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Handling Qualities of Unstable Highly Augmented Aircraft

Demanding requirements for performance and handling qualitiestogether with extendedflight
envelopes lead to use of new technologies like active control and control configured unstable vehicles.
The review of the handling qualities issues of unstable aircraft which by necessityare highly
augmented Is the theme of this report. In general the handling qualitles criteria for highly augmented
stable aircraft are equally applicable to the specialized case of unstable aircraft. Accordingly, this
report contains a review of existing highly augmented aircraft, both stable and unstable. Handling
qualities criteria for both large and small amplitude longitudinal maneuvers are presented. Other
areas of Interest are also considered: basic aerodynamic design, specific issues relating to the feel
system and control sensitivity, evaluation techniques and the handling qualities design and evaluation
process. The subjects of carefree handling, lateral-directional criteria and agility are presented in
separate appendices. Where possiblethe lessonsto be learnedfrom the combined experiences of the
working group are highlighted.
This document is directed at the special problems of vehicles which are highly augmented because
they are statically unstable longitudinally. Statically unstable aircraft are not new; for example the
Wright Flyerwas statically unstable and the pilot providedthe control "augmentation". As knowledge
of the balance between stability and control improved, aircraft were balanced stable to allow safe
piloted control for demanding or protracted tasks. Why do we again today relax stability? "If the
designer is permitted to ignorethe customer requirementfor natural weathercockstability in pitch and
In yaw, he will be able to produce configurations with substantially increased performance" (Pinsker,
1979). With today's technology we now have the advantage of actuation, sensor and computing devices
to augment, with full authority, the pilot’s effort. Demandingrequirementsfor performance and
handling qualities together with extended flight envelopes lead to use of new technologies like active
control and control configured unstablevehicles, Benefits of task-tailored handling, carefree
handling and automatic functions and control modes outweigh penalties like larger actuators with high
power consumption, high sensor performance, redundant controls and demanding computer speed and
capacity requirements.