Showing papers on "Pitching moment published in 2016"

A predictive quasi-steady model of aerodynamic loads on flapping wings

Abstract: Quasi-steady aerodynamic models play an important role in evaluating aerodynamic performance and conducting design and optimization of flapping wings. The kinematics of flapping wings is generally a resultant motion of wing translation (yaw) and rotation (pitch and roll). Most quasi-steady models are aimed at predicting the lift and thrust generation of flapping wings with prescribed kinematics. Nevertheless, it is insufficient to limit flapping wings to prescribed kinematics only since passive pitching motion is widely observed in natural flapping flights and preferred for the wing design of flapping wing micro air vehicles (FWMAVs). In addition to the aerodynamic forces, an accurate estimation of the aerodynamic torque about the pitching axis is required to study the passive pitching motion of flapping flights. The unsteadiness arising from the wing’s rotation complicates the estimation of the centre of pressure (CP) and the aerodynamic torque within the context of quasi-steady analysis. Although there are a few attempts in literature to model the torque analytically, the involved problems are still not completely solved. In this work, we present an analytical quasi-steady model by including four aerodynamic loading terms. The loads result from the wings translation, rotation, their coupling as well as the added-mass effect. The necessity of including all the four terms in a quasi-steady model in order to predict both the aerodynamic force and torque is demonstrated. Validations indicate a good accuracy of predicting the CP, the aerodynamic loads and the passive pitching motion for various Reynolds numbers. Moreover, compared to the existing quasi-steady models, the presented model does not rely on any empirical parameters and thus is more predictive, which enables application to the shape and kinematics optimization of flapping wings.

Euler-Equation-Based Drag Minimization of Unconventional Aircraft Configurations

Abstract: This study investigates the potential of unconventional aircraft transports through numerical optimization. Three distinct configurations are investigated: a box wing, a C-tip blended wing–body, and a braced wing. Each transport is sized for the same regional mission and is subjected to the same optimization strategy based on the Euler equations. The figure of merit is inviscid pressure drag at transonic speed; the nonlinear constraints are lift, pitching moment, and internal volume. The design variables include the section shape and twist distribution of the main lifting surfaces. It is found that the box-wing, C-tip blended-wing–body, and braced-wing configurations investigated here are, respectively, 34.1, 36.2, and 40.3% more efficient than a similarly optimized conventional tube-and-wing configuration. Each optimization revealed, in one way or another, the importance of accounting for flow nonlinearity during the early stages of unconventional aircraft design. For the blended wing–body, the C tip doe...

A novel multi-component strain-gauge external balance for wind tunnel tests: Simulation and experiment

Abstract: This paper presents a novel multi-component strain-gauge external balance to measure lift and drag forces as well as pitching moment in wind tunnel experiments. First, an innovative structure is proposed and its geometry is determined through a tedious trial and error scheme using finite element (FE) simulation. The sensor dimensions are thus chosen so as to acquire acceptable sensitivity and negligible interference error among the components considering maximum loading capacities. Appropriate locations of the strain gauges on the structure are determined via simulation. Then, sensitivities of the balance components are found using the FE analysis. Finally, the designed external balance is constructed and calibrated. It is found that the interference error among the balance components is less than 2.01%. Furthermore, the measured sensitivities of the sensor components are in a good agreement with the simulation results through which the design procedure is validated.

Aerodynamic parameters from distributed heterogeneous CNT hair sensors with a feedforward neural network.

