-> AERONAUTICS AND SPACE FLIGHT
(Total: 88 journals)
- Correction of aerodynamic influence matrices for transonic flow
- Abstract: Abstract
The authors present a novel correction approach for the doublet-lattice method in this paper. The doublet-lattice method is a standard tool for calculating unsteady aerodynamic loads in aeroelasticity. It solves the linear potential equations and is thus valid only at subsonic flow conditions. Hence, corrections have to be applied for transonic flow. The proposed correction method, CREAM (CorREction of Aerodynamic Matrices), uses surface pressure distributions obtained using computational fluid dynamics (CFD) simulations for the correction. It is based on a Taylor expansion of the aerodynamic influence coefficient matrix, where the Taylor coefficients are corrected successively. The approach can be applied to quasi-steady as well as to unsteady aerodynamic calculations. The method is demonstrated on the AGARD LANN wing at transonic attached flow conditions and compared to linearized unsteady CFD computations. Two different correction orders are examined: a “zeroth order correction” with a quasi-steady CFD sample as correction input and a “first order correction” with an additional unsteady CFD sample. It is shown that CREAM gives improved results for small reduced frequencies, where the first-order correction is always superior to the zeroth order correction.
- Vortical flow prediction for the design of a wind tunnel experiment with a
pitching lambda wing
- Abstract: Abstract
The IWEX wind tunnel model (Instationäres Wirbelexperiment, German for: Unsteady Vortex Experiment) was developed to study vortical flow at static and oscillating angles of attack. Numerical computations were performed beforehand to study important aerodynamic aspects. The focus of this paper is on those results that were affecting the design of the model. The experimental concept of a new test rig, wind tunnel walls and the new half wing model is briefly described. The lambda wing has a purely round leading edge with a constant nose radius to chord ratio of 0.5 %. The flow characteristics of the main vortex, generated at the nose of the main wing, and the smaller tip vortex are specified. The inboard motion of the main vortex with increasing angle of attack and the consequences for the load distribution are described. The free stream Mach number range of the test envelope is from 0.3 to 0.7. Therefore, transonic effects, especially shock–vortex interactions triggering vortex development, had to be analyzed for a safe design of the model. A peniche was designed to minimize the differences of the flow character compared to reference results of the model without wind tunnel walls. The effects of corner separation and displacement are specified at different stages of the iterative process. Finally, basic results for a pitching motion are discussed, based on global and distributed coefficients.
- Model order reduction for steady aerodynamics of high-lift configurations
- Abstract: Abstract
In aerodynamic applications, many model reduction methods use proper orthogonal decomposition (POD). In this work, a POD-based method, called missing point estimation (MPE), is modified and applied to steady-state flows with variation of the angle of attack. The main idea of MPE is to select a subset of the computational grid points (control volumes) and to limit the governing equations to this subset. Subsequently, the limited equations are projected onto the POD subspace. This approach has the advantage that the nonlinear right-hand side of the governing equations has to be evaluated only for a small number of points (control volumes) in contrast to POD, for which the full right-hand side has to be evaluated. An error estimation for MPE in the continuous ODE setting is tackled. Numerical results are presented for the Navier–Stokes equations for two different industrially relevant, two-element high-lift airfoils, one which is normally adopted during landing and the other during take-off.
- Semi-empirical modeling of fuselage–rotor interference for
comprehensive codes: the fundamental model
- Abstract: Abstract
The flow field around the isolated HART II fuselage is computed by a computational fluid dynamics code. Velocities normal to the rotor rotational plane are extracted in a volume around the rotor as a data basis. A simple semi-empirical analytical formulation of the fuselage-induced velocities, based on parameter identification from computational fluid dynamics or measured data, is developed for use in comprehensive rotor codes. This model allows the computation of fuselage–rotor interferences on the rotor blade element level. It also allows the prediction of the rotor wake geometry deformation due to the presence of the fuselage in both prescribed wake and free-wake codes. Its impact on rotor thrust, power and trim is evaluated analytically using blade element momentum theory and by DLR’s comprehensive rotor code.
