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Motivated by the unsatisfactory handling qualities of a conceptual Blended-Wing-Body (BWB) aircraft uncovered during recent assessments, flying qualities requirements for relaxed static stability (RSS) next-generation civilian aircraft are developed through a series of flight simulation experiments. The flying qualities of a regional-class MDO-based BWB are examined to illustrate the shortcomings of existing designs. A flight simulation experiment is then conducted on a large transport aircraft model with variable short-period and Dutch Roll dynamics to establish envelopes of acceptable flying qualities based on Cooper-Harper Handling Qualities Ratings and NASA Task Load Index (NASA-TLX) scores. Controllability limits for RSS aircraft are suggested based on feedback from test pilots and commercial pilots, alongside Dutch Roll requirements in the presence of unstable pitch dynamics. Implications of the results for the design of next-generation BWB aircraft are then discussed, as is their implementation as Stability and Control constraints in MDO algorithms. Results represent a step towards enabling next-generation RSS aircraft with sufficient handling qualities to enable safe landings in the event of a stability augmentation system failure, thus achieving the trim drag benefits inherent to RSS while mitigating the risks of certifying civilian aircraft requiring full stability augmentation.