A flying aircraft requires some control. The control can be delivered by a human or a computer. In this case, a computer is much safer, faster and reliable in controlling a flying an aircraft than a human being. A Stability Augmentation System manages the control surfaces of an aircraft and alters their movement as necessary to make the aircraft more stable at all times. SAS is used to eliminate unfavorable effects such as short period oscillations, Phugoid oscillations, Dutch rolls and spiral modes. Stability Augmentation system together with a rate control system constitutes a flight management system. Rate control system manage the pilots demand for movement of a control interface that remains within the working conditions of the actuator by controlling the sensitivity of the movement of the surface in relation to the stick inputs. Longitudinal static stability of modern high speed aircrafts is greatly influenced with aircrafts flight envelope expansion. When the aircraft has sufficient static stability in supersonic flight stage, it’s far much unstable in subsonic stage thereby necessitating the design of stability augmentation system.
This report explores the methods used by Boeing 727 to create controllers for Stability Augmentation System and rate control. It is found that closed loop feedback systems gives the best response for Stability Augmentation and rate control for both stable and unstable flight conditions.
Conventionally aircraft control is conducted by the flight control system. The use of fly-by-wire systems allows electrical signals to be transmitted to the control services. A flight control system contains a Flight Control Computer that allows automatic alteration of the pilot parameters. The use of a flight control computer provides the much needed high reaction velocity that cannot be achieved by the pilot. Flight control system has the ability to compute the current state of the aircraft and assess how the pilot stick input is affecting it. Thus, the control demand of the pilot is altered by the damping response and control rate of movement of the surface in respect to the input desired by the pilot.
However effective the control flight control system can be, they are only suited for a certain envelope such that when the aircraft is operating outside that range, the system is obsolete.
This paper is about the Stability Augmentation System of Boeing 727 aircraft. Boeing B-727 is a three engine commercial plane carrying 150 passengers. The plane has a swept wing of T-nail. The three low-bypass JT8D turbofan engines are mounted on the aft fuselage. Two on-board engines are located short pylons; while the center engine is mounted in the aft fuselage and consist of an inlet above the fuselage.
B-727 is characterized by a conventional flight control system controlling cables traversing hydraulically-boosted control surfaces. High-rate electric trim is used for the horizontal stabilizer. Engine response is characterized by idle to a slow power setting, then to a fast setting to reach full thrust.
B-727 TOC evaluation conducted reveals the following characteristics. Under total hydraulic turnoff, there was total loss of the control system. In the evaluation of the engines-only roll rate with outboard engines at full differential thrust, roll rates of 3.5 degrees per second were found. There was an appreciable 1-sec lag roll rate. From an initial wings level, it took 11 seconds to make a 30 degrees band angle. The bank angle recorded at the fourth minute was 12 degrees. Its roll capability is much less than other aircrafts such as F-15 and B-720 with considerably large values bending due to their fuselage mounting on the engines.
An evaluation of the pitch control reveal a small pitching moment due to the thrust offset, and a significant high pitching authority due to speed stability. With the airplane trimmed and the throttles set for level flight, noseup pitch rates account for full thrust at approximately 0.75degrees per second. Nosedown pitch is maintained at o.4 degrees per second.
The brief analysis allow us to conduct a probe of how these values were obtained during simulation and how the Stability Augmentation System works to stabilize the plan in real-world scenarios.
The aircraft flying qualities can be improved by using the stability augmentation system in order to facilitate the different stages of flight and to accomplish various missions. Most modern high performance commercial and military aircrafts static stability is not satisfactory, therefore the design of SAS is essential. A longitudinal aircraft stability augmentation designed to improve the longitudinal stability and flying quality is discussed below. The longitudinal modes comprise of the short period mode and the long period mode. When the properties of the two modes do not meet the requirements, a designed SAS is needed n to satisfy the short-period natural frequency and damping factors.
Stability Augmentation System
Stability Augmentation System ensures that the stability of the aircraft is controlled such that control surfaces are utilized to make the aircraft more stable. SAS are used to eliminate effects such as phugoid, short period oscillation, lateral modes of spiral, roll and Dutch roll. This is achieved through the use of the following controllers.
Fig. 1 Stability Augmentation System
LONGOTUDINAL AND LATERAL AUGMENTATION
An aircraft requires feedback in order to be statistically stable. In order to achieve this stability three augmentation functions based on fly-by-wire aircraft are used. Angle of attack feedback, pitch rate feedback and angle of attack and pitch rate feedback.
