Propulsion Airframe Research Articles

A Review of Novel and Non-Conventional Propulsion Integrations for Next-Generation Aircraft

The aim of this review paper is to collect and discuss the most relevant and updated contributions in the literature regarding studies on new or non-conventional technologies for propulsion–airframe integration. Specifically, the focus is given to both evolutionary technologies, such as ultra-high bypass ratio turbofan engines, and breakthrough propulsive concepts, represented in this frame by boundary layer ingestion engines and distributed propulsion architectures. The discussion focuses mainly on the integration effects of these propulsion technologies, with the aim of defining performance interactions with the overall aircraft, in terms of aerodynamic, propulsive, operating and mission performance. Hence, this work aims to analyse these technologies from a general perspective, related to the effects they have on overall aircraft design and performance, primarily considering the fuel consumption as a main metric. Potential advantages but also possible drawbacks or detected showstoppers are proposed and discussed with the aim of providing as broad a framework as possible for the aircraft design development roadmap for these emerging propulsive technologies.

Propulsion–Airframe Integration for Conceptual Redesign of a Low-Boom Supersonic Transport

A low-boom supersonic transport was generated in a previous low-boom multidisciplinary optimization study that used computational fluid dynamics (CFD) off-body pressure to compute the undertrack sonic boom ground signature and calibrated low-fidelity aerodynamic analyses to compute the mission performance metrics. This low-boom aircraft, referred to as the Mach 1.7 40-PAX concept, can carry 40 passengers for a low-boom overland mission with cruise Mach 1.7 for the airport pairs over the continental United States and has the potential to achieve an undertrack sonic boom ground noise level below 70 perceived level of decibels at the start of overland cruise (SOC). The engine for the Mach 1.7 40-PAX concept was designed using the Numerical Propulsion System Simulation (NPSS) for the mission analysis and modeled as a flow-through nacelle for CFD-based sonic boom analysis. To understand how the engine plume affects the undertrack ground signature, the Mach 1.7 40-PAX concept is redesigned in this paper after replacing the flow-through nacelles with CFD engines for sonic boom analysis using aeropropulsive CFD simulation. A process is developed for approximation of the NPSS engine at SOC by a CFD engine for aeropropulsive CFD simulation. The generated CFD engine has the identical nozzle boundary conditions and approximately the same mass flow and thrust as those of the NPSS engine at SOC. Then, the outer mold line of the configuration with the CFD engines is redesigned to approximately restore the low-boom characteristics of the Mach 1.7 40-PAX concept at SOC. Finally, the CFD simulation data for the redesigned concept with the CFD engines are used to calibrate the low-fidelity aerodynamic analyses for mission analysis of this concept. The cyclic dependency of the involved disciplinary analyses is resolved using an iteration method for a consistent coupling of the mission analysis, NPSS engine analysis, and low-boom redesign using the aeropropulsive CFD simulation. This low-boom redesign study is used as an example to demonstrate how the propulsion–airframe integration could be implemented for conceptual design of supersonic transports that satisfy both the low-boom and mission performance requirements.

Data-Driven Multiobjective Optimization of Wave-Packets for Near-Field Subsonic Jet Noise

Aeroacoustics of innovative aircraft cannot disregard the development of low-order models for the jet noise source, which are essential to assess the propulsion–airframe interactions from the conceptual design stage. The main scope of this work is to provide a noise source model that can be coupled with relatively low computational cost methods for aeroacoustic scattering. To this end, this paper presents for the first time a multiobjective optimization of the zeroth-mode wave-packet in the jet near field. The importance of calibrating the model with near-field pressure data stems from the fact that in new aircraft, the engine nacelles are typically positioned at a few diameters from the wing or fuselage. In this work, the near field of a high subsonic jet at a Mach number of 0.9 is represented as a cylindrical surface radiating the pressure disturbances of a wave-packet source, which is optimized using large-eddy simulation data from three lines at different radial distances. The optimized model has been tested at Strouhal numbers between 0.25 and one, and the optimized solutions have been chosen using a Pareto-ranking criterion considering the wave-packet prediction over an extra line. A good agreement is achieved between the reference data and model predictions for multiple near-field radial distances.

