Pitch Stability Research Articles

High aerodynamic lift from the tail reduces drag in gliding raptors.

ABSTRACTMany functions have been postulated for the aerodynamic role of the avian tail during steady-state flight. By analogy with conventional aircraft, the tail might provide passive pitch stability if it produced very low or negative lift. Alternatively, aeronautical principles might suggest strategies that allow the tail to reduce inviscid, induced drag: if the wings and tail act in different horizontal planes, they might benefit from biplane-like aerodynamics; if they act in the same plane, lift from the tail might compensate for lift lost over the fuselage (body), reducing induced drag with a more even downwash profile. However, textbook aeronautical principles should be applied with caution because birds have highly capable sensing and active control, presumably reducing the demand for passive aerodynamic stability, and, because of their small size and low flight speeds, operate at Reynolds numbers two orders of magnitude below those of light aircraft. Here, by tracking up to 20,000, 0.3 mm neutrally buoyant soap bubbles behind a gliding barn owl, tawny owl and goshawk, we found that downwash velocity due to the body/tail consistently exceeds that due to the wings. The downwash measured behind the centreline is quantitatively consistent with an alternative hypothesis: that of constant lift production per planform area, a requirement for minimizing viscous, profile drag. Gliding raptors use lift distributions that compromise both inviscid induced drag minimization and static pitch stability, instead adopting a strategy that reduces the viscous drag, which is of proportionately greater importance to lower Reynolds number fliers.

Performance evaluation of a submerged tidal energy device with a single mooring line

A submerged tidal energy device with contra-rotating Diffuser Augmented Tidal Turbines (DATTs) has been investigated in this paper. The device is moored to the seabed with a single mooring line, which limit it to operate at mean water depth, but otherwise allows it to float freely with the tidal current, like a kite in the wind, to harness tidal current energy. This research focuses on the evaluation of stability and power generation of the submerged tidal energy device based on 1:5th scaled model tests and the full-scaled prototype sea trials.A 1:5th scaled model had been manufactured and tested in a circulating water channel to observe the power generation performance and working attitude around the designed inflow velocity. A full-scaled prototype was manufactured and tested near the CHU Island in Shandong Province, China. The results show that the device can change its direction automatically to make the DATTs face the tidal inflow, as the tidal current changes direction. The device has a good stability in pitch and roll motions. But the device's stability in yaw motions is worse than the other two, which will significantly affect the power generation performance and introduce more demanding structural requirements.

Stability analysis of Hoist Vertical Shiplift chamber under pitching motion

Aiming at the stability problem of large Hoist Vertical Shiplift chamber under pitching motion. A fluid dynamic model based on the Housner theory is built to study the sloshing in the ship chamber. The theoretical formulas of the capsizing moments are derived to research the effect of sloshing. To study the whole shiplift pitching stability, the system coupled by pitching motion, torsional vibration of main hoist and sloshing is built. Then, the critical distance of lifting points is obtained by using the Hurwitz theory. Taking seven built typical Hoist Vertical Shiplifts as examples, it is verified that the expression about the critical distance of lifting points is rational. The calculation results show that the sloshing in the ship chamber and the capsizing moment produced by the convective pressure acting on the sidewall of ship chamber could not be neglected in the design of the Hoist Vertical Shiplift. According to the successful experience of the Hoist Vertical Shiplift, it is suggested that the safety factor S of the shiplift under pitching motion should be more than 1.5.

Investigation into the vibration characteristics of agricultural wheeled tractor-implement system with hydro-pneumatic suspension on the front axle

Before predicting the vibration characteristics of a tractor/implement system with front axle hydro-pneumatic suspension, it is essential to develop a dynamic model. The nonlinear stiffness and damping calculation equations for hydro-pneumatic suspension are derived and the complete three dimensional multibody dynamic model of the wheeled tractor/implement system is presented in this paper. The construction of the model considers the effect of implement and passive rub cabin suspension as well as front axle hydro-pneumatic suspension together. The vibration responses and the corresponding power spectral densities (PSD) of driver seat, cabin, chassis and implement under different forward speed and field road conditions gathered from the simulation using the models are in good agreement with experimental results, demonstrating the effectiveness of the proposed model. A comparative investigation on the dynamic effects of a passive cabin suspension, implement and front axle hydro-pneumatic suspension under four cases was made. The effects of front axle suspension and the implement on the rollover limit chart for the tractor system were also studied. Simulation results showed that the front axle hydro-pneumatic suspension will deteriorate the ride comfort of drivers but will improve pitch and roll stability. Passive cabin suspension plays a more important role in improving the ride comfort of drivers than the front axle hydro-pneumatic suspension does, while compromising the pitch stability to some extent. The implement will raise the riding comfort of drivers and the pitch stability of tractors, but dramatically worsen the roll stability. The presence of the front axle suspension and the implement improves the minimum slope angle and reduces the maximum slope angle. Furthermore, the effect of the initial nitrogen volume, the pressure of energy accumulators, and the orifice diameter of the small throttle valves on the vibration characteristics of the tractor system with the large throttle valve closed or fully opened is also investigated.

