25-01-2013, 11:52 AM
STABILITY AUGMENTATION SYSTEM (SAS) FOR AIRCRAFT
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IntroductionTo Stability Augmentation System (SAS)
Stability augmentation system (SAS) were generally the first feedback control system design intended to improve dynamic stability characteristic. There were also referred to as damper, stabilizers, and stability augmenters. Aircraft such as the F-104 had SAS. This system generally fed back an aircraft motion parameter, such as pitch rate, to provide a control deflection that opposed the motion and increase damping characteristic. The SAS had to be integrated with the primary mechanical control system of the aircraft consisting of the stick, pushrods, cables, and bellcranks leading to the control surface or the hydraulic actuator that activated the control surface. The control authority (percentage of full surface deflection avaible)
Understanding of Stability Augmentation System (SAS)
Stability augmentation system is system that alters of the inherent behaviour of a system. This system is used on aircraft. This is usually achieved by a system which controls one or more flight-control surfaces (or engines) automatically without inputs from the pilot. The inherent stability and response behaviour of many modern airplanes tends toward low damping or even instability. The physical reasons have to do with the configuration of the airplane and the combination of flight speed and altitude at which the airplane is operated. Several modern fighters and even some transports are intentionally designed with no or little inherent stability. There are a number of reasons for such a design condition. In the case of fighters, excellent manoeuvre ability in combat is essential. By making a fighter intentionally inherently unstable, it is easy to design the control system so that load factors in pull-ups or in turns can be built up rapidly. In the case of transports, the motivation to design for little or no inherent stability is to lower the size of the tail and thereby achieve a reduction in drag and weight
For Dynamic and Static Stability
The main goal Stability augmentation systems (SAS) areto make the aircraft more stable. There are SASs are divide into two parts that are dynamic stability (whether the eigen motions don’t diverge) and the static stability (whether the equilibrium position itself is stable).
Dampers – Acquiring dynamic stability.
An airplane has several eigenmotions. When the properties of these eigenmotions don’t comply with the requirements, we need an SAS. The SAS is mostly used to damp the eigenmotions. Therefore, we will now examine how various eigenmotions are damped. For this part,SAS consist of :
1. The yaw damper: modelling important systems
2. The yaw damper: determining the transfer function
3. The pitch damper
4. The phugoid damper
The yaw damper: modelling important systems
When an aircraft has a low speed at a high altitude, the Dutch roll properties of the aircraft deteriorate. To prevent this, a yaw damper is used. An overview of this system can be seen in figure 1. The yaw damper gets its input (feedback) from the yaw rate gyro. It then sends a signal to the rudder servo. The rudder is then moved in such a way that the Dutch roll is damped much more quickly than usual(1). As a designer, the only parts that can be influence is the yaw damper.