Aircraft Propulsion Systems Research Articles

Friction Stir Welding of Metal Matrix Composites for use in aerospace structures

Friction Stir Welding (FSW) is a relatively nascent solid state joining technique developed at The Welding Institute (TWI) in 1991. The process was first used at NASA to weld the super lightweight external tank for the Space Shuttle. Today FSW is used to join structural components of the Delta IV, Atlas V, and Falcon IX rockets as well as the Orion Crew Exploration Vehicle. A current focus of FSW research is to extend the process to new materials which are difficult to weld using conventional fusion techniques. Metal Matrix Composites (MMCs) consist of a metal alloy reinforced with ceramics and have a very high strength to weight ratio, a property which makes them attractive for use in aerospace and defense applications. MMCs have found use in the space shuttle orbiter's structural tubing, the Hubble Space Telescope's antenna mast, control surfaces and propulsion systems for aircraft, and tank armors. The size of MMC components is severely limited by difficulties encountered in joining these materials using fusion welding. Melting of the material results in formation of an undesirable phase (formed when molten Aluminum reacts with the reinforcement) which leaves a strength depleted region along the joint line. Since FSW occurs below the melting point of the workpiece material, this deleterious phase is absent in FSW-ed MMC joints. FSW of MMCs is, however, plagued by rapid wear of the welding tool, a consequence of the large discrepancy in hardness between the steel tool and the reinforcement material. This work characterizes the effect of process parameters (spindle speed, traverse rate, and length of joint) on the wear process. Based on the results of these experiments, a phenomenological model of the wear process was constructed based on the rotating plug model for FSW. The effectiveness of harder tool materials (such as Tungsten Carbide, high speed steel, and tools with diamond coatings) to combat abrasive wear is explored. In-process force, torque, and vibration signals are analyzed to assess the feasibility of on-line monitoring of tool shape changes as a result of wear (an advancement which would eliminate the need for off-line evaluation of tool condition during joining). Monitoring, controlling, and reducing tool wear in FSW of MMCs is essential to the implementation of these materials in structures (such as launch vehicles) where they would be of maximum benefit.

Research on Parameters Matching of Ultralight Electric Aircraft Propulsion System

The propulsion system of ultralight electric aircraft is one of the general aviation technology development directions. It has the advantages such as light pollution, low noise, high energy utilization ratio, simple structure, easy maintenance, high reliability, less heat radiation, little operation cost and so on. Combined with the certain type of ultralight aircraft design parameters, the layout of aircraft electric propulsion, the principles and steps of the parameter matching of electric propulsion system were presented. The method of parameter matching and performance verification of electric propulsion system was put forward. The feasibility of the system is verified from the point of dynamic property. The study of parameter matching of electric propulsion system could not only provide basis for the integrated optimization for electric power system, but also evaluate the performance of the system simulation as reference.

Dynamic Analysis of Damage to Aircraft Propulsion System Impacted by Exploding Missile

In this paper, an attempt is made to analytically predict the damage to a running turbofan jet engine if it gets hit by an incoming heat-seeking missile. The predictive methodology is based upon developing a LS-DYNA® finite-element model of the entire engine, as well as the exploding missile. The analytical simulation considers both the effect of ballistic energy, as well as the pressure pulse generated as a result of the detonation of the warhead in the missile. The engine model accurately captures the complex dynamic effect of the transient imbalance forces and gyroscopic moments that are caused in a dual concentric rotor system that is spinning at two different speeds. The numerical results presented here are directly applicable in determining the structural damage to individual components of a typical large turbofan jet engine that is used in commercial aviation. The high-velocity impact damage to engine components can involve penetration, fragmentation, and even the uncontained liberation of debris from the rotors. In general, the heat-seeking missile always has a tendency to go toward the engine core, but the impact results in large dynamic loads on the engine mount system.

A Review of Current Prognostics and Health Management System Related Standards

Prognostics and health management(PHM) is a promising technology to reduce the overall life cycle cost and improve flight readiness in the civil and military aviation field. As the prognostic algorithms are applied into monitoring condition of aircraft structures, avionics, wiring, control actuators, power supplies, propulsion systems and etc, more attention has been focused on establishing a common standard to validate whether the PHM algorithms or systems can reach the design requirements and compare the efficiency of the diagnostics and prognostics algorithms. Currently, there’re many standards in the diagnostics and test regions, such as condition monitoring and diagnosis standard in ISO, the engine health management standards in SAE and so on. All these PHM related standards may help developing the new standard. First, this paper identifies the specification of PHM standards from different applications. And the state-of-the-art in PHM related standards is also given. Second, the current PHM-related standards in various standardizations and agencies are overviewed and analyzed according to the CBM, PHM, IVHM and HUMS classification; Lastly, some suggestions are given for PHM standardization development, and the focus of the future work and directions are also proposed.

