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All helicopters must have this capability to be certified. Innormal powered flight, airis drawn into the main rotor system from above and exhausted downward. During autorotation, airflow onters the rotor disc from below as the helicopter descends, creating an aerodynamic force that drives the rotor and keeps it turning. This is known as tho autorotative force.
Figure ] Figure demonstrates the way this autorotative force comes about. Air flowing upward through the rotor produces an angle of attack such as wo see hore. From our study of basic aerodynamics, we know that the lift produced by an airfoil acts per- pendicularly to the relative wind, while the induced drag acts parailel, but in opposite direc- tion, to the relative wind.
If we consider the com- ponents of lift and drag that act along the plane of rotation of the rotor, we see that there is a net force acting in the direction of rotor rotation along this, plane. This part of the rotor disc is called the autorotation region. When the angle between the relative wind and, the axis of rotation is high, the resultant lift wil be ahead of the axis of rotation, and there will be an autorotative force that pulls the rotor in its direction of rotation.
The drag force that acts along the plane of rotation is greater than the component of the lift that acts along this plane. The result is a force that tries to deceler- ate the rotor, This is called an anti-autorotative Figure During autorotation, the upward flow of relative wind permits the main rotor blades to rotate at their normal speed.
If the rotor disc is tilted forward, the aircraft will fly forward; if it is tilted back, it will fly backward; and tilting it to the side will cause it to fly sideways. The early gyro- planes and some of the simplest rotary wing aircraft, mainly amatour-built, have a control bar that directly tilts the rotor head.
This form of direct con- trol usually requires the control forces to be quite light. The swash plate is used to transmit control inputs from the cockpit controls to the main rotor blades. It consists of two main parts: the stationary swash plate and the rotating swash plate. The sta- tionary swash plate is mounted to the main rotor mast and connected to the cyclic and collective con- trols in the cockpit by a series of pushrods.
It is restrained from rotating but is able to tilt in all directions and move vertically. The rotating swash plate is mounted to the stationary swash plate by means of a bearing and is allowed to rotate with the main rotor mast. Both swash plates tilt and move up Aircraft Structural Assembly and Rigging and down as a unit. The rotating swash plate is con- nected to the main rotor grips by the pitch links.
Control rods transmit collective and cyclic control inputs to the stationary swash plate , causing It to tilt or move vertically. The pitch links, attached from the rotating swash plate to the pitch arms on the rotor blades, transmit these movements to the blades. This causes all the rotor blades to increase or decrease blade pitch angle by the same amount, or collectively as the name implies.
As the collective pitch control is raised, there is a simultaneous and equal increase in pitch angle of all main rotor blades, and lift increases. As it is lowered, there is a simultaneous and equal decrease in pitch angle, and lift is decreased. This is done through a series of mechenical linkages, and the amount of movement in the collective lever will determine the amount of blade pitch change.
A friction control is adjusted by the pilot to prevent inadvertent collective pitch movement. Figure ] Changing the pitch angle on the blades changes the blade angle of attack and lift.
With a change in angle of attack end lift comes a change in drag, and the speed or rp. As the pitch angle of the blades is increased, the angle of attack and drag increase, while the rotor rp.
Decreasing pitch angle decreases both angle of attack and drag, and rotor p. To maintain a constant rotor r. This system maintains rp. Once the rotor p. Governors are common on all turbine helicopters Some helicopters do not use correlators or governors and require the pilot to coordinate all collective and throttle movements together.
When the collective is raised, the throttle must be increased; when the col: loctive is lowered, the throttle must be decreased. Some tur ine helicopters have the throttles mounted on the overhead pane! Some turbine helicopters have throttles mounted on the overhead panel or on the floor of the cockpit. The function of the throttle is to regulate engine p.
If the correlator system does not maintain the desired r. However, if the governor fails, the pilot will have to uso the throttle to maintain np. When the main rotor disc is tilted, the horizontal compo- nent of lift will move the helicopter in the direction of tilt. The cyclic can pivot in, all directions. An increase in pitch angle will increase angle of attack; a decrease in pitch angle will decrease angle of attack.
