Pitching Moment Research Articles

Computational Investigation of Multirotor Interactional Aerodynamics with Hub Lateral and Longitudinal Canting

This study investigates the interactional aerodynamics for laterally and longitudinally canted two-rotor systems with a front rotor and an aft rotor aligned with the flow. The -diameter, three-bladed fixed-pitched rotors are simulated using computational fluid dynamics at a targeted disk loading and 30 kt edgewise freestream. Simulations are performed using the commercial Navier—Stokes solver AcuSolve with a detached eddy simulation model. In addition to an uncanted case, two laterally canted cases (10° advancing sides up and 10° advancing sides down) as well as two longitudinally canted cases (10° inward and 10° outward) are simulated. Aft rotor performance is compared to isolated rotors operating at the same revolutions per minute, speed, and shaft tilt angle in order to quantify the effect of rotor–rotor aerodynamic interaction. For all configurations, the aft rotors experience a lift deficit at the front of the rotor disk, which also results in a nose-down pitching moment relative to an isolated rotor. The lift deficit for the uncanted rotor was around 15%. Lateral canting only slightly increases the lift deficit (to 16–18%) but also produces 28–38% change in roll moments. Change in aft rotor nose-up pitching moments for the uncanted and laterally canted rotors were in the 55–64% range. Longitudinal canting produces larger changes in the magnitude of the lift deficit and pitching moment, but has minimal effect on roll moments. In particular, canting inward results in a lift deficit as high as 21% and a 94% change in pitching moment. Canting outward, on the other hand, reduces the aft rotor lift deficit to 11% and the pitching moment change to 19%. The paper explains the changes in the flowfield and the underlying physics for the different cases in detail.

Modeling and linearization of longitudinal dynamics for a flapping wing micro aerial vehicle dragonfly-like with active rigid tail

The main purpose of this paper is to model and simulate flight dynamics for a flapping wing micro aerial vehicle dragonfly-like with two pair clap and fling mechanism and active rigid tail. This article simulates the flight dynamics of a micro aerial vehicle dragonfly-like that can also use tail movements for longitudinal stability. Initially, using Kane's method, the equations of motion of the longitudinal mode are obtained. Then aerodynamics forces of Delfly II micro aerial vehicle and gearbox simulation are added to the equations of motion. Also, a novel design for a flapping wing micro aerial vehicle dragonfly-like is presented, in which tail movement is similar to the movement of insect tails in longitudinal mode. In this work, the tail movement is not used as an elevator, but the rigid tail movement is used as a control torque. The difference in brush motor rpm leads to differential thrust and pitch moment generation, similar to a quadrotor. Finally, the dynamic equations and aerodynamics and gearbox are all linearized and presented as state-space equations. Also, the response of the open-loop linearized model is compared with the nonlinear response by creating suitable initial conditions for the brush motor rpm.

An Experimental Analysis of Active Pitch Control for an Assault Amphibious Vehicle Considering Waterjet-Hydrofoil Interaction Effect

The present study aims to reduce the pitch motion of an assault amphibious vehicle system in seaways by waterjet impeller revolution rate control. A series of seakeeping tests were performed in a towing tank with a 1/4.5-scale model. This vehicle is manufactured as a box-shaped hull, and since an appendage that generates lift force is attached, the amount of change in pitch motion is large according to the forward speed. For pitch motion reduction, the impeller revolution rate and resultant pitch moment were controlled through a proportional-integral-derivative controller. Improvements in seakeeping performance were examined in both regular and irregular conditions by the model tests in terms of root mean square of pitch motion. The tuned controller decreased pitch motion by more than 60%.

Nonlinear wave loads on a stationary cylindrical-type oscillating water column wave energy converter

Based on the potential flow theory, a second-order Higher-Order Boundary Element Method model is developed to simulate hydrodynamic loads and pressure on a stationary cylindrical-type oscillating water column (OWC) wave energy converter. Quadratic pneumatic damping and additional viscous damping are introduced to simulate the effects of the power take-off and the viscous dissipation inside the chamber, respectively. The proposed model is verified against a carefully instrumented scaled experiment. The effects of the chamber geometry and the wave environment on the hydrodynamic loads of the OWC device are then explored using the validated model. The resonance excited by the second-order wave component is observed with the fundamental frequency being half of the natural frequency. This resembles a sloshing mode and leads to a significant increase in hydrodynamic pressure and surge force as well as pitch moment. The effects of the second-order waves cannot be ignored under the action of the low-frequency long waves. In addition, the surge force and the pitch moment are not sensitive to the variation in the orifice opening ratios, while the heave force increases with the opening ratio at a resonant heave frequency.

