Changes in Propeller Performance Due to Rotor and Ceiling Proximity in Forward Flight

Date of Award

5-5-2024

Degree Name

Ph.D. in Engineering

Department

Department of Mechanical and Aerospace Engineering

Advisor/Chair

Sidaard Gunasekaran

Abstract

With the increasing interest in electric vertical takeoff and landing air vehicles and small-scale Unmanned Air Vehicles, many novel design concepts favor the fixed-pitch-propeller as the primary propulsion system due to its simplicity and reliability. This expands the application scenario of the fixed-pitch propeller from axial forward flight to edgewise flight conditions. The current study investigated the changes in its performance when operating at higher incidence angle conditions as well as the proximity effects of the propellers in these conditions. It is hypothesized that the propeller performance under various conditions and proximities can be reasonably predicted by modeling the changes in the inflow angle of the propeller. This hypothesis was tested using three major steps. First, a relationship between inflow angle, propeller inclination angle, and advance ratio was established using a series of experimental investigations. Second, this relationship was used to predict the performance of two propellers in tandem configuration with various horizontal and vertical offset distances. Third, the same model was used to predict the ceiling effect of the propeller at different incidence angles and advance ratios. All experiments were conducted at the University of Dayton Low-Speed Wind Tunnel (UD-LSWT) Laboratory under its open jet configuration. Force-based experiments, flow visualization as well as phase-locked Particle Image Velocimetry (PIV) experiments were conducted for all investigations. The changes in propeller performance at various flight conditions were quantified and several normalization methods were successfully employed indicating the predictability of various propeller forces and moments. A novel propeller axial thrust prediction model was proposed considering the propeller performance as a summation of propeller-like components and wing-like component, with an overall error of less than 8.3%. Flow visualization and PIV results confirmed the changes in propeller performance were strongly related to the propeller inflow angle. A model relating the propeller advance ratio, incidence angle, and inflow angle was developed, which can be applied to predict the propeller performance in proximity to another propeller and in proximity to the ceiling. The investigation of the changes in propeller performance in the negatively staggered configuration, where the rear propeller was placed above the plane of the front propeller, and in the positively staggered configuration, where the rear propeller was placed beneath the plane of the front propeller, was then conducted. The changes in propeller performance at different propeller vertical and horizontal spacings, incidence angles, and advance ratios were quantified. A reduction in the back propeller thrust generation was measured, resulting in a lower overall efficiency of the tandem propeller system. Flow visualization and PIV experiment confirmed a reduction of the back propeller inflow angle due to the interaction with the front propeller, resulting in the reduction of its thrust generation. The linear superposition model developed from the standalone propeller flowfield showed good agreement with the PIV results from the tandem propeller configuration, indicating the linear superposition model can be applied to predict the trend of the changes in propeller performance in tandem. The investigation continued with the changes in propeller performance in ceiling effect in forward flight. In general, an increment in propeller thrust generation and a reduction in power consumption are found in the ceiling effect. For propellers operating in edgewise flight, a higher ceiling effect benefit was observed when compared to propellers operating at lower incidence angles. Performance similarity in propeller ceiling effect at different distances to the ceiling is found. An in-ceiling-effect performance prediction method is proposed, which combines the ceiling-effect hovering prediction model and the propeller forward flight prediction model. Phase-locked PIV confirmed an increase in the propeller inflow angle in the ceiling effect, resulting in higher thrust generation. Additionally, PIV and surface flow visualization reveal the presence of a nodal point on the ceiling plate indicating the propeller flowfield in the ceiling effect is highly three-dimensional. The current study established a strong connection between the changes in propeller performance in forward flight and its inflow angle. The proposed prediction model provides a quick estimation of the propeller axial thrust in edgewise flight conditions using only the axial flight performance data. The validation of the propeller inflow linear superposition model presents the potential of using a single propeller performance and flowfield to estimate the performance of a multi-propeller vehicle system. This provides a design guideline for multi-rotor systems at the initial design stage. Meanwhile, the current study also reveals the changes in propeller performance and flowfield in ceiling effect forward flight and provides a performance estimation method when designing a vehicle operating in the ceiling effect.

Keywords

Fixed-pitch Propeller, Proximity Effect, Rotor to Rotor Interaction, Ceiling Effect, Forward Flight, Rotor Aerodynamics, Experimental Aerodynamics

Rights Statement

Copyright 2024, author

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