CFD/Quasi-Steady Coupled Trim Analysis of Diptera-type Flapping Wing MAV in Steady Flight

The nuances in flapping wing aerodynamics are not yet fully understood to the extent where concepts can be translated to practical designs. Trimmed flight is a fundamental concept for aircraft in general. It describes the flight condition when there are no accelerations on the vehicle. From an engineering perspective, trim estimation is essential for performance analysis and flapping wing vehicle design. Without an efficient trim algorithm, trial-and-error based identification of the trimmed wing kinematics is computationally expensive for any flight condition, because the large number of simulations required make the process impractical. In a global sense the nature of forces produced by flapping wings closely resemble those on a helicopter blade, such that an analogy can be drawn between the two. Therefore, techniques developed for helicopter performance calculations are adapted and applied to the flapping wing platform particularly for analyzing steady flight. Using a flight dynamic model of the insect, which comes embedded with simplified quasi- steady wing aerodynamics and is coupled to high-fidelity CFD analysis, trim solutions are obtained in realistic time frames. This procedure is analogous to rotorcraft periodic coupling for trim. This multi-fidelity approach, where many quasi-steady calculations are combined with a judicious number of CFD simulations, may be used in parametric sweeps and design studies to improve hover and cruise performance. It was shown that the coupled trim methodology based on the QS model is capable of driving the CFD towards a stable trim solution. In forward flight the trim procedure tilts the stroke plane resulting in lift generation during downstroke and propulsive force during upstroke. The airloads, thrust and power are affected by the trim parameters, and the CFD/QS methodology accurately accounted for these inter-dependencies. Also it is observed that power initially decreases as an insect goes from hover to forward flight. Furthermore, the lift-to-power ratio versus average lift was identified as a principal efficiency metric to assess the performance of flapping-wing vehicles for a given geometry and kinematic parameters

The Suction Panel - xHLFC and Structural Solution for Energy Efficient Aviation

Future energy-efficient aircraft requires a further drastic reduction in drag and weight. Is it contradictory to improve both at the same time? Is it possible to design a highly efficient HLFC system to be weight-neutral? The present study, performed within the Cluster of Excellence SE2A – Sustainable and Energy-Efficient Aviation, summarizes aspects and considerations of the contributing disciplines to derive a solution for a suction-based system on short-range aircraft wings with maximum efficiency, i.e. hybrid laminar flow control application capabilities at minimum weight penalty. Several new features – novel wing design and simulation tools, the potentials of thin plies for weigth saving and the 3D-printing possibilities for ventable core structures – are investigated to achive this goal

Multidisciplinary design optimization of transonic wings with boundary layer suction

A quasi-three-dimensional aerodynamic solver is developed for aerodynamic analysis of wings in a transonic regime, able to capture the effect of Boundary Layer Suction (BLS) in Hybrid Laminar Flow Control (HLFC) application or transition to turbulent flow for Natural Laminar Flow (NLF). The tool provides accurate results but without the high computational cost of high-fidelity tools. The solver combines the use of an Euler flow solver characterized by an integral boundary layer method and Linear Stability Analysis using a 2.75D approximation for transition prediction. In particular, a conical transformation is adopted, including the determination of the shock-wave position. The solver is implemented in a Multidisciplinary Design Optimization (MDO) framework, including wing weights estimation and aircraft performance analysis. The framework consists of different modules: aerodynamics, structure, suction system analysis, and performance evaluation. Using a genetic algorithm and considering HLFC technology, wing MDO has been performed to find the optimum wing planform and airfoil shape. A backward swept wing aircraft, developed inside the Cluster of Excellence SE2A (Sustainable and Energy Efficient Aviation) is studied. Novel technologies such as active flow control, limited maximum load factor due to load alleviation and novel materials allow a fuel weight reduction of 6%

Design of Hybrid-Laminar-Flow-Control Wing and Suction System for Transonic Midrange Aircraft

