helicopter | Facts, History, & Types | bestwebdirectory.info
velop the guidelines is wind tunnel tests of a tail rotor helicopter model. These tests right front, and main rotor height-to-diameter ratio of 0. Procedures are . Citation: () "Tail rotorless helicopters", Aircraft Engineering and Aerospace which interacts with the main rotor downwash to supplement the side thrust. Helicopter designers have long dreamed of ways to eliminate the tail rotor but had The obvious way to overcome main-rotor torque without a tail rotor is to use a . from tail rotor cling to the surface of the tail by creating a pressure difference.
In both piston and turbine powered helicopters, the main rotor and the tail rotor are mechanically connected through a freewheeling clutch systemwhich allows the rotors to keep turning in the event of an engine failure by mechanically de-linking the engine from both the main and tail rotors.
During autorotationthe momentum of the main rotor continues to power the tail rotor and allow directional control. To optimize its function for forward flight, the blades of a tail rotor have no twist to reduce the profile drag, because the tail rotor is mounted with its axis of rotation perpendicular to the direction of flight.
Reliability and safety[ edit ] Many tail rotors are protected from ground strikes by a skid plate or by a steel guard, such as on this Bell The tail rotor and the systems that provide power and control for it are considered critically important for safe flight. As with many parts on a helicopter, the tail rotor, its transmission, and many parts in the drive system are often life-limited, meaning they are arbitrarily replaced after a certain number of flight hours, regardless of condition.
Between replacements, parts are subject to frequent inspections utilizing visual as well as chemical methods such as fluorescent penetrant inspection to detect weak parts before they fail completely. Despite the emphasis on reducing failures, they do occasionally occur, most often due to hard landings and tailstrikesor foreign object damage. Though the tail rotor is considered essential for safe flight, the loss of tail rotor function does not necessarily result in a fatal crash.Helicopter Loss of Tail Rotor Effectiveness and Collision With Terrain
In cases where the failure occurs due to contact with the ground, the aircraft is already at low altitude and the pilot may be able to reduce collective and land the helicopter before it spins completely out of control.
Should the tail rotor fail randomly during cruise flight, forward momentum will often provide some directional stability, as many helicopters are equipped with a vertical stabilizer.
The pilot would then be forced to autorotate and make an emergency landing with significant forward airspeed, which is known as a running landing or roll-on landing. The tail rotor itself is a hazard to ground crews working near a running helicopter.
aircraft design - How can a helicopter be designed without a tail rotor? - Aviation Stack Exchange
In considering helicopter flight, the relative wind can be affected by the rotation of the blades, the horizontal movement of the helicopter, the flapping of the rotor blades, and wind speed and direction.
In flight, the relative wind is a combination of the rotation of the rotor blade and the movement of the helicopter. Like a propeller, the rotor has a pitch angle, which is the angle between the horizontal plane of rotation of the rotor disc and the chord line of the airfoil. The pilot uses the collective and cyclic pitch control see below to vary this pitch angle. In a fixed-wing aircraft, the angle of attack the angle of the wing in relation to the relative wind is important in determining lift.
The same is true in a helicopter, where the angle of attack is the angle at which the relative wind meets the chord line of the rotor blade. Angle of attack and pitch angle are two distinct conditions. Varying the pitch angle of a rotor blade changes its angle of attack and hence its lift. A higher pitch angle up to the point of stall will increase lift; a lower pitch angle will decrease it. Individual blades of a rotor have their pitch angles adjusted individually.
Rotor speed also controls lift—the higher the revolutions per minute rpmthe higher the lift. However, the pilot will generally attempt to maintain a constant rotor rpm and will change the lift force by varying the angle of attack. As with fixed-wing aircraft, air density the result of air temperature, humidity, and pressure affects helicopter performance.
The higher the density, the more lift will be generated; the lower the density, the less lift will be generated. Just as in fixed-wing aircraft, a change in lift also results in a change in drag. When lift is increased by enlarging the angle of pitch and thus the angle of attack, drag will increase and slow down the rotor rpm.
