Unducted Fan Research Articles

Determining patterns in the influence of the number of blades in the ducted and unducted propfans on propfan thrust

The efficiency of an aircraft engine is estimated by many parameters, one of which is the thrust force. Improving the efficiency of aircraft engines is an important task for engine building. However, questions remain regarding the effect of the number of blades on the change in the thrust of the ducted and unducted fans. In this work, the object of study is a propfan. 3 variants of the propfan with 8, 10, and 12 blades were investigated. The study was conducted by the method of numerical experiment. The aim of the work was to compile recommendations for choosing the number of blades in the ducted and unducted fans for motors with an ultra-high bypass ratio. That could make it possible to improve the efficiency of an aircraft engine with a propfan. Studies have shown that the number of blades in a propfan significantly affects the thrust force that it creates, as well as efficiency. With an increase in the blades of the ducted fan from 8 to 12, the thrust force increases to 38 %. With an increase in the blades of the propfan from 8 to 12, the thrust force increases to 36.9 %. An increase in the blades from 8 to 12 in the ducted fan leads to an increase in its performance, thereby improving efficiency by 2.4–5.7 %. When flowing around a propfan, it is possible to note the peculiarity that occurs when all three variants are streamlined – vortex traces of the blades in the peripheral parts. Visualization of current lines when flowing around an unducted fan with 8, 10, and 12 blades demonstrates a similar flow character. On the periphery, there are zones of higher speed but there are no zones with eddy formations. The resulting regularities of the influence of the number of blades on a change in the thrust of the ducted and unducted fans could improve the efficiency of the aviation power plant with an engine whose bypass ratio is ultra-high.

Aerodynamic design and evaluation of a ducted fan lift system for vertical takeoff and landing flying cars

Urban air mobility is a relatively new concept that has been proposed in recent years as a means of transporting passengers and goods in urban areas. It encompasses a diverse range of Vertical TakeOff and Landing (VTOL) vehicles that function more like passenger-carrying drones for on-demand transportation. Among them, the car-like VTOL is advantageous due to its compact configuration, safe rotors, high user affinity, and technological fashion. These characteristics are frequently derived from the flying car’s Ducted Fan Lift System (DFLS). This study aims to develop a method for the rapid design and the evaluation of the aerodynamic performance of the DFLS, to support the preliminary scheme demonstration of the ducted fan flying car. The proposed method uses blade element theory to design the unducted fan and applies momentum theory to calculate the aerodynamic thrust of the DFLS. The DFLS of a 1:3 scale verifier for a flying car scheme was designed and evaluated using the proposed method and a numerical method, respectively. To validate the proposed method, a prototype of the scale DFLS was manufactured and tested, and the result was compared with those of the proposed theoretical method and the numerical method. This study demonstrates that while both the theoretical and numerical methods are capable of designing an unducted fan accurately, the theoretical method is simpler and faster. Compared to the DFLS test results, the theoretical method’s average difference is approximately 1.9%. When evaluating the DFLS, the accuracy of the numerical calculation is reduced, and the difference is greater than 30% at low power. The theoretical method presented in this paper can be used to improve the aerodynamic design and evaluation efficiency of the DFLS and to aid in the configuration evaluation of VTOLs equipped with ducted fans.

Assessment of the broadband noise from an unducted axial fan including the effect of the inflow turbulence

This paper presents an assessment of the broadband noise computed by using the zonal LES coupled with the Ffowcs Williams - Hawkings (FWH) acoustic analogy for an unducted low speed axial fan. The results of the aerodynamic and aeroacoustic simulations are benchmarked with Particle Image Velocimetry (PIV) and noise measurements from the authors. An in-focus look is dedicated to Constant Temperature Anemometry (CTA) measurements for providing the numerical solver with the correct level of inflow turbulence. Far field noise results are presented for both laminar and turbulent inlet in a frequency range from 0.1 to 10 kHz. A specific analysis is carried out to detect noise sources on the blade surface, showing that broadband noise from 0.4 to 2 kHz is mainly due to the boundary layer scattering at the trailing edge and to the interaction of the tip vortex with the blade trailing edge, resulting rather non-sensitive to the inflow turbulence. At frequencies above 2 kHz, the main noise sources are detected on the front portion of the blade suction side, and their intensity is found to depend significantly on the inflow turbulence.