Aero-Propulsive Modeling for Propeller Aircraft Using Flight Data

This paper describes methods to identify an integrated propulsion–airframe aerodynamic model and a decoupled propulsion model for fixed-wing aircraft with propellers using flight data. Propulsion aerodynamics and airframe aerodynamics for propeller aircraft are usually modeled separately, which fails to describe unavoidable interaction effects and propeller performance deviations when integrated on an aircraft. Two novel flight test system identification approaches are presented to develop flight dynamics models with improved characterization of propeller aerodynamics compared to conventional methods. Orthogonal phase-optimized multisine inputs are applied to both the control surfaces and propulsion system to generate data with high-quality information content for model identification. Propulsion explanatory variables derived from propeller aerodynamics theory combined with traditional aircraft modeling variables yield accurate aero-propulsive modeling results and provide propeller performance estimates, which are compared to isolated propeller wind tunnel data. An assessment of model adequacy using flight maneuvers withheld from model identification indicates that the models have good prediction capability. The paper describes application of these methods to a small unmanned aircraft, but the methods are generalizable to many propeller-driven aircraft.

Systematic validation of the PAAShA shielding prediction method

Based on classical diffraction theories with modifications and extensions in analytical formulations and numerical implementations, a new code has been developed at NASA for the prediction of aircraft noise shielding, named as Propulsion Airframe Aeroacoustic Shielding Attenuation (PAAShA). The code is developed primarily for aircraft system noise predictions, although it may also be useable in other applications with acoustic shielding. The requirements for this code are driven by the need for a robust, capable code to use with NASA’s Aircraft Noise Prediction Program (ANOPP) for aircraft integration and system noise research. The requirements are met and include capabilities to use a wide range of aircraft geometries, rapid calculation times consistent with aircraft system noise problems, and the flexibility to model realistic noise source characteristics and distributions. The accuracy and robustness of the method are demonstrated in this paper with a set of problems, including a cylinder, a finite plate, a symmetrical two-dimensional airfoil, and a full three-dimensional hybrid wing body aircraft model tested in a wind tunnel. This range of problems demonstrates both smooth and sharp edge diffraction capabilities for a wide range of frequencies and low Mach number flow effects at low angles of attack. Predictions are shown to be accurate to within 1–4 dB over a wide range of the most significant frequencies and directivity angles. This is determined by comparing with data, which have experimental uncertainties, particularly at high frequencies, high angles, and source characteristics. The accuracy diminishes for geometries that include a significant reflection component, which is not calculated by the code. Accuracy can also be somewhat diminished for high azimuthal angles. Accurate modeling of the noise source, particularly its frequency and directivity characteristics, is essential to obtaining accurate results.

Aero-Propulsive Modeling for Tilt-Wing, Distributed Propulsion Aircraft Using Wind Tunnel Data

A high-fidelity, integrated propulsion–airframe aerodynamic model was developed for the Langley Aerodrome No. 8 (LA-8) tandem tilt-wing, distributed electric propulsion, vertical takeoff and landing aircraft. Electric vertical takeoff and landing (eVTOL) vehicle configurations exhibit aerodynamic characteristics of both fixed-wing and rotary-wing aircraft as well as complex vehicle-specific phenomena, such as propeller–wing interactions and high-incidence-angle propeller aerodynamics. Consequently, conventional aircraft aerodynamic modeling strategies require modification when applied to eVTOL aircraft. Two novel system identification-based approaches are used to develop an aero-propulsive model for the LA-8 aircraft configuration using wind tunnel data collected with design of experiments techniques. The modeling strategies are compared by assessing their predictive performance for validation data acquired separately from the data used to identify the model and are shown to have sufficient predictive capability. Research findings are presented with a discussion of unique eVTOL aerodynamic modeling characteristics and practical strategies to inform future aero-propulsive modeling efforts for eVTOL aircraft.

