Subsonic Transport Aircraft Research Articles

Comparison of Overwing and Underwing Nacelle Aeropropulsion Optimization for Subsonic Transport Aircraft

This research compares a forward-mounted overwing nacelle configuration to a conventional underwing nacelle for a single-aisle transport aircraft. We focus on characterizing the impact of the overwing placement on cruise fuel burn. To this end, we optimize both configurations under similar constraints and design processes. We use coupled computational fluid dynamics and propulsion cycle analysis with Gaussian process-based adaptive sampling in a multistage design process. The particular overwing design has a 4% higher fuel flow than the underwing design at cruise conditions. However, a postoptimization nacelle placement study illustrates that this fuel burn difference can drop to 2–3% if the overwing nacelle is moved farther forward in our design space without accounting for any resulting structural weight changes.

Application of Noise Certification Regulations within Conceptual Aircraft Design

ICAO Annex 16 regulations are used to certify the acoustic performance of subsonic transport aircraft. Each aircraft is classified according to the measured EPNL levels at specific certification locations along the approach and departure. By simulating this certification process, it becomes possible to identify all relevant parameters and assess promising measures to reduce the noise certification levels in compliance with the underlying ICAO regulations, i.e., allowable operating conditions of the aircraft. Furthermore, simulation is the only way to enable an assessment of novel technology and non-existing vehicle concepts, which is the main motivation behind the presented research activities. Consequently, the ICAO Annex 16 regulations are integrated into an existing noise simulation framework at DLR, and the virtual noise certification of novel aircraft concepts is realized at the conceptual design phase. The predicted certification levels can be directly selected as design objectives in order to realize an advantageous ICAO noise category for a new aircraft design, i.e., simultaneously accounting for the design and the resulting flight performance. A detailed assessment and identification of operational limits and allowable flight procedures for each conceptual aircraft design under consideration is enabled. Sensitivity studies can be performed for the relevant input parameters that influence the predicted noise certification levels. Specific noise sources with a dominating impact on the certification noise levels can be identified, and promising additional low-noise measures can be applied within the conceptual design phase. The overall simulation process is applied to existing vehicles in order to assess the validity of the simulation resultsfcompared to published data. Thereafter, the process is applied to some DLR low-noise aircraft concepts to evaluate their noise certification levels. These results can then be compared to other standard noise metrics that are typically applied in order to describe aircraft noise, e.g., SEL isocontour areas. It can be demonstrated that certain technologies can significantly reduce the noise impact along most of an approach or departure flight track but have only a limited influence on the noise certification levels and vice versa. Finally, an outlook of the ongoing developments is provided, in order to apply the new simulation process to supersonic aircraft. Newly proposed regulations for such concepts are implemented into the process in order to evaluate these new regulations and enable direct comparison with existing regulations.

Utilization of Low-Potential Energy Through to the Use in Aircraft Fuel System

The article discusses the possibility of improving the main characteristics (efficiency, specific thrust) of a gas turbine engine by using liquefied natural gas (LNG) as fuel. This possibility is considered on the example of using as fuel instead of aviation kerosene on PS-90A and NK-93 engines. The parameters of the engine were determined at various stages in the cruise flight mode when operating on kerosene and LNG. The parameters evaluating the fuel consumption of the engine are calculated and compared for both fuels. The performance characteristics for kerosene and LNG were evaluated in the system of the Tu-330 subsonic transport aircraft. Energy losses during storage of cryogenic fuel on board the aircraft are estimated.

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.

Monte Carlo Information-Reuse Approach to Aircraft Conceptual Design Optimization Under Uncertainty

This paper develops a multi-information source formulation for aerospace design under uncertainty problems. As a specific demonstration of the approach, it presents the optimization under uncertainty of an advanced subsonic transport aircraft developed to meet the NASA goals and shows how the multi-information source approach enables practical turnaround time for this conceptual aircraft optimization under uncertainty problem. In the conceptual design phase, there are often uncertainties about future developments of the underlying technologies. An aircraft design that is robust to uncertainty is more likely to meet performance requirements as the technologies mature in the intermediate and detailed design phases, reducing the need for expensive redesigns. In the particular example selected here to present the new approach, the multi-information source approach uses an information-reuse estimator that takes advantage of the correlation of the aircraft model in the design space to reduce the number of model evaluations needed to achieve a given standard error in the Monte Carlo estimates of the relevant design statistics (mean and variance). Another contribution of the paper is to extend the approach to reuse information during trade studies that involve solving multiple optimization under uncertainty problems, enabling the analysis of the risk–performance tradeoff in optimal aircraft designs.