Additional power will then be required to sustain a desired rpm. Thus, while a helicopter is affected like a conventional aircraft by the forces of lift, thrustweight, and drag, its mode of flight induces additional effects. In a helicopter, the total lift and thrust forces generated by the rotor are exerted perpendicular to its plane of rotation.
When a helicopter hovers in a windless condition, the plane of rotation of the rotor the tip-path plane is parallel to the ground, and the sum of the weight and drag forces are exactly balanced by the sum of the thrust and lift forces.
In vertical flight, the components of weight and drag are combined in a single vector that is directed straight down; the components of lift and thrust are combined in a single vector that is directed straight up.
To achieve forward flight in a helicopter, the plane of rotation of the rotor is tipped forward. For sideward flight, the plane of the rotation of the rotor is tilted in the direction desired. For rearward flight, the plane of the rotation of the rotor is tilted rearward.
Because the rotor is powered, there is an equal and opposite torque reaction, which tends to rotate the fuselage in a direction opposite to the rotor. This torque is offset by the tail rotor antitorque rotor located at the end of the fuselage.
The pilot controls the thrust of the tail rotor by means of foot pedals, neutralizing torque as required. There are other forces acting upon a helicopter not found in a conventional aircraft. These include the gyroscopic precession effect of the rotor—that is, the dissymmetry of lift created by the forward movement of the helicopter, resulting in the advancing blade having more lift and the retreating blade less.
This occurs because the advancing blade has a combined speed of the blade velocity and the speed of the helicopter in forward flight, while the retreating blade has the difference between the blade velocity and the speed of the helicopter. This difference in speed causes a difference in lift—the advancing blade is moving faster and hence is generating more lift.
If uncontrolled, this would result in the helicopter rolling. However, the difference in lift is compensated for by the blade flapping and by cyclic feathering changing the angle of pitch. Because the blades are attached to a rotor hub by horizontal flapping hinges, which permit their movement in a vertical plane, the advancing blade flaps up, decreasing its angle of attack, while the retreating blade flaps down, increasing its angle of attack.
This combination of effects equalizes the lift. Blades also are attached to the hub by a vertical hinge, which permits each blade to move back and forth in the plane of rotation. The vertical hinge dampens out vibration and absorbs the effect of acceleration or deceleration.
In addition, in forward flight, the position of the cyclic pitch control causes a similar effect, contributing to the equalization of lift. Other forces acting upon helicopters include coning, the upward bending effect on blades caused by centrifugal force; Coriolis effect, the acceleration or deceleration of the blades caused by the flapping movement bringing them closer to acceleration or farther away from deceleration the axis of rotation; and drift, the tendency of the tail rotor thrust to move the helicopter in hover.
Control functions A helicopter has four controls: Raising or lowering the pitch control increases or decreases the pitch angle on all blades by the same amount.
An increase in the pitch angle will increase the angle of attack, causing both lift and drag to increase and causing the rpm of the rotor and the engine to decrease. The reverse happens with a decrease in pitch angle.
Because it is necessary to keep rotor rpm as constant as possible, the collective pitch control is linked to the throttle to automatically increase power when the pitch lever is raised and decrease it when the pitch lever is lowered. The collective pitch control thus acts as the primary control both for altitude and for power.
The throttle control is used in conjunction with the collective pitch control and is an integral part of its assembly.
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The throttle control is twisted outboard to increase rotor rpm and inboard to decrease rpm. The antitorque controls are pedals linked to operate a pitch change mechanism in the tail rotor gearbox.
A change in pedal position changes the pitch angle of the tail rotor to offset torque. As torque varies with every change of flight condition, the pilot is required to change pedal position accordingly.
The Active Flapping The active flapping of blades is the key for the Ornicopter concept. In this manner, the blade can generate a propulsive force to rotate itself, and hence the shaft torque is not needed. This results in a torque-less main rotor, i. Update Concerning the question on scalability posed in one of the comments: I found a PhD thesis on the topic download which did a feasibility study on a Bo scale ornicopter. This optimal design results in an enlarged flight envelope due to the reduced rotor stall area and improved yaw stability in forward flight.
To compensate for the higher profile power needed for the Ornicopter's optimal design, a larger rotor radius is required in order to reduce the induced power and keep the increase in the total required power to a minimum.