Hybrid Laminar Flow Control (HLFC) has shown significant promise in the viscous drag reduction of aircraft. However, the use of HLFC for commercial applications requires further simplification. The current study proposes tools for the conceptual design of transonic HLFC wing and suction system. In the first part of the study, airfoil sections for the wing are optimized for minimum total drag using a multi-objective genetic algorithm approach at six span-wise locations. The induced drag of the wing is estimated using a vortex lattice method solver. In the second part of the study, suction system design is performed using ASPeCT, an in-house solver for HLFC system design. A simplified inner structure for the suction system is proposed, which can be integrated easily within the wing structure. A total drag penalty approach is proposed to establish a trade-off between matching the target suction distribution and the complexity of the suction system. Finally, the additional weight and off-design performance of the suction system are analyzed for a +/- 0.1 change in the design lift coefficient. A maximum fuel reduction of 7 % can be expected with the HLFC system taking into account the additional weight added and power off-take from the engine

Comparison of Gradient-Based and Genetic Algorithms for Laminar Airfoil Shape Optimization

Reducing airfoil drag is a common objective to decrease fuel burn and emissions in aviation. Shape optimization tools have been shown to effectively design low drag airfoils. Two different approaches for laminar airfoil shape optimization are currently under investigation within the German research cluster SE2A (Sustainable and Energy Efficient Aviation). One is based on a gradient-free genetic algorithm and the other one uses a gradient-based adjoint algorithm. The performance of these two optimization toolchains is compared. In addition, a combination of the two methods by using the former in exploring the design space and the latter for further refinement of the solution is inspected. The RAE2822 airfoil was chosen as baseline airfoil.Two design conditions at transonic speeds and one at subsonic speed were investigated. The result of the adjoint method was found to be dependent on the initial reference airfoil, which limited the extent of drag reduction. In comparison, the genetic algorithm could explore diverse geometries and produce designs with longer laminar flow and weaker shocks, but was more computationally expensive. The combination of the two methods utilized the strengths of both and resulted in maximum drag reduction

Analysis and uncertainty quantification of a hybrid laminar flow control system

This study aims at quantification of suction velocities for a hybrid laminar flow control (HLFC) application by taking data and model uncertainties into account. Since suction velocities cannot be measured directly, they are calculated using the Darcy-Forchheimer model. The model uses the pressure distribution and the porosity characteristics of the suction skin as boundary conditions that incorporate various sources of uncertainties. This paper attempts to quantify these uncertainties and to propagate them through the Darcy-Forchheimer model. The approach uses a sampling method to provide uncertainties associated with the suction velocities. In addition, the modelling error of the Darcy-Forchheimer model is estimated and also taken into account. It turns out that information on the local porosity characteristics of the suction skin is essential to reduce uncertainty of the estimated suction velocities for the application studied. In order to identify the most influential contributions to uncertainties of the suction velocities, a sensitivity study is carried out. The most influential contribution shows to be the uncertainty in the porosity characteristics, followed by uncertainties in the pressure distribution. Contributions with negligible influence are the model error and the uncertainty in reference pressur

Design and power calculation of HLFC suction system for a subsonic short-range aircraft

Hybrid laminar flow control (HLFC) can be a possible solution for future sustainable energy-efficient aviation. The current study proposes a MATLAB-based numerical tool for the design of the suction system for an airfoil optimized for a subsonic short-range HLFC application. Considerable energy losses may occur when the air passes through the perforated metallic outer surface and the inner structure of the suction system. A semi-empirical approach is used to design a layout that provides a target suction velocity based on measured pressure losses through porous medium and substructures. Flowbench measurements were performed on 3D-printed internal core test samples to quantify the pressure losses that can be used to create a lower pressure below the porous sheet matching the target suction velocity. The actual suction realized on the airfoil using this substructure concept has a discrete nature that increases with the distance between two adjacent walls. Finally, the suction system’s power requirement is calculated. The power requirement for distributed suction accounts for the pressure loss characteristics of the porous material, the internal core structure, and throttling holes. However, the study does not include the ducting losses from the substructure to the compressor. Approximately 80% of the total suction power is utilized to eject the sucked air back to the freestream conditions for a system with a compressor and propulsive system efficiency equal to one. The study analyses the performance of the designed internal core layout to different flight conditions and addresses the suction power requirement variation with lift coefficient and flight altitude.Aerodynamic