A Comprehensive Robust Adaptive Controller for Gust Load Alleviation

The objective of this paper is the implementation and validation of an adaptive controller for aircraft gust load alleviation. The contribution of this paper is the design of a robust controller that guarantees the reduction of the gust loads, even when the nominal conditions change. Some preliminary results are presented, considering the symmetric aileron deflection as control device. The proposed approach is validated on subsonic transport aircraft for different mass and flight conditions. Moreover, if the controller parameters are tuned for a specific gust model, even if the gust frequency changes, no parameter retuning is required.

Hydrogen fuel tanks for subsonic transport aircraft

Hydrogen is since long seen as an outstanding candidate for an environmentally acceptable, future aviation fuel. Since the first studies, the design of light yet highly insulated tanks for cryogenic liquid hydrogen has been identified as one of the key enabling technologies. Despite this early recognition, the design of the tanks is nowadays still seen as crucial as aircraft tanks differ significantly from existing tanks in the automotive or aerospace sector. To enable system level feasibility studies of hydrogen fueled aircraft, a preliminary design model for aircraft liquid hydrogen tanks is developed for both foam and multilayer insulations. This model is then used to design tanks for a small regional airliner as well as a large long range transport aircraft. Foam based and multilayer insulations are compared and the sensitivity of the tank weight to the fuselage diameter and the mission fuel load is assessed. The influence of a ‘hold’ period before take-off is analyzed too. As the developed model is intended for use in the preliminary aircraft design phase, structural design or attachment issues are not addressed.

Evaluation of Bolt Bearing Behavior of Highly Loaded Composite Joints at Elevated Temperature

The high load intensities and elevated temperature in the wing and fuselage of a supersonic transport aircraft will result in bolted joints stack-up thickness much greater than the current subsonic transport aircraft. Bolt bearing– bypass tests were conducted to develop a database and a bearing–bypass stress interaction for the design community. All test coupons are IM7/PETI-5 quasi-isotropic laminates (64 plies) with a bolt-hole for a 0.95 cm diameter fastener. Static tests were conducted at room temperature as well as at 177 C. Thermo-mechanical fatigue tests were conducted using a typical supersonic transport flight spectrum loading. Short-term creep tests,of 250 h,indicated that IM7/PETI-5 quasi-isotropic (64 plies) laminates has improved properties over IM7/K3B. Long-term creep tests, of 10,000 h, demonstrate that IM7/PETI-5 is highly creep resistant for long periods. Test results also show that these composites experience very little hole elongation before failing in a bearing mode.

Reduction of Uncertainties in Prediction of Wake-Vortex Locations

Lift-generated vortex wakes of subsonic transport aircraft are known to pose a rolling-moment hazard to in-trail following aircraft. Hazardous wake encounters are now avoided by maintaining in-trail separation distances that are larger than needed for other operational considerations. To increase airport capacity, considerable effort has been devoted to the development of techniques and procedures by which vortex wakes can be accurately located as a function of time, so that following aircraft can avoid them with less in-trail spacings. To improve the accuracy with which vortex wakes can be located, a study of the uncertainties associated with the determination of the time-dependent location of vortex wakes and on ways to reduce the uncertainties are reported

Overview of Wake-Vortex Hazards During Cruise

A large amount of research effort has been devoted to the hazard posed by lift-generated vortex wakes of subsonic transport aircraft during approach and departure at airports. In contrast, only a small amount of effort has gone into the corresponding potential hazard at cruise altitudes. Because more frequent encounters with wakes may occur as the density of aircraft increases with time, improved knowledge is needed of the persistence and magnitude of the hazard posed by lift-generated wakes at cruise altitudes. The purpose here is to review and then to extend previous work to characterize properly the hazard posed by vortex wakes at cruise altitudes. Photographs are presented of the various fluid dynamic stages that vortex wakes usually go through as they age. Estimates made of the potential hazard that each stage poses indicate that the rolling-moment hazard during an axial encounter of a vortex wake can be as severe at cruise as for approach at airports; however, as in the airport-approach case, the hazard only persists for several minutes (about 20 n mile) behind the generating aircraft. However, the hazard posed by the downwash in the wake persists for up to tens of minutes (about 200 n mile) behind the generating aircraft