Abstract: Distributed arrays of artificial hair sensors have bio-like sensing capabilities to obtain spatial and temporal surface flow information which is an important aspect of an effective fly-by-feel system. The spatiotemporal surface flow measurement enables further exploration of additional flow features such as flow stagnation, separation, and reattachment points. Due to their inherent robustness and fault tolerant capability, distributed arrays of hair sensors are well equipped to assess the aerodynamic and flow states in adverse conditions. In this paper, a local flow measurement from an array of artificial hair sensors in a wind tunnel experiment is used with a feedforward artificial neural network to predict aerodynamic parameters such as lift coefficient, moment coefficient, free-stream velocity, and angle of attack on an airfoil. We find the prediction error within 6% and 10% for lift and moment coefficients. The error for free-stream velocity and angle of attack were within 0.12 mph and 0.37 degrees. Knowledge of these parameters are key to finding the real time forces and moments which paves the way for effective control design to increase flight agility, stability, and maneuverability.

Numerical Investigation of the Performance of Pitching Airfoils at High Amplitudes

Abstract: Recently, a renewed interest toward vertical-axis wind turbines has developed, due to the increasing need of a rational utilization of renewable energy resources. In vertical-axis wind turbines, each blade undergoes a wide variation of angle of attack during one rotation, with important dynamic stall phenomena. Neglecting wake-to-wake interactions and curvature effects, a pitching airfoil is a good representation of the flow experienced by the blades of a vertical-axis wind turbine. In the present work, the flow around an Eppler 387 airfoil in pure pitching is investigated, exploring a parametric space tailored to vertical-axis wind-turbine applications. In such a case, the angle of attack α varies in the range −40 deg<α<40 deg, resulting in large-amplitude pitching oscillations. The Reynolds number, based on the freestream velocity and the chord length, is set to Rec=3×104. In this sense, the present work differs from earlier studies, where typically the combined heaving and pitching or pure pitching f...

Longitudinal Aerodynamic Modeling of the Adaptive Compliant Trailing Edge Flaps on a GIII Airplane and Comparisons to Flight Data

Abstract: A pair of compliant trailing edge flaps was flown on a modified GIII airplane. Prior to flight test, multiple analysis tools of various levels of complexity were used to predict the aerodynamic effects of the flaps. Vortex lattice, full potential flow, and full Navier-Stokes aerodynamic analysis software programs were used for prediction, in addition to another program that used empirical data. After the flight-test series, lift and pitching moment coefficient increments due to the flaps were estimated from flight data and compared to the results of the predictive tools. The predicted lift increments matched flight data well for all predictive tools for small flap deflections. All tools over-predicted lift increments for large flap deflections. The potential flow and Navier-Stokes programs predicted pitching moment coefficient increments better than the other tools.

Dynamic stall analysis of S809 pitching airfoil in unsteady free stream velocity

Abstract: In this paper, the dynamic stall of S809 airfoil that widely used in horizontal axis wind turbine, in different reduced frequencies is investigated. The simulation was carried out numerically handling Navier-Stokes equations. For this purpose, the segregated solver with SIMPLE algorithm was chosen to solve the momentum equations. The effect of turbulence on the flow field is taken into account using Shear Stress Transport (SST) K-ω turbulence model. The obtained numerical results are compared with experimental and a few numerical results. The results indicate that the K-ω SST model is in good agreement with experimental results for both steady and unsteady conditions. Furthermore, a non-dimensional parameter, relating the acceleration of unsteady free stream velocity and acceleration of pitch motion (known as reduced frequency), is also investigated. In addition, the results show that any increase in the reduced frequency increases the instantaneous aerodynamic characteristics of oscillating airfoil.

Measurement of unsteady transition on a pitching airfoil using dynamic pressure sensors

Abstract: Unsteady boundary layer transition on a pitching OA209 airfoil in a wind tunnel was detected by using pressure fluctuation measurement at different oscillation frequency. Thirty Kulite dynamic pressure transducers flush-mounted on the airfoil surface recorded pressure signatures, and root mean square of pressure signatures were calculated. Results indicated that the criterion of transition for static airfoil defined as the peak of root mean square of pressure fluctuation was still suitable for detection of transition on a pitching airfoil. Fixed transition experiment for pitching airfoil was performed to validate the conclusion. Effect of oscillation frequency on transition was investigated. For small reduced frequency, the hysteresis loop is larger near leading edge. With increasing in the oscillation frequencies, the transition was promoted and relaminarization was enhanced.