Research on windmill starting characteristics of MTE-D micro turbine engine

Micro turbine engine (MTE) is an important kind of propulsion system for miniature unmanned aircraft or missiles, because of its better high-speed performance (than propeller propulsion) and higher propulsion efficiency (obviously than rockets). Windmill start is a common air-starting mode used in micro turbine engine. The windmill starting characteristics are important to the practical use of micro turbine engine. In this paper, the windmill starting characteristics research for a 12cm diameter (MTE-D) micro turbine engine is carried out by experiment and numerical simulation. The characteristic of rotor mechanical losses at low-speed condition is studied, and the engine common working line of windmill starting process is obtained. Based on the engine windmill characteristics, the propane ignition characteristics under different inflow conditions are researched, and the envelope of propane ignition and propane flameout is determined. The experimental research of fuel supply and ignition characteristics is completed, and the envelope of fuel supply and ignition is obtained. The windmill stage, propane ignition stage, fuel ignition stage and acceleration process from idling-speed to 80% full speed of MTE-D micro turbine engine is optimized, and the optimization windmill starting parameters are collected. The successful windmill starting experiment under this condition with engine speed up to 80% full speed indicates that these starting parameters are reasonable. All the starting parameters of MTE-D micro turbine engine obtained in this work are dimensionless parameters, and the conclusions obtained in this study have some reference to other micro turbine engines with the similar structural form and starting process.

Environmental impact assessment of a turboprop engine with the aid of exergy

To develop approaches that effectively reduce engine environmental effect of aircrafts, it is necessary to understand the mechanisms that have enabled improvements in thermodynamic efficiency of aircraft engines. In the present work, a turboprop engine used in regional aircrafts that produces 1948 shp and 640 N.m torque is examined using exergo-environmental method. The results show compressor, combustion chamber, gas generator turbine, power turbine and exhaust nozzle create 9%, 69%, 13%, 7%, 2% of total environmental impact of the engine, respectively. According to rates, the compressor and gas turbine can be considered first to improve in case of component related environmental impact. Furthermore, total component related environmental impact for the turboprop engine is found to be 2.26 mPts/s for the constructional phase and 2.34 mPts/s for the operation/maintenance phases. Accordingly, it is suggested that, in order to estimate environmental impact metric of aircrafts, the exergo-environmental analysis can be employed for aircraft propulsion systems.

Development of a 3D Sizing Model for All-Superconducting Machines for Turbo-Electric Aircraft Propulsion

Distributed propulsion in aircraft brings many advantages in terms of efficiency and noise reduction. While the distribution can be done mechanically through the use of gears and transmissions, electrical propulsion allows for lower maintenance needs, higher efficiency, and lower emissions through the complete decoupling of the gas turbines and the propulsion fans. Such systems have been investigated in the past and NASA is executing on a development plan to bring turbo-electric propulsion systems for transportation aircraft by 2035. The very high specific power required for the airborne generators and motors can only be achieved by using superconductors. Analytical 2-D sizing models have been created and showed very promising results. NASA is now funding the development of higher fidelity models for superconducting machines in which an actual 3-D representation of the geometry is considered. The magnetic flux distribution is calculated using Biot-Savart's law coupled with the magnetic moment method for the backiron. The code also includes thermal and mechanical models allowing for a full and accurate design. The paper describes the model architecture and the methods used to perform high-temperature superconducting machine sizing and optimization.

Aircraft Propulsion System Flight Test—Analysis and Evaluation Challenges and Integrated Solutions

The paper describes the challenges and solution methodologies associated with the flight test and evaluation of the propulsion system for a twin-engine military legacy aircraft. Recent flight-test programs evaluated the effects of temperature distortion and biased engine inlet total temperature measurement (TT2) on engine scheduling and compressor stability margin. The challenges are associated with the limited instrumentation and the repeatability of the flight-test points. During the test program it was necessary to employ techniques to extend the usefulness of the data beyond that provided by the acquired and reduced data sets to address the challenges associated with flight-test analysis. The challenges were addressed using mathematical models, engine cycle decks, uninstalled ground-test data, computational fluid dynamics, or some combination of these. Several specific challenges and solutions are described in detail in the paper.