Pitch stability analysis of high-lift wire rope hoist vertical shiplift under shallow water sloshing–structure interaction

Pitch stability under shallow water sloshing–structure interaction has always been the most concerned issue in the design of the high-lift wire rope hoist vertical shiplift, which brings great challenges to the operational safety. A semi-analytical method including the developed modal system and the new coupled dynamics model is presented for pitch stability analysis. Based on the linear modal theory, the modal system is developed to describe the shallow water sloshing in the shiplift chamber, and the hydrodynamic moment associated with infinite set of modal functions is reasonably simplified by only retaining the lowest mode. Then a new 9-DOF coupled dynamics model of the complete main hoist system, shiplift chamber motion, and shallow water sloshing is established as dynamic equations by using the Lagrange equation of the second kind. Subsequently, the coefficient matrix of the dynamic equations and the Lyapunov motion stability theory are used in combination to numerically obtain the critical distance of suspension points. Taking four typical high-lift wire rope hoist shiplifts as an example, the results indicate that the proposed scheme improves the computational accuracy by 7.0–20.8% with respect to previous methods. Furthermore, for the being designed 200 m level wire rope hoist vertical shiplift, the preliminary design parameters can ensure the pitch stability safety factor not less than 1.3, increasing the wire rope stiffness or the synchronous shaft stiffness can effectively enhance the pitch stability.

Aerodynamic Sails for Passive Guidance of High-Altitude Balloons: Static-Stability and Equilibrium Performance

Balloon trajectory control remains a sought-after goal for the current scientific ballooning community. In this work, a trajectory control system capable of passively guiding a high-altitude balloon is discussed. Using convention common to aircraft literature, the pitch stability and maximum possible guiding force of a given sail geometry are derived in closed form. A system model is then presented, which facilitates a study of the performance of the system. Using the full model, system performance is compared for various parameters, such as tether location and sail mass. It is found that the tether location influences the pitch stability and affects control authority. Furthermore, sail performance is found to be a function of the sail mass, and the ratio of spanwise center of pressure and spanwise center of mass locations.

Open and closed loop gust loads analyses for a flying wing configuration with variable longitudinal stability

This work presents results of dynamic “one-minus-cosine” gust load simulations for a flying wing configuration. The in-house toolbox Loads Kernel is used for the loads analysis of the free flying aircraft. Flight mechanical characteristics are captured by application of a non-linear equation of motion in the time domain. The underlying aerodynamic methods are the Vortex Lattice and the Doublet Lattice Method with a rational function approximation (RFA) for unsteady simulations in the time domain. The structural model was created using DLR's parametric ModGen/Nastran design process. The structure is optimized for minimum structural weight with typical design load cases including maneuver, gust and landing loads. In this article, the focus lies on gust encounters and the flight characteristics of a flying wing configuration. It differs from classical configurations (wing-fuselage-empennage) due to a pronounced nose-up pitching motion when the aircraft enters the gust field. Finally, a flight controller is designed to increase the pitching stability. This is essential for the flight of a naturally unstable configuration. It is shown that the loads during a gust encounter increase significantly. The influence on the structural weight is small as the layout is very robust and the required material thickness is below the minimum thickness in most areas.

Intermittent theta burst stimulation over right somatosensory larynx cortex enhances vocal pitch-regulation in nonsingers.

While the significance of auditory cortical regions for the development and maintenance of speech motor coordination is well established, the contribution of somatosensory brain areas to learned vocalizations such as singing is less well understood. To address these mechanisms, we applied intermittent theta burst stimulation (iTBS), a facilitatory repetitive transcranial magnetic stimulation (rTMS) protocol, over right somatosensory larynx cortex (S1) and a nonvocal dorsal S1 control area in participants without singing experience. A pitch-matching singing task was performed before and after iTBS to assess corresponding effects on vocal pitch regulation. When participants could monitor auditory feedback from their own voice during singing (Experiment I), no difference in pitch-matching performance was found between iTBS sessions. However, when auditory feedback was masked with noise (Experiment II), only larynx-S1 iTBS enhanced pitch accuracy (50-250 ms after sound onset) and pitch stability (>250 ms after sound onset until the end). Results indicate that somatosensory feedback plays a dominant role in vocal pitch regulation when acoustic feedback is masked. The acoustic changes moreover suggest that right larynx-S1 stimulation affected the preparation and involuntary regulation of vocal pitch accuracy, and that kinesthetic-proprioceptive processes play a role in the voluntary control of pitch stability in nonsingers. Together, these data provide evidence for a causal involvement of right larynx-S1 in vocal pitch regulation during singing.

