Assessment of Conceptual Hybrid-Electric Regional Aircraft with Boundary-Layer Ingestion Propeller

Abstract

Boundary Layer Ingestion (BLI) is a promising propulsion concept for aviation, aiming to reduce fuel burn. This configuration involves using the propulsor to ingest the boundary layer from the fuselage or wing. Typically, aircraft with BLI propulsors also integrate a hybrid-electric powertrain. This integrated setup has shown potential in fuel burn reduction. This thesis investigates the interactive effects of BLI-induced power-saving benefits and aircraft design parameters on overall performance, focusing on a fuselage tail-mounted BLI propeller.

The thesis has three primary objectives. First, it seeks to enhance the fidelity of the existing BLI model within a conceptual aircraft design tool. The improvement involves transitioning from the actuator disk theory to the blade element theory (BET) with gradient-based optimization. Additionally, a surrogate model is established with this improvement through multidisciplinary design optimization (MDO), design of experiment (DoE), and response surface methodology (RSM) to predict power-saving benefits based on fuselage geometric and operational parameters. Comparison reveals discrepancies between the existing BLI model and the surrogate model, emphasizing the influence of blade aerodynamics.

The second and third objectives delve into the sensitivity of aircraft-level performance and powertrain settings. The conceptual aircraft design tool, Aircraft Design Initiator (Initiator), is used for sizing, with the surrogate model integrated. The study uses a regional turboprop ATR-72 as a reference conventional aircraft design and employs a partial-turboelectric (PTE) architecture for the hybrid-electric powertrain in the radical aircraft design. A sensitivity analysis varying design parameters by 20%, including fuselage slenderness ratio, propeller size ratio, and shaft-power ratio, reveals their impact on BLI effects and aircraft-level performance.

The study reveals that fuselage length significantly impacts fuel weight and, consequently, aircraft performance. Surprisingly, aero-propulsive benefits from BLI do not directly enhance overall aircraft performance, attributed to an associated weight penalty. Instead, the shaft-power ratio and propeller size ratio prove significant at the system level. Further investigation into powertrain settings suggests that radical BLI-equipped aircraft designs are not able to surpass the energy efficiency of conventional counterparts. Additionally, the proposed surrogate model exhibits a limited applicable range, especially under high BLI propeller disk loading, rooted in underlying physical constraints in propeller design. These findings prompt a critical examination of the feasibility of hybrid-electric powertrains with BLI for regional turboprop aircraft, with a note of caution regarding potential variations in modeling methods and assumptions.