Pitch dampers are used to alter the effects of longitudinal movement where phugoid and short period motion can be eliminated. According to the control law, the pitch damper is designed using the pitching moment due to AOA and tail. Stability augmentation is arrived at by constructing an output matrix C using the input matrix A and state matrix B. The matrix C is used to attain stability control using two mechanisms: AOA feedback and q feedback. Airlines companies such as Boeing are using Q feedback because it is easy to obtain the information from the state space equation.
A closed loop eliminates the phugoid oscillations as well as increasing short period oscillations. Pitch controller response is adequate as it reduces the undesired effects of short period oscillation and automatically eliminates the phugoid oscillations without the input of the pilot. A controller dampens the response yielding a smooth curve to signify the effective damping system and subsequently increased pitch stability.
Angle of attack of feedback
For angle of feedback, a proportional gain given by constant K is used. The control law is given by δ = Ke α and is described: The angle of attack keeps constant and the elevator is in equilibrium while the aircraft is in stable condition. The deviation of the angle of attack is detected by a sensor and a signal is send back to the elevator servo via the feedback loop. When the angle of attack is altered due to a disturbance, correspondingly the elevator deflects an angle to make the deviation decrease until the angle of attack returns to the value and the elevator would change to a new equilibrium position. The proportional constant K can be altered to get satisfied short-period natural frequency.
The controller for the pitch damper was initially built using AOA feedback. However, it is worth noting that a large damping ration will significantly increase the control actuator. Thus, this effect is not required in practical systems and a minimal value should be maintained. A parallel structure is desired for aircraft dynamics with feedback compensator. By making the system a closed loop, the feedback control is improved significantly. This is achieved by using a feedback inline with the aircraft dynamics and then creating a feedback loop around both systems. This gives room for the use of a PI system which corrects all the errors. A tuning process can be further conducted to improve the system using a proportional feedback.
Fig. 2 The working principle of angle of attack feedback
Pitch rate feedback
The pitch rate is utilized as feedback in this mechanism. The working principle utilizes feedback control gain L and according to the control law is expressed as δ = Lq. Only the proportional gain L is adjusted in this case.
The working relationship is diagrammatically represented in the following form
Angle of attack and pitch rate combined feedback
The working principle of angle of attack and pitch rate combined feedback is given diagrammatically as shown;
Fig. 4 Angle of attack and pitch rate combined feedback
The control rate for the angle of attack and pitch rate combined feedback is given by the relation δ = α + LqKe where the inner loop is the angle of attack feedback while the outer loop is the pitch rate feedback. There are two parameters adjusted to get the desired the short-period natural frequency and damping ration at the same timed. At L= -0.42 AND K=-2.75, the short-period natural frequency is given by 0.7 and the damping ration is equivalent to six. This is the recommended requirements for aircraft flying quality at the same time.
Fig. 5 Combined step response of the pitch rate
A viable solution for attaining stable augmented system sin aircraft lies in using the angle of feedback and pitch rate combined feedback. Aircraft manufacturers use this strategy for attaining longitudinal stability because it presents two feedback loops, the pitch rate and the angle of attack values. Thus, a short period pitch damper is increased as well as the natural frequency thereby increasing the longitudinal stability of the aircraft.
Boeing 727 uses yaw dampers to reduce the rolling and yawing oscillations as a result of Dutch roll mode. Dutch Roll is an inherent characteristic of all swept wing aircrafts. It is characterized by the nose of the aircraft making a constant eight shaped motion in flight and if left unattended, the motion gets constantly larger and more pronounced until the aircraft finally become unstable.
It uses yaw rate sensors and processors that sense and process the signal to be transmitted to an actuator connected to the rudder. Yaw dampers are necessary requirements in this aircraft to provide stability and a better ride for passengers onboard.
Yaw dampers use same servos as autopilots to introduce rudder deflection to eliminate the rolling motion. With non-functional yaw dampers, the aircraft have to be navigated at low altitudes and low speeds in order to maintain safety.
A moderate amount of Dutch roll is usually not dangerous but it tends to cause unnecessary feelings such as nausea in passengers. It then requires that it should be eliminated completely. Rolling and yawing motions are associated with Dutch roll. The wingtip yaws forward, then rolls up, then yaws backwards, then rolls backwards, then repeats. The opposite wings execute the same processes 180 degrees out of phase. Slip-rolling is a common challenge to designers where the roll-wise stability is increased but Dutch roll damping is decreased.