Aeropropulsive Coupling Effects on a General-Aviation Aircraft with Distributed Electric Propulsion

As distributed electric propulsion has continued to gain popularity as an enabling aircraft technology, further research is required to understand the complete implications of aeropropulsive coupling effects for distributed propulsion systems integrated into actual aircraft geometries. A set of low-speed wind-tunnel tests was performed at the University of Illinois at Urbana-Champaign to characterize the aerodynamic performance of a semispan Cirrus SR22T wing-body model that was equipped with an array of four ducted fans integrated into the upper-surface, trailing edge of the wing. Aerodynamic performance and aeropropulsive coupling effects were investigated through six-component load-cell measurements of the forces and moments experienced by the model, supplemented by chordwise pressure distributions at four stations along the span of the wing. Additionally, a set of unsteady force and moment measurements were taken to investigate time constants associated with the response of aerodynamic forces and moments to step changes in fan speed. It was found that the fan tip-speed ratio dictates the aerodynamic performance of the model, with primary effects including an increased lifting force, yawing moment, rolling moment, and nose-down pitching moment as the tip-speed ratio increased. Additionally, the lift-curve slope changed with tip-speed ratio as a function of thrust setting in relation to thrust required.

THE IMPACT OF DIFFERENT OUTWARD AND INWARD PROTRUSION POSITIONS ON THE NACA 1410 AIRFOIL SECTION AT VARIOUS ANGLES OF ATTACK

The current work aims to increase the NACA 1410 airfoil's aerodynamic efficiency by altering its surface qualities for various angles of attack. The NACA 1410 airfoil's lower surface has semicircular outward and inward protrusions for a more advantageous profile. The lift and drag coefficients were calculated using CFD, at various attack angles, from -2º to 18º. An ANSYS Fluent Workbench model of the NACA 1410 airfoil was used to investigate flow characteristics at 3 x 105 Reynolds numbers. The semicircular protrusions caused turbulence, reducing pressure drag and increasing lift, delaying flow separation. The NACA1410 profile has protrusions facing outward, increasing the pitch moment coefficient. The inward protrusion was found to be more advantageous in the study of the effects of surface alterations on airfoils at various angles of attack.

Experimental Validation of Wave Induced Disturbances for Predictive Station Keeping of a Remotely Operated Vehicle

Predictive control methods can substantially improve the performance of Unmanned Underwater Vehicles (UUVs), particularly in shallow water environments or near the free surface where wave induced disturbance are of magnitude comparable to the vehicle characteristic inertia. To facilitate the adoption of these methods, a fast estimation of the time evolution of hydrodynamic forces acting on a vehicle is required. To this end, we perform experiments in a wave tank with an ROV to validate the use of Linear Wave Theory (LWT) to capture the time history of surge, heave and pitch wave induced forces and moments. Validation is performed for various sea states, reconstructed with a mean correlation of 0.9138 in comparison to experimental measurements, displaying a maximum normalised mean error deviation between simulation and experimental data of 0.16 and 0.27 respectively for surge and heave forces, and 0.34 for pitch moment. The effectiveness of employing real-time wave disturbance forecasting for the purpose of anticipatory control is then assessed by incorporating the predicted loads within a Model Predictive Controller. Results display a mean RMS positional error reduction of 47.32% in comparison to a standard PD controller. This presents evidence that accurate, near real-time predictions of the wave-generated forces and moments on an ROV can be produced, laying the foundation for developing model-based predictive control strategies that better suit operation in harsh environments.