Aircraft Turbine Engine Control Research at NASA Glenn Research Center

This paper provides an overview of the aircraft turbine engine control research at NASA Glenn Research Center (GRC). A brief introduction to the engine control problem is first provided with a description of the state-of-the-art control law structure. A historical aspect of engine control development since the 1940s is then provided with a special emphasis on the contributions of GRC. With the increased emphasis on aircraft safety, enhanced performance, and affordability, as well as the need to reduce the environmental impact of aircraft, there are many new challenges being faced by the designers of aircraft propulsion systems. The Controls and Dynamics Branch (CDB) at the GRC is leading and participating in various projects to develop advanced propulsion controls and diagnostics technologies that will help meet the challenging goals of NASA Aeronautics Research Mission programs. The rest of the paper provides an overview of the various CDB technology development activities in aircraft engine control and diagnostics, both current and some accomplished in the recent past. The motivation for each of the research efforts, the research approach, technical challenges, and the key progress to date are summarized.

Robust Control for Static Loading of Electro-hydraulic Load Simulator with Friction Compensation

Load simulator is a key test equipment for aircraft actuation systems in hardware-in-the-loop-simulation. Static loading is an essential function of the load simulator and widely used in the static/dynamic stiffness test of aircraft actuation systems. The tracking performance of the static loading is studied in this paper. Firstly, the nonlinear mathematical models of the hydraulic load simulator are derived, and the feedback linearization method is employed to construct a feed-forward controller to improve the force tracking performance. Considering the effect of the friction, a LuGre model based friction compensation is synthesized, in which the unmeasurable state is estimated by a dual state observer via a controlled learning mechanism to guarantee that the estimation is bounded. The modeling errors are attenuated by a well-designed robust controller with a control accuracy measured by a design parameter. Employing the dual state observer is to capture the different effects of the unmeasured state and hence can improve the friction compensation accuracy. The tracking performance is summarized by a derived theorem. Experimental results are also obtained to verify the high performance nature of the proposed control strategy.

Low Pressure Turbine Secondary Vortices: Reynolds Lapse

A two-stage turbine is tested in a cooperation between the Institute of Aircraft Propulsion Systems (ILA) and MTU Aero Engines GmbH (MTU). The experimental results taken in the Altitude Test Facility (ATF) are used to assess the impact of cavity flow and leakage on vortex structures. The analysis focuses on a range of small Reynolds numbers, from as low as 35,000 up to 88,000. The five hole probe area traverse data is compared to steady multistage CFD predictions behind the second vane. The numerical model compares computations without and with cavities modeled. The simulation with cavities is superior to the approach without cavities. The vortex induced blockage is found to be inversely proportional to the Reynolds number. The circulation of the vortices is dependent on the Reynolds number showing a reversing trend to the smallest Reynolds numbers. The steady numerical model as of yet is unsuitable to predict these trends. A first unsteady simulation suggests major improvements.

Creep Resistance of WE43 Magnesium Alloy Joints

Magnesium alloys of Mg-Y-RE-Zr series are characterized by creep resistance up to a temperature of 250 ° C, and can work up to a temperature of 300oC. These properties allow for the application of alloys of Mg-Y-RE-Zr series for the elements of racing car engines operating in the conditions of high loads and temperatures. The requirement of high reliability components of aircraft propulsion system, with high strength and corrosion resistance, also led to the use of these alloys in the aerospace industry. Welding technologies in cast magnesium alloys are applied in order to repair defects in castings, occurring in the casting process, as well as to regenerate worn out castings. Joints made of magnesium alloys should have at least the same properties as a finished casting. The literature lacks information on the properties of joints welded of cast magnesium alloys.This work includes examination of influence of heat treatment on creep resistance of alloy WE43. Material for the study comprised joints made by the TIG method, welded in the cast state. Creep tests were carried out on joints without heat treatment and joints after heat treatment. The tests were performed at the temperatures of 200 ° C and 250 ° C during 100h. It was found that there is an increase in creep resistance of the joints after heat treatment.

Robust Surge Avoidance Control for a Low Pressure Compressor of a Turbojet Engine

The aim of this paper is to improve the efficiency of a low pressure compressor within a turbojet engine. For this purpose a new feedback controller will be introduced which compares the actual compressor surge margin to a given reference value. Then, this new feedback controller calculates a deviation on the existing nozzle area schedule such that the overall efficiency of the controller is improved at steady state condition. The major contribution of this work consists in a robust control design approach leading to improved robustness properties and reduced calibration efforts for the controllers of future unmanned aircraft propulsion systems.

