ICA B "Flight Physics and Vehicle Systems"

The Junior Research Group (JRG) “Flow Physics of Laminar Wing and Fuselage” main focus is to obtain laminar flow over wing airfoil by applying suction based system. Based on conceptual design studies, flow laminarization of the wing, fuselage and empennage by Boundary Layer (BL) suction offers large gains in overall aircraft efficiency. Leveraging these gains for future aircraft calls for new knowledge on laminar flow control design of a 3D wing and fuselage and required a multi-disciplinary research. Experimental wind tunnel and fluid bench testing incorporated with numerical simulation efforts are required to translate the acquired knowledge into empirical based ROMs to inform the external and internal design of the suction system within the structural and manufacturing limitations. In the first phase of this research, the new knowledge of suction-BL will be applied for a short-range and medium-range aircraft. Ultimately, the numerical and empirical approach used for suction-based on finite wing will be extended for suction on the fuselage.

JRG-B1

Aircraft wings are subject to dynamic loads caused by unsteady gusts and flight maneuvers, which reduce passenger comfort and induce structural wing deformations that are typically countered by sturdier and, consequently, heavier wing designs. This project will investigate dedicated flow actuators as part of novel Active Load Reduction systems that aim to alleviate dynamic wing loads during unsteady maneuvers or gust encounters by altering the lift distribution over the entire wingspan, potentially enabling lighter wing designs. The approach is to conduct unsteady flow simulations of flow control on 2D wing segments and 3D wings of finite span for a range of actuation and geometric parameters, and for both subsonic and transonic flow conditions. The simulation results will be validated by wind tunnel experiments on representative 2D and 3D wing models in wind tunnels at the TU Braunschweig and DLR Braunschweig. Based on the acquired flow field and load data, Reduced Order Models (ROMs) will be derived that represent the actuator functionality depending on wing operating conditions, actuation parameters, and actuator geometry. These ROMs will be made available for the load alleviation system comparison in project ICA-2.3 and overall aircraft design process in ICA-B5.1.

JRG-B2

In aviation, a significant reduction of fuel consumption and emission requires new, more revolutionary concepts. A large number of propellers along the wing, called distributed propulsion (DP), is such a concept. Due to the distributed propeller slipstreams, the flow over the wing is enhanced compared to an unblown wing, resulting in a higher lift. Therefore, less wing span and area is necessary during take-off and landing. Thus, the wings can be optimised for cruise flight, giving leverage for less wing drag and weight. So far, electric DP was only investigated for small aircrafts of the general aviation in a low-speed range. Therefore, the next step of research should be the investigation of the feasibility of DP for a short-haul aircraft with 30 – 40 tons MTOW in cruise flight conditions. This is the objective of the project B1.1 „Propeller and wing aerodynamics of distributed propulsion„.

To achieve this, a parametrical study of the DP and wing configuration will be conducted with steady and unsteady simulations. First, an automated process for the fast assessment of a high number of configurations will be developed. The propellers will be modelled with actuator disks and varied regarding their radius, rotational speed, number and positioning relative to the wing. Additionally, high fidelity simulations with full 3D blades will be performed to examine the interactions of propeller and wing as well as between the propellers themselves. Finally, the applicability of DP for commercial short-haul aircrafts will be evaluated for different operating points and the most promising DP configurations will be identified. Consequently, this research project shall provide the aerodynamic basis for the application of DP in commercial short-haul aircrafts.

B1.1

To reduce the carbon footprint of the future air transport system, it is inevitable to radically improve the aircraft and propulsion efficiency. This should not only reduce the fuel consumption but also enable the usage of new game-changing technologies (i.e. electric engines). For significantly reducing the total energy demand of the aircraft, new approaches have to be taken account of. Boundary layer ingestion (BLI) is considered to be such a promising method to improve the efficiency of future aircraft. By increasing the momentum of the aircraft boundary layer flow the propulsion efficiency can increase and thereby save propulsive energy. Due to the close coupling of BLI to other aircraft drag reduction measures like aircraft active laminar flow control (LFC) on both, wing and aircraft body, both effects has to be assessed in parallel for maximising the overall aircraft benefit. As this directly effects the propagation of the airframe boundary layer sucked into the embedded propulsion system, obviously, an interaction has to be considered. On the other hand BLI-based power saving does only work if the propulsor itself, i.e., the fan, will not be adversely affected be the incoming inhomogeneous flow. Therefore, detailed integrated studies of BLI will help to achieve the targets. The proposed research project covers investigations with analytical tools and detailed high-fidelity numerical simulations of asymmetric BLI configurations. The synergies of BLI and LFC are mainly examined with an enhanced parallel compressor method in order to identify an optimum configuration. State-of-the-art RANS simulations are carried out to analyse the sensitivities of boundary layer ingestion on the aircraft and the propulsion side of a blended wing body (BWB) aircraft configuration with a rear mounted propulsor array.
Due to the unsteady interaction of the propulsor with a local incoming BLI flow field, asymmetric BLI represents also a new noise source on propulsion and aircraft level. Hence, URANS simulations of selected configurations will also generate high-fidelity flow field data for acoustic analysis within SE²A.