Lift-generated vortex wakes of subsonic transport aircraft

The flow fields of vortices, whether buoyancy-driven or lift-generated, are fascinating fluid-dynamic phenomena which often possess intense swirl velocities and complex time-dependent behavior. As part of the on-going study of vortex behavior, this paper presents a historical overview of research conducted on the structure and modification of the vortices generated by the lifting surfaces of subsonic transport aircraft. Although primarily presented from an experimental point of view, the derivation and use of relatively compact theoretical formulations and concepts are included. Experience with lift-generated wakes has shown that they are so complex that progress requires application of a combined theoretical and experimental research program, because either alone often leads to incorrect conclusions. The primary purpose of the research to be described is to find a way to reduce the hazard potential of lift-generated vortices shed by subsonic transport aircraft in the vicinity of airports during landing and takeoff operations. The information presented points out that the characteristics of lift-generated vortices are related to the aerodynamic shapes that produce them and that various arrangements of surfaces can be used to produce quite different vortex structures. It is concluded that a satisfactory aerodynamic solution to the wake-vortex problem at airports has not yet been found, but a reduction in the impact of the wake-vortex hazard on airport capacity may soon become available through wake-vortex avoidance concepts currently under study.

Aerodynamic design of a natural laminar flow nacelle and the design validation by flight testing

The laminar flow technology is one of the key technologies in aeronautics offering substantial improvements in the areas of economy and ecology. This paper describes the aerodynamic design methodology for a natural laminar flow (NLF) nacelle and the subsequent verification of the design quality by flight tests with a subsonic transport aircraft. The aerodynamic design was a contribution within the framework of a European collaborative programme to flight test a natural laminar flow nacelle. The participants in this programme were the aero-engine manufacturers Rolls-Royce and MTU as well as the Institute for Design Aerodynamics of the DLR. The nacelle manufacture was undertaken by Hurel-Dubois (UK) Ltd. and some components of instrumentation were provided and operated by the University of Oxford. The flight trials, apart from having the goal of validating the aerodynamic design of the nacelle, had the major objective of obtaining realistic experience with the operational aspects of the NLF nacelle. In the first series of flight trials, which began in September 1992, the flight test results confirmed the suitability of the aerodynamic design tools used.

Rear Fuselage Transonic Flow Characteristics for a Complete Wing-Body Configuration

The study presents results for experiments conducted in the transonic pressurized T2 wind tunnel of Centre d'Etudes et de Recherches de Toulouse. The 1/80th scale model of a modern subsonic transport aircraft was mounted in the test section using a fin-sting. It consisted of a fuselage, a detachable horizontal rear stabilizer, a belly fairing, and tip-truncated wings. The tip-truncated wings were designed by Aerospatiale to reproduce the correct downwash in the rear fuselage region. Tests were conducted at a freestream Mach number of 0.82 and a Reynolds number evaluated with the model chord length of approximately 2.5 x 10 6 . Various measurements were performed, mainly in the vicinity of the fuselage downstream rear part: oil-flow visualizations, pressure distributions, boundary-layer surveys along the fuselage symmetry lines and some lateral lines using a laser Doppler anemometry system, and near-wake surveys with both pressure and velocity measurements. These experiments were designed to allow a fine description of the flows developing along the rear part of a fuselage in a configuration similar to actual flight applications.

Box and Gaussian plume models of the exhaust composition evolution of subsonic transport aircraft in- and out of the flight corridor

Abstract. A box and a Gaussian plume model including gas-phase photochemistry and with plume dispersion parameters estimated from the few available plume observations are proposed and used for evaluation of photochemical transformations of exhausts from a single subsonic transport aircraft. The effects of concentration inhomogeneities in the plume cross section on the photochemical sources and sinks in the plume are analyzed for various groups of compounds. The influence of these inhomogeneities on the rate and on the mass of ambient air entrainment into the plume are studied also by comparing the box and the Gaussian plume model simulations during the first hours of their "life''. Due to the enterance of HOx and NOx from ambient air into the plume with rates varying from the wind shear and turbulence conditions, the rate of emitted NOx oxidation in the plume is dependent on these and also on the background concentration levels of HOx and NOx.

Box and Gaussian plume models of the exhaust composition evolution of subsonic transport aircraft in- and out of the flight corridor

A box and a Gaussian plume model including gas-phase photochemistry and with plume dispersion parameters estimated from the few available plume observations are proposed and used for evaluation of photochemical transformations of exhausts from a single subsonic transport aircraft. The effects of concentration inhomogeneities in the plume cross section on the photochemical sources and sinks in the plume are analyzed for various groups of compounds. The influence of these inhomogeneities on the rate and on the mass of ambient air entrainment into the plume are studied also by comparing the box and the Gaussian plume model simulations during the first hours of their "life''. Due to the enterance of HOx and NOx from ambient air into the plume with rates varying from the wind shear and turbulence conditions, the rate of emitted NOx oxidation in the plume is dependent on these and also on the background concentration levels of HOx and NOx.

