Comparative analysis of flight maneuver loads between flexible and rigid aircraft

ifasd2024 Tracking Number 117

The analysis of aircraft loads during flight maneuvers plays a pivotal role in ensuring structural integrity, safety and the design of lighter and more fuel-efficient structures. This study focuses on a comparative analysis of internal load diagrams for a flexible aircraft and its rigid-body counterpart, emphasizing the impact of structural flexibility on flight dynamics and loads. The research employs a dynamically-coupled formulation for the flexible model, considering small deformations and inertially coupled equations of motion. The aerodynamic loads are calculated with a quasi-steady VLM model, and the structural dynamics is represented by a linear FEM model. The rigid-body model is obtained by neglecting structural flexibility, setting the number of elastic modes to zero. To calculate the internal loads, the force summation method is employed. Three maneuvers from CS-25 specifications are simulated: the symmetrical unchecked (or maximum pitch control displacement) and checked maneuvers, and the roll maneuver. For the unchecked maneuver, the flexible model exhibits a slightly slower response and reduced wing and horizontal tail loads compared to the rigid model, attributed to inertial, aerodynamic, and propulsive coupling. In the checked maneuver, the flexible model displays nuanced differences in loads, because the elevator profile is characterized by a sinusoidal input with a frequency matching the short period mode, which significantly differs from the frequency of the first elastic mode. The roll maneuver reveals a slower response and steady roll rate in the flexible model due to structural deformations and aileron control effectiveness, consequently the wing internal loads of flexible model is smaller. Detailed comparisons of pertinent parameters and wing and horizontal tail internal loads for all maneuvers highlight differences in aerodynamic, inertial, and propulsive contributions. Despite variations, the study emphasizes the importance of considering structural flexibility in analyzing flight maneuver loads and the need for more precise and efficient methods in load analysis to address the evolving landscape of aircraft design. The framework developed may be employed to calculate any flight and ground condition loads if some upgrades are performed, e.g., the implementation of an unsteady aerodynamic model to calculate turbulence encounter loads, and control laws to design a load alleviation system.