B1.2

With an ever more increasing integration of propulsor and airframe at modern and future aircraft, new sources of sound occur, which can neither be classically categorized into engine noise nor airframe noise. Instead, a new category of sound source occurs, which is due to the fact, that two or more aircraft components are in mutual aerodynamics interaction. These sources are named “installation sources”, which occur only as a result of installing components at an aircraft and which would not be present, when considering these components in isolation. For instance, if a propeller is installed as a pusher configuration downstream of a wing, the wake of the wing produces unsteadiness on the propeller blade loading and thus additional excess noise with quite different characteristics as propeller-alone sound. This type of sound source is characteristic for all aircraft configurations considered in SE2A. In view EU’s “Flightpath 2050” goal for an environmentally more friendly future aviation it is mandatory to assess the noise impact of any future technology and aircraft concepts proposed to reach the aggressive objectives of the EU. Such aircraft configurations are characterized by an unconventional propulsion integration, no matter if for a short, medium, or long range mission. A new, necessarily non-empiric prediction approach is proposed for the quantification of sound generated as a result of the integration of propulsors (propeller, fan) at the aircraft. Since –by nature- empirical noise models do not exist for new engine integrations, the challenge to overcome is to enable predictions for largely arbitrary (tight) arrangements of propulsor and airframe, while being fast enough to cover a respective design space. The proposed prediction concept therefore rests upon a non-empiric approach in the sense that modelling is restricted to mostly universal features of fluid mechanics and acoustics. This means turbulence modelling in the sense of a RANS approach to aerodynamics, as well as actuator disk [5] approaches to model the effect of a propulsor aerodynamically. Moreover, by representing aerodynamic sound sources at rotor blades by respective (unsteady) forces the source wise installation as well as radiation wise installation may be described appropriately. In that way the modelling is configuration independent and the working hypothesis of the proposal is that with this aeroacoustic prediction approach the excess noise of arbitrary propulsor integration may be quantified.

B1.3

The present research proposal contributes to the SE2A objectives by filling a methodological gap that exists in physics-based prediction of laminar-turbulent transition and the design of hybrid laminar flow control concepts for the fully three-dimensional (3D) boundary layers on blended-wing-body (BWB) configurations. The (direct) 3D parabolized stability equations (PSE-3D) will be solved for efficient but still physics-based transition prediction. The necessary know-how in the application of this transition prediction tool for fully 3D boundary layers will be gathered and a suitable transition prediction strategy will be developed and validated. This direct PSE-3D approach will be combined with a newly developed solver for the corresponding adjoint PSE-3D and integrated together with a direct and adjoint flow solver into an efficient gradient-based optimization tool chain for laminar drag reduction on BWB configurations by adjusting its 3D shape and optimizing the 3D suction distribution on its surface.

