Devaiah Nalianda

Methodology to assess the performance of an aircraft concept with distributed propulsion and boundary layer ingestion using a parametric approach

The performance benefits of boundary layer ingestion in aircraft with distributed propulsion have been extensively studied in the past. These studies have indicated that propulsion system integration issues such as distortion and intake pressure losses could mitigate the expected benefits. This paper introduces and develops a methodology that enables the assessment of different propulsion system designs, which are optimized to be less sensitive to the effects of the aforementioned issues. The study models the propulsor array and main engine performance at design point using a parametric approach, and further at component level, the study focuses on identifying optimum propulsor configurations, in terms of propulsor pressure ratio and BL capture sheet height. At a system level, the study assesses the effects of splitting the thrust between the propulsor array and main engines. The figure of merit used in the optimization is the TSFC. The suitability of the concepts is further assessed using performance predictions for HTS electrical motors. For the purpose of this study, the NASA N3-X aircraft concept is selected as baseline configuration, where the different propulsion designs are tested. As the study focuses on performance assessment of the propulsion system, sizing implication issues and aircraft performance installations effects have not been included in the analysis. The results from the parametric analysis corroborated previous studies regarding the high sensitivity of the propulsion system performance to intake losses and BL inlet conditions. As the study found low-power consumption configurations at these operating conditions, this may be considered as a major issue. The system analysis from the study indicated that splitting the thrust between propulsors and main engines results in improved system efficiency with beneficial effects in fuel savings. When a 2% increase in intake pressure losses and a similar reduction in fan efficiency were assumed due to boundary layer ingestion, the study found an optimum configuration with 65% of thrust delivered by the propulsor array. To summarize, the present work built on past research further contributes to the field through the inclusion of the thrust split as a key variable in the propulsion system design. The thrust split, when introduced, enabled reduction of the detrimental effects of intake losses on the overall system performance. Additionally, as it reduces the power required for the propulsor array, it is expected to reduce the operating power of HTS and cooling systems and therefore improve the effectiveness of the concept.

Turbo-electric distributed propulsion – opportunities, benefits and challenges

Purpose – With the predicted rise in air traffic, a growing need exists to make the aviation industry more environmentally sustainable in the long-term future. Research has shown that the turbo-electric distributed propulsion system (TeDP) could be the next disruptive technology that has the potential to meet the ambitious environmental goals set for the N + 3 time frame. This however will require the use of superconductivity, application of high-temperature superconducting materials and cryogenic liquids. This paper provides a brief overview of the technology and further discusses the benefits, advantages and new opportunities that may arise from the application of the technology. Design/methodology/approach – This paper provides a brief overview of the technology and further discusses the benefits, advantages and new opportunities that may arise from the application of the technology. Findings – Implementation of superconducting technology is currently one of the greater challenges faced and hence this article also reviews some of the key considerations to enable utilisation of cryogenic fuels in the future. Originality/value – This paper provides a viewpoint and reviews some of the work undertaken in the field. It also provides a perspective on some new possibilities and advantages from using TeDP with cryogenic fuels.

Aero engine compressor fouling effects for short- and long-haul missions

The impact of compressor fouling on civil aero engines unlike the industrial stationary application has not been widely investigated or available in open literature. There are questions about the impact of fouling for short- and long-haul missions comparatively, given their unique operational requirements and market. The aim of this study is to quantify the effects of different levels of fouling degradation on the fan, for two different aircraft with different two-spool engine models for their respective typical missions. Firstly, the study shows the increase in turbine entry temperature for both aircraft engines, to maintain the same level of thrust as their clean condition. The highest penalty observed is during take-off and climb, when the thrust setting is the highest. Despite take-off and climb segment being a larger proportion in the short-haul mission compared to the long-haul mission, the percentage increase in fuel burn due to fouling are similar, except in the worst case fouling level were the former is higher by 0.8% points. In addition to this, for all the cases, the additional fuel burn due to fouling and its cost is shown to be small. Likewise, the increase in turbine entry temperature for both missions at take-off are similar, except in the worst case fouling level for the short-haul mission were the turbine entry temperature is 7 K higher than the corresponding long-haul mission for the same level of degradation. The study infers that the penalty due to rise in temperature is of more concern than the additional fuel burn. Hence the blade technology (cooling and material) and engine thrust rating are key factors in determining the extent to which blade fouling would affect aero engine performance in short- and long-haul missions.