- Investigation and analysis of deterioration in high pressure compressors
due to operation
- Abstract: Abstract
Due to environmental and operational effects, the deterioration of all gas path-related jet engine components is a highly influencing parameter leading to an increase in exhaust gas temperature and specific fuel consumption over time. As a particularly strained engine module, the high-pressure compressor (HPC) is prone to a variety of abrasive and deforming effects that are responsible for a considerable part of overall engine performance losses. During HPC overhaul, new airfoils are typically mixed with reused parts whose refurbishment workscopes typically range from “passed inspection” up to “full leading and/or trailing edge restoration” as well as “tip weld repair”. Hence, a wide spread of airfoil geometries with a distinct statistical distribution can be found within an engine after operation. To allow a statistical analysis, a geometric survey on two full ex-service HPC blade sets and an assortment of equivalent new production parts was conducted. The blades were digitalized by a structured-light 3D scanner in conjunction with a photogrammetry system. Based on the measured three-dimensional data, a CAD model was generated for each blade. Afterwards, airfoil sections on different blade heights were analyzed to generate aerodynamic design parameters such as leading and trailing edge geometries (radius and thickness), their maximum profile thickness, their chord length and their stagger angle. By assessing the statistical results of both used and new parts, the effects of manufacture variations and in-service wear on HPC blade geometry can be compared in detail.
- Development of a wind tunnel experiment for vortex dominated flow at a
pitching Lambda wing
- Abstract: A half wing model, a test rig and new wind tunnel walls were designed to study the vortex development at a lambda wing. The model has a sweep angle of 53° and a round leading edge. It is designed for pitching oscillations around a mean angle of attack of up to 20° up to a free stream Mach number of 0.7. Unsteady aerodynamic load data shall be delivered for aeroelastic simulations of Unmanned Combat Aerial Vehicles. Due to the highly nonlinear aerodynamic character, the design and sizing of the model had to take into account load cases with beginning and fully developed vortices. Furthermore, the different characters at subsonic and transonic speeds had to be included. Coupled simulations with a finite element model including the mounting and the connection to the actuation system were performed to assess the stability and the dynamic response of the model. Furthermore, the test concept and the process of the design of the model will be described.
- SFB 880: aeroacoustic research for low noise take-off and landing
- Abstract: This paper gives an overview about prediction capabilities and the development of noise reduction technologies appropriate to reduce high lift noise and propeller noise radiation for future low noise transport aircraft with short take-off and landing capabilities. The work is embedded in the collaborative research centre SFB 880 in Braunschweig, Germany. Results are presented from all the acoustics related projects of SFB 880 which cover the aeroacoustic simulation of the effect of flow permeable materials, the characterization, development, manufacturing and operation of (porous) materials especially tailored to aeroacoustics, new propeller arrangements for minimum exterior noise due to acoustic shielding as well as the prediction of vibration excitation of aircraft structures, reduced by porous materials.
- Comparison of real-time flight loads estimation methods
- Abstract: Nowadays flight load exceedance monitoring is an important task to both: the aircraft manufacturer as well as the operator. The estimation of flight loads is required in several phases during development and operation of an aircraft. The requirements are usually different, for e.g. calculation of design loads for certification and operational loads monitoring of stress and fatigue. The ability to determine aircraft operational loads (more) precisely may reduce the time in maintenance. Being able to detect critical load exceedance events during flight or in a post-process is also an enabler for e.g. loads/fatigue monitoring at operator level. In this paper, a novel system identification method named local model networks is applied to the field of flight loads estimation and compared to approaches based on artificial neural networks as found in the literature. The presented approach tries to overcome some limitations with respect to model creation, robustness, inter- and extrapolation.