Rapid Multi-Objective Aerodynamic Design Using Co-Kriging and Space Mapping

Abstract: In this paper, a procedure for computationally feasible multi-objective design optimization of aerodynamic surfaces is presented. Our approach exploits multi-fidelity aerodynamics models as well as a multi-objective evolutionary algorithm (MOEA). For the sake of cost reduction, the initial Pareto front is obtained by optimizing a fast kriging surrogate model using MOEA. The surrogate is constructed from sampled low-fidelity model which is pre-conditioned using high-fidelity model data and space mapping. The surrogate is then iteratively refined by enhancing it using high-fidelity model data points sampled along the Pareto set using co-kriging. The process is continued until the Pareto front representation produced by the surrogate aligns with the high-fidelity verification samples. The proposed method allows us to obtain—at a low computational cost—a set of aerodynamic geometries representing trade-offs between the figures of merit. Our approach is illustrated on the design of airfoil shapes in transonic flow at constant lift and obtaining the Pareto front for the drag and pitching moment coefficients.

Free Flight Testing in Hypersonic Flows: HEXAFLY-INT EFTV

Abstract: This study reports the application of free flight testing in a hypersonic wind tunnel for the aerodynamic characterization of a hypersonic vehicle. Sub-scale models of ESA’s HEXAFLY-INT EFTV geometry were released into hypersonic flow and motion was measured through a combination of high-speed video analysis and on-board gyroscopes and accelerometers. These measurements allow aerodynamic coefficients to be determined and give insight into the stability of the design. Models were fabricated using both 3D printing and CNC milling techniques. A light-weight instrumentation system was assembled to measure angular movements (pitch, roll and yaw) and accelerations and transmit the sensor data via a Bluetooth transceiver to an off-board computer for post processing. The University of Southern Queensland’s short-duration hypersonic wind tunnel (TUSQ) was used for testing. It produces a quasi-steady test flow of approximately 200 ms at nominally Mach 6. Results are presented from two separate test campaigns. The first is a comprehensive repeatability study of the free flight technique and the second is the preliminary results of testing a new model design with interchangeable flap angles. The repeatability study showed that the free flight technique is inherently repeatable, despite large variations in model movement between experiments. As expected, lift coefficient showed less scatter than pitching moment coefficient, due to accelerations being measured directly as opposed to angular accelerations which required differentiation of the gyroscope data. Preliminary data from the new hybrid metallic-plastic models showed static stability in all cases. The models themselves proved to be robust, surviving all experiments with minimal damage.

Dynamic CFD simulations of the MEADS II ballistic range test model

Abstract: Dynamic, viscous, free-to-oscillate simulations of the Mars Entry Atmospheric Data System (MEADS) ballistic range model are performed using two different ow solvers, OVERFLOW and US3D. At the time of publication, data from the ballistic range experiment was not yet available, so the current work serves as a code-to-code exercise. Results from the analysis show good agreement between the predicted static aerodynamic coefficients for each solver. Both codes predict damped pitch oscillations for Mach 3:0 with initial amplitudes of 5⁰ and 30⁰, as well as for Mach 1:5 with initial amplitude of 30⁰. The two solvers predict undamped pitch oscillations for Mach 1:5 with initial amplitude of 5⁰. For most cases, US3D predicts less damping than OVERFLOW. The difference is attributed to higher pressures in the separated region of the wake, and the resultant effect on the backshell contribution to the pitching moment.

Whiteflies stabilize their take-off with closed wings.

