Ph.D. (Engg): A Slotted Wing Configuration for STOL Applications

The rapid development of Urban Air Mobility (UAM) has led to the emergence of electric vertical take-off and landing (eVTOL) aircraft, with many current concepts relying on tilt-rotor or other complex propulsion-conversion mechanisms to achieve vertical take-off and landing followed by efficient forward flight. While these configurations provide the required operational flexibility, the mechanical complexity associated with tilting propulsion systems can introduce challenges related to durability, power efficiency, system integration, and flight stability. This thesis explores an alternative approach based on short take-off and landing (STOL) capability, with the objective of developing a compact, mechanically simple, and aerodynamically efficient wing configuration that can reduce the dependence on such complex mechanisms. The proposed design is CFD based and the required computations have been carried out using the CFD solver HiFUN. All computations have been performed using standard SA turbulence model. The propellers are simulated using varying levels of fidelity available in the solver, such as the Blade Element Method and sliding mesh simulations. Moving mesh capability available in the solver is made use of in simulating synthetic jets.
A novel fixed-wing configuration is developed specifically for STOL operation in the context of urban air mobility. Instead of relying on conventional high-lift devices, the proposed configuration combines a highly cambered wing, distributed propulsion, and active flow control to achieve the high lift required during take-off and landing. A NACA four-digit formulation is used to define the airfoil, with an unconventionally high camber and a slot introduced at the location of maximum camber. To maintain vehicle compactness while improving aerodynamic efficiency, a low aspect-ratio wing with optimized end plates is employed. The influence of end-plate size on the aerodynamic performance is investigated, showing a progressive reduction in trailing-vortex strength and associated induced effects on lift and drag with increasing end-plate size. Based on the investigated configurations, an end plate with a width equal to 20% of the wing aerodynamic chord is selected.
Distributed propulsion is subsequently integrated into the wing configuration to exploit propeller slipstream effects during STOL operation. An enhanced actuator-disc model, coupled with blade-element analysis, is used to represent the propellers, with a NACA 640 four-bladed propeller selected for the study. The propeller thrust requirements are determined from the take-off drag of the unpowered wing and distributed among four equally spaced propellers. The interaction between an individual propeller and the wing is first investigated to determine the optimum propeller location. The resulting slipstream–wing interaction substantially increases the wing lift, thereby reducing the required take-off velocity. The complete powered configuration is then investigated using high-fidelity sliding-mesh simulations with counter-rotating propellers arranged on either side of the wing.

The high camber required to achieve the desired lift characteristics introduces a separation bubble in the rear portion of the wing downstream of the slot. To address this limitation without introducing conventional high-lift mechanisms, a zero-net-mass-flux active flow-control system based on synthetic jets is incorporated. A dynamic-mesh methodology is first validated against the NASA Langley Research Center standard hump test case with oscillatory control. The validated methodology is then used to establish the actuator momentum coefficient and excitation frequency. To reduce the computational cost of the full-wing simulations, an unsteady boundary-condition model is developed and validated and subsequently employed to investigate the effectiveness of synthetic-jet actuation in suppressing the separation bubble and improving the aerodynamic performance of the wing. Discrete synthetic-jet slits are positioned between the propeller slipstreams, allowing flow control to be concentrated in regions outside the primary propeller wakes while reducing the additional power requirement.
Overall, the thesis demonstrates the feasibility of combining fixed-wing STOL aerodynamics, distributed propulsion, and localized active flow control as an alternative pathway for compact urban air-mobility vehicles.

Speaker : S Amsha

Research Supervisor : N. Balakrishnan