Propeller Effects on Very Flexible Aircraft

Abstract

High Altitude Long Endurance aircraft are unmanned air vehicles with the capability of performing long-duration flights and can be used in many applications, such as communication and data relay, Earth observation, etc. To achieve their higher aerodynamic performance, HALE aircraft are typically high-aspect-ratio configurations, resulting in a very flexible structure. Moreover, the typical low cruise speeds often require a propeller-engine combination for propulsion.

Although many studies have been done in propeller-wing interaction, propeller effects on very flexible aircraft (VFA) have not received any attention. State-of-the-art nonlinear aeroelastic frameworks lack complete propeller modeling and, instead, use concentrated forces to model its thrust.

This work aims to fill this gap by incorporating propeller aerodynamics and inertial effects into a coupled nonlinear aeroelastic-flight dynamics framework. For that, the University of Michigan's Nonlinear Aeroelastic Simulation Toolbox (UM/NAST) is enhanced with an Unsteady Vortex Lattice for the lifting surfaces and a Lifting Line and Viscous Vortex Particle (LL/VVPM) methods to model the propeller aerodynamics. Furthermore, inertia effects associated with the rotating rigid blades are also incorporated.

Verification tests are performed for each of the new components added to the enhanced framework. Results for static and dynamic aeroelastic analysis with the coupled UM/NAST and Vortex Lattice model, isolated propeller, propeller-wing interaction, and the gyroscopic loads calculation are compared with results from other codes or published numerical and experimental data available in the literature.

Additionally, an approach based on system identification and proper orthogonal decomposition is introduced and verified for the stability analysis of VFA with propellers based on a time-series signal. The method is successfully verified against UM/NAST modal analysis for a purely structural case.

Based on the new developments, investigation of propeller effects on the aeroelastic response and stability of a VFA is presented and discussed. The results showed that the presence of propellers can influence the aeroelastic static and dynamic response of a VFA, as well as modify the aeroelastic modes and affect the flutter onset. Therefore, the additional propeller effects (besides just the incorporation of thrust) should be included in aeroelastic simulations.

Although focused on very flexible aircraft applications, it should be noted that the new enhanced framework can also be used to analyze new propeller-driven aircraft concepts such as the ones being proposed for Urban Air Mobility and distributed propulsion configurations.