Angle-of-attack effects on counter-rotating propellers at take-off

Due to their potential for significant fuel consumption savings, Counter-Rotating Open Rotors (CRORs) are currently being considered as an alternative to high-bypass turbofans. When CRORs are mounted on an aircraft, several 'installation effects' arise which are not present when the engine is operated in isolation. This paper investigates how flow features arising from one such effect - The angle-of-attack of the engine centre-line relative to the oncoming flow - can influence the design of CROR engines. Three-dimensional full-annulus unsteady CFD simulations are used to predict the time-varying flow field experienced by each rotor and emphasis is put on the interaction of the frontrotor wake and tip vortex with the rear-rotor. A parametric study is presented that quantifies the rotorrotor interaction as a function of the angle-of-attack. It is shown that angle-of-attack operation significantly changes the flow field and the unsteady lift on both rotors. In particular, a frequency analysis shows that the unsteady lift exhibits sidebands around the rotor-rotor interaction frequencies. Further, a non-linear increase in the total rear-rotor tip unsteadiness is observed for moderate and high angles-of-attack. The results presented in this paper demonstrate that common techniques used to mitigate CROR noise, such as modifying the rotor-rotor axial spacing and rear-rotor crop, can not be applied correctly unless angle-of-attack effects are taken into account. Copyright © 2012 by ASME.

Hybrid-electric propulsion for aircraft

Against a background of increasing energy demand and rising fuel prices, hybrid-electric propulsion systems (HEPS) have the potential to significantly reduce fuel consumption in the aviation industry, particularly in the lighter sectors. By taking advantage of both Electric Motor (EM) and Internal Combustion Engine (ICE), HEPS provide not only a benefit in fuel saving but also a reduction in take-off noise and the emission levels. This research considers the design and sizing process of a hybrid-electric propulsion system for a single-seat demonstrator aircraft, the experimental derivation of the ICE map and the EM parameters. In addition to the experimental data, a novel modeling approach including several linked desktop PC software packages is presented to analyze and optimize hybrid-electric technology for aircraft. Further to the analysis of a parallel hybrid-electric, mid-scale aircraft, this paper also presents a scaling approach for a 20 kg UAV and a 50 tonne inter-city airliner. At the smaller scale, two different mission profiles are analyzed: an ISR mission profile, where the simulation routine optimizes the component size of the hybrid-electric propulsion system with respect to fuel saving, and a maximum duration profile; where the flight endurance is determined as a function of payload weight. At the larger scale, the performance of a 50 tonne inter-city airliner is modeled, based on a hybrid-electric gas-turbine, assuming a range of electric boost powers and battery masses.

Vortex filament simulation of trailing vortex merger

Aircraft in high-lift configuration shed multiple vortices. These generally merge to form a downstream wake consisting of two counter-rotating vortices of equal strength. The understanding of the merger of two co-rotating trailing vortices is important in evaluating the separation criteria for different aircraft to prevent wake vortex hazards during landing and take-off. There is no existing theoretical method on the basis of which such norms can be set. The present study is aimed at gaining a better understanding of the behaviour of wake vortices behind the aircraft. Two dimensional studies are carried out using the vortex blob method and compared with Bertenyi's experiment. It is shown that inviscid two dimensional effects are insufficient to explain the observations. Three dimensional studies, using the vortex filament method, are applied to the same test case. Two Lamb-Oseen profile vortices of the same dimensions and initial separation as the experiment are allowed to evolve from a straight starting condition until a converged steady flow is achieved. The results obtained show good agreement with the experimental distance to merger. Core radius and separation behaviour is qualitatively similar to experiment, with the exception of rapid increases at first. This may be partially attributable to the choice of filament-element length, and recommended further work includes a convergence study for this parameter. Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Investigation of a radial-inflow bleed as a potential for compressor clearance control

The mismatch in thermal response between a High Pressure Compressor (HPC) drum and casing is a limiting factor in the reduction of compressor clearance. An experimental test rig has been used to demonstrate the concept of radial inflow to reduce the thermal time constant of HPC discs. The testing uses a simulated idle - Maximum Take Off (MTO) - idle transient in order to measure the thermal response directly. The testing is fully scaled in the dimensionless sense to engine conditions. A simple closure model based on lumped capacitance is used to illustrate the scope of potential benefits. The proof-of-concept testing shows that HPC disc time constant reductions of the order 2 are feasible with a radial-inflow bleed of only 4% of bore flow at scaled MTO conditions. Using the experimental results, the simple closure modelling suggests that for a stage with a significant mismatch in thermal response, reductions in 2D axis-symmetric clearance of as much as 50% at MTO conditions may be possible along with significant scope for improvements at cruise conditions. Copyright © 2013 by ASME.