Handling qualities assessment of short period mode for fly-by-wire passenger airliner with relaxed static stability design

The handling qualities design of a fly-by-wire passenger airliner places great emphasis on safety and comfort. Lower short period frequency, control sensitivity, bandwidth and higher time delay are expected on airliners due to its design and operating characteristics. Moreover, the control law designs differ from military airlift aircraft. Developing a handling qualities and airworthiness assessment method, which take military standards as reference, is crucial to the flight control law design for a passenger aircraft. In this paper a flight control law with C ^* mode, load factor mode and angle of attack mode are developed to improve the short period handling qualities of a relaxed static stability passenger jet. Handling qualities and airworthiness assessment, based on handling qualities rating method (HQRM) from advisory curriculum AC25-7A, is used to examine the short period handling qualities of the augmented airplane. Low order equivalent systems method, bandwidth criterion and pitch rate response criterion are used in the assessment. Results indicate that time delay and band width restrictions from military specifications should be relaxed when applied to passenger aircraft. For conventional response type such as the angle of attack mode, all three criteria are applicable. Low order equivalent systems and pitch rate criteria are suitable for load factor mode. C ^* mode features an unconventional response type, thus only bandwidth and pitch rate response criteria are applicable.