Influence of tension layer quality on mechanical properties of timing belts

Reinforcement of the tooth belts is the element responsible for the transmission of the torque. Its structure is related to the dynamics of the belt and such important parameters as the stiffness of running stability on pulleys and above all, the pitch stability. The axis of symmetry of the reinforcement in the vertical direction of the belt is the reference point of the belt geometry as well as the neutral axis of stresses caused by bending. On the arc of the contact between belt and pulley, over the reinforcement, the belt is stretched, and below it is compressed. In the tooth belts, there are various solutions of the carrier layer, in terms of the materials used, but also types of fibers, quantity of the fibers, etc. The paper presents the results of analysis of the carrier layer of the most popular tooth belts with T10 and AT10 trapezoidal teeth. The paper shows possible direction of choice of the construction materials of the carrier layer and their importance for the dynamics of the gear with the tooth belt.

Wing morphing allows gulls to modulate static pitch stability during gliding.

A gliding bird's ability to stabilize its flight path is as critical as its ability to produce sufficient lift. In flight, birds often morph the shape of their wings, but the consequences of avian wing morphing on flight stability are not well understood. Here, we investigate how morphing the gull elbow joint in gliding flight affects their static pitch stability. First, we combined observations of freely gliding gulls and measurements from gull wing cadavers to identify the wing configurations used during gliding flight. These measurements revealed that, as wind speed and gusts increased, gulls flexed their elbows to adopt wing shapes characterized by increased spanwise camber. To determine the static pitch stability characteristics of these wing shapes, we prepared gull wings over the anatomical elbow range and measured the developed pitching moments in a wind tunnel. Wings prepared with extended elbow angles had low spanwise camber and high passive stability, meaning that mild perturbations could be negated without active control. Wings with flexed elbow angles had increased spanwise camber and reduced static pitch stability. Collectively, these results demonstrate that gliding gulls can transition across a broad range of static pitch stability characteristics using the motion of a single joint angle.

Characteristics of Ground Effects of an Unmanned Aerial Vehicle with High-aspect-ratio Flying Wings during Take-Off

In view of the take-off process of an unmanned aerial vehicle (UAV) with high-aspect-ratio flying wing configurations, the influences of ground effects on longitudinal aerodynamic characteristics of the UAV were investigated. By using a numerical simulation method, the effects of different angles of attack, heights from the ground, and take-off velocity on the UAV were studied. Moreover, based on comparisons with test data in previous literature, the numerical calculation method was verified to be accurate. The height from the ground is the primary factor influencing longitudinal aerodynamic characteristics of the UAV. With the increase in height from the ground, the lift and drag coefficients of the UAV, and its pitching stability, increased. When the angle of attack was small, a large suction area was generated on the lower surface of wings with a large suction amplitude therewith. This results in a large range of downwash. As the angle of attack increased, the lower surface of the trailing edge for wings presents weakening downwash. As the take-off velocity rose, the suction area on the lower surface of the UAV, and its amplitude, gradually decreased, and the negative pressure on the upper surface also decreased to a certain extent. The research results can provide a certain theoretical basis for the safe design of take-off operations for high-aspect-ratio UAVs.DOI: http://dx.doi.org/10.5755/j01.mech.24.5.20934

Parametric Study on Design Parameters of Water-running Robot Based on Dynamic Simulation

There are many design parameters to determine a performance of a robot. Especially, in the case of the mobile robots, they require complicated motion at various environments to get high performances. In this sense, the analysis of the design parameters is the most important work to design an efficient mobile robot. In this study, we analyze the design parameters for the water-running robot. From the parametric study, we find some solutions to improve the performances of the robot. We derive dynamic equations for the water-running motion, and do a sensitivity analysis to understand the relationships between the parameters (frequency of the leg, stiffness of torsional springs that connects multi frames and mass of frames) and the performance of the water-running motion such as running speed and pitching stability. We use an orthogonal array to make various combinations of the parameters, and to reduce the number of a simulation process. As results, we summarize some solutions to improve the water-running motion. We are expecting that this study is going to be used to design robots that are operated on the water.