Aircraft System Noise of the NASA D8 Subsonic Transport Concept

A vehicle-level noise assessment has been performed for the NASA D8 (ND8) concept aircraft in the NASA Advanced Air Transport Technology Project portfolio. The NASA research-level Aircraft Noise Prediction Program was used to predict the noise from each source component on the ND8 to build up a noise estimate for the full aircraft. The propulsion airframe aeroacoustic effects of the ND8 (namely, boundary-layer ingestion, with its influence on fan noise; and the noise shielding, reflection, and diffraction mechanisms of the unconventional airframe) were empirically modeled using experimental data. Far-term noise reduction technology concepts were modeled and added to the noise prediction to evaluate the low-noise potential of the ND8 in the far-term timeframe. Results indicate that the increase in fan noise due to boundary-layer ingestion, as well as the lack of aft shielding, prevents the aircraft from approaching NASA’s noise goals for either the midterm or far term. The aircraft achieves margins to Stage 4 of only 9.4 and 17.3 effective perceived noise in decibels (EPNdB) in the midterm and far-term configurations, respectively, compared with goals of 32–42 EPNdB in the midterm and 42-52 EPNdB in the far term.

System Noise Prediction Uncertainty Quantification for a Hybrid Wing–Body Transport Concept

Aircraft system noise prediction for advanced, unconventional concepts has undergone significant improvement over the past two decades. Prediction model uncertainty quantification is necessary to reliably assess the potential benefits of unconventional configurations, which can be outside the range of empirical models. This paper builds on previous work to improve estimates of element prediction uncertainties where the model has improved, or new validation data are available. These element-level uncertainties are propagated via a Monte Carlo method to the system level for predictions at each of the three certification points. Although the quantitative uncertainty values contained here are specific to the hybrid wing–body configuration presented, the underlying process is the same regardless of configuration. Comparisons with previous work show a reduction of 1.1 EPNdB in the 95% coverage interval of the cumulative noise level. The largest impediment for continued uncertainty reduction is the need for validation experiments and improved modeling of fan source noise, propulsion airframe aeroacoustic effects, and the Krueger flap source noise, which comprise the bulk of the uncertainty in the cumulative certification noise level.

Aerodynamic design of integrated propulsion–airframe configuration of a hybrid wing body aircraft

A hybrid wing body (HWB) concept is being considered by NASA as a potential subsonic transport aircraft that meets aerodynamic, fuel, emission, and noise goals in the time frame beyond 2035. While the concept promises advantages over a conventional wing-and-tube aircraft, it poses unknowns and risks, thus requiring in-depth and broad assessments. Specifically, the configuration entails a tight integration of the airframe and propulsion geometries; the aerodynamic impact has to be carefully evaluated. With the propulsion nacelle installed on the (upper) body, the lift and drag are affected by the mutual interference effects between the airframe and nacelle. The static margin for longitudinal stability is also adversely changed. In the present paper, a design approach is developed in which the integrated geometries of airframe (HWB) and propulsion are accounted for simultaneously in a simple algebraic manner, via parameterization of the planform and airfoils at the design sections of the wing body. This paper presents a design of a 300-passenger aircraft that employs distributed electric fans for the propulsion. The trim condition for stability is achieved through the use of the wing tip twist angle. The geometric shape variables are determined through the adjoint optimization method by minimizing the drag while subjecting them to lift, pitching moment, and geometry constraints. An Euler model-based aerodynamic shape optimization is employed to save the design cost for the evaluation of the static margin and longitudinal stability, while the performance of the optimized configuration is evaluated by the RANS model coupled with a drag decomposition method to assess the true aerodynamic performance. The design results clearly show the influence on the aerodynamic characteristics of the installed nacelle and trimming for stability. A drag minimization with the trim constraint yields a reduction of 10 counts in the drag coefficient from the baseline design N3-X configuration, which is comparable with 2000 lbs more payload on a conventional subsonic civil transport airplane.

