This study presents an experimental aeroacoustic evaluation of a tip-joined blade (TJB) propeller and compares its performance with conventional two-and four-bladed configurations under similar operating conditions. Acoustic measurements are conducted in the anechoic wind tunnel at Delft University of Technology using an eight-microphone far-field directivity arc. All propellers are designed to deliver equivalent thrust while maintaining identical diameter and chord distributions, enabling a consistent comparison at an advance ratio of = 0.4 and a rotational speed of 4,000 rpm. The TJB propeller satisfies the thrust requirement but exhibits a propulsive efficiency approximately 4% lower than the four-bladed configuration, primarily due to increased torque associated with the closed-loop geometry. The acoustic results show that the TJB does not provide a uniform reduction across the entire noise spectrum. Broadband noise levels are reduced relative to the four-bladed propeller and fall below those of the two-bladed configuration above approximately 9 kHz, yielding broadband overall sound pressure levels comparable to the two-bladed baseline. In contrast, tonal levels are increased, with the blade-passing frequency peak exceeding those of the two-and four-bladed propellers by approximately 3 dB and up to 17 dB, respectively, at the = 90 ^◦ observer position. Consequently, the total overall sound pressure level of the TJB propeller lies between those of the two-bladed and four-bladed propellers. These findings indicate that the TJB geometry provides effective broadband noise mitigation while exhibiting increased tonal components, highlighting both the potential and limitations of tip-joined blade concepts for propeller noise reduction.

The growing demand for urban air mobility (UAM) has accelerated the development of hybrid/electric vertical takeoff and landing (H-/eVTOL) aircraft, which use distributed propulsion systems that generate complex aerodynamic interactions during forward and transition flight. This study investigates the aerodynamic interactions between two propellers arranged in a one-after-another (OAA) configuration, using unsteady Reynolds-averaged Navier–Stokes (URANS) simulations coupled with the k-ω shear stress transport (SST) turbulence model. The numerical setup was validated against experimental data for the isolated propeller across all investigated advance ratios, showing good agreement. Results reveal that, in forward flight, the rear propeller experiences up to 24% thrust and 20% power loss due to the disturbed airflow created by the wake of the front propeller. This is accompanied by only a 21% increase in axial velocity at the rear propeller location, indicating less effective energy transfer and lower thrust. Pressure coefficient analysis at the rear propeller midspan within the overlap region indicates increased flow separation and lower local loading. During transition, the overlap decreases with tilt, yet at 30° and 60°, the rear propeller still loses up to 11% thrust and 10% power due to slipstream deflection. As the propeller tilt angle increases, the thrust in the propeller axis direction also increases; however, this requires more power and generates a more complex slipstream. These findings provide valuable insights into wake interactions and their influence on rear propeller performance, offering practical design guidelines to improve the efficiency and reliability of UAM vehicles.