Position tracking control of a helicopter in ground effect using nonlinear disturbance observer-based incremental backstepping approach

Helicopters are highly nonlinear and internally unstable air vehicles, the dynamic response of which can be strongly influenced by flight conditions (wind gust, ground effect, etc.). Therefore, it is a challenging task to design a reliable flight control system (FCS) with all safety and performance requirements satisfied. This paper investigates the robust position tracking problem of a helicopter in ground effect (IGE). Based on a highly reliable and computational efficient finite state representation of rotor flow field IGE, a novel nonlinear disturbance observer-based incremental backstepping (NDOIBS) controller is designed to track position commands (all derivatives of the reference trajectory are known) under the influence of system uncertainties and external disturbances. Without requiring the exact knowledge of helicopter dynamics, the NDOIBS approach guarantees that instant control increments are derived in terms of Lyapunov theory and ensures robustness in the presence of mismatched disturbances whose first derivatives are bounded. It is shown that all state variables of the closed-loop system are semi-globally uniformly ultimately bounded (SGUUB). In addition, to further improve the horizontal position tracking performance, rotor state feedback (RSF) technique and a disturbance feedback strategy are applied to develop a pitch stability augmentation system (SAS). Finally, controller performance is demonstrated through numerical simulations using the Bo-105 utility helicopter. With the efficiency and robustness properties verified, the suggested NDOIBS control scheme shows great potential for implementing advanced FCS designs in existing helicopters.

Control of Pitch Attitude by Abdomen During Forward Flight of Two-Dimensional Butterfly

The objective of this paper is to understand roles of abdominal motion in the pitch stability of flapping flights of butterflies numerically, and a two-dimensional butterfly model has thoracic pitch instability in the periodically flapping flight. Because the obtained uncontrolled flight is unstable, the hierarchical sliding-mode control is used to design the control input for thorax–abdomen joint torque. It is found in the controlled flight that the abdominal motion is sufficient to counteract the thoracic pitch instability and can maintain flapping flights with short recovery time to artificial thoracic torque perturbations. On the other hand, it is also found that the long-term pitch stabilization is not possible only by the abdominal control for all flapping trajectories. However, with the introduction of a wing lead–lag control, the long-term pitch stabilization is obtained. Hence, these observations indicate that the abdomen is used for a short-term control and the wing for a long-term control in stabilizing the pitch of the butterfly’s flight.

‘OF THE TEMPERAMENT OF THOSE MUSICAL INSTRUMENTS’: CONSIDERING TIBERIUS CAVALLO AND THE SCIENTIFIC OBSERVATION OF MUSICAL SOUNDS IN LATE EIGHTEENTH-CENTURY LONDON

The connection of music to scientific exploration in late Enlightenment London can be considered from various perspectives, perhaps most evidently through the binary of amateur–professional. These two realms intersected within natural philosophical observation, a practice that often served concurrently as entertainment and as study. The development of scientific instruments for the observation of various phenomena appeared in both professional and amateur contexts, contributing to technological growth and research. Natural philosopher Tiberius Cavallo (1749–1809) and his 1788 article on musical temperament (‘Of the Temperament of Those Musical Instruments, in Which the Tones, Keys, or Frets, are Fixed, as in the Harpsichord, Organ, Guitar, &c’) provide a captivating example of amateur interest overlapping effectively with the professional domain; as an amateur musician and professional scientist, Cavallo observed equal temperament in both mathematical and aesthetic terms. Consideration of his work promotes a more nuanced view of London as a place where scientific and musical ideas could meet and be ‘instrumentalized’, emphasizing the city's status as a vibrant arena for the interaction of scientific exploration, artistic endeavour and professional identities. In this sense, Cavallo's work on temperament was not merely a scientific activity; it reflected technological change during a stimulating period of scientific and musical progress in late eighteenth-century London. For example, instrument builders were actively developing ways to improve pitch control and tuning stability, as witnessed by numerous British patents for harp mechanisms, the addition of flute keys and keyboard construction.

A New Methodology for Aerodynamic Design and Analysis of a Small Scale Blended Wing Body