Preliminary Sizing Method for Hybrid-Electric Distributed-Propulsion Aircraft

The use of hybrid-electric propulsion (HEP) entails several potential benefits such as the distribution of power along the airframe, which enables synergistic configurations with improved aerodynamic and propulsive efficiency. This paper presents a comprehensive preliminary sizing method suitable for the conceptual design process of hybrid-electric aircraft, taking into account the powertrain architecture and associated propulsion–airframe integration effects. To this end, the flight-performance equations are modified to account for aeropropulsive interaction. A series of component-oriented constraint diagrams are used to provide a visual representation of the design space. A HEP-compatible mission analysis and weight estimation are then carried out to compute the wing area, powerplant size, and takeoff weight. The resulting method is applicable to a wide range of electric and hybrid-electric aircraft configurations and can be used to estimate the optimal power-control profiles. For demonstration purposes, the method is applied to a regional HEP aircraft featuring leading-edge distributed propulsion (DP). Three powertrain architectures are compared, showing how the aeropropulsive effects are included in the model. Results indicate that DP significantly increases wing loading and improves the cruise lift-to-drag ratio by 6%, although the growth in aircraft weight leads to an energy consumption increase of 3% for the considered mission.

Wavepacket modeling of the jet noise source

This study is motivated by the need for physical models for the jet noise source to be used in practical noise prediction schemes for propulsion–airframe integration concepts. The basis for the source model is an amplitude-modulated traveling wave—the wavepacket. The source is parameterized and the parameters are determined by minimizing the difference between the modeled and experimental sound intensity distributions in the far field. Even though the pressure signal that reaches the far field is highly filtered, sufficient information is available to construct a wavepacket with reasonable physical characteristics. A simple stochastic extension of this concept shows a connection between the shape of the far-field sound pressure level spectrum and the emission polar angle. It suggests that the broadening of the spectrum with increasing polar angle from the downstream axis can be explained on the basis of a single noise source (the wavepacket), rather than the prevailing model of two distinct noise sources, one coherent and the other incoherent.

Chemical-Equilibrium Analysis with Adjoint Derivatives for Propulsion Cycle Analysis

The design optimization of aircraft engines considering their integration with the airframe has been limited by challenges with existing propulsion modeling tools. Gradient-based optimization with derivatives computed using adjoint methods has been successful in solving aerodynamic and structural shape optimization problems but has not yet been applied to coupled propulsion–airframe optimization, partly because existing tools lack analytic derivative computation. As a step toward obtaining a full cycle analysis with efficient analytic derivative computation, a new chemical-equilibrium thermodynamics solver is developed for propulsion applications. This solver provides a continuous formulation that enables analytic derivative computation using a coupled adjoint approach. The results from this solver are verified against a well-established chemical-equilibrium code. The analytic derivatives are also verified by comparing them with finite-difference approximations. The performance of the analytic derivative computations is tested using two optimizations: combustion temperature maximization with respect to equivalence ratio, and combustion temperature maximization with respect to air pressure. The results show clear speed and numerical stability benefits when comparing the proposed method against finite-difference approximations. It is now possible to use this new solver as the foundation for further development of a complete propulsion analysis for integrated propulsion–airframe design optimization.

DEVELOPMENT OF THE CONCEPT OF CURVILINEAR CHANNELS FOR PROPULSION SYSTEM INLETS AND NOZZLES EMBEDDED IN THE AIRFRAME

The propulsion airframe integrated configuration of a tailless aircraft is considered in this study. Several concepts of embedded inlets and nozzles are presented. The results of the numerical flow simulation in the presented inlets and nozzles are obtained.

Auralization of Hybrid Wing–Body Aircraft Flyover Noise from System Noise Predictions

System noise assessments of a state-of-the-art reference aircraft (similar to a Boeing 777-200ER with GE90-like turbofan engines) and several hybrid wing–body aircraft configurations were recently performed using NASA engine and aircraft system analysis tools. The hybrid wing–body aircraft were sized to an equivalent mission as the reference aircraft and assessments were performed using measurements of airframe shielding from a series of propulsion airframe aeroacoustic experiments. The focus of this work is to auralize flyover noise from the reference aircraft and the best hybrid wing–body configuration using source noise predictions and shielding data based largely on the earlier assessments. Here, auralization refers to the process by which numerical predictions are converted into audible pressure time histories. It entails synthesis of the source noise and propagation of that noise to a receiver on the ground. For each aircraft, three flyover conditions are auralized. These correspond to approach, sideline, and cutback operating states, but flown in straight and level flight trajectories. The auralizations are performed using synthesis and simulation tools developed at NASA. Audio and visual presentations are provided to allow the reader to experience the flyover from the perspective of a listener in the simulated environment.