Optimization of open counter – rotation fan blades on the basis of solving a 3D Navier-Stokes inverse problem with the aim of reducing tonal noise

The paper presents some results of gas-dynamic and aero-acoustic optimization of the blade profile of an unducted counter-rotation fan (CRF) by using a 3D inverse problem. It was established on the basis of unsteady-state 3D Navier-Stokes equations that the interaction of tip vortices of the first and second rotors as well as potential interaction of the rotors is one of the key sources of tonal noise. Using the 3D solver of the inverse problem, aerodynamic loads are redistributed along the height of the blades of R1 and R2 rotors so as to reduce tip vortex intensity and potential rotor interaction in case of possible increase of the CRF thrust. To check the acoustic characteristics of the modified CRF, tonal noise modeling was carried out for the original and modified CRFs using CIAMs aero-acoustic 3DAS solver for the solution of unsteady-state equations. The near acoustic field and directivity diagrams in the far field were found. The fan tonal noise in take-off and landing was decreased by 4 dB without any thrust or efficiency losses.

An industrial onsite methodology for combined acoustic-aerodynamic diagnostics of axial fans, involving the phased array microphone technique

A combined acoustic-aerodynamic diagnostics methodology is presented herein, involving the phased array microphone technique and an in-house rotating source identifier processing algorithm. The methodology makes possible the industrial onsite investigation of unducted or short-ducted rotor-only axial fans. Its application is illustrated in a case study of a free-inlet, free-exhausting fan, surveyed from the upstream direction. A methodology is outlined for guaranteeing the appropriateness of averaging of the phased array microphone records. A technique is proposed for eliminating the ambiguity of the angular position of the rotor noise sources. A complete set of noise source maps of various frequencies have been evaluated. Noise sources, such as tip leakage flow and suction side blade boundary layer flow, have been identified. Empirical correlations have been pointed out between indicators of aerodynamic noise and aerodynamic loss along the blade span. Such correlations may contribute to redesign guidelines for simultaneous reduction of noise and improvement of efficiency at prescribed fan performance.

Progress in open rotor propulsors: The FAA/GE/NASA open rotor test campaign

AbstractModel scale tests of modern ‘open rotor’ propulsor concepts that have potential for significant fuel burn reduction for aircraft applications were completed at NASA Glenn Research Center. The recent test campaign was a collaboration between NASA, FAA, and General Electric (GE). GE was the primary industrial partner, but other organisations were involved such as Boeing and Airbus who provided additional hardware for fuselage simulations. The open rotor is a modern version of the UnDucted Fan (UDF®) that was flight tested in the late 1980s through a partnership between NASA and GE. The UDF® was memorable for its scimitar shaped propeller blades and its unique noise signature. Design methods of the time were not able to optimise for both high aerodynamic efficiency and low noise simultaneously. Contemporary CFD/CAA based design methods can produce open rotor blade designs that maintain efficiency with acceptable acoustic signatures. Tests of two generations of new open rotor designs were conducted in the 9’ × 15’ Low Speed Wind Tunnel and the 8’ × 6’ Supersonic Wind Tunnel starting in late 2009 and completed in early 2012. Aerodynamic performance and acoustic data were obtained for take-off, approach and cruise conditions in isolated and semi-installed configurations. Additional detailed flow diagnostic measurements and acoustic measurements, including canonical shielding configurations, were obtained by NASA. NASA and GE conducted joint systems analysis to evaluate the performance of the new blade designs on a Boeing 737 class aircraft. The program demonstrated a 2-3% improvement in overall net efficiency relative to the best efficiency designs of the 1980s while nominally achieving 15-17 EPNdB noise margin to Chapter 4 (at a Technology Readiness Level of 5) for a notional aircraft system defined by NASA.