Abstract: The transition from ground to air in flying animals is often assisted by the legs pushing against the ground as the wings start to flap. Here, we show that when tiny whiteflies (Bemisia tabaci, body length ca. 1 mm) perform take-off jumps with closed wings, the abrupt push against the ground sends the insect into the air rotating forward in the sagittal (pitch) plane. However, in the air, B. tabaci can recover from this rotation remarkably fast (less than 11 ms), even before spreading its wings and flapping. The timing of body rotation in air, a simplified biomechanical model and take-off in insects with removed wings all suggest that the wings, resting backwards alongside the body, stabilize motion through air to prevent somersaulting. The increased aerodynamic force at the posterior tip of the body results in a pitching moment that stops body rotation. Wing deployment increases the pitching moment further, returning the body to a suitable angle for flight. This inherent stabilizing mechanism is made possible by the wing shape and size, in which half of the wing area is located behind the posterior tip of the abdomen.

Sensor based closed loop control of active aerodynamic elements

Abstract: A method of controlling an active aerodynamic element for a vehicle includes determining a target position for the active aerodynamic element from a target aerodynamic force, which may be a given value that is provided based on dynamic conditions of the vehicle. The method actuates the active aerodynamic element to the target position and senses an aerodynamic response characteristic of the active aerodynamic element while actuated to the target position. An estimated applied aerodynamic force is determined from the aerodynamic response characteristic, and is compared to the target aerodynamic force. A force error is determined from the comparison of the estimated applied aerodynamic force and the target aerodynamic force, and a modified position for the active aerodynamic element is determined from the force error and the target aerodynamic force. The active aerodynamic element is actuated to the modified position.

CFD Analysis of the F/A-18E Super Hornet during Aircraft- Carrier Landing High-Lift Aerodynamic Conditions

Abstract: The goal of the current study was to determine if computational fluid dynamics is capable of accurately predicting the forces and moments on the F/A-18E Super Hornet at the high-lift aerodynamic conditions usually encountered during carrier landing. Past F/A18E computational studies have mainly focused on wind-tunnel scale calculations with the results being compared to existing wind-tunnel data. The calculations for this study were conducted at full scale and the results were compared to F/A-18E sim data where possible. The computational geometry included all of the flaps, control surfaces, shrouds and gaps usually present on the actual aircraft. During this study, static calculations were completed at various F/A-18E conditions along the path of an untrimmed longitudinal stick doublet maneuver. These calculations represent small, medium and large trailing-edge flap deflections. The results were compared to F/A-18E sim data. In addition, several static calculations were conducted for the F/A-18E trimmed with two different sets of control laws at three different longitudinal stick positions. Once again, these results were compared to F/A-18E sim data. Finally, a study was conducted to determine the effect of the trailing-edge flap deflection, aileron deflection and angle of attack on the F/A-18E forces and moments. A wide range of trailing-edge flap deflections was considered. Overall, the comparisons between the computational results and the sim data were favorable. However, the correlations for the pitching moment indicate that there is room for improvement. Traditionally, accurately predicting the pitching moment on the F/A-18E has been very challenging.

Aerodynamic Parameter Prediction on a Airfoil with Flap via Artificial Hair Sensors and Feedforward Neural Network

Abstract: Gust alleviation and flutter suppression are essential elements of an effective fly-by-feel system. Knowledge of real-time forces and moments can have huge effect on designing an effective controller for flutter suppression and gust rejection. One unique method of predicting forces and moments is to use distributed arrays of artificial hair sensors that are capable of sensing the environment and therefore capturing important flow features. In this paper, the local flow measurement from the artificial hair sensor is used with feed-forward neural network to predict the aerodynamic parameters (angle of attack, freestream velocity, lifte coefficient and moment coefficient per unit span, and flap angle) on an airfoil containing control surface. These aerodynamic parameters can be combined with the airfoil’s physical parameters to predict the real time lift and moment. Also, the effect of artificial hair sensor integration location on prediction of aerodynamic parameters is studied.

