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Control Systems 102: GNC, Control

Posted by Harry Seldon on May 04, 2009

This post is part of a series about Guidance, Navigation and Control. See the table of contents here.

Stability and Control

Control answers the question ”How can the vehicle be stable?”. For an aircraft, it means “how can the aircraft accomplish basic moves such as flying straight, climbing, descending?”. A more technical definition of stability would be “the tendency of the vehicle to maintain or deviate from an established flight condition”. Control is the ability of the vehicle to be manoeuvred or steered from one flight condition to another.
It is very important to notice that questions regarding the stability (as opposed to a crash) are mainly addressed by the control and not the guidance nor the navigation. That is why one often speaks about Stability and Control System (SCS) and not only Control System.
The SCS is made of two parts: Stability Augmentation System (SAS) that stabilizes the aircraft (if it is naturally unstable) and improves its handling qualities. Then, the Control Augmentation System (CAS) typically allows the vehicle to maintain its altitude or heading. These functions are called altitude hold and heading hold modes in the AutoPilot (AP). The SCS creates the low level orders directly sent to the actuators (ailerons, rudder, elevator, engines, etc.). It is also sometimes called the Piloting System, meaning piloting is associated to low level, stabilization work.
Thus, how to fly is known by the control. Once it is done, higher level objectives can be achieved such as following a trajectory, that is going from a point A to a point B. High level orders will be sent by the guidance system.


Next chapter will be about Guidance.

Stability and Control
About GNC written GN&C or GCN written GC&N
About the human pilot
About control loops

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