Showing papers on "Flapping published in 1997"

Parametric Investigation of a High-Lift Airfoil at High Reynolds Numbers

Abstract: A new two-dimensional, three-element, advanced high-lift research airfoil has been tested in the NASA Langley Research Center s Low-Turbulence Pressure Tunnel at a chord Reynolds number up to 1.6 x 107. The components of this high-lift airfoil have been designed using a incompressible computational code (INS2D). The design was to provide high maximum-lift values while maintaining attached flow on the single-segment flap at landing conditions. The performance of the new NASA research airfoil is compared to a similar reference high-lift airfoil. On the new high-lift airfoil the effects of Reynolds number on slat and flap rigging have been studied experimentally, as well as the Mach number effects. The performance trend of the high-lift design is comparable to that predicted by INS2D over much of the angle-of-attack range. However, the code did not accurately predict the airfoil performance or the configuration-based trends near maximum lift where the compressibility effect could play a major role.

Broadband tonpilz underwater acoustic transducers based on multimode optimization

Abstract: Head flapping has often been considered to be deleterious for obtaining a tonpilz transducer with broadband, high power performance. In the present work, broadband, high power tonpilz transducers have been designed using the finite element (FE) method. Optimized vibrational modes including the flapping mode of the head are effectively used to achieve the broadband performance. The behavior of the transducer in its longitudinal piston mode and in its flapping mode is analysed for in-air and in-water situations. For the 37.8% bandwidth of the center frequency from 28.5 to 41.8 kHz, the amplitude variation of the transmitting voltage response (TVR) does not exceed /spl plusmn/2 dB and a maximum TVR of 146.8 dB (ref. 1 /spl mu/Pa/volt at 1 meter) is obtained. Reasonable agreement between the experimental results and the numerical results is achieved. A maximum sound pressure level of 214.8 dB can be expected. Further numerical calculation indicates that there is potential for increasing the bandwidth by varying other parameters in the design.

Application of Wavelet Cross-Correlation Analysis to a Plane Turbulent Jet

Abstract: A new cross-correlation method, which is called wavelet cross-correlation analysis and is used to express the statistical cross-correlation of two arbitrary signals in terms of scale and time delay, is proposed and its main properties are presented. By analyzing two test signals, it is shown that wavelet cross-correlation does not have the limitations of classical cross-correlation. As a practical application to fluid mechanics, wavelet cross-correlation is employed to determine the cross-correlation relationships between the x-components of the fluctuation velocities at two points on opposite sides of the centerline amd along the centerline of a plane turbulent jet in terms of period and time delay. In the distributions of the wavelet cross-correlation coefficients, similar structures with various scales are observed instantaneously, and the period of eddy and apparent flapping motions can be determined easily in terms of period and time delay. It is found that the apparent flapping behavior appears first in region with an intermediate period. It is also revealed that a similar structure with a high period consists of similar structures with a low period, i.e., a large eddy contains small eddies.

Rotor blade swashplate-axis rotation and gyroscopic moments compensator

Abstract: An apparatus for compensating for spatial phase angle phase errors in the flapping angle oscillations of a rotor blade is disclosed. The apparatus receives a measured aircraft roll and a measured aircraft pitch. Gyroscopic moments resulting from these measured rolls and pitches are determined, and compensating gyroscopic moments are then generated and summed with commanded pitch and roll accelerations. Flapping angle phase lags of the rotor blade swashplate are added to the summed signals to cancel any flapping angle phase leads, caused by changes in aircraft or rotor blade speed caused by installation configurations of the rotor blade swashplate.

Effects of dynamic stall phenomena on propulsive efficiency and thrust of a flapping airfoil

Abstract: Numerical simulation of dynamic stall phenomena around an airfoil oscillating in a coupled mode, where the pitching and heaving oscillations have some phase differences, has been performed using the Navier-Stokes code. The propulsive efficiency and the thrust have been calculated for various combinations of the phase difference and the reduced frequency for two different amplitude ratios, and the effects of the dynamic stall phenomena on the behaviors of the propulsive efficiency and thrust are discussed in detail by examining each flow pattern obtained. The highest efficiency has been observed for the case where the pitching oscillation advances 90 deg. ahead of the heaving oscillation, for which the flow separation is confined in the small region on the airfoil surface near the trailing edge in spite of the large amplitude oscillations. For phase angles other than 90 deg. the efficiency is degraded by the occurrence of the large scale leading edge separation.

The effect of an intermittant flapping motion on the properties of merging plumes

Abstract: Experiments with merging buoyant jets revealed the existence of an intermittent flapping motion in the flow trajectory. The flapping commences beyond the point where the initially axi-symmetric buoyant jets merge to form a plane plume. The size and relative positions of large-scale vortices on either boundary of the plane plume determine whether the flapping flow state occurs. The effect of the flapping on the short term timeaveraged flow properties was measured and the flapping was found to increase the spreading rate and also change the shape of the concentration fluctuation distribution in the plane plumes. The intermittent presence of flapping in plane plumes is shown to be the cause of widely accepted differences between plane and axisymmetric plumes. The new findings on the flapping behaviour were used to improve an integral model which describes merging buoyant jets.

Power requirements for large-amplitude flapping flight

Abstract: In this paper, a method is presented for computing the circulation distribution along the span of a e apping wing that minimizes the power required to generate a prