This paper presents a method for modeling ducted fan propulsion systems for hybrid-electrically driven small aircraft based on mean line flow analysis methods. Studies of the essential design parameters provide the basis for a discussion of potential advantages when compared to free propellers, as well as possible new fields of application for the ducted fan. A two-seater aircraft, equipped with two ducted fans powered by a combustion engine and additionally by a parallel hybrid electric start-boost drive, is used as a reference application case. The fan performance characteristics are described with the aid of aero mean line flow analysis methods and semi-empirical loss correlations. Physics-based methods are applied to assess the nacelle drag and noise emissions. The combustion engine and electric motor performance are described using maps. The ducted fan design variables are investigated to identify the main trade-offs and favorable designs for the target aircraft mission, with special attention to noise. The results show that the performance of fixed pitch fans benefits strongly from hybridization. Ducted fans can also operate considerably more quietly than is required by current certification standards. The physics-based design method presented here can be used for conceptual design and performance prediction of ducted fan propulsion systems, which may be especially interesting if low noise emissions are required.
This publication presents an analytical method for the aerodynamic and acoustic pre-design of ducted fans for small aircraft. Based on studies of the primary design variables, the paper discusses the physical sound generation phenomena, as well as the interdisciplinary relationships of ducted fan design. First, the fan design and analysis methods are described. On the basis of an aerodynamic mean line method, the radial distribution of the flow velocities is used to determine the steady blade flow. Unsteady aerodynamic excitations are calculated by means of Sears’ blade response function. To determine the generation and propagation of sound within the ducted fan, Goldstein’s acoustic analogy is solved analytically. The methods are applied to a reference case, for which studies show that ducted fans offer significant potential in reducing sound emission, compared to free propellers. Since the rotor-alone noise of subsonic ducted fans is always cut-off, tonal sound is predominantly excited by rotor–stator interactions. These sources can be significantly reduced by a proper selection of the rotor blade and stator vane numbers, as well as optimal lean and sweep of the stator vanes. Large diameters and axial gaps are acoustically advantageous, but reduce fan stage efficiency and increase nacelle drag due to large wetted areas, especially at cruise. As a result, the importance of considering these complex interdependencies in a comprehensive design approach is shown. The pre-design method presented can be used to determine an optimal ducted fan design, taking into account interdisciplinary trade-offs, with an emphasis on noise. Analysis of the ducted fan concept shows that the ducted fan can be a promising alternative to the free propeller, especially if low noise emission is required.
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