Designers of Boeing 727 have resolved this dilemma by realizing that roll-wise stability is a factor of slip-roll coupling and long-tail slip effect. Therefore, the solution lies in decreasing slip-roll coupling and increasing long-tail slip effect. This is achieved by making the tail boom longer while reducing the ruder area.
Effects that require yaw for correction operate in the lateral direction. The prime function of the system is to reduce Dutch roll by providing automatic rudder displacement proportional to and opposite the amount of yaw experienced. One yaw damper will control the upper rudder while the other controls the lower. Each damper has an associated coupler which operates as a gyro and senses yaw. The rudder system then institutes the required rudder movements to oppose and damp out the yaw. Rudder displacement as a result of the yaw damper input is constrained to 5 deg avoids full rudder application in the event of a yaw damper malfunction. The lower damper is powered by electrically from the Essential Radio Bus, while the upper yaw damper is powered from number two Radio Bus.
Damper warning flags located on the rudder and elevator position indicator are in a biased mode out of view when the respective yaw damper is engaged. Aircrafts with green lights instead of the warning lights turns green up when the damper is engaged.
The damper controls are associated with the rudder through a transfer valve located on the rudder power unit. The lower rudder utilizes system A while the upper rudder utilize system B. In the event that either of the hydraulic system pressure is lost, then the associated yaw damper is lost. Under this circumstance, the loss damper disengages without giving a warning. A common circuitry between the yaw damper and the autopilot is an interlock that requires one of the yaw damper to be on in flight.
Fig. 6 Yaw Damper control system
Roll rate controller and Yaw rate controller
The roll rate controller just like the yaw rate controller acts in the lateral direction. Roll rate control tend to decrease beyond the recommended safe level in flights. In order to correct this decrease, a proportional gain is induced. Also by increasing the feed forward control, the rise time of the system is improved. Roll rate controllers are set up to stabilize the aircraft for long periods. This increases the general stability of the system.
In yaw rate control normally, the system oscillate to any step input but slowly damps the resulting response. In this case, a fine tuning of the controller would output the desired results.
Differential thrust may be used in yaw rate controls. This always depends on the engine selection. All aircraft with multiple engines have a usable amount of yaw control subsequently resulting in roll control. Differential throttle inputs may, however, excite the Dutch roll-roll oscillatory mode and may be applied when fundamentally necessary alone.
Boeing B-727 is designed to deal with bank control using differential thrust between left and right engines. The resultant yaw causes a roll rate that\ can be modulated to give a desired bank angle. The desired bank angle is a factor of engine spoolup lag, amount of dihedral effect, roll due to yaw coupling, roll damping and the distance between the engines and the aircraft centerline. On aircraft with more than two engines such as the B-727, reduction of thrust on onboard engines finally improving the rate of roll control response. It was found that Dutch-roll oscillations are usually aggravated by throttle inputs, thus, it is important to allow natural damping. This helps minimize the probability of inducing them through small differential throttle inputs pulses stemming from changing bank angles.
Bank angles are always recommended to be minimal keeping them preferably between 10 and 15 degrees in order to avoid interaction with pitch control. This refers to the pilot aspect of stabilizing. In an even where the airplane is not laterally trimmed, then holding a steady throttle is essential to maintain the wing levels. Lateral mistrims more than 3 degrees of rudder are accommodated.
It is evident that the use of controllers integrated with Flight Control System can aide in the identification and preservation of the stability characteristics of a Boeing B-727. The controllers have the beneficial effects of removing unwanted modes of oscillations in longitudinal and lateral directions. The use of Stability Augmentation System simplifies the pilot’s workload as it implies that the pilot is less required to manually correct the unpleasant effects such as phugoid oscillations and Dutch roll.
The use of the system can, however, be detrimental to the pilot if not configured correctly. This is specifically applicable to roll and Yaw rate controllers. Control systems can institute unfavorable effects if they are not aligned to an appropriate gain schedule or provide same handling characteristics under different circumstances such as altitude and Mach number. By setting the controllers in an appropriate manner, they can be used to provide stability augmentation in unstable flight conditions. This rescue the pilot in having to configure the system manually as the flight control system will function effectively in managing stability and comfort.
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