Depending on the complexity of the helicopter, the cyclic and collective may be linked together by a ''mixing uni ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Unequal Rotor Lift Distribution
Unequal rotor lift distribution is an effect where the blades of a helicopter rotor generate more lift at the rotor tips than at the rotor hub. A helicopter rotor blade is an aerofoil, which is driven through the air to create lift. The lift generated is proportional to the square of the speed. Because the tips of the rotating blades travel much faster through the air than the parts of the blades near the hub, they generate much more lift. If not mitigated, this effect would cause large bending stresses in the blade. In addition, the tip would have to be made stronger to handle the increased load. Helicopter manufacturers use the following techniques to equalise lift across the blade: * Washout is a geometric twist in the blade, such that the blade root near the hub has a higher angle-of-attack, thus higher lift. * Varying the aerofoil cross-section, such as flattening the aerofoil towards the tip, or tapering the blade towards the tip, which reduces its surface area thus red ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Rotorcraft
A rotorcraft or rotary-wing aircraft is a heavier-than-air aircraft with rotary wings or rotor blades, which generate lift by rotating around a vertical mast. Several rotor blades mounted on a single mast are referred to as a rotor. The International Civil Aviation Organization (ICAO) defines a rotorcraft as "supported in flight by the reactions of the air on one or more rotors". Rotorcraft generally include aircraft where one or more rotors provide lift throughout the entire flight, such as helicopters, autogyros, and gyrodynes. Compound rotorcraft augment the rotor with additional thrust engines, propellers, or static lifting surfaces. Some types, such as helicopters are capable of vertical takeoff and landing. An aircraft which uses rotor lift for vertical flight but changes to solely fixed-wing lift in horizontal flight is not a rotorcraft but a convertiplane. Classes of rotorcraft Helicopter A helicopter is a rotorcraft whose rotors are driven by the engine(s) throughout ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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