For example, if the cyclic is moved forward, the angle of attack decreases as the rotor blade passes the right side of the helicopter and increases on the left side. Because of gyroscopic procession, this results in. This assumes that the direction of rotor blade rote- tion is counterclockwise as viewed from above, which is the case of single rotor helicopters manu- factured in the United States.
Without this provision, the fuselage will assume an excessive nose-down pitch at high airspeeds, but the elevator holds it at a more level attitude. Helicopters that do not have synchronized elevators instead may havo a fixed horizontal stabilizer, that has a very pronounced airfoil section, mounted on. This serves two main purposes: to overcome high control forces and to prevent vibrations from the rotor system from being fed back into the controls. The hydraulic sys- tem consists of actuators, also called servos, on each flight control, a pump that is usually driven by the main rotor gearbox, and a reservoir to store the hydraulic fluid.
A switch in the cockpit can turn the system off, although it is left on under normal con- ditions. A pressure indicator in the cockpit may be installed to monitor the system, [Figure ] When a control input is made, a valve inside the servo directs hydraulic fluid under pressure to the piston to change the rotor pitch.
This type of servo uses a system of check valves to make the system irreversible; that is, to make it so the pilot can con- trol the pitch of the rotor blades, but vibration from. This gives the pilot enough time to land the helicopter with normal control. When the engine drives the rotor, the same force that spins the rotor tries to spin the fuselage. If the helicopter is to be controlled about its vertical axis, there must be some way to counteract this torque.
Some designers have driven the rotor with a jet of air from the rotor blade tip, thus eliminating any torque from a fuselage-mounted transmission drive. Others used two rotors, either on coaxial shafts, or mounted side by side or fore and aft. Despite all of the different attempts to com- Figure A typical hydraulic system for helicopters inthe light to medium range is shown here.
The pilot changes the pitch of the tall rotor to correct for torque by moving the pedals in the cockpit. Some of the flapping hinges incorporate a Delta-three hinge, whose hinge line is angled with respect to the rotor span, When the blade flaps, it also changes its effective pitch angle and can correct for the dissymmetry of lift without a severe flap- ping angle.
The pilot varies the thrust to rotate the fuselage while hovering or to compensate for any changes in the torque as power and speed are changed. This system uses a sories of rotating blades shrouded within a vertical tail.
Because the blades are located within a circular duct, they are less likely to come in contact with people or objects. This system uses low- pressure air, which is forced into the tailboom by a fan mounted within the helicopter. The air is forced Aircraft Structural Assembly and Rigging through horizontal slots, located on the right side of the tailboom, to a controllable rotating nozzle, pro- viding antitorque and directional control.
Directing the thrust from the control- lable rotating nozzle creates the rest. An airplane has both positive static and dynamic stability about all three of its axes, but the helicopter is not so fortunate, One of the developments pioneered by Bell Helicopter was the stabilizer bar. The control rods from the swash plate attach to the stabilizer bar, and pitch con- trol links connect the blade pitch arms to the stabi- lizer bar. In flight, the stabilizer bar acts as a very effective gyro, having rigidity in spaco; that is it does not want to depart from its plane of rotation.
If the helicopter tilts, the stabilizer bar remains in its origi- nal plane of rotation and an angular difference is formed between the bar and the mast. This is trans- mitted to the rotor blades as a pitch change in the cor- rect direction to right the helicopter.
If the rotor system in figure tilts to the right in flight, the angle of attack of the descending rotor blade will increase and the blade will flap up. As it flaps, the offset hinge will cause its pitch angle to increase and automatically produce lift in the direction needed to right the helicopter. It has the same effect as the pilot applying corrective use of the cyclic control, but this is automatic.