The blended wing body (BWB) concept is a relatively new concept of an aircraft. The wings and the fuselage blend into one integral structure greatly reduce drag and increases lift thus making it a highly efficient design. The aim of the research was to design a radio controlled small scale BWB aircraft for use over long ranges at low altitudes in order to deliver payloads. The BWB was divided into the center body and the outer wing. Four airfoils, HS522, LA2573A, NACA 25111 and MH78 were analyzed in XFLR5. In consideration of their lift and moment characteristics, NACA 25111 and MH78 were selected for the center body and the wing respectively. The stall speed and wing loading were the primary factors used in determining the area and size of the aircraft which converged to a design having a five feet wingspan. Center of gravity was placed ahead of aerodynamic center to provide static and dynamic stability in pitch. Twist, dihedral and sweep were given to increase stability and controllability. The final design was tested in XFLR5 for stability and in commercial computational fluid dynamic code ANSYS-Fluent for comparison. These simulation results were compared to wind tunnel tests of a 20% scaled down prototype. 3D Panel Method results in XFLR5 were found to be very close to wind tunnel results but CFD results were seen to be not conforming to the wind tunnel results after 10° angle of attack. Thus, CFD was deemed to be unnecessary for designing a plane of this size. Ultimately, a larger test prototype was made out of polystyrene foam and a successful flight was achieved.

Well Production Enhancement Using an Underpump Stem Subjected to Load

The method of dynamic loading of sucker-rod pumps to act upon the producing well bottom zone is described. The effect is exerted through an underpump stem set on the well bottom. Under the weight of the flow string, the underpump stem loses pitch stability and is pressed to the inner surface of casing pipes. The operating sucker-rod pump induces axial vibration in the string, which generates lateral stresses transmitted to the adjacent rock mass along the spiral contact line between the underpump stem and the casing pipes. This dynamic impact on inactive or slightly active interlayers invokes fluid flow in them and results in enhanced production of wells. The field test data are presented to illustrate the described effect in wells in different geological conditions.

THE INFLUENCE OF WINGLETS ON THE AERODYNAMIC CHARACTERISTICS AIRCRAFT WITH THE TANDEM SCHEME

«Tandem» aerodynamic aircraft scheme with comparable areas of front and rear wings is well-known in aviation.«Tandem» scheme allows increasing in lift and lifting capacity with limited wingspan. So, it has found a new application in modern unmanned aerial vehicles.However, an increase in the number of lifting surfaces also leads to aircraft drag increasing. To solve this problem on classical aerodynamic schemes winglets, small additional elements at the wingtips which reduce the induced drag and increase the lift, are used. Such an effect can be achieved by selecting the optimal geometric characteristics of the winglets.Winglets are almost not used for tandem-scheme aircraft. Theoretically, winglets installing on the front wing of the tandem aircraft allows reducing the induced drag of both the front wing and the rear wing. But the open sources do not contain the experimental confirmation. Thus, the research task of the winglets effect on the tandem aircraft aerodynamics remains unsolved.Presented investigation allows estimation of the winglets effect on lift-drag ratio, pitch, roll and yaw static stability coefficients for tandem-scheme aircraft.

DYNAMIC STABILITY OF FOILBORNE HYDROFOIL/SWATH WITH ANHEDRAL FOIL CONFIGURATION

The hybrid hydrofoil/SWATH (Small Waterplane Area Twin Hull) designed and patented by Stefano Brizzolara, is a novel vehicle that is optimized to operate in both a high-speed foilborne mode and a displacement mode. The deployable hydrofoils on the vehicle take on a unique four surface piercing anhedral foil configuration. This foilborne design had previously undergone only preliminary assessment of stability characteristics. A six degree of freedom model of the foilborne vehicle dynamics is introduced as a framework to study vehicle stability and maneuvering. Stability criteria derived from multiple linearized models of the vehicle dynamics are compared to the six degree of freedom results in both the vertical and horizontal planes. Foil configuration design criteria are developed for pitch equilibrium, pitch stability, and directional stability.

Fluid–dynamic analysis of a multi–blade gravity damper

This paper addresses a computational fluid–dynamics study of a multi–blade inlet damper for gas turbine power plants. This device is used in the aspiration duct of the aforementioned plants with the aim to control the air mass flow rate, to dump flow turbulence and to guarantee the plant security.Nowadays the flat plate is the standard shape for damper blades. Clearly the device is rather simple but the operating conditions may prove burdensome. This is related to the vortex shedding developing behind the damper blades. If the shedding frequency of the vortices is close to the resonance frequency of the structure it generates a synchronism that involves the oscillation growth. This can lead to the structure damage. In this research work we have devised a new airfoil shaped blade for inlet dampers in order to suppress the vortex shedding phenomenon behind the damper blades.A proper numerical analysis of the damper blades pitching stability requires to solve a two-way fluid–rigid body interaction problem. Moreover the overset mesh method is strictly needed to investigate the blades motion due to aerodynamic interactions; as a matter of fact other techniques cannot be advocated for this problem since blades closure requirements produce the overlap of moving zones. For this reason we have developed a specific approach, based on the above mentioned techniques, to investigate the dynamic behaviour of the blades of a inlet damper.The obtained results show as the new blades have a very good pitching stability.