B1.4

Due to the aeroelastic coupling between aerodynamics and structure, static and dynamic loads of an aircraft are determined by the elastic properties of its primary components. The vibrations and the resulting dynamic loads depend primarily on the damping level of the wing structure but also on the damping levels of components like e.g. the horizontal and vertical stabilizer, which are characterized by similar mechanical design as compared to wing structures. Current structural design of wings and stabilizers features a linear deformation behavior and a very low structural damping. In ICA B-2, the potential of different candidate technologies for load reduction will be investigated. These candidate technologies adressed in ICA B-2 shall contribute to a reduction of the sizing loads on the overall aircraft structure. The ideal combination of these load reduction approaches in an optimization process depends on the selected SE²A target reference aircraft configuration and on the physical sources of the dimensioning loads (i.e. maneuver and gust loads, ground loads including landing impact or even crash) for each of its structural components. The potential of a structural design concept with tailored nonlinear stiffness will be evaluated in this proposal with the intention to reduce structural loads in comparison to conventional linear structures. This can be achieved e.g. by a shift of the lift distribution during maneuver or gust encounter further inboard. Deformation dependent nonlinear stiffness design of wing structures is a key enabler for this approach to load alleviation. Next to nonlinear stiffness design, the potential of additional damping elements in wing or stabilizer structures will be evaluated in view of dynamic load reduction with the objective of overall aircraft weight reduction. One candidate damping technology will address tailored damping from designated friction dampers or nonlinear vibration absorbers requiring nonlinear normal mode analysis and efficient nonlinear transient response analysis. Another candidate damping technology is based on the incorporation of materials with strong inherent material damping in the structural design of wings and stabilizer structures. In particular, an analytical approach for designing structural damping levels by introduction of a number of local metal-elastomer-laminate dampers at optimized positions will be addressed.

B2.1

Currently, load alleviation is commonly realized by developing new control laws and new moveable layouts for aircraft wings. However, the potential of structural technologies is neglected. Within this project, concrete structural designs and technologies will be developed to enable passive as well as active load alleviation for new aircraft wings. For passive approaches, tailoring technologies will be structurally investigated. Furthermore, the potential of CFRP structures in a post-buckling regime for load alleviation will be investigated. Both approaches can significantly enhance the application area of composites in aerospace. In addition, a structural design for active load control will be developed. This new design will enable the improved moveable layouts and control laws. The assumptions of the potential will be validated using a functional demonstrator. On aircraft level, the benefits in terms of load alleviation, and therefore mass reduction, will be determined.

B2.2

This project aims at significantly reducing gust and manoeuvre loads down to a level equivalent to steady 1 to 1.5g flight, which implies that the aircraft needs to actively alleviate loads at a high level of reliability at all time and all flight conditions. In order to enable wing structural sizing to be based on the load level as close as possible to the loads experienced during 1g steady flight, with the most efficient lift distribution, various new technologies need to be developed and maturated. These technologies can be passive (e.g. aeroelastic tailoring) or active (e.g. gust and manoeuvre load alleviation functions). With the drastically reduced weight, the wing is expected to be much more flexible which raises the need of having multiple actuators along the wing. Based on the aircraft configuration, such actuators could be standard actuators such as ailerons, flaps, spoilers, but for future advanced aircraft configurations also active flow control, morphing surfaces, flexible control surfaces or distributed propulsion.

B2.3

An effective load alleviation enables significant weight-savings for aircraft structures. The current developments of aero-elastic research try to improve the conventional designs incrementally incorporating the existent aerodynamic and structural nonlinearities in the analysis and adding localized devices for damping or active flow control. This proposal tackles the limitation of the weak nonlinear behavior of the elementary structural components – stiffeners, shells, etc. – and discovers a new aero-elastic design space wherein stronger nonlinearities are feasible. Stronger nonlinearities enable a passive (weight-saving) control of the load-deflection-behavior and can be used for load alleviation. Furthermore, buckling and snap-through effects provide increased damping properties to the structure.

In the beginning, a basic understanding of representative structural components will be evaluated detecting the design driver regarding a desired nonlinear behavior. Variations of geometry, pre-stress, material are intended. Based on the reference configurations of the SE2A first, the nonlinear components will be assembled into a 2D-aerofoil model to investigate the gust load reduction capabilities. In focus are the unsteady loading-unloading fluid-structure-interaction (FSI) processes, its damping under realistic aerodynamic conditions and the effectiveness for load alleviation. Second, the extension to a 3D-wing adds the promising stronger nonlinear span-wise bending-torsion to the design space and a best suited wing for load alleviation will be designed taking the flutter requirements into account. Finally, the potential and limitations of strong nonlinear structural components for gust and load alleviation will be demonstrated and weight saving will be quantified.

B2.4

This proposal addresses the subjects initially planned for the Junior Research Group B.3 “Structural architecture for active flow control”.
“A Junior Research Group shall embed new design technology into an overall design framework that provides the setup for compliant overall design of wing and fuselage shell structures. Parametric models of the fluid functions provided by computational fluid dynamics models in ICA-B1 will be connected to parametric representations of the CRFP sandwich, along with appropriate models for the skin, the inner shell and additional requirements. The ability to optimise the topology of the sandwich core, shells, and stiffening elements will be important.” Since this JRG was not established, Peter Horst and Christian Hühne now took over the issue.
The proposed project addresses the call at three scales, namely, global, building block concept and local.

