High Altitude Long Endurance (HALE) aircraft are a class of unmanned air vehicle with low energy consumption and a wide variety of promising applications. Basically, HALE must have high aerodynamic performance. Modern day UAVs achieve this by having high-aspect-ratio configuration that unfortunately result in a very flexible structure, which imposes additional challenges to aircraft stability and control. In addition, since such UAVs cruise at low speeds, a propeller-motor combination is often the choice of propulsion. The presence of the propellers results in different kinds of loads transmitted to an aircraft: i.e,. aerodynamic loads and inertial loads. Despite there being different loads, in the context of very flexible aircraft, the modeling of propeller effects is usually reduced to just a concentrated force (thrust), and little has been explored about the influence of the other propeller effects on the aeroelastic stability of such very flexible structures. A thorough review of existing literature further reveals that, although many studies with rigid configurations have demonstrated that propellers can significantly influence an aircraft performance and flight dynamics stability, few investigations have been conducted on the effects of propellers on the aeroelastic stability of very flexible aircraft.
Additionally, state-of-the-art nonlinear coupled aeroelastic-flight dynamics frameworks for very flexible configurations usually follow similar modeling approach for the propeller effects, and just the thrust is typically included. To address these issues, Dr. Patricia Teixeira and Professor Carlos Cesnik from the Department of Aerospace Engineering at the University of Michigan investigated the impact of propeller effects on the aeroelastic stability of very flexible aircraft. Their goal was to gain a better insight on the isolated contribution of the different kinds of loads transmitted by the propeller. Their research work is currently published in The Aeronautical Journal.
Their approach employed a previously developed framework to generate time-data series. In their work a method based on proper orthogonal decomposition (POD) plus system identification to extract frequency, damping and mode shapes from the time-series data was successfully verified for a purely structural case for which a reference solution was available. The method was then used to analyze the effect of the propeller on the aeroelastic stability of the very flexible aircraft based on a set of generated snapshots.
From the analysis of aeroelastic cases and the clamped model considered, the authors observed that the presence of propeller aerodynamics and gyroscopic effects influenced the values of damping and frequency of some modes and could influence the stability boundary. Moreover, an increase in phase delay and differences in amplitude in response to a perturbation close to flutter were shown as compared to the case with just thrust. Further, a reduction of flutter boundary was reported when there was an increase in propeller RPM.
In summary, the Teixeira-Cesnik study successfully looked at the influence of propeller aerodynamics and gyroscopic effects on the aeroelastic stability of very flexible aircraft. Their approach entailed the application of an enhanced aeroelastic framework with propellers developed in prior studies to extract time-data snapshots of the clamped modified sample (i.e., the X-HALE UAS) HALE aircraft. Overall, the results obtained showed that the presence of propellers can influence the aeroelastic stability of a Very Flexible Aircraft.
P. C. Teixeira, C. E. S. Cesnik. Propeller influence on the aeroelastic stability of High-Altitude Long Endurance aircraft. The Aeronautical Journal 2020 Volume 124 No 1275.