Aeroelastic gust and maneuver loads have driven the sizing of primary aircraft structures since the beginning of aviation. Methodologies for determining the gust loads on aircraft have evolved over the last 100 years. There are three general approaches to gust loads analysis: quasi-static, transient, and continuous methods. Quasi-static analysis offers the greatest computational efficiency. A quasi-static formulation referred to as Pratt's Method is the current practice for FAR Part 23 certification requirements. Assumptions made in the derivation of Pratt's Method are acceptable for many conventional aircraft, but additional fidelity from transient and continuous analysis are required to certify FAR Part 25 aircraft.
This work provides an assessment of the usability of Pratt's Method for unconventional high altitude long endurance (HALE) aircraft. Derivation Pratt's Method is reviewed and all assumptions are identified. Error of a key curve fit equation is quantified directly. Application dependent errors are quantified by comparing loads calculated using Pratt's Method to loads calculated from transient analysis. To facilitate this effort, a state of the art nonlinear aeroelastic code has been modified to more accurately capture the transient gust response.
Application dependent errors are presented in the context of a SensorCraft inspired joined-wing HALE model, and a Helios inspired flying wing HALE model. Recommendations are made on the usability of Pratt's Method for aircraft similar to the two HALE models. It is concluded that Pratt's Method is useful for preliminary design of the joined-wing HALE model, but inadequate for the analysis of the flying wing model. Additional recommendations are made corresponding to subtleties in the implementation of Pratt's Method for unconventional configurations.