Output-feedback control of underwater gliders by buoyancy and pitching moment control: Feedback linearization approach

Abstract: This paper addresses an output-feedback control problem of an underwater glider. The buoyancy and the pitching moment that are generated by the net mass variation and the elevator control, respectively, are used as control inputs. Additional forces induced by the elevator control increase nonlinearity of the plant dynamics, which make controller design difficult. By using the feedback linearization technique, we convert the concerned nonlinear dynamics to a linear time-invariant model, and based on this, design an observer-based output-feedback controller. A simulation result is shown to verify the effectiveness of the proposed technique.

Turbine Powered Simulator Calibration and Testing for Hybrid Wing Body Powered Airframe Integration

Abstract: Propulsion airframe integration testing on a 5.75% scale hybrid wing body model us- ing turbine powered simulators was completed at the National Full-Scale Aerodynamics Complex 40- by 80-foot test section. Four rear control surface con gurations including a no control surface de ection con guration were tested with the turbine powered simulator units to investigate how the jet exhaust in uenced the control surface performance as re- lated to the resultant forces and moments on the model. Compared to ow-through nacelle testing on the same hybrid wing body model, the control surface e ectiveness was found to increase with the turbine powered simulator units operating. This was true for pitching moment, lift, and drag although pitching moment was the parameter of greatest interest for this project. With the turbine powered simulator units operating, the model pitching moment was seen to increase when compared to the ow-through nacelle con guration indicating that the center elevon and vertical tail control authority increased with the jet exhaust from the turbine powered simulator units.

Aeroelasticity of the PrandtlPlane: Body Freedom Flutter, Freeplay, and Limit Cycle Oscillation

Abstract: Aeroelasticity of PrandtlPlane configurations is a yet unexplored field. The overconstrained structural system and the mutual aerodynamic interference of the wings enhance the complexity of the aeroelastic response. In this work the aeroelastic behavior of several models based on wing system of 250-seat PrandtlPlane design is studied. When an aluminum version of the structure is considered, flutter is associated with a coalescence of the first two elastic modes, the first being characterized by a classic upward bending of both wings, and the second one being associated with an out-of-phase bending of the two wings and tilting of the lateral joint. Analyses show that energy is injected in the structure mainly at the tip of the front wing, close to the aileron. Effects of freeplay of mobile surfaces are evaluated, showing how, in some cases, an increase in the flutter speed is observed. When flutter analyses are repeated considering the configuration free to pitch and plunge, flutter speed does increase due to a particular interaction between rigid-body pitching and elastic modes. Several of the above findings are demonstrated on more detailed structural models considering also the local stiffness distribution, and taking also into consideration compressibility effects. When composite materials are employed, flutter issues are completely overcome.

Numerical investigation of an advanced aircraft model during pitching motion at high incidence

Abstract: An unsteady Reynolds averaged Navier–Stokes (URANS) method combined with a rigid dynamic mesh technique was developed to simulate unsteady flows around complex configurations during pitching motion. First, a test case with the NACA0012 airfoil was selected to validate the numerical methods and our in-house codes. Then, we evaluated the unsteady flows around an advanced aircraft model during harmonic pitching motion at high incidence. The effects of pitching motion on the hysteresis of aerodynamic force, the evolution of the leading-edge vortex, and the distribution of pressure on the model’s surface were analyzed in detail. The roles of several significant parameters such as the reduced frequency and pitching amplitude were revealed. Several conclusions were found: pitching motion would delay the initiation of the leading-edge vortex, strengthen the vorticity, postpone the occurrence of vortex breakdown, and weaken the massively separated flows, thus causing additional aerodynamic force. Two categories of critical reduced frequency have been found, which divide the influence of reduced frequency on aerodynamic force into three stages, called the linear increasing range, slowly increasing range, and constant range. The first-order phase lag between the aerodynamic force and the incidence is a constant that is independent of the amplitude when the reduced frequency is sufficiently high. A scaled maximum value of CL is proposed; it depends only on the reduced frequency (instead of the amplitude), and increases linearly when the reduced frequency is sufficiently low.