AVIONICS. III. The automatic control the flight route (5)

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Aircraft navigation is the science about the means and tools of controlling aircrafts while flying from one point on earth’s surface to another according to the trajectory selected in time and space. Automatic operation provides complete control of most operations; pilot performs only general management of equipment.  (Begining  No.20 )

 Automatic flight control systems are expensive and complex, so not every aircraft uses them. According to the method of use of these devices can be divided into two groups:
– Devices that help automate management of aircraft aerodynamic surfaces;
– Devices that help to identify deviations from the selected navigation parameters.

The simplest is a manual control of the aircraft. In this case, manual drives can be used to power steering, monitoring of devices and navigational calculations are carried out by the pilot.

When aircrafts started to fly over longer distances, time-consuming manual operations, consisting mainly of maintaining plane angular positions, very distressed pilots. Aircrafts started to use autopilots – automated systems for stabilizing plane‘s angular positions. Soon autopilot not only become a necessary equipment of the airplane, but has expanded its functions. Height, speed and vertical speed stabilization, automatic horizontal alignment after spatial orientation was lost, coordinated turn, change of a flight altitude and other maneuvers have appeared. Also limiting of dangerously large roll, pitch and angles of attack, stall and too high airspeed (CAS, M), compensation of engine thrust asymmetry.
Autopilots were able not only to stabilize the plane, but also to perform simple maneuvers. They were called Automatic Flight Control Systems (AFCS). Next to the control system was included the engine control levers – machine to maintain a flight speed and engine trust. Planes were able to fly on instruments in bad visibility. The increasing number of aircrafts caused the need to start planning the flight routes and stick to them. Navigation calculations became more complex.
A further step in the automation of the management of aircraft was its flight route control according to a pre-established program. Program (flight plan loaded into the computer) is a series of waypoints in space and their connecting sections. Flight path management software has instrumental in providing the control system functions, which were difficult to carry out for a pilot, for example , automatic change of flight altitude or direction, automatic navigation and radio frequency change in the default location , the ability to change the scheduled flight trajectory to the nearest airfield, take-off and landing at the airport schemes SID (standard instrument departure) and STAR (standard arrival ) in a good and limited visibility , and so on.

In order to automatically fly the chosen route, one must be set up flight program before the flight and to have facilities which executes this program. When flight circumstances change, the program should be changed too. This can be done only by using computer technology. This way qualitatively new flight control system was created. Their main innovation was programmed flight from the takeoff to landing, and many of the individual aircraft navigation and control measures were integrated into one large single system referred to as aircraft flight control system FMS (Flight Management System). FMS has to determine the aircraft location and position in the space, compare the data with the flight program (the navigation database), measure the existing tolerances and send signals to the aircraft control surfaces. Flight management system performs all the basic functions of automatic control, depending on the air temperature and wind.

FMS can determine the location of the plane and perform flight program, it provides data for various aircraft navigation instruments. The most important of these are the inertial and radio navigation systems. Pilots monitor the airplane cockpit devices, where flight program data (chosen data) and navigation tools data (the actual data) are shown. These data pilots can compare and identify existing deviations. If the FMS does not send signals to control surfaces, pilot instead can control the plane himself. Then the plane control signals are received from the cockpit controls. Some aircraft may have FMS which can not automatically manage the aircraft surfaces, but provides the route and time deviations. Such systems are called FDS (flight director system). When turning off the auto- or flight director systems, pilot can control the airplane manually.

In the older aircraft FDA and AFCS were independent units, or they could act independently or together (like FMS). Newer aircraft FMS is one device, but it can make a lower -level management, for example, act as the AFCS or even as a simple autopilot.

Airplane ailerons, rudders, altitude and direction wheels are controlled independently. Each equipment manages only one aerodynamic surface, it is called control channel. Most modern FMS have three channels. Channels usually called by the same name as the primary control surface – the height wheel ( stabilizer ) channel , rudder channel, aileron channel . The height wheel is used to control flight height and speed. Aileron channel controls horizontal angle and route, rudder channel removes drift. Almost every major aircraft rudder is also used to suppress route deviations. In some airplanes route deviations are suppressed by stabilizer, and tilt – by ailerons. Very often, these three channels known as rake roll and yaw channels. Important is the speed control channel – power traction (throttle) machine.

The main purpose of the aircraft FMS trajectory control during the flight. FMS differs from autopilots, which only support (stabilize) position of the aircraft. If one wants to change the flight altitude or course, the pilot should perform calculations himself, turn off the autopilot, perform maneuver, and only after reaching steady position of the new aircraft to turn on stabilization. FMS performs all calculations and flight maneuvers automatically.

Throttle machines automatically manages the engine thrust from the take-off to landing, reducing the pilots’ workload. Traction machine is a computer-controlled electromechanical system that operates in conjunction with the aircraft AFCS automatic control system and computer of the flight management system FMS computer. Throttle machine AT (auto – throttle) is also associated with an inertial system INS and other systems. For example, engine throttle control needs static air pressure and temperature signals. Throttle machine is run by the computer; it receives signals from other systems. AT computer processes the received signals and manages electromechanical fuel supply servomechanisms. Servo motors operates levers on each engine. Levers may have position sensors whose signals are required for other systems, such as flight data recording apparatus.

With increase of speeds and airplane surfaces operated by electromechanical and hydraulic power amplifiers, to achieve satisfactory aircraft stability and maneurability, become only possible by improving FMS with new automated equipment, as SAS ( Stability Augmentation System), automatic stabilizer control , flight stabilization depending on Mach speed of the airplane, stabilization at low speeds and high traction , roll damping , etc. . Improvement the stability of the aircraft is widely used in automatic rocking suppression devices (dampers). Rocking dampers are installed in the aircraft yaw, pitch, and in some cases in angle channels. Rudders in lager aircrafts can suppress dodder due to air “holes” under the wings, engines thrust imbalance and other factors on the plane “shuffling“ by a few degrees. Rocking as they can be within a few seconds. Rocking dampers automatically controls the rudder surface. Some airplanes suppress inclination and tilt, and the same dampers automatically controls the height and (or) rudder surfaces. The main elements of such dampers are gyroscopic angular rate sensors with their signal amplifiers and ailerons, direction or height steering mechanisms. When aircraft inclines in any direction, the sensor deviation signals occur, proportional to inclination or tilt size and speed. They are amplified and sent to the respective surfaces of control mechanisms.

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