History of Low-Order Equivalent Systems for Aircraft Flying Qualities

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Introduction F ROM the beginnings of the evolution of flight, there evolved an appreciation of the advantages of an aircraft that was easy for the pilot to fly. Among the insights for which they are famous is the Wright brothers’ realization that when the ability to balance and steer became understood, then aviation would truly have arrived. As the balance between stability and control became better understood, aircraft emerged with sufficiently good flying (or handling) qualities to allow air-to-air combat, reliable passenger transport, and a host of other uses. As aeronautical engineers examined the measurable factors that seemed to bestow good flying qualities, they followed the urge of all engineers, which is to write down those lessons, rules of thumb, and guidelines that make the job easier the next time. From the observations of these engineers, and especially from dedicated research using special-purpose variable-stability aircraft, the community began to document characteristics that were preferred for good flying qualities, along with characteristics that would indicate poor qualities. Most of the early flying-qualities work involved essentially steady-state characteristics like stick force variation with steady, trimmed speed. Dynamic behavior was not so well understood, though early aviators did comprehend the slow tradeoff between altitude and speed in Lanchester’s phugoid mode. They also knew all too well the sinister origins of the slow divergence called the spiral mode. The quick lateral-directional oscillations of the Dutch roll were commonly experienced. Appreciation of the rigidbody modes of motion greatly improved in the first half of the 20th century, including the faster lateral roll mode and the quick angle-of-attack oscillations of the short-period mode. By the middle of the 20th century, engineers realized that the time constants, damping ratios, and natural frequencies of all of these modes could be correlated with flying qualities, good or bad. The stage was set for formal requirements, in the form of military specifications, to define the good and bad values of these modal parameters.