The simplest of these systoms is a force trim system, which uses a mag- notic clutch and springs to hold the cyclic control in the position where it was released. More advanced systoms use electric servos that actually move the flight controls. These servos receive control com- mands from a computer that senses helicopter alti- tude. The SAS may be overridden or disconnected by the pilot at any time. Stability augmentation systems allow the pilot more time and concentration to accomplish other duties.
It improves basic aircraft control harmony and reduces outside disturbances, thus reducing pilot workload. These systems are useful when pilots are required to perform other duties such as sling load- ing and search and rescue operations. The autopilot can actually fly the heli- copter and perform certain functions selected by the pilot. The most advanced autopilots can fly an instrument approach to a hover without any additional pilot input once the initial functions have been selected.
The autopilot system consists of electric actuators servos connected to the flight controls. The num- ber and location of these servos depend on the type of system installed, A two-axis autopilot controls the helicopter in pitch and roll; one servo controls fore and aft cyclic and another controls left and right cyclic.
A three-axis autopilot has an additional servo connected to the antitorque pedals and con- trols the helicopter in yaw. A four-axis system uses a fourth servo, which controls the collective power. A control panel in the cockpit has a number of switches that allows the pilot to select the desired functions and engage the autopilot An automatic disengage feature is usually included for safety purposes, which disconnects the autopilot in heavy turbulence or when oxtreme flight atti tudes are reachod.
Even though all autopilots can be overridden by the pilot, thoro is also an autopilot disengage button located on the cyclic or collective which allows you to completely disengage the autopilot without removing your hands from the controls.
Because autopilot systems and installa- tions differ from one helicopter to another, it is important that you refer to the autopilot operating procedures located in the Rotorcraft Flight Manual. Although not all vibration can ever be completely eliminated, irreversible con- trols and special vibration-absorbing engine and transmission mounts have minimized its effects.
To keep it simple and use- ful, we categorize them basically into two frequency ranges, two modes, and two conditions. The break between low and medium, and between medium and high is rather nebulous, so in this text we lump them all into either low or high frequencies. Those are normally associated with the main rotor systom. They may have a ratio of , , or with the main rotor, depending on the rotor configura- tion, and may be caused by either a static or a dynamic unbalance condition or by acrodynamic forces acting upon the rotor.
Any component that turns at a high p. Vibration Modes Vibrations can cause the helicopter to jump up and down in what is called a vertical vibration, or to shake sideways, which is called lateral vibration. Vertical vibrations are generally caused by some dis- parity in lift as the rotor spins and generally points to an out-of-track condition. Lateral vibrations are most often caused by an out-of-balance condition of the rotor.
Modes of helicopter vibration are shown here. Once an observation is made, it is plotted ona chart, which then directs us to make a move to correct the track or balance. Generally, only one cor- rective move is made at a tim Some of the newer systems use infrared sensors together with computers that direct us to which cor- rective moves to make.
In these systems, more than one move can be made each time, thus accelerating, the track and balance procedure. In almost all of these systems, an electronic pickup is used to key the system each time a rotor blade passes a reference point.
An accelerometer or velometer measures the amplitude of the vibration and marks the position in the blade path where this vibration occurs. Static bal- ance refers to balancing a blade while it is not mov- ing and is generally removed from the helicopter. Dynamic balance is accomplished with the blades mounted on the helicopter and turning. Before helicopter rotor is mounted on the mast, it mu be balanced both chordwise and spanwise. After this aligument, or chordwise balanc is accomplished, balance the rotor spanwise by adding.
Some helicopters, specifically those with three or more blades, may require the hub to be balanced without the blades, and then the blades are installed that have been balanced against a master blade.
Old methods made use of a marking stick, which was raised until it Figur sists of aligning the blades so that they are straight across the hub, just contacted the bottom of the rotor blade at the tip, or a flag that was moved into the tip plane until the tip just touched the flag.
