Trim-Constrained Aero-Propulsive Assessment of Distributed Electric Ducted Fans for a Fixed-Wing Unmanned Aerial Vehicle

Technical Summary: Trim-Constrained Aero-Propulsive Assessment of Distributed Electric Ducted Fans for a Fixed-Wing UAV

Problem Statement
The electrification of fixed-wing Unmanned Aerial Vehicles (UAVs) transforms propulsion sizing into a coupled vehicle-integration challenge. While Distributed Electric Propulsion (DEP), specifically using Electric Ducted Fans (EDFs), offers the potential to enhance low-speed lift through slipstream energization, it introduces complex aero-propulsive trade-offs. Previous studies often isolate lift augmentation without simultaneously accounting for the associated drag penalties, spanwise loading distortions, and the critical impact on longitudinal trim and control authority. For small and medium UAVs, a configuration that increases lift may be operationally unusable if it induces excessive drag, creates non-elliptical loading, or saturates the elevator, particularly given the sensitivity of stability to Center of Gravity (CG) positioning. This paper addresses the gap in assessing distributed EDF integration not merely as a lift-generation mechanism, but as a system where efficiency and control margins must be preserved.

Methodology
The study utilizes the LSU 05 NG fixed-wing UAV as a representative platform, integrating twelve 70 mm EDFs (six per semi-span) along the wing. The research employs a two-tiered assessment framework:

1. Reduced-Order Aero-Propulsive Model: A simplified slipstream-loading model was developed to screen the full twelve-EDF layout. This model treats the EDF-induced velocity increment as a Gaussian superposition of wakes. It calculates local dynamic pressure, integrates spanwise lift, and evaluates drag components (profile, induced, and installation). Crucially, this model incorporates a longitudinal trim analysis to determine the elevator deflection ($\delta_e$) required to balance pitching moments across various flight regimes (cruise, takeoff, landing, stall) and CG positions ($x_{CG}/MAC = 0.30, 0.35, 0.37$).
2. CFD Diagnostic Workflow: To support the reduced-order findings, a Computational Fluid Dynamics (CFD) workflow was established for a representative wing section equipped with three EDFs. This workflow compares paired "power-off" (baseline) and "power-on" (35,000 rpm) simulations across an angle-of-attack ($\alpha$) sweep from $0^\circ$ to $20^\circ$. The CFD analysis focuses on quantifying incremental changes in lift ($\Delta C_L$), drag ($\Delta C_D$), and efficiency ($\Delta (C_L/C_D)$), alongside rotor diagnostics (thrust, torque, power).

Key Contributions
The paper's primary contribution is a unified, trim-constrained framework that evaluates DEP performance through a combined metric-level lens rather than isolated force coefficients. Specifically:

• Integrated Metrics: It couples lift augmentation with drag growth, lift-to-drag ($L/D$) degradation, and residual elevator margin within a single conceptual design workflow.
• CG Sensitivity Analysis: It explicitly screens the impact of CG location on trim feasibility, demonstrating that while slipstream lift gain is relatively insensitive to CG, the required elevator authority is highly sensitive.
• Design-Support Template: It provides a structured template for inserting high-fidelity CFD data (paired power-off/on results) to validate the physical origins of reduced-order predictions without conflating design support with formal validation of the full aircraft model.

Key Results

• Aerodynamic Trade-offs: The reduced-order analysis of the twelve-EDF configuration indicates a lift augmentation ($G_L$) of 47.0–60.2%. However, this is accompanied by a significant drag increase ($G_D$) of 88.6–101.2%, resulting in a degradation of the lift-to-drag ratio ($R_\eta$) by 16.15–24.32%.
• Trim and Control Limits:
  • Forward CG ($x_{CG} = 0.30$): The aircraft reaches elevator limits during landing and stall, rendering the configuration trim-limited.
  • Mid CG ($x_{CG} = 0.35$): Landing trim is recoverable with a 4.27° margin, but stall cases remain elevator-saturated.
  • Aft CG ($x_{CG} = 0.37$): This configuration offers the best compromise, retaining a 5.80° landing elevator margin and the lowest takeoff efficiency degradation (17.53%).
  • Stall Condition: In all CG cases, the stall regime reaches the elevator deflection limit ($\delta_{e,lim} = -25^\circ$), indicating that the powered-lift benefit cannot be safely utilized in deep stall without further modifications to tail authority or thrust scheduling.
• CFD Insights: The CFD analysis of the three-EDF section reveals that DEP is most effective in the attached-flow regime. At $\alpha = 5^\circ$, the power-on configuration yields a massive relative lift increase (+697.5%) and a significant efficiency gain (+177.2%) because the slipstream re-energizes the boundary layer. However, as $\alpha$ approaches stall ($17^\circ$), the efficiency gain collapses to near zero (+1.9%), as the additional lift is almost entirely offset by drag penalties and flow separation. The CFD confirms that DEP acts as a low-to-moderate angle-of-attack lift mechanism rather than a high-angle-of-attack stall cure.

Significance and Claims
The paper claims that the value of DEP integration lies not in maximizing lift in isolation, but in identifying configurations where aerodynamic efficiency and control authority remain acceptable. The study demonstrates that for the LSU 05 NG, the "best" design is not the one with maximum lift, but the one (specifically the aft-CG case) that preserves usable trim margins and minimizes efficiency loss. The authors assert that DEP design must incorporate CG envelope analysis and tail-authority constraints from the earliest conceptual stages. The proposed framework serves as a reproducible workflow to screen layouts and guide future optimization, such as smoothing spanwise blowing distributions and scheduling EDF RPM based on trim state, before proceeding to wind-tunnel or flight testing.