An MDO Framework for Relaxed Static Stability Constraint in Tail Sizing of High Performance Airplanes.

How unstable the aiframe can be?

Historically, airplane designers have relied on conservative static stability margins, leading to oversized tail areas that increase trim drag, despite modern flight control systems capable of mitigating stability issues. By relaxing these static stability constraints, it becomes possible to reduce tail size, decrease drag, and enhance maneuverability. Our study explores the potential benefits of relaxed static stability (RSS) in high-performance aircraft, supported by case studies of advanced designs like the X-29 and F-117, which achieved superior performance with significantly unstable static margins. A focus is placed on the interplay between horizontal tail design, center of gravity (CG) placement, and control feedback systems, addressing critical balance, stability, and flying quality requirements. The study seeks to quantify the extent to which feedback control can compensate for reduced static stability, enabling optimized designs with smaller tail areas without compromising performance.