- Revised approach procedures to support optimal descents into Malta
- Abstract: This paper presents a proposal for new approach procedures for the most commonly used runways at Malta International Airport, runways 31 and 13, as a basis to facilitate the introduction of optimal descents in Maltese airspace. In addition, a standard arrival, one from the north from where the large majority of aircraft approach Malta, linking to runway 31 is proposed. The design, following ICAO recommendations, is described in detail. Current practice of aircraft inbound from the same entry point, observed using ADS-B recorded data, is also presented in a discussion leading to the identification of gains that could result from flying the proposed arrival and approach routes optimally.
- Design and aeroelastic assessment of a forward-swept wing aircraft
- Abstract: Aeroelastic effects strongly influence the design of an aircraft. To be able to assess those effects early on, reliable simulation models representing the global aeroelastic properties of a new design are required. At a conceptual or a pre-design stage, an intelligent parameterization concept allows for limited changes of the configuration while the simulation models are adapted accordingly. In the DLR project Integrated Green Aircraft, the goal was to investigate the impact of technologies for the reduction of fuel consumption on the aeroelastic properties of aircraft. One main aspect was the influence of laminar wing design on divergence, flutter and dynamic loads. As the reference aircraft in the project, the concept of a forward-swept wing aircraft with rear-mounted engines has been analysed. An aeroelastic model has been built up in the project. The model design procedure is based on the DLR in-house tool set MONA (ModGen/NASTRAN). Focus of this design process is the generation of a parameterized structural model, representing the global dynamic properties of the elastic aircraft, but as detailed as reasonable to capture relevant local effects and to result in a feasible structural design. In the article, the aircraft design is presented. The modelling and sizing process for the structure is described. Results of the loads analysis as well as of the aeroelastic stability analyses are discussed.
- Assessment of leading-edge devices for stall delay on an airfoil with
active circulation control
- Abstract: The use of active, internally blown high-lift flaps causes the reduction of the stall angle of attack, because of the strong suction peak generated at the leading-edge. This problem is usually addressed by employing movable leading-edge devices, which improve the pressure distribution, increase the stall angle of attack, and also enhance the maximum lift coefficient. Classical leading-edge devices are the hinged droop nose or the more effective slat with a gap. The flow distortions generated by the gap become an important source of noise during approach and landing phases. Based on these considerations, the present work aims at evaluating the potentials of gap-less droop nose devices designed for improving the aerodynamics of airfoils with active high lift. Both conventional leading-edge flaps and flexible droop noses are investigated. Flexible droop nose configurations are obtained by smoothly morphing the baseline leading-edge shape. Increasing the stall angle of attack and reducing the power required by the active high-lift system are the main objectives. The sensitivities of the investigated geometries are described, as well as the physical phenomena that rule the aerodynamic performance. The most promising droop nose configurations are compared with a conventional slat device as well as with the clean leading-edge. The response of the different configurations to different blowing rates and angles of attack are compared and the stalling mechanisms are analyzed.
- ALLFlight: blob-based approach to detect dangerous drift velocities during
helicopter landing approaches
- Abstract: The Institute of Flight Systems at the German Aerospace Center (DLR) site in Braunschweig is dedicating much effort to the goal of making helicopter flying safe. This paper concentrates on decreasing the danger of accidents caused by so-called “dynamic rollovers” which may result from undetected lateral velocities during the landing approach. Especially in degraded visual environment situations where pilots cannot visually evaluate the horizontal movement, accidents are likely to occur. While under normal circumstances the combination of INS and GPS is usually sufficient to detect critical lateral drift velocities, during the landing process, the INS drift may reach a dangerously high value quite soon after the GPS signal has been lost (for instance, in canyons, cities, or due to jamming). The vision algorithm that is proposed here uses a blob-based approach to solve this problem. Blobs are regions in an image that fit together bound by a specific criterion. In this paper, we will describe how an image is segmented, possible blobs are selected and characterized, how they are tracked, and how the velocity of the helicopter is calculated. Finally, the accuracy of this algorithm will be analyzed using data that have been recorded from flight tests conducted by the DLR’s flying helicopter simulator EC135 (ACT/FHS).
