A 3D unsteady RANS simulation utilizing the Spalart-Allmaras (SA) turbulence model was conducted to investigate aerodynamic interactions within a propeller-wing-flap system. The research specifically examines the complex flow field around a slotted flap, highlighting the interaction between the propeller slipstream and the main wing and flap during powered high-lift conditions. Operating conditions include a chord-based Reynolds number of 2 million, with thrust and flap settings reflecting take-off conditions (J = 0.765, Tc = 1.267, 5/= 15°) at an angle of attack a = 8.3°. Chordwise pressure distributions and surface shear stress contours show strong agreement with previous experimental measurements and oil flow visualizations of the same geometry. Findings indicate that a portion of the propeller slipstream transfers from the pressure side of the main wing to the upper side of the flap through the cove, dominating the flap flow field. Although the upper side of the main wing experiences fluctuating flow originating from the propeller slipstream, this flow does not induce unsteadiness or penetrate the flap upper side boundary layer along the wing span. Furthermore, it is shown that the shedding of vortices from the propeller root, along with the resulting vortices on the lower side of the geometry, weakens the flap boundary layer as this flow is transferred through the cove area, consequently inducing flap flow separation. Overall, the findings provide valuable insights into propeller-wing-flap interactions, which had not been visualized before in this detail, yet emphasizing the need for further research to confirm and expand on these results.

This paper explores the influence of the characteristics of the helical vortex system of a propeller slipstream on the resulting propeller–wing interaction, with a particular focus on how variations in helix angle impact slipstream deformation. Slipstream characteristics are changed by controlling the thrust and torque coefficient of the propeller through adjustments in blade pitch, advance ratio, and blade count. We conducted experimental measurements of a propeller–wing–flap model using seven-hole pressure probes, oil flow visualization, and infrared thermography in both cruise and high-lift configurations (with deployed slotted flap). The results presented in this paper demonstrate the dominance of the torque coefficient, and thereby longitudinal vorticity in the slipstream, on the slipstream deformation. The paper also underscores the role of the nacelle integration in the development of the slipstream, as well as the flow on the wing surface. The insights into the slipstream deformation provided in this work are essential for future closely coupled propeller–wing designs, particularly when it comes to high-lift configurations.

Experiments were performed using a wall-to-wall unswept and untapered wing with a single slotted flap and a propeller, to obtain a validation dataset and gain insight into primary flow phenomena in propeller-wing-flap interactions. Measurements were taken using pressure taps, a wake rake and oil flow visualization, for several flap deflections (0, 15 and 30 degrees) and thrust settings (unpowered, J = 0.8 / T _c = 1.05 and J = 1.0 / T _c = 0.45). Similarity of the measured data to similar experiments was poor, which was believed to be due to the low Reynolds number of Re = 6e5 and sensitivity of local measurements due to occurrence of stall cells. Oil flow visualizations showed significant induction of flow separation from nacelle-wing interactions in unpowered conditions, traced to boundary layer growth. For powered cases it was shown that both sides of the deployed flap are immersed in the part of the slipstream that passes the pressure side of the main element. This part of the slipstream deforms significantly before it reaches the flap and thus results in complex spanwise variations for the flap flow. This stresses the need to investigate slipstream development in propeller-wing-flap systems and the effects on flap flow specifically to gain in-depth understanding of the interactions. The results presented in this paper expose the inherent complexity of investigating propeller-wing-flap systems and gaining viable validation data, and might serve to guide for future investigations of propeller-wing-flap systems.

The use of formation flight to achieve aerodynamic benefit as applied to rotorcraft is, unlike its fixed-wing counterpart, an unproven principle. This document presents a proof-of-concept of rotorcraft formation flight through a numerical research study, supported by results from an independent wind-tunnel experiment. In both cases, two helicopters are placed in an echelon formation aligned on the advancing side of the main rotor, though they do not simulate directly comparable flight conditions. The vertical and lateral alignment is varied in order to observe the achievable reductions in main rotor power required during cruise flight. The wind-tunnel experiment data yields an estimated maximum total power reduction for the secondary aircraft of 24%, while the numerical models yield reductions between 20% and 34% dependent on flight velocity. Both experiments predict a higher potential for aerodynamic benefit than observed for fixed-wing formations, which is contributed to the asymmetric upwash profile in the rotor wake. Optimal lateral alignment of both experimental and numerical results is found to feature overlap of the rotor disk areas due to circular area effects. Experimental data shows an optimal vertical alignment of the secondary rotorcraft below the primary, due to wake displacement. This is not present in the numerical simulations as a result of the applied leader wake modeling.

The use of formation flight to achieve aerodynamic benefit applied to rotorcraft has, unlike its fixed-wing counterpart, received little attention in the literature. This document presents a proof-of-concept of rotorcraft formation flight from two independent investigations: a numerical study of a fully articulated helicopter influenced by an upstream helicopter wake and a wind-tunnel experiment featuring two small-scale helicopter models with fixed-pitch blades. Both cases feature a representation of two helicopters in a diagonal, staggered formation aligned on the advancing side of the main rotor, but do not simulate directly comparable flight conditions. The vertical and lateral alignment of the two helicopters is varied in order to observe the achievable reductions in main rotor power required during cruise flight. The wind-tunnel experiment data yield an estimated maximum total power reduction for the secondary aircraft of approximately 24%, while the numerical models yield reductions between 20% and 34% dependent on flight velocity. Both experiments predict a higher potential for aerodynamic benefit than generally observed for fixed-wing formations, which is attributed to the asymmetric velocity profile induced by the wake of the upstream rotor. Optimal lateral alignment of both experimental and numerical results is found to feature overlap of the rotor disk areas, rather than tip-to-tip alignment, as a result of the circular rotor disk area. Experimental data show an optimal vertical alignment of the secondary rotorcraft below the primary, due to the self-induced vertical displacement of the rotor wake, which is absent from the numerical results due to the application of a flat wake assumption. The results show a promising potential for rotorcraft formation flight, though due to the limited nature of the models used, conclusions cannot be generalized. The potential aerodynamic benefit indicated by the present study invites further research in the field of rotorcraft formation flight.