Primary Closed-Loop Flight Control Systems

The search for a solution of the problem of achieving stable equilibrium flight conditions was already started in the decades preceding the first successful powered flight. Two schools of thought in those early days can be distinguished in relation to the search for stability (Ref. 3.1). The first school expressed the opinion that stability could be created by a kind of, what might be called in the present terminology, automatic control system, while the second one argued that an aircraft could be made inherently stable by proper design.