- Joined-wing test bed UAV
- Abstract: The future green aircraft will be required to meet demanding constraints, including weight reduction, high energy and aerodynamic efficiencies and high performance, to be compliant with pollutant emissions and noise generation regulations. The joined-wing concept is considered a trade-off variant for a green aircraft design with a lower cruise drag and lower structural weight. In addition, the requirements for low pollution and noise could be met using an all-electric aircraft. Hence, the aim of the present study is to design and produce a joined-wing unmanned aircraft test bed or flight laboratory. The basic design incorporates tip-joined front and rear wings with wing-tip vertical joints. The airframe is mainly composed of carbon and glass fibre composite materials. The power plant consists of an electric ducted fan, speed controller and Li-Po batteries. The aircraft integrates the Piccolo II Flight Management System, which offers a state-of-the-art navigation and flight data acquisition. Prior to production and flight testing of the prototype, aircraft aerodynamics and flight dynamics were analysed. Potential models with wind tunnel tests have been used to determine aircraft aerodynamics. One of the major problems found during simulation and flight experiments is the Dutch roll effect. This is thoroughly discussed in the paper. Some problems that concern autopilot tuning are also described.
- POD approach for unsteady aerodynamic model updating
- Abstract: A method for aerodynamic model updating is proposed in this paper. The approach is based upon a correction of the eigenvalues of the reduced-order unsteady aerodynamic matrix through an optimization with objective function defined through the difference in the generalized aerodynamic forces or on the aeroelastic poles. The high-fidelity model in reduced-order form is obtained by the proper orthogonal decomposition (POD) technique applied to the computational fluid dynamics Euler-based formulation. Many of the methods that have been developed in the past years for simpler aeroelastic models that use, for example, doublet-lattice method aerodynamics, can be adopted for this purpose as well. However, this model is not able to capture shocks and flow separation in transonic flow. The proposed approach performs the updating of the aerodynamic model by imposing the minimization of a global error between target aerodynamic performances, namely experimental performances, and an aerodynamic model in reduced-order form via POD approach. After a general presentation of the application of the POD method to the linearized Euler equations, the optimization strategy is presented. First, a simple test on a 2D wing section with theoretical biased data is performed, and then, the performances of different optimization strategies are tested on a 3D model updated by wind tunnel data.
- Analysis of trimmable conditions for a civil aircraft with active
- Abstract: Abstract
This paper outlines recent flight dynamic analysis results of a civil aircraft with active high-lift system using blown Coandă flaps. The main focus lies in the trim analysis of the aircraft. Therefore, the basic structure and core elements of the nonlinear model, describing the dynamic behavior of an aircraft with this specific type of active high-lift system are presented. The center of gravity range allowing controllability and static stability of the aircraft is determined, and the resulting characteristics of the aerodynamic model and their impact on the trim results of the aircraft are analyzed. The results show specific flight dynamic difficulties related to the active high-lift system, namely in the flying characteristics necessary for safe take-off and approach procedures. The flight physics is explained and discussed. The necessity of the application of a wing leading edge device is outlined by preliminary studies and further remedial means are proposed.
- A predictive envelope protection system using linear, parameter-varying
- Abstract: Abstract
A parameter-varying, model-predictive envelope protection system is developed simplifying the controller structures required to keep the aircraft within a safe angle-of-attack and normal load factor envelope. The idea of a quasi-steady flight condition is used to map the flight envelope limits onto the setpoint values of a single flight control law. Since no mode switching is required, the selected level of automation, i.e., autopilot and flight management functionalities, is independent from the proximity to any angle-of-attack and normal load factor limit. In contrast to previous approaches, the proposed algorithm makes use of a quasi-linear, parameter-varying control loop model to adapt to the true nonlinear aircraft behavior. A variance-based sensitivity analysis highlights the most significant scheduling variables within this control loop model and, therefore, indicates the option of model reduction and improvement in efficiency, respectively. The proposed envelope protection system is evaluated throughout virtual flight tests with the unmanned flight test platform ULTRA-Dimona showing promising overall performance also in the presence of wind and turbulence.
