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Making use of advantages of ducted fan, i.e. compactness and approachability, ducted fan VTOL can be used for many purposes, such as construction and servicing of the tall structures, rescue, etc.. In this paper the development of a working platform which utilizes the ducted fan VTOL concept is tried. Two types of ducted fan VTOL research models are developed. One is driven by electric motors and the other by an engine. The aim of the development is to investigate the ability of the ducted fan VTOL as a platform for working tasks at a high position. One of the most important problems for the development of the working platform in air is the development of control system which enable precision hovering. In this paper mainly the automatic flight control system for ducted fan VTOL is investigated and the control problems are studied which will arise when the VTOL is utilized as a working platform.

It is often realized that the control system design utilizing modern state space theory produces unstable controllers. Such unstable controllers which should stabilize the plant theoretically, do not really stabilize the plants when they are implemented. Unstable controllers are not preferable from the view point of robustness and safety. Even if an unstable controller can stabilize the plant, it works well only when it is operating with the original plant. When some accident occurs and the feedback loop is cut, unstable motion of controller brings the worst results. In the design of control system for many aerospace vehicles and space structures, classical design methods of compensator are not applicable and only the modern techniques can be utilized, where the number of states, inputs and outputs are very large. In such cases, controllers should include high dimensional complex dynamics.