Large Lift Coefficients Research Articles

Experiments on the Weis-Fogh mechanism of lift generation by insects in hovering flight. Part 1. Dynamics of the ‘fling’

From a series of experiments using simplified mechanical models we suggest certain minor modifications to the Weis-Fogh (1973)–Lighthill (1973) explanation of the so-called ‘clap and fling’ mechanism for the generation of large lift coefficients by insects in hovering flight. Of particular importance is the production and motion of a leading edge, separation vortex that accounts for virtually all of the circulation generated during the initial phase of the ‘fling’ process. The magnitude of this circulation is substantially larger than that calculated using inviscid theory. During the motion that subsequently separates the wings, the vorticity over each of them is convected and combined to become a tip vortex of uniform circulation spanning the space between them. This combined vortex moves downwards as a part of a ring, of large impulse, that is then continuously fed from quasi-steady separation bubbles that move with the wings as they continue to open at a large angle of attack. Such effects are able to account for the large lift forces generated by the insect.

Theory and Design of Axial Flow Fan (Part II)

It is important to know in which section the stall of impeller and guide vane occurs at first. It seems that the stall is closely connected with the distribution of lift on the blade. Though at the root solidity is small, large lift coefficient is frequently required, we also introduce the expression for stall condition at the root and prepare certain diagrams convenient for application to design. By using the diagrams we can easily know whether the flow and pressure coefficient at the design point are safe or not for stall. And we introduce a method of assuming the pressure-and efficiency-discharge characteristic curve, especially at once predicting stall point and show examples.

Aerodynamics of flapping flight with application to insects.

ABSTRACT General formulae are derived giving the lift, thrust and power when the wing motion is specified. The formulae are applied to twenty-five insects for which quantitative data are available. Average values for lift and drag coefficients, CL and CD, are derived by equating the weight to the vertical force and the thrust to the horizontal drag of the body. The large drag and lift coefficients obtained for insect flight are attributed to acceleration effects. There is a distinct correlation between and the ratio of the flapping velocity of the wings to the linear velocity of flight. When this ratio and therefore the accelerations are small, the force coefficients do not exceed those to be expected for flat plates. Owing to the nature of the assumptions and approximations made, the values derived for CD, CL and CD/CL are minimum values. Other characteristics of insect flight are discussed. In general, insects fly in such a way as to minimize the mechanical power required. In most, but not all cases, the useful force is the one perpendicular rather than parallel to the relative wind. The wing tips should move in a figure 8, the down beat should be slower than the up beat, and the majority of the necessary force must be supplied on the down beat. Figures are given using the data from the twenty-five insects considered, showing average relations between power, specific power, mass, acceleration forces, force coefficients and geometrical dimensions. The power per gram, the ‘wasted power’, and the force coefficients all increase as the importance of the acceleration forces increases. When plotted as functions of mass, quantities involving the power show much less dispersion than quantities involving the geometrical dimensions. This is taken to mean that despite the diversity of insect form, as ‘power plants’, they are all essentially similar. A table of the observed or adopted flight parameters (frequency of beating, mass, wing area, velocity of flight, amplitude and orientation of wing motion) is appended.

Experiments with positive rays

Wing kinematics in forward-flying fruit-flies was measured using high-speed cameras and flows of the flapping wing were calculated numerically. The large lift and thrust coefficients produced by the wing were explained. The wing flaps along a forward-tilting stroke plane. In the starting portion of a half-stroke (an upstroke or downstroke), the wing pitches down to a small pitch angle; during the mid portion (the wing has built up its speed), it first fast pitches up to a large pitch angle and then maintains the pitch angle; in the ending portion, the wing pitches up further. A large aerodynamic force (normal to the wing surface) is produced during the mid portion of a half-stroke. The large force is produced by the fast-pitching-up rotation and delayed-stall mechanisms. As a result of the orientation of wing, the thrust that propels the insect is produced by the upstroke and the major part of the vertical force that supports the weight is produced by the downstroke. In producing the thrust the upstroke leaves a “vortex ring” that is almost vertical, and in producing the vertical force the downstroke leaves a “vortex ring” that is almost horizontal.