Effect of Lift-offset Rotor Hub Vibratory Load Components on Airframe Vibration Responses of High-Speed Compound Unmanned Rotorcrafts

This paper investigates numerically the effect of rotor hub vibratory load components on the airframe vibration responses of high-speed compound unmanned rotorcraft (HCUR) using a lift-offset coaxial rotor, wings, and two propellers. The rotor hub vibratory loads are predicted using a rotorcraft comprehensive analysis code, CAMRAD II, and the airframe vibration responses are calculated by a finite element analysis software, MSC.NASTRAN. It is shown that the vibratory hub pitch moment of a lift-offset coaxial rotor is the most dominant component for both the longitudinal and vertical vibration responses at four specified locations of the airframe. Keywords: HCUR, Lift-offset ë™ì¶•ë°˜ì „ 로터, 로터 진동 하중, 기체 진동 응답 Keywords: High-speed Compound Unmanned Rotorcraft, Lift-offset Coaxial Rotor, Rotor Hub Vibratory Loads, Airframe Vibration Responses

HYDRODYNAMIC LOADS AND FLOW STRUCTURE ANALYSIS FOR A SURFACED SUBMARINE USING CAPTIVE MODEL EXPERIMENTS

Submarines are required to operate on the surface for significant periods of time, for example during transit in shallow waters. However, as they are designed primarily for submerged operations, their surfaced sea-keeping behaviour may be poor. Furthermore, the wave making resistance of the vehicle increases in the surfaced state, substantially impacting on its powering requirements. The bow wave encountered when operating in a surfaced state applies a downward heave force and a bow down pitching moment, impacting the vessel’s sea-keeping performance. To provide insight into these effects, a captive model experimental program was undertaken at the Australian Maritime College Towing Tank facility to generate data at model scale for a 1.69m BB2 generic submarine geometry operating on the surface in calm water. The data generated includes the hydrodynamic loads and detailed, time-averaged flow structures on the surface, which will be used to study the behaviour of the vehicle and validate numerical tools.

Analysis on controllability and stability of quad-tilt-rotor aircraft in helicopter mode

The controllability and stability of quad-tilt-rotor aircraft in helicopter mode are modeled and analyzed, which will provide a theoretical guidance for the subsequent control system design. First of all, the flight dynamics model is established considering rotor-wing interference and verified with relevant experiments. Then, a control strategy for helicopter mode is proposed with trim characteristic analysis. Finally, corresponding control efficiency and cross coupling are calculated and analyzed along with characteristics of the stability derivatives and eigenvalues. The results show that the value of heading control efficiency is much smaller than that of other channels. The longitudinal force and pitch moment caused by vertical control input increase with the increase of the velocity. Yawing moment caused by lateral control input shows similar variations. The velocity stability becomes worse with the increase of the velocity. The stability of all other modes is augmented as velocity increases except the spiral mode.

Study of added resistance and seakeeping of KVLCC2 in waves with and without propeller

Numerical simulation of a full-scale ship model, KVLCC2, has been conducted applying the Reynolds Averaged Navier-Stokes (RANS) approach using the STARCCM+ commercial computational fluid dynamics (CFD) software to calculate total resistance, seakeeping and Pitch Moments. Results are obtained for the speed of 15.5 Knots under different sea conditions (calm water, regular waves and irregular waves), The total resistance calculated for the KVLCC2 ship hull in calm water is in a good agreement with the results from experiments and the results for motion (heave and pitch) and added resistance in waves were compared to numerical and experimental findings from previous research with good agreement. In addition to wave excitations, the full-scale ship model was subjected to propeller excitations using the virtual disk model from the CFD software. The body force propeller method, which simplified the full propeller characteristic of the KVLCC2 into a resultant body force, is applied to the virtual disk model. Results are compared with results from the hull-only model. A comparison of the wake results with previous work is also presented.

Experimental study on the maneuvering derivatives of a half-scale SUBOFF model in the vertical plane

The maneuverability of a submersible in the vertical plane is investigated. For this purpose, extensive experiments are performed on a half-scale SUBOFF model in the circulating water channel (CWC) with the planar motion mechanism (PMM). Oblique towing tests containing high attack angles, pure heave tests and pure pitch tests are involved. The validity of the experiments is identified by the accordance of the present hydrodynamic coefficients and the ones of Defense Advanced Research Projects Agency (DARPA) . In order to fit the measured data for cases with high attack angles, this paper develops the expression of dimensionless pitch moment M′ by the piecewise function. Besides, the blockage effects are corrected by Tamura equation. In addition, the uncertainty analysis (UA) is conducted referring to the recommended procedure 7.5-02-06-04 by 27th ITTC (2014b). The results show that, the developed equation turns out to be superior to the standard one. And the total uncertainties of the hydrodynamics are less than 2%, which reveals the accuracy and credibility of the experimental measurement.

