Evaluation of Quasi-Static Gust Loads Certification Methods for High-Altitude Long-Endurance Aircraft

Abstract

Aeroelastic gust loads analysis can be approached using quasi-static, transient, or continuous methodologies. Pratt method is a quasi-static approach that forms the basis for Federal Aviation Regulation Part 23 gust loads certification requirements. This work evaluates the usefulness of Pratt method for unconventional high-altitude long-endurance aircraft. The derivation of Pratt method is reviewed, and all assumptions are identified. Error of a key curve fit equation is quantified directly. A state-of-the-art nonlinear aeroelastic code was upgraded and used to facilitate quantification of application-dependent errors by comparing quasi-static results to results from nonlinear transient analysis. Application-dependent errors are presented in the context of a SensorCraft-inspired joined-wing model and a Helios-aircraft-inspired flying-wing model. Recommendations are made on the usability of Pratt method for aircraft similar to the two high-altitude long-endurance models. It is concluded that Pratt method is useful for preliminary design of the joined-wing model but inadequate for the analysis of the flying-wing model. Further recommendations are made regarding the subtleties in the implementation of Pratt method for unconventional configurations.