Hybrid Wing Body Aircraft System Noise Assessment with Propulsion Airframe Aeroacoustic Experiments

A system noise assessment of a hybrid wing body configuration was performed using NASA's best available aircraft models, engine model, and system noise assessment method. A propulsion airframe aeroacoustic effects experimental database for key noise sources and interaction effects was used to provide data directly in the noise assessment where prediction methods are inadequate. NASA engine and aircraft system models were created to define the hybrid wing body aircraft concept as a twin engine aircraft with a 7500 nautical mile mission. The engines were modeled as existing technology, in production, bypass ratio seven turbofans. The baseline hybrid wing body aircraft was assessed at 26.4 dB cumulative below the FAA Stage 4 certification level. To determine the potential for noise reduction with relatively near term technologies, seven other configurations were assessed beginning with moving the engines two fan nozzle diameters upstream of the trailing edge and then adding technologies for reduction of the highest noise sources. Aft radiated noise was expected to be the most challenging to reduce and, therefore, the experimental database focused on jet nozzle and pylon configurations that could reduce jet noise through a combination of source reduction and shielding effectiveness. The best configuration for reduction of jet noise used state-of-the-art technology chevrons with a pylon above the engine in the crown position. This configuration resulted in jet source noise reduction, favorable azimuthal directivity, and noise source relocation upstream where it is more effectively shielded by the limited airframe surface, and additional fan noise attenuation from acoustic liner on the crown pylon internal surfaces. Vertical and elevon surfaces were also assessed to add shielding effectiveness. The elevon deflection above the trailing edge showed some small additional noise reduction whereas vertical surfaces resulted in a slight noise increase. With the effects of the configurations from the database included, the best available noise reduction was 41.5 dB cumulative. Projected effects from additional technologies were assessed for an advanced noise reduction configuration including landing gear fairings and advanced pylon and chevron nozzles. Incorporating the three additional technology improvements, an aircraft noise is projected of 42.9 dB cumulative below the Stage 4 level.

Propulsion Airframe Aeroacoustic Integration Effects for a Hybrid Wing Body Aircraft Configuration

An extensive experimental investigation was performed to study the propulsion airframe aeroacoustic effects of a high bypass ratio engine for a hybrid wing body aircraft configuration where the engine is installed above the wing. The objective was to provide an understanding of the jet noise shielding effectiveness as a function of engine gas condition and location as well as nozzle configuration. A 4.7% scale nozzle of a bypass ratio seven engine was run at characteristic cycle points under static and forward flight conditions. The effect of the pylon and its orientation on jet noise was also studied as a function of bypass ratio and cycle condition. The addition of a pylon yielded significant spectral changes lowering jet noise by up to 4 dB at high polar angles and increasing it by 2 to 3 dB at forward angles. In order to assess jet noise shielding, a planform representation of the airframe model, also at 4.7% scale was traversed such that the jet nozzle was positioned from downstream of to several diameters upstream of the airframe model trailing edge. Installations at two fan diameters upstream of the wing trailing edge provided only limited shielding in the forward arc at high frequencies for both the axisymmetric and a conventional round nozzle with pylon. This was consistent with phased array measurements suggesting that the high frequency sources are predominantly located near the nozzle exit and, consequently, are amenable to shielding. The mid to low frequency sources were observed further downstream and shielding was insignificant. Chevrons were designed and used to impact the distribution of sources with the more aggressive design showing a significant upstream migration of the sources in the mid frequency range. Furthermore, the chevrons reduced the low frequency source levels and the typical high frequency increase due to the application of chevron nozzles was successfully shielded. The pylon was further modified with a technology that injects air through the shelf of the pylon which was effective in reducing low frequency noise and moving jet noise sources closer to the nozzle exit. In general, shielding effectiveness varied as a function of cycle condition with the cutback condition producing higher shielding compared to sideline power. The configuration with a more strongly immersed chevron and a pylon oriented opposite to the microphones produced the largest reduction in jet noise. In addition to the jet noise source, the shielding of a broadband point noise source was documented with up to 20 dB of noise reduction at directivity angles directly under the shielding surface.

NASA’s propulsion airframe aeroacoustics research

The integration of propulsion and airframe is a fundamental consideration in the design of an aircraft system. Many considerations influence the integration, such as structural, aerodynamic, and maintenance factors. In the future, a focus on the aerodynamic and acoustic interaction effects of installation, propulsion airframe aeroacoustics will become more important as noise reduction targets become more difficult to achieve. In addition to continued fundamental component reduction efforts, a system level approach that includes propulsion airframe aeroacoustics will be required in order to achieve the 20-dB noise reductions envisioned by the aggressive NASA goals. This emphasis on the aeroacoustics of propulsion airframe integration is a new part of NASA’s ongoing acoustics research. The presentation will review current efforts and highlight technical challenges and approaches.

Propulsion airframe aeroacoustics practices at Honeywell

Honeywell has been developing and applying acoustic models of propulsion airframe aeroacoustic phenomena for over 20 years. The initial application of a wing-shielding model was developed for the NASA General Aviation Synthesis Program in 1982. Since that time, more sophisticated models of wing shielding and reflection have been developed with internal and NASA funding. Recent work has involved models of wing shielding for aft mounted engines and wing reflection for wing mounted engines. These methods are described in the presentation. Comparisons with the Raynoise Code by LMS and measured aircraft fly over noise data are made to show the effectiveness of the model. The attenuation of the inlet noise by the wing of an aft mounted engine and the amplification of the noise by wing mounted engines are evaluated.

Diagnostics and reduction of propulsion airframe aeroacoustic interactions

In the past, significant reduction in aircraft noise has been achieved by studying engine noise and airframe noise in isolation as two separate components. However, when an engine is installed on an aircraft the flow and acoustic interactions between them produce a total noise signature which is often different than the sum of the component noise signatures. While take-off conditions are typically dominated by engine noise, approach conditions are characterized by both engine and airframe noise. Nevertheless, propulsion airframe aeroacoustic (PAA) interactions are present under all conditions and gain in importance when engine or airframe noise is further reduced. This paper focuses on the diagnostics and reduction of the flow-acoustic interaction effects between an engine exhaust and the airframe, especially the high-lift system on wings. Flow and acoustic results are presented for an isolated scale-model nozzle in a free jet, and also when it is installed under a wing at take-off and approach conditions. The difference between these two noise signatures, in the far field and at the source, captures the PAA effect. In particular, the PAA effect due to changes in nozzles, pylons, and flaperons is presented and the overall noise reducing mechanisms are postulated.

Propulsion airframe integration design, analysis and challenges going into the 21st century

AbstractPropulsion airframe integration design and analysis has many challenges as we move into the 21st century. Along with the conventional challenges of integrating a propulsion system on an aircraft, new technologies and new business drivers dictate innovative approaches to the design process be developed and applied.On the technical side, new innovations in jet noise suppression and new aircraft concepts such as the blended wing body (BWB) will give rise to many challenges in propulsion system performance, operability, and meeting system requirements such as thrust reverse capability. Aircraft engine and aircraft manufacturers must have the appropriate design and analysis tools in place which provide the ability to react quickly to inevitable design changes, driven by constantly changing requirements, during the product development cycle.On the business side, the rapid globalisation of the business dictates that the latest electronic technology be utilised to enable speed in communication with global customers as well as revenue sharing partners. More than ever, cost and schedules dictate the use of analytical methods to minimise the amount of qualification testing. Design and analysis software must be flexible and capable of integrating CAD/CAM and CAE tools while maintaining configuration control of the product.The following paper describes some of the new technical challenges facing the industry. Innovative methods of addressing those challenges are described.