Investigation of Non-Linear Ship Hydroelasticity by CFD-FEM Coupling Method

With the increase of ship size, the stiffness of the hull structure becomes smaller. This means that the frequency of wave excitation tends to be closer to the natural frequency of the hull vibration, which in turn makes the hydroelastic responses more significant. An accurate assessment of the wave loads and motion responses of hulls is the key to ship design and safety assessment. In this paper, the coupled CFD (Computational Fluid Dynamics)-FEM (Finite Element Method) method is used to investigate the non-linear hydroelasticity effect of a 6750-TEU (Twenty-foot Equivalent Unit) container ship. First, by comparing the heave, pitch, and vertical bending moment at midship section (VBM4) against experimental results reported in the literature, the validity of the numerical method in this paper is illustrated. Secondly, the ship responses under different wave length–ship length ratio, wave frequency-structure natural frequency, wave steepness, and ship speeds are studied. It is found that the wave length–ship length ratio has a more important influence on the hydroelastic response than that from wave frequency-structure natural frequency ratio, and the effect of wave non-linearity will behave differently under different wave length–ship length ratio. The increase of rigid body motion caused by forward speed will not correspondingly increase the non-linearity of the hydroelastic response.

Parameter computation for a Lagrangian mechanical system model of a submerged vessel moving near a free surface

This paper describes the computation of parameters for a Lagrangian mechanical system model of a submerged vessel moving near an otherwise calm free surface using a medium-fidelity potential flow code. The software uses the boundary element method to solve for the flow potential on the body and the free surface. The model, a system of integro-differential equations involving functions of the velocity potential, is the “maneuvering” component of a Lagrangian nonlinear maneuvering and seakeeping model that was introduced in earlier work. Here, this nonlinear maneuvering model is reformulated and the potential flow software is modified to support parameter computations. Parameters are computed for a prolate spheroid moving parallel to a calm free surface at a constant forward speed. This motion induces a steady flow with respect to the body, which results in a steady surge force, heave force and pitch moment. These longitudinal forces and moment are computed using the reformulated Lagrangian nonlinear maneuvering model and the results are compared with basic panel code solutions for various depths and Froude numbers and for various computational parameter values.

CFD Prediction of Ship-Bank Interaction

Ships operating in different shallow-water areas are at risk of grounding due to squat, heel and wave-induced motions. The paper aims not only at assessing the ship squat in any bathymetry, but also to reveal the waves influence on the environment. A 3D numerical model for predicting ship resistance in confined waters by taking the ship sinkage into account is proposed. A time-domain model is used to predict unsteady squat and dynamic acceleration effects for a vessel travelling in non-uniform water depth. A quasi-steady approach where the prediction at each time step is based on steady-state heave force and pitch moment in uniform water depth is used. Resistance, sinkage and trim, the free-surface elevation as well as the wake flow structure are considered in the ship performance analysis. The results show the wave pattern in the subcritical, critical and supercritical regions. The total resistance/drag is calculated at various ship speeds in shallow water using CFD and compared with the calculated deep water resistance. Attention is paid to the verification and validation based on a grid convergence study, as to the exploration of the modelling error in RANS computations to enable more accurate and reliable predictions of the bank effects.

Effects of upstream rotor tilt on a downstream floating wind turbine

This work quantifies the relationship between the design tilt of an upstream rotor and the structural response of a spar floating offshore wind turbine located in its wake. Three wind speed scenarios are considered: below, at, and above rated operation. The inflow is generated with the Mann model and the wake and loads are simulated with FAST.Farm. As the upstream rotor tilt goes from 0° to 6°, we find that the mean wake is displaced upward by more than 0.2 rotor diameters (D) by a downstream distance of 7 D. The vertical velocities increase by up to 35 cm/s in the center of the wake. As a result, the downstream rotor is partially waked and experiences a rotated velocity vector. With a higher upstream rotor tilt, the velocities and moments on the downstream turbine increase their mean axial value and their lateral and vertical standard deviation. These changes affect the blade and tower loading and the floater motion primarily in the out-of-plane direction: the damage-equivalent loads for the tower pitch moment and blade-root moment increase by up to 10% because of higher variability at the first mode for the tower and at one blade passing frequency for the blade root. Lesser effects are observed for the roll moments and for floater sway and heave. When the joint effect of rotor tilt and platform motion is considered, the load response on the downstream system is amplified primarily for tower pitch and blade out-of-plane moment.

