Abstract:
Concorde's unique low-speed handling qualities are acceptable when flown in a rigidly
procedural manner by experienced pilots. However, to be commercially viable and
environmentally acceptable more numerous second generation supersonic transport
(SST) aircraft would have increased passenger carrying capacity, range and the
flexibility to integrate with sub-sonic air traffic. Their much larger size, weight and
inertia compared to Concorde's, combined with increasing levels of relaxed longitudinal
stability to improve aerodynamic efficiency, results in unstable dynamics and degraded
handling qualities on the final approach, where precise manual flightpath control is
required.
Modern fly-by-wire command and stability augmentation systems can restore stability,
provide task tailored command laws and an associated level of handling qualities. Nonlinear
Dynamic Inversion (NDI) enables control law prototyping and analysis for the
rapid assessment, of conceptual designs to identify control power and command
response requirements using both off-line and real-time simulation. This study has
developed and applied NDI, and its realisable form (RNDI), in a novel way to design
flight control laws specifically addressing handling quality requirements using selected
criteria.
Piloted validation has demonstrated that NDI pitch rate command will consistently
provide Level 1 low-speed handling qualities in both steady and turbulent conditions.
However, the best handling qualities can be achieved through a second order pitch rate
response, generated by pre-filters, designed to author-suggested constraints on control
anticipation parameter (CAP). The SST pitch rate criterion envelope, modified to ensure
positive pitch attitude dropback, can then be applied to verify the time response. The
resulting pre-filters are easily applied to RNDI inner loop controllers and would be
straightforward to implement with simple and proven sensor requirements.
Carefully designed NDI normal acceleration command laws are also capable of
generating Level 1 low-speed handling qualities in steady conditions. However, their
degraded performance in turbulence was exacerbated, relative to the pitch rate
command laws, by the use of a fixed base simulator for pilot evaluation. Further motion
based simulation studies would provide, in addition to pitching motion, the normal
acceleration response cues necessary for a fair command law comparison to be made.
