This paper outlines the design and performance analysis of a composite sandwich flap for high-temperature applications, which is intended for use in advanced aerospace platforms. The composite sandwich flap has been designed as a multi-layer sandwich composite, which is capable of providing the required performance against the combined effects of aerodynamic, high-temperature, and structural loading conditions, which are encountered at high-speed flight profiles. A novel thermo-adaptive composite sandwich flap has been proposed, which comprises an IM7/BMI outer skin, an IM7/PEKK inner skin, and a titanium carbide (TiC) honeycomb core material. The proposed composite sandwich flap has been validated by aero-thermal simulations at Mach 2.2 and has resulted in a specific stiffness of 56.4 MJ/kg, which enables a 12.5% mass reduction compared to conventional titanium-based materials while maintaining a factor of safety of 1.6. The composite flap has effectively eliminated chordwise bowing by reducing it to 0.85 mm, thereby preventing the occurrence of premature flow separation. This has resulted in the aircraft maintaining its lift distribution, thereby enhancing its performance by 4.8% in the form of an improved L/D ratio of 4.25, which is a measure of efficiency. The results indicate that the advanced composite flap has provided the required aeroelastic integrity for relaxed stability aircraft at supersonic flight speeds.