Center Of Lift Research Articles

Hydrodynamic performance of windsurfing boards with bottom modifications at high speed in calm water

Hydrodynamic performance of windsurfing boards is greatly affected by geometrical details of their bottom surfaces. In this study, computational fluid dynamics is applied to simulate flow around boards with several bottom modifications, including deadrise, rocker, step, wedge, and wing-tail profile. These simulations are conducted in steady-state, calm-water regimes at high planing speeds. It is found that at similar board trim angles and vertical positions, the largest lift is produced by setups with the bottom wedge and the wing-tail profile that enhance the downward deflection of the incident water flow. The highest lift-drag ratio is achieved by the variants with the bottom step and wedge that enable partial air ventilation of the bottom surface, which leads to reduced wetted surface and lower frictional resistance. The center of lift is located more forward on the rocker and stepped configurations, while it is positioned more rearward for the wedge and wing-tail setups. The obtained results can be used for guiding board designs aimed at maximizing the board hydrodynamic performance in calm water.

Influence of flow-induced bend–twist coupling on the natural vibration responses of flexible hydrofoils

For flexible hydrofoils, the natural flow-induced bending and twisting deformations are coupled if the center of lift is away from the elastic axis; this coupling may be influenced by viscous effects due to movement in the center of lift. The objective of this work is to investigate the role of flow-induced bend–twist coupling effects, as well as their dependence on the reduced velocity and solid-to-fluid added mass ratio. A secondary objective is to compare inviscid and viscous fluid–structure interaction (FSI) simulations of the natural flow-induced vibration responses of flexible cantilevered hydrofoils in water. The focus is on attached flow conditions in fully turbulent regimes at low angles of attack, where inviscid FSI method should be relatively accurate, and the foil response should be dominated by the foil's natural frequencies. The viscous FSI model is formulated by coupling a two-dimensional (2D) unsteady Reynolds-averaged Navier–Stokes (URANS) model with a two-degrees-of-freedom (2-DOF) model representing the spanwise tip bending and twisting deformations. The results show that the flow-induced bend–twist coupling terms are important for accurate prediction of the natural vibration frequencies and damping characteristics, particularly for the twisting motion, and their relative importance grows when the relative velocity increases and the solid-to-fluid added mass ratio decreases.

The aerodynamics of feather asymmetry and implications for paravian flight (13.4)

The presence of asymmetric feathers has been regularly used as an indicator of aerodynamic function and flight capacity in fossil birds and non‐avian dinosaurs. However, most of the flight feathers in living flying birds are not highly asymmetric. Only the most lateral primaries are typically highly asymmetric, and secondaries are typically symmetrical. The effects of feather asymmetry are more complex than simple presence or absence metrics indicate. Complex twisting occurs in the lateral primaries of living birds, which delays stall, improves aeroelastic stability, and promotes dynamic maneuvers and rigorous flapping flight at low speeds. Modeling feathers as simple, straight, flat plates suggests that they will only twist in the stabilizing “wash out” manner if vane depth ratios exceed 4:1. Because the center of lift on a flat plate sits near 1/4 chord, feathers with vane depth ratios of 4:1 or less are loaded primarily in bending. I demonstrate that the lateral primaries in most species are asymmetric enough to twist in wash out, as anticipated under the straight plate model. However, a substantial number of species do not meet expectations, which demonstrates that some combination of feather overlap, curvature, and orientation works to help stabilize these feathers under aerodynamic load. These patterns can be applied to fossil taxa to draw more precise conclusions about wing, hindwing, and tail functions.

2A2-F01 蝶型はばたき飛行ロボットに関する研究 : リード・ラグ運動による旋回手法の提案(飛行ロボット・メカトロニクス(2))

In this paper, we investigate the relationship between lead-lag motion of a flapping robot and yaw turning motion. Although flapping flight used by insects has attractive characteristics such as right angle turn, hovering, and high acceleration, the mechanism is not yet clarified. Here we focus on the lead-lag motion of a butterfly which changes the center of lift and wing area by overlapping the fore and hind wings and analyze the mechanism using two types of butterfly-style flapping robot. The case 1 robot can move the right forewing forward and backward according to the flapping motion, while the swept forward angle of the right wing for the case 2 robot is fixed. The results showed that the case 1 robot turned more largely than that of case 2 robot by the difference of the left and right wing reaction forces and that the dynamic shift of the wing reaction force enhanced the yaw moment by the feathering effect.