Computational analysis of the effects of a boundary layer ingesting propulsion system in transonic flow

Continuous requirements for more efficient aircrafts lead to the design and analysis of novel propulsion configurations, with an example being the boundary layer ingestion. The complexity and integration challenges in such aircraft synergistic propulsion system characterize the research in this field, driven by the potential benefits. The aim of this article is to investigate the effects of boundary layer ingestion on the aerodynamics of a transonic wing, together with the quality of the flow ingested by the propulsion system. A two-dimensional computational model of a transonic airfoil with boundary layer ingesting propulsion system is developed in order to assess boundary layer ingestion for a commercial air transport at cruise conditions and highlight the complex integration issues arising from such configuration. A parametric analysis of the effects of flight conditions, nacelle geometry and engine operating point, on lift, pressure recovery, distortion, total pressure and velocity distribution at the intake, comes to enhance understanding of the performance of this configuration. The pressure distribution around the airfoil and the boundary layer growth are both substantially affected by the engine operating condition, which is represented by the mass flow ratio, with a direct impact on pressure recovery and lift. Mach number and angle of attack influences on lift and drag ingested are also investigated. Intake size and position on the airfoil appear to have significant effects on lift and losses ingested. In general, the results of this study include several aspects related to wing aerodynamics and ingested flow quality, which may facilitate design and integration of the boundary layer ingestion propulsion system for future commercial aircraft.

Lifing Requirement Based Turbine Airfoil Mass Estimation Method in Conceptual Aero-Engine Design

The presented paper describes a method developed by MTU Aero Engines to estimate the mass of turbine blades during multidisciplinary conceptual design studies based on a prescribed airfoil lifetime (Klingels, 2009, “Vordimensionierung von Turbinenschaufeln unter Berücksichtigung der Missionsanforderungen,” MTU Aero Engines, Technical Paper No. M09TET-0028, and Weisser, M., 2010, Erstellung und Implementierung eines erweiterten regelbasierten Verfahrens zur Schaufelauslegung mit Kühlluftmengenbestimmung in der Triebwerksvorauslegung, Diploma thesis, Institute of Aircraft Propulsion Systems, Universität Stuttgart, Stuttgart). For a given material, the target lifetime can be translated into a maximally allowable material temperature and stress level. While the latter has to be maintained by an appropriate mechanical design of the turbine blades, the material temperature needs to be established by sufficient cooling air. The predominant life-limiting effects are taken into account to determine the allowable temperatures and stresses as an accumulation of the varying operating condition over a flight cycle. The applicable stress levels are then used to calculate the necessary radial area distribution of the airfoil and by this a prediction of its mass is possible. Furthermore, the methodology estimates the required amount of cooling air per airfoil cascade from the computed material temperatures. Example calculations are presented and discussed which illustrate design trends and the benefits which are gained from the proposed method.

Technological assessment of high temperature sensing systems under extreme environment

PurposePresently there exists no way to directly measure strain at high temperatures in engine components such as the combustion chamber, exhaust nozzle, propellant lines, and turbine blades and shaft. The purpose of this paper is to address this issue.Design/methodology/approachThermomechanical fatigue (TMF) prediction, which is a critical element for a blade design, is a strong function of the temperature and strain profiles. Major uncertainties arise from the inability of current instrumentation to measure temperature and strain at critical locations. This prevents the structural designer from optimizing the blade design for high temperature environments, which is a significantly challenging problem in engine design.FindingsBeing able to directly measure strains in different high temperature zones would deeply enhance the effectiveness of aircraft propulsion systems for fatigue damage assessment and life prediction. The state of the art for harsh environment, high temperature sensors has improved considerably over the past few years.Originality/valueThis paper lays down specifications for high temperature sensors and provides a technological assessment of these new sensing technologies. The paper also reviews recent advances made in harsh environment sensing systems and takes a peek at the future of such technologies.

Research on hypersonic aircraft using pre-cooled turbojet engines

Systems analysis of a Mach 5 class hypersonic aircraft is performed. The aircraft can fly across the Pacific Ocean in 2h. A multidisciplinary optimization program for aerodynamics, structure, propulsion, and trajectory is used in the analysis. The result of each element model is improved using higher accuracy analysis tools. The aerodynamic performance of the hypersonic aircraft is examined through hypersonic wind tunnel tests. A thermal management system based on the data of the wind tunnel tests is proposed. A pre-cooled turbojet engine is adopted as the propulsion system for the hypersonic aircraft. The engine can be operated continuously from take-off to Mach 5. This engine uses a pre-cooling cycle using cryogenic liquid hydrogen. The high temperature inlet air of hypersonic flight would be cooled by the same liquid hydrogen used as fuel. The engine is tested under sea level static conditions. The engine is installed on a flight test vehicle. Both liquid hydrogen fuel and gaseous hydrogen fuel are supplied to the engine from a tank and cylinders installed within the vehicle. The designed operation of major components of the engine is confirmed. A large amount of liquid hydrogen is supplied to the pre-cooler in order to make its performance sufficient for Mach 5 flight. Thus, fuel rich combustion is adopted at the afterburner. The experiments are carried out under the conditions that the engine is mounted upon an experimental airframe with both set up either horizontally or vertically. As a result, the operating procedure of the pre-cooled turbojet engine is demonstrated.