Rotor-rotor interaction for counter-rotating fans. I - Three-dimensional flowfield measurements

The rotor wake/vortex flowfield generated in a scale model simulator of General Electric's counter-rotating unducted fan (UDF®) engine was investigated using three-dimensional hot-wire anemometry. The purpose was to obtain a set of benchmark experimental aerodynamic data defining the rotor wake and vortex structure, particularly in the tip region, and to relate this observed flow structure to its acoustic signature. The tests were conducted in a large, freejet anechoic chamber. Measurements of the three components of velocity were made at axial stations upstream and downstream of each rotor for conditions that simulate takeoff, cutback, and approach power

Optical Measurements of Unducted Fan Flutter

The paper describes a nonintrusive optical method for measuring flutter vibrations in unducted fan or propeller rotors and provides detailed spectral results for two flutter modes of a scaled unducted fan. The measurements were obtained in a high-speed wind tunnel. A single-rotor and a dual-rotor counterrotating configuration of the model were tested; however, only the forward rotor of the counterrotating configuration fluttered. Conventional strain gages were used to obtain flutter frequency; optical data provided complete phase results and an indication of the flutter mode shape through the ratio of the leading- to trailing-edge flutter amplitudes near the blade tip. In the transonic regime the flutter exhibited some features that are usually associated with nonlinear vibrations. Experimental mode shape and frequencies were compared with calculated values that included centrifugal effects.

Optical Measurement of Unducted Fan Blade Deflections

A nonintrusive optical method for measuring unducted fan (or propeller) blade deflections is described and evaluated. The measurement does not depend on blade surface reflectivity. Deflection of a point at the leading edge and a point at the trailing edge in a plane nearly perpendicular to the pitch axis is obtained with a single light beam generated by a low-power, helium-neon laser. Quantitative analyses are performed from taped signals on a digital computer. Averaging techniques are employed to reduce random errors. Measured static deflections from a series of high-speed wind tunnel tests of a counterrotating unducted fan model are compared with available predicted deflections, which also are used to evaluate systematic errors.

Aerodynamic Performance of a Scale-Model, Counterrotating Unducted Fan

The aerodynamic performance of a scale-model, counterrotating unducted fan has been determined and the results are discussed herein. Experimental investigations were conducted using the scale-model propulsor simulator and uniquely shaped fan blades designed by the General Electric Company under contract to NASA. The blades, designed for a high disk loading at Mach 0.72, 35,000 ft altitude max climb condition, are aft-mounted on the simulator in a “pusher” configuration. Data are compared with analytical predictions at the design point and show good agreement.

Acoustic characteristics of counterrotating unducted fans from modelscale tests

Development of an acoustic technology that describes the noise characteristics of counterrotating fans is an important phase in the design of fuel-efficient, unducted fan engines. In order to obtain the needed data, measurements were made in an anechoic facility with approximately 1/5-scale counterrotating model fan blades. Tests were conducted over a range of power settings to determine the effects of variations in blade number, tip speeds, and rotor-to-rotor spacings on community noise. Selected results are presented and discussed in this paper. Significant overall acoustic benefits were measured with an increase in blade number, a reduction in tip speed for a given thrust, increased rotor-to-rotor spacing, and clipped aft blades.

News and Views

Dowty Rotol and GE Aircraft Engines signed an agreement to collaborate in a joint programme covering the design, development, and manufacture of composite fan blades for the GE36 Unducted Fan (UDF) engine.

Unducted Fan Aerodynamic Design

A method being used to design unducted fans, which are counterrotating propellers with unusually high disk loadings, is described. The quasi-three-dimensional approach employed is similar to that used for ducted fan designs; it consists of a circumferential-average flowfield analysis followed by cascade designs that include secondary flow and sweep effects. The open-tip condition enters as a significant secondary flow contribution. Blade shapes defined by this process are analyzed by a three-dimensional Euler solver and final adjustments are made as necessary. Examples of application of the method are presented to elucidate the magnitudes of the phenomena involved.

Engines for Commercial Aircraft

CHARACTERISED by fierce competition among the major producers, the civil powerplant market has recently had to come to terms with a new type of propulsion system which, if all its claims are fully realised, will bring large benefits in terms of fuel efficiency and economy of operation. The most effective size of these engines, whether of the unducted fan or geared fan configuration, has yet to be proved but it appears that they will not be produced for applications requiring very high thrust, at least not for the foreseeable future.