Development of semispan model test techniques

A computational investigation was performed to support the development of a semispan model test capability in the NASA Langley Research Center's National Transonic Facility. This capability is desirable for the testing of advanced subsonic transport aircraft at full-scale Reynolds numbers. A state-of-the-art three-dimensional Navier-Stokes solver was used to examine methods to improve the flow over a semi-span configuration. First, a parametric study is conducted to examine the influence of the stand-off height on the flow over the semispan model. It is found that decreasing the stand-off height, below the maximum fuselage radius, improves the aerodynamic characteristics of the semi-span model. Next, active sidewall boundary layer control techniques are examined. Juncture region blowing jets, upstream tangential blowing, and sidewall suction are found to improve the flow over the aft portion of the semispan model. Both upstream blowing and suction are found to reduce the sidewall boundary layer separation. The resulting near surface streamline patterns are improved, and found to be quite similar to the full-span results. Both techniques however adversely affect the pitching moment coefficient.

Validation of vortex-lattice method for loads on wings in lift-generated wakes

A study is described that evaluates the accuracy of vortex-lattice methods when they are used to compute the loads induced on aircraft as they encounter lift-generated wakes. The evaluation is accomplished by the use of measurements made in the 80 by 120 ft Wind Tunnel of the lift, rolling moment, and downwash in the wake of three configurations of a model of a subsonic transport aircraft. The downwash measurements are used as input for a vortex-lattice code in order to compute the lift and rolling moment induced on wings that have a span of 0.186, 0.510, or 1.022 times the span of the wake-generating model. Comparison of the computed results with the measured lift and rolling-moment distributions the vortex-lattice method is very reliable as long as the span of the encountering or following wing is less than about 0.2 of the generator span. As the span of the following wing increases above 0.2, the vortex-lattice method continues to correctly predict the trends and nature of the induced loads, but it overpredicts the magnitude of the loads by increasing amounts.

Analysis of an Optimal Flight Path of Subsonic Transport Aircraft for Fuel Saving

Analysis of an Optimal Flight Path of Subsonic Transport Aircraft for Fuel Saving

Application of circulation control to advanced subsonic transport aircraft. Part II - Transport application

An experimental/ analytical research program was undertaken to develop advanced versions of circulation control wing (CCW) airfoils and to address specific issues related to the application of these blown high-lift devices to subsonic transport aircraft. The primary goal was to determine the feasibility and potential of these pneumatic configurations to increase high-lift system performance in the terminal area while reducing system complexity and aircraft noise. A four-phase program was completed, including 1) experimental development and evaluation of advanced CCW high-lift configurations; 2) development of effective pneumatic leading-edge devices; 3) computational evaluation of CCW airfoil designs plus high-lift and cruise capabilities; and 4) the investigation of the terminal-area performance of transport aircraft employing these airfoils. The first three phases were presented in Part I of this article. This segment, Part II, describes the fourth phase of the program. Experimental lift coefficient values approaching 8.0 at zero incidence were demonstrated by two-dimension al CCW configurations and were reported in Part I. These were used to predict 70-80% reductions in takeoff and landing distances for a three-dimensional advanced subsonic transport configuration employing a simplified pneumatic high-lift system. These results and the methodology used to obtain them will be presented in greater detail in the following discussions.

Application of Circulation Control to Advanced Subsonic Transport Aircraft, Part I - Airfoil Development

An experimental/ analytical research program was undertaken to develop advanced versions of circulation control wing (CCW) blown high-lift airfoils, and to address specific issues related to their application to subsonic transport aircraft. The primary goal was to determine the feasibility and potential of these pneumatic airfoils to increase high-lift system performance in the terminal area while reducing system complexity. A four-phase program was completed, including 1) experimental development and evaluation of advanced CCW high-lift configurations, 2) development of effective pneumatic leading-edge devices, 3) computational evaluation of CCW airfoil designs plus high-lift and cruise capabilities, and 4) investigation of the terminal-area performance of transport aircraft employing these airfoils. The first three phases of this program are described in Part I of this article. Applications to the high-lift and control systems of advanced subsonic transport aircraft and resulting performance are discussed in the continuation of this article, Part II. Experimental lift coefficient values approaching 8.0 at zero incidence and low blowing rates were demonstrated by two-dimensional CCW configurations that promised minimal degradation of the airfoil's performance during cruise. These results and experimental/CFD methods will be presented in greater detail in the following discussions.