A Coupled Mathematical Framework for Structural Load Estimation and Material Selection in a Hybrid STOL Aircraft.

Hybrid-electric Short Takeoff and Landing (STOL) aircraft with a Maximum Takeoff Mass (MTOM) of 2000 kg require lightweight structures capable of maintaining adequate stiffness and strength under demanding low-speed maneuver and gust loads. This study develops a deterministic analytical framework for preliminary structural load assessment and multi-criteria material selection. A coupled mathematical model integrates flight-envelope and gust-load equations with a Multi-Criteria Decision Matrix (MCDM) to evaluate structural material candidates. The V–n diagram is formulated to determine critical load factors, bending moments, and shear stresses acting on primary lifting structures. Candidate materials, including Al-Li alloys, Ti-6Al-4V, and carbon-fiber-reinforced polymer (CFRP), are evaluated using specific stiffness, fatigue resistance, and thermal tolerance. CFRP laminate configurations, including [0°/90°] and [±45°], are further assessed for directional load transfer. Experimental laminate-level characterization through tensile, open-hole tensile, and three-point bending tests is used to support the CFRP selection. The analytical framework identifies the primary wing spar and fuselage shell as regions suited to [0°/90°] CFRP because of their bending-dominated loading and high specific stiffness requirements. Ti-6Al-4V is identified as a suitable material for localized distributed electric propulsion (DEP) mounts exposed to concentrated mechanical and thermal loading. The framework provides a load-informed basis for preliminary structural material selection in hybrid-electric STOL aircraft. While laminate-level experimental results support the material-selection component, full experimental validation of the aerodynamic and structural framework remains necessary and is proposed for future work.