- Alternative fuels in aviation
- Abstract: Abstract
During the last years, the aviation sector has been looking into alternatives to kerosene from crude oil, to combat climate change by reduction of greenhouse gas (GHG) emissions and to ensure security of supply at affordable prices. The efforts are also a reaction to commitments and policy packages. Currently, a wide range of possible fuel candidates and fuel blends are discussed in the triple feedstock, process, and product. Any (synthetic) aviation fuel must be certified; hence, a profound knowledge on its properties, in particular thermophysical and chemical, is inevitable. In the present paper, an overview is given on alternative jet fuels, looking into the short-term and long-term perspective. Examples focusing on experimental and modeling work of combustion properties of existing—coal to liquid, gas to liquid (GtL)—and possible alternative fuels—GtL + 20 % 1-hexanol, GtL + 50 % naphthenic cut—are presented. Ignition delay times and laminar flame speeds were measured for different alternative aviation fuels over a range of temperatures, pressures, and fuel–air ratios. The data are used for the validation of a detailed chemical reaction mechanism following the concept of a surrogate. Such validated reaction models able to describe and to predict reliably important combustion properties of jet fuels are needed to further promote the development of even more sophisticated jet engines and to optimize synthetic jet fuel mixtures in practical combustors.
- Aerodynamic power and mechanical efficiency of bat airframes using a
- Abstract: Abstract
Previous bat aerodynamic power models are refined by (1) varying the value of wing profile drag with lift coefficient, which varies with both flight speed and Reynolds number, (2) allowing for the aerodynamic cleanliness of head, body, ears and tail in calculating parasitic drag values at various speeds and according to airframe type, (3) incorporating models of wingbeat amplitude and frequency in the power calculations, and, (4) upgrading the allometric, phylogenetically corrected relationship between basal metabolic rate and body mass using data from 98 bat species. The fidelity of the aerodynamic power model is assessed using published wind tunnel data on a bat in steady glide. By comparing empirical published metabolic power (P
met) values with values derived using the new aerodynamic model, we update estimates of in-flight musculoskeletal mechanical efficiency (η) for the airframes of eight bat species at steady level flight speeds. Furthermore, we calculate the increase in η at high speeds. The bats assessed range in body mass from 0.01 to 1 kg, and the comparison covers the speed range normally used by free-flying bats during their excursions. At their best endurance flight speeds (V
end), η = 1.52 Ln (m
bat) + 11.44 (%). At speeds > V
end, η = η@
Vend + 1.3 (V−V
end) (%). These equations yield accurate P
met estimates for flight speeds within the range used for the steady level flight.
- Comparing explicit and implicit modeling of rotor flapping dynamics for
the EC 135
- Abstract: Abstract
To be valid up to higher frequencies, state space models derived by system identification methods have to include rotor states describing the dynamics of flapping and sometimes even downwash and regressive lag motion. In the case of flapping, the rigid body equations are usually augmented by the explicit differential equations for the longitudinal and lateral flapping angles which are then coupled to the rigid body equations through the corresponding force and moment terms. Alternatively, an implicit formulation can be used that includes the pitch and roll acceleration as model states. The paper compares the two modeling approaches for the EC 135 helicopter. For this helicopter, the formulation with flapping angles in its classical hybrid form shows some higher order deficiency in the match of the longitudinal and lateral accelerations, which are not apparent in the implicit model. After ruling out other effects, the discrepancies can be attributed to a vertical offset of the center of gravity from its assumed position. Once this offset is accounted for in the data, results from the two modeling approaches can be compared to each other and to theoretical predictions.
- Hans-Peter Kreplin, retired