Passive flow control of shock-induced dynamic stall via surface-based trapped vortex generators

A novel passive approach for controlling the flow in a 2D dynamic stall at variabl freestream is investigated. 2D computational fluid dynamics simulations of an SC1095 airfoil with surface-based trapped vortex generator (STVG) type passive flow control were conducted. The airfoil was exposed to a fluctuating freestream of Mach 0.537 ± 0.205 and Re = 6.1 × 106 (based on the mean Mach number) and experienced a 10° ± 10° pitch oscillation with a frequency of 4.25 Hz. These conditions were selected as an approximation to the flow experienced by a UH-60A helicopter rotor airfoil section in an actual fast forward flight test case. The baseline simulations were cautiously validated with experimental data for both transonic flow and dynamic stall under the variable freestream. Then, 20 different local STVGs type geometry modifications were investigated as a means of passive flow control. Modifications were examined on both the airfoil’s upper and lower surfaces. Results showed that the STVGs were able to mitigate the negative effects of shock-induced dynamic stall. The best geometries could reduce the peak negative pitching moment by as much as 9–23% during the transonic phase of a cycle and by as much as 19–71% during the dynamic stall phase. Also, they were able to reduce peak drag by 8–20% in the transonic phase and by 15–44% in the dynamic stall phase. On the other hand, the lift-to-drag ratio was significantly increased by 3–28% per one rotor cycle. All the above advantages came at virtually no penalty in the lift.

Analytic solution of in-plane vortex\u2013rotor interactions with arbitrary orientation and its impact on rotor trim

The general aerodynamic problem of arbitrarily oriented in-plane vortex-rotor interaction was investigated in the past only by numerical simulation. Just one special case of in-plane vortex-rotor interaction with the vortex axis in flight direction was recently solved analytically. In this article, the analytical solution for arbitrary in-plane vortex orientation and position relative to the rotor is given for the first time. The solution of the integrals involved as derived here encompasses and simplifies the previous derivation of the special case significantly. Results provide the vortex impact on rotor trim (thrust, aerodynamic rolling and pitching moments about the hub) and the rotor controls required to mitigate these disturbances. For the special case with the vortex axis in flight direction, the results are identical to the former solution and results for the other in-plane vortex orientations and positions agree with the numerical results obtained so far.

Motion Analysis of Butterfly-Style Flapping Robot Using CFD Based on 3D-CAD Model and Experimental Flight Data

In this paper, a computational fluid dynamics (CFD) analysis system based on a 3D-CAD model of a butterfly-style flapping robot using its experimental flight data is proposed. The butterfly-style flapping robot can control its attitude by changing its flapping and lead-lag angles; however, measuring the lift, thrust, and body pitch moment directly during flight is difficult. In the case of the flight motion analysis of insects, the state of flight has been photographed, and numerical analysis has been performed to obtain the flow field around the wings. However, when performing the motion analysis of hardware, it is difficult to reflect the shape of the body accurately using this method. In this study, a CFD analysis system considered the shape of the developed butterfly-style flapping robot as 3D-CAD data and analyzed the flow field around the wings using the experimental flight data of the hardware. The results of motion analysis showed that the attitude during flight differs due to the difference in lifts and body pitch moments in the flight experiment data of the hardware with different neutral angles of the flapping wings.

Control law design to improve the unexpected pitch motion in slow down turn maneuver

The highly maneuverable fighter aircraft is exposed to unexpected pitch motion such as over-Nz (normal acceleration) and Nz-drop characteristics in transonic and supersonic flight conditions with moderate angle of attack. These characteristics not only degrade flying qualities by destabilizing the aircraft but also threaten flight safety by increasing the structural load. This article proposes an additional augmentation control in the incremental nonlinear dynamic inversion structure, which feeds back the error of pitch angular acceleration to mitigate unexpected pitch motion in slow down turn maneuver. We evaluate the stability, flying qualities, and robustness of the proposed control system by performing the frequency-domain linear analysis and the time-domain numerical simulations based on the mathematical model of advanced trainer aircraft. As a result of the evaluation, the additional augmentation control further improves flying qualities and deceleration performance of the aircraft by decreasing over-Nz and Nz-drop characteristics in high-Nz maneuvering in the transonic flight condition as well as ensuring the stability and robustness of the control system against the major uncertainty factors of the aircraft system compared to the existing transonic pitching moment compensation (TPMC) control in which the predefined scheduling for Nz feedback is used.