Economic Viability Assessment of NASA’s Blended Wing Body N3-X Aircraft

Numerous novel aircraft concepts are under development that aim to achieve dramatic increases in efficiency and reductions in emissions in comparison to current aircraft. Research into these concepts typically focuses on performance aspects to establish whether the aircraft will be capable of meeting developmental goals. However, the final goal of such concepts is to progress to viable commercial products. Economic viability assessments are therefore an integral part of the development process to ensure a sustainable industry. The key question to address is whether a high efficiency aircraft concept can translate into an attractive product from an economic perspective. This research performed an economic viability assessment of NASA’s N3-X aircraft, a blended wing body aircraft with a distributed boundary layer ingesting propulsion system. The sensitivity of the aircraft’s direct operating cost to changes in acquisition price and maintenance cost was predicted to establish maximum cost margins for the aircraft. In a May 2017 fuel price scenario, the N3-X could be no more than 25% more expensive than the baseline aircraft to remain economically viable. Introducing a carbon tax or fuel price jump widens the margin for increased costs. Aircraft cost estimates for the aircraft predict an acquisition cost from 11–37% more expensive than the baseline. In combination with the direct operating cost sensitivity analysis, the N3-X is predicted to need to capture 30% of the aircraft market up to 2035.

Installed performance assessment of an array of distributed propulsors ingesting boundary layer flow

Conventional propulsion systems are typically represented as uninstalled system to suit the simple separation between airframe and engine in a podded configuration. However, boundary layer ingesting systems are inherently integrated, and require a different perspective for performance analysis. Simulations of boundary layer ingesting propulsions systems must represent the change in inlet flow characteristics which result from different local flow conditions. In addition, a suitable accounting system is required to split the airframe forces from the propulsion system forces. The research assesses the performance of a conceptual vehicle which applies a boundary layer ingesting propulsion system NASA’s N3-X blended wing body aircraft as a case study. The performance of the aircraft’s distributed propulsor array is assessed using a performance method which accounts for installation terms resulting from the boundary layer ingesting nature of the system. A ‘thrust split’ option is considered which splits the source of thrust between the aircraft’s main turbojet engines and the distributed propulsor array. An optimum thrust split for a specific fuel consumption at design point is found to occur for a thrust split value of 94.1%. In comparison, the optimum thrust split with respect to fuel consumption for the design 7500 nmi mission is found to be 93.6%, leading to a 1.5% fuel saving for the configuration considered. ∗Address all correspondence to this author.

Performance assessment of a boundary layer ingesting distributed propulsion system at off-design

As research on boundary layer ingesting aircraft concepts progresses, it becomes important to develop methods that may be used to model such propulsion systems not only at design point, but also over the full flight envelope. This research presents a methodology and framework for simulating the performance of boundary layer ingesting propulsion systems at off-design conditions. The method is intended for use as a preliminary design tool that may be used to explore the design space and identify design challenges or potential optimum configurations. The method presented in this research enables the rapid analysis of novel BLI configurations at a preliminary design stage. The method was applied to a case study of NASA’s N3-X aircraft, a blended wing body concept with a distributed propulsor array ingesting the airframe boundary layer. The performance of two propulsor in the array was compare, one at the airframe centreline and one at the extreme edge of the array. Due to difference in flow conditions, the centreline propulsor was shown to be more efficient at off-design than the end propulsor. However, this difference in efficiency disappeared at sea level static where the boundary layer thickness is negligible and mass flow ratio is high. Difference in thrust produce by the two propulsors was instead due their different sizes. Performance of the propulsor array as a whole was also presented both independently and including a link to a pair of turbogenerators to provide power. At off design, it was found that there was a discrepancy between the maximum power available from the turbogenerators at off-design operating points and that demanded by the propulsor array operating at 100% fan rotational speed. This discrepancy means that the propulsor array’s performance is limited by the turbogenerators at off-design, particularly for low speed, low altitude operation.

Recursive least squares for online dynamic identification on gas turbine engines

NLINE identification for a gas turbine engine is vital for health monitoring and control decisions because the engine electronic control system uses the identified model to analyze the performance for optimization of fuel consumption, a response to the pilot command, as well as engine life protection. Since a gas turbine engine is a complex system and operating at variant working conditions, it behaves nonlinearly through different power transition levels and at different operating points. An adaptive approach is required to capture the dynamics of its performance. Dynamic identification for gas turbine engines is mostly carried by frequency analysis through the experiments with sinusoidal fuel input. From the research by Evans et al. [1], different frequency responses are shown at different operating points [1]. A set of estimated functions is used for representation over the full operating range. For the process of simplification, an adaptive approach needs to be implemented so that the estimated model can be evolved along with the change of engine dynamics. Isermann et al. [2] compared six methods commonly used in the industry, and most of the online methods were based on the theory of least squares and likelihood [2]. These methods are particularly favored to the online identification because of their simplicity and computing efficiency. They did not require iterations and training like neural networks, but the accuracy of these methods was sometimes compromised. The recursive least squares (RLS) algorithm is well known for tracking dynamic systems. Torres et al. [3] attempted to identify the dynamic of the gas turbine engine offline, mainly at steady states with stochastic signals. Arkov et al. [4] focused on real-time identification for transient operations and concluded that an engine system could be averaged to a time-invariant firstor second-order transfer function by the extended RLS [4]. The tracking speed and accuracy for the RLS could be improved with a different design of forgetting factors. The effect of using a forgetting factor was to shift the estimating average toward the most recent data, such as that in the work by Paleologu et al. [5]. In this paper, classic and modified RLS