They left a mark of their respec- tive color when the blade tips touched the flag, If the rotor was in track, the colored marks were superim- posed. The drawbacks to these two methods is that the helicopter can only be tracked on the ground and not in the air where it is most important, Modern tracking systems either use a strobe light held by the technician or an infra-red light mounted on the helicopter to determine blade track. The rotor is turned at the proper p.
A special reflector is installed on the tip of each blade, and they form a distinctive pattern as the strobe illu- minates each reflector. If the blades are in track, the images will be all in line, but if they are not in track, tho images will be staggered up and down.
The dis- tinctive mark on each reflector tells the technician which blade is out of track, in which direction, and how much. Checking blade track using a strobe light may 'be done on the ground and in fight. If the blade is out of track, as seen during an in-flight track check, it is usually corrected by slightly bending one of the rotor blade trim tabs. Piston engines are generally used in smaller heli- copters because they are relatively simple and inex- pensive to operate.
Turbine engines are more power- ful and are used in a wide range of helicopters. They produce a tremendous amount of power for their size but are more expensive to operate. While these engines are made by the same manufacturers and carry the same basic designation as those used in airplanes, they differ in operational details.
In some helicopters, the engine is mounted verti- cally so the transmission and the rotor mast can mount directly on the engine crankshaft. When this is done, the lubrication system must be changed. They are generally converted from a wet sump sys- tem, in which the oil is carried inside the engine itself, to a dry sump system, where the oil is carried in an external tank. The oil is pumped through the engine for lubrication and cooling and returned to the tank.
Other helicopters have the engine mounted horizontally and are connected to the transmission with several V-belts. The lack of a propeller to serve as a flywheel and to provide cooling air gives a helicopter two problems thet the airplane does not have.
Since the engine car- ries the load of the rotor system, it must be operated ata much higher idling speed, so there is not nearly as wide a range of engine rp.
Since there is no propeller, a fan blows air over the engine cooling fins to remove the heat, Engine power control is entirely different on a heli- copter compared to that on an airplane.
Rather than having complete control of the engine speed, the collective pitch control governs the helicopter power output. The rotor rp. As more load is placed on the rotor by increasing the blade angle with the collective pitch control, the throttle must be increased to maintain r.
This is either done auto- matically by correlators or governors, or manually accomplished by the pilot. The pilot determines the power that the engine is producing primarily by the relationship between the tachometer and the mani- fold pressure gauge.
The turbine operates best at a con- stant speed, which is perfect for the helicopter, but the most outstanding advantage is its tremen- dous power output for its small size and light weight. Helicopters use turboshaft engines, which differ from turbojets in that they have extra stages in their turbine section to extract a maximum amount of energy from the expanding gases. Very little thrust is, obtained from the exiting gases in a turboshaft engine.
The direct-shaft engine uses a set of turbine wheels mounted on a single shaft that drives both the com- pressor in the gas generator and the output shaft Reduction gearing reduces the p. The free turbine engine has one set of turbines to drive the compressor for the gas generator and a separate set of turbines for the transmission and rotor. The exhaust gases do not leave a turboshaft engine in a straight line from the turbine section as, they do in turbojet engine, so it is possible to drive the output shaft from either the hot or the cold end.
There are two tachometers on free turbine engines: N1 shows the speed of the compressor and N2 the spoed of the power turbine. The main difference between a turboshaft and a turbojet engine is that most of the energy produced from the expand- ing gasses Is used to drive a turbine rather than producing thrust through propulsion of exhaust gasses. The main components of the transmis- sion system are the main rotor transmission, tail rotor drive system, clutch, and freewheeling unit Helicopter transmissions are normally lubricated and cooled with their own oil supply.
A sight gauge is provided to check the oil level. Achieving a rotor speed of p. A 9 to 1 reduction would mean that the rotor would turn at r-p. A dual-neodle tachometer shows both engine and rotor rp.
The rotor r. When the needles are superimposed or married, the ratio of the engine rp. There are various types of dual-needie tachometers. When the needles are superimposed or mar- Fed, the ratio of the engine rp.
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