B3.1

The research approach of the proposal lies in the investigation of 3D printing for the production of suction panels as a modular element for laminar flow control in aircraft skins. The project B3.1 will develop local designs from a global aircraft design, which will be implemented in
this project using an innovative printing technology. The latter includes multimaterial capabilities and realizes a fiber-reinforced backbone structure with three-dimensional lay-up guiding the internal flow of the sucked air. The outer skin is to be printed in metal with holes of sub-millimeter size. The chamber structure with the necessary connections to the unperforated outer skin will be printed on it with fiberreinforced thermoplastics using the fused deposition modeling process. This approach requires detailed investigations and modelling of the materials, processes and transition zones between the materials, also at extreme temperature differences. The expected residual stresses must be determined and the necessary simulations validated experimentally. Finally the initial concept for a smart suction panel will be developed.

B3.2

To realize an ultralight energy efficient and sustainable aircraft, beside the questions related to the right energy resources, further significant decrease of energy consumption by improved aerodynamic and minimum structural weight has to be achieved. One option, especially for wing structures already today is the use of CFRP-structures instead of metal. But there are some sever drawbacks today, which need to be resolved. One are the so called Compression-After-Impact (CAI-) allowables, these strength allowables are much lower than the static ones and must be used in all cases of so called barely visible damages. CAI allowables thus are a clear weight driver. Another obstacle are the todays production costs of CFRP components. Whereas CAI-properties can be significantly improved by thin-ply prepregs, the production cost increases significantly.
One of the challenges of the introduction of new aircraft technologies on design level in the past was the issue of production. To be able to realize an ultralight smart HLFC wing structure as envisaged within the integrated cluster area B3 requires new materials together with technologies to efficiently realize structures made of those new materials.
This project will bridge a gap between basic research and envisaged industrial application and will demonstrate that an ultralight and energy efficient aircraft can also be produced in an efficient manner.

B3.3

Failure and fatigue analysis capability for composite components is a key aspect for the development of future novel aircraft configurations. A partner consortium within the current ICA-B3 Call will develop a suction panel configuration for laminar flow control, making use of multi-material 3D-printing technology. The present project contributes by developing new failure and fatigue approaches for the configuration envisaged within the consortium research theme, both for the printed thermoplastic backbone structure supporting the perforated metal outer skin and for the interface between the backbone structure and the outer skin. Complicating factors inherently involved through the 3D-printing process, such as residual stresses and spatially varying material properties, are addressed within the development of these approaches. The resulting models are used at structural level for the fatigue analysis of the suction panel configuration, including load sequence effects.

B3.4

A multi-layer framework is proposed for aircraft conceptual design and multidisciplinary optimization. This design engine includes four different layers. The first layer is a full aircraft conceptual design tool, including all the relevant disciplines in aircraft design, however with lower/medium level of fidelity. This tool is used for full aircraft conceptual design and provides general outputs such as aircraft geometry, weight breakdown, and energy consumption. The second layer is a surrogate modelling toolbox, which is used for connecting the results of higher fidelity analysis to the low fidelity design tool. This toolbox is also used for multi-fidelity optimization. The third layer, is a physics-based coupled-adjoint multidisciplinary design optimization (MDO) toolbox, with medium level of fidelity. This toolbox is used for more detailed design refinements of aircraft and providing outputs, such as aircraft loads for detailed structural design. Eventually the forth layer is a high fidelity MDO toolbox for design optimization of selected dominant aircraft components with advanced technologies such as boundary layer suction. In order to enhance the future MDO driven design of aircraft by assessing the consequences of design changes on the aircraft noise, an additional research activity is proposed on the prediction of excess noise due to unconventional propulsor integration. While the respective computation method is developed, validated and demonstrated in a companion research proposal (ICA-B1), here, a systematic study on propulsion integration excess noise as a function of relevant design parameters is proposed
by applying the mentioned method.

B5.1

The goal of the research is to describe the relation between thermal strain and IFF-density at cryogenic conditions by measuring the laminate elongation by means of FBG-sensors in a first step. Subsequently, the relation between IFF-density and permeation rates is investigated in order to assess the suitability of strain measurement for prediction of permeation rates.

JRP