전동기 일체형 편로드 유압액추에이터의 기동 및 정지특성해석

전동기 일체형 유압액추에이터(EHA)는 전기모터와 직결되어 구동되는 유압펌프를 이용하여 유압 피스톤의 변위나 속도를 제어할 수 있도록 모듈화된 유압기기로 효율이 높고 독립된 동력원을 갖는 장점으로 인하여 항공기의 전자구동시스템에 적용되어 왔다. 최근에는 다양한 형태의 유압시스템이 적용되고 있는 건설중장비 분야에서 엔진과 전기모터를 결합한 하이브리드 기술이 개발됨에 따라 EHA의 활용범위가 확대될 것으로 예상된다. 본 연구에서는 편로드 유압실린더를 갖는 EHA가 적용된 유압시스템을 대상으로 펌프작동에 따른 유압실린더의 기동 및 정지특성을 고찰하기 위하여 EHA 내부 유압회로의 기능을 파악하고 동특성 모델을 유도하였으며 파일럿작동 체크밸브의 크래킹압력을 분석하여 개방 및 폐쇄특성을 예측하였으며 시뮬레이션을 통하여 분석된 결과를 검증하였다.

Propelling the future - the meaning of ACARE VISION 2050 for the future development of propulsion systems for aircraft

In the year 2000, a group of personalities of aerospace industry, government and research institutes formed the ACARE ("Advisory Committee on Aerospace Research in Europe”) and established the well-known VISION 2020. This vision reflects the targets to reduce the environmental impact of air traffic: reducing both carbon dioxide and noise emissions by half and nitrogen oxide emissions even by 80%. Industry and universitypartners since then jointly defined measures to achieve these challenging goals. Many research programs were implemented funded by national and international bodies, such as the European Union within the Framework program. However, the current status still inhibits further needs to install intense research work.

A Fuel Cell Based Propulsion System for General Aviation Aircraft: The ENFICA-FC Experience

The hydrogen and fuel cell power based technologies that are rapidly emerging can be exploited to start a new generation of propulsion systems for light aircraft and small commuter aircraft. Different studies were undertaken in recent years on fuel cells in aeronautics. Boeing Research & Technology Centre (Madrid) successfully flew its converted Super Dimona in 2008 relying on a fuel cell based system. DLR flew in July 2009 with the motor-glider Antares powered by fuel cells. The goal of the ENFICA-FC project (ENvironmentally Friendly Inter City Aircraft powered by Fuel Cells -European Commission funded project coordinated by Prof.Giulio Romeo) was to develop and validate new concepts of fuel cell based power systems for more/all electric aircrafts belonging to a inter-city segment of the market. As a result of the project, a technology demonstrator was designed, manufactured and tested: a new proton exchange membrane (PEM) fuel cell based power system was installed in the Skyleader RAPID200 that had its demonstrative flights in 2010. One of the most critical, if not the most critical, aspect of the all-electric power system adopted for demonstrator is undoubtedly prediction of fuel cell behaviour, specifically in terms of temperature control since reduced volumes and low weights (needed for the aeronautical applications) are not beneficial from a heat-management point of view. Moreover temperature control of PEM fuel cell is extremely important for a correct employment of the cell itself and, since the fuel cell is the main power generating device on the aircraft (batteries are however present as emergency back-up), temperature control becomes a very important safety issue too. In particular, as the ionic conduction of the polymeric membrane is a function of its degree of humidification, the stack temperature has to be carefully controlled to avoid phenomena of water evaporation causing an increase of ohmic drop and a decrease of stack performances. The output power of the fuel cells system is affected considerably by the change of the stack temperature. The paper will report a description of the research activity that lead to the fuel cell based power system developed for the ENFICA-FC project: the main topics will regard the design phase, including numerical and theoretical modelling (validated by flight tests) of the fuel cell system and its cooling, the results of ground testing and installation and, finally, the results of flight testing