Unsteady aerodynamics analysis and modelling of a Slingsby Firefly aircraft: Detached-Eddy Simulation model and flight test validation

Abstract

This paper presents unsteady stall characteristics of a Slingsby T67M260 Firefly light aircraft from both a computational fluid dynamics (CFD) half model and flight tests. Initial results from the steady CFD, based on a RANS k − ω SST turbulence model, established the critical angle of attack of the stall to be α stall = 16 • , with a maximum lift coefficient of CLmax = 1.2. Comparisons with straight and level flight test data were comparable up to α = 12 • -14 • , with the increasing deviation at higher α attributed to the effect of the propeller slipstream under these flight conditions. The RANS CFD model was then extended to an unsteady Detached-Eddy Simulation (DES) model for three angles of attack at pre-stall and stall condition (α = 14 • , 16 • , 18 • ), with analysis of the vortex shedding frequency. Further comparisons were then made with flight test data taken using on-board accelerometers and wing tuft surface flow visualization, at a stalled condition at equivalent α. These unsteady CFD data established a dominant shedding frequency ranging from 11.7 Hz -8.74 Hz with increasing α and a Strouhal number based on wing chord of St = 0.11, which when compared to flight test accelerometer spectra matched within 2.9% of the measured frequency.

DES of a Slingsby Firefly Aircraft: Unsteady Flow Feature Extraction Using POD and HODMD

“…Around 1.5s later, the aircraft is stalled and the dominant frequencies switch from the engine to the aerodynamic and structural interactions, indicated in Figure2b, where the tailplane structural natural frequencies are excited at 9.4Hz and 40.5Hz. In this case, the DES model was predicting aerodynamic wake shedding frequencies close to the lower frequency of the tailplane structure(Neves et al 2020), as confirmed by the video footage of the tail. In the last two stages of the sequence, a large change and reduction in acceleration can be seen which coincides with the 'wing drop' in a heavy stall.…”

“…Previous work by the authors described the DES model based on k − ω SST formulation, and its application in modelling the unsteady flow field around the Slingsby in the stalled condition (Neves et al 2020). The following section summarises the main details of the CFD simulation and outlines the additional data extracted from the computed unsteady flow field, to apply the proper orthogonal decomposition (POD) and HODMD methods to the DES wake data.…”

“…The complex nature of the coefficients and the possibility of spin generally lead to a requirement for extensive wind tunnel and flight testing (Hall et al 2004) to verify the behaviour of the aircraft in a heavy stall (Heinz 2020). Recent developments of advanced numerical methods, such as Detached Eddy Simulation (DES) and unsteady RANS methods (Zhou et al 2019a;Zhou et al 2019b), now offer the opportunity to predict these complex aerodynamic behaviours with greater fidelity (Wang and Fu 2017;Casadei et al 2019;Neves et al 2020), supplementing the existing experimental data and providing design fixes, ahead of the flight testing program.…”

“…Then the unsteady pressure monitors were activated and the flow simulations were run for further 8500 time steps, to study the spectral contents of the pressure signals at various locations. Further details on the mesh generation and the CFD setup can be found in(Neves et al 2020). FLIGHT TEST DATAThis section summarises previous flight test work by the authors(Neves et al 2020) which was taken from a T 67M 260 Slingsby Firefly light aircraft and used to validate the DES model.…”

“…Further details on the mesh generation and the CFD setup can be found in(Neves et al 2020). FLIGHT TEST DATAThis section summarises previous flight test work by the authors(Neves et al 2020) which was taken from a T 67M 260 Slingsby Firefly light aircraft and used to validate the DES model. The flight tests were designed to aerodynamically stall the aircraft, whilst accelerometer, video and surface flow data were recorded.…”