1P1-F06 翅脈を有する蝶型はぼたきロボットの開発(飛行ロボット・メカトロニクス)

In this study, we constructed a butterfly-style flapping robot with wing vein and investigated the flight characteristics for various design parameters such as swept-forward wing angle and position of center of mass (COM). Here, the wing length of the constructed hardware is 53mm, the total mass is 640mg, and the flapping frequency is 10 Hz. The results showed that the angle of attack was controlled by swept-forward wing angle and the relative position between COM and center of lift (COL), and that the model with swept-forward wing and rear-COM is more suitable than that with sweepback wing and mid-COM for butterfly-style flight.

Low Aspect Ratio Aerodynamics at Low Reynolds Numbers

The recent interest in the development of small unmanned aerial vehicles (UAVs) and micro air vehicles has revealed a need for a more thorough understanding of the aerodynamics of small airplanes flying at low speeds. In response to this need, a study of the lift, drag, and pitching moment characteristics of wings of low aspect ratio operating at low Reynolds numbers are presented. Wind-tunnel tests of wings with aspect ratios between 0.5 and 2.0, four distinct planforms, thickness-to-chord ratios of ≈ 2%, and 5-to-1 elliptical leading edges have been conducted as part of this research. The Reynolds numbers considered were in the range of 7 × × 10 4 to 2 × × 10 5 . Analysis of the data includes comparison of lift-curve slope, nonlinear equation approximations, maximum lift coefficient, and center of lift.

Mechanical properties of bat wing membrane skin

The skin of the bat wing in functionally unique among mammals: it serves as a major locomotor organ in addition to its protective and regulatory functions. We used tensile testing to investigate the mechanical capabilities of wing membrane skin, and compared stiffness, strength, load at failure, and energy absorption among specific wing regions and among a variety of bat taxa. We related these characteristics to the highly architectural fibrous supporting network of the wing membrane. We found that all material properties showed a strong anisotropy. In particular, wing membrane skin shows maximum stiffness and stregth parallel to the wing skeleton, and greatest extensibility parallel to the wing's trailing edge. We also found significant variation among wing regions. The uropatagium (tail membrane) supported the greatest load at failure, and the plagiopatagium (proximal wing membrane between laterl body wall and hand skeleton) is the weakest and most extensible part of the wing. We believe that the increased load bearing ability of the uropatagium relats to its key role in capture of insect prey, and that the great extensibility of the plagiopatagium promotes development of camber near the wing's centre of lift. In interspecific comparisons, energy absorpion and load to failure were greatest in Artibeus jamaicensis, the largest bat in our sample and the species with the highest wing loading, suggesting that wing loading may play a role in dictating the fuctional design of wing membranes.

On the aerodynamics of birds’ tails

The aerodynamic properties of a bird’s tail, and the forces produced by it, can be predicted by using slender lifting surface theory. The results of the model show that unlike conventional wings, which generate lift proportional to their area, the lift generated by the tail is proportional to the square of its maximum continuous span. Lift is unaffected by substantial variations in tail shape provided that the tail initially expands in width along the direction of flow. Behind the point of maximum width of the tail the flow is dominated by the wake of the forward section. Any area behind this point therefore causes only drag, not lift. The centre of lift is at the centre of area of the part of the tail in front of the point of maximum width. The moment arm of the tail, about its apex, is therefore more than twice the moment arm of a conventional wing about its leading edge. The drag of the tail is a combination of induced drag proportional to lift, and profile drag proportional to surface area. Induced drag can be halved by drooping the outer tail feathers to generate leading edge suction. This may be used for control, particularly in slow flight when both wings and tail are generating maximum lift. The slender lifting surface model is very accurate at angles of attack below about 15°. At higher angles of attack vortex formation at the leading edge can stabilize the flow over the tail and thereby generate increased lift by a detached vortex mechanism. Asymmetry in the orientation of the leading edges with relation to the freestream (either in roll, yaw or caused by asymmetry in the planform) is amplified in the flow field and leads to large rolling and yawing forces that could be used for control in turning manoeuvres. The slender lifting surface model can be used to examine the effect of variations in tail shape and tail spread on the aerodynamic performance of the tail. A forked tail that has a triangular planform when spread to just over 120° gives the best aerodynamic performance and this may be close to a universal optimum, in terms of aerodynamic efficiency, for a means to control pitch and yaw. However, natural selection may act to optimise the performance of the tail when it is not widely spread. The tail is normally only widely spread during manoeuvres, or at low speeds, selection may act to improve the efficiency of the tail when it is spread to only a relatively narrow angle - for example to maximize the bird’s overall lift to drag ratio - the optimum shape at any angle of spread is that which gives a straight trailing edge to the tail. This will always give a slightly forked planform, but fork depth will depend on how widely the tail is spread when selection acts, and this depends on the criteria for optimization under natural selection. A forked tail is more sensitive to changes in angle of attack and angle of spread, than other tail types. Forked tails are more susceptible to damage than other tail morphologies, and suffer a greater loss of performance following damage. Forked tails also confer less inherent stability than any other type of tail. Aerodynamic performance may not be an im portant optimization criterion for birds that fly in a cluttered environment, or do not fly very much. Natural selection, under these conditions, may favour tails of other shapes. The aerodynamic costs of sexually selected elongated tails can be predicted from the model. These predictions can be used to distinguish between the various models for the evolution of elongated tails. Elongated graduated tails and pintails could have evolved either through a Fisherian or H andicap mechanism. The evolution of long forked tails can be initially favoured by natural selection, the pattern of feather elongation seen in sexually selected forked tails is predicted by the Fisher hypothesis (Fisher 1930) but not by any of the other theories of sexual selection.

The X-29: is it coming or going?

The X-29 is an experimental aircraft built by the Grumman Aerospace Corp. for the US Defense Research Projects Agency. Relaxed static stability, one of the aircraft's key design features, means that the pilot must depend exclusively on computers to fly the plane. The X-29's computers monitor and adjust the positions of the plane's control surfaces every 25 milliseconds to follow the pilot's directions. Because it is designed to be unstable, the X-29 has its center of lift ahead of the center of gravity rather than behind it, unlike most stable aircraft. The way that the aircraft's canards and flaperons contribute to maneuverability is explained, and an account is given of how a computer-aided design and manufacturing system was used to design the composite wing. The aircraft has three digital computers backed up by three analog computers. The three digital computers are linked to one another by data buses in a ring-like configuration.

Two-dimensional impervious sails: experimental results compared with theory

Experiments have been made on quasi two-dimensional sails of small camber and at small incidence. Four excess-length ratios have been tested at a Reynolds number of 1.2 x 105. The results for lift, tension, centre of lift, maximum camber and its position, and leading- and trailing-edge membrane angles have been compared with existing inviscid theories and show poor agreement in general. This is attributed to leading- and trailing-edge flow separations as indicated by supplementary flow-visualization experiments. The optimum incidences in particular are much greater than the theoretical value of 0°. Luffing occurs at slightly negative incidences and appears to be a dynamic instability. The highest lift-to-drag ratio obtained was 16.5 on a membrane with an excess-length ratio of 0.03.

Fuel measuring and control systems

Fuel measuring and control systems have become progressively more sophisticated as aircraft performance has been extended both in the number of operating regimes and in operational capability. The fuel tank arrangements for the conventional aircraft types of the pre‐supersonic era, both piston and jet‐engined, allowed fairly straight‐forward fuel gauging and management. The tank dispositions were usually in line from left to right across the span of the wing. The advent of the swept wing aircraft introduced the complexities of fore and aft fuel balance in order to limit excursions of the CG about aft of the centre of lift. At the same time the operating economics of both military and civil aircraft, with massive fuel rate demand, required the development of fuel management systems capable of measuring and indicating fuel consumption and quantities to higher orders of accuracy.

Antisymmetric problems in shock diffraction theory

Various aspects of the diffraction of plane shock waves by moving antisymmetric thin bodies are examined. A method is presented for removing the inhomogeneity in the shock condition. This is done without knowledge of the explicit form of the solution, and is applicable to both symmetric and antisymmetric problems. With this reduction, explicit solutions have been previously obtained for symmetric bodies. For the antisymmetric case, the problem has been previously reduced to a complicated set of integrodifferential equations. In the cases where the flow field exhibits conical properties or when the body is slender, solutions may be obtained without recourse to solving the governing integral equations. In this paper it is shown that two-dimensional conical solutions are obtainable for moving antisymmetric wings only in the restricted case of the wing Mach numbers relative to the stream ahead of and behind the shock being the same. An analytical solution is presented for the above problem. In the case of slender bodies, it is shown that the classical steady slender body theory may be extended to shock diffraction problems and again analytical solutions are presented. It is shown that for general slender bodies, the lift and moment coefficients, and therefore the center of lift is influenced by the shock wave only through the Mach number change across the shock. Results are presented for the pressure distribution on the body, the lift and moment coefficients, and the center of lift for various slender body configurations at angle of attack.

U.S. Patent Specifications

An aircraft includes a body, an air compressor located within the body and having a discharge duct directed downwardly to develop a lift reaction, and a second air compressor located within the body and having a discharge duct directed downwardly for developing a lift reaction, both of the ducts having their outlets located adjacent to one another and the centre of area of the outlets of all of the ducts being substantially under the fore‐and‐aft centre line of the aircraft so that the centre of lift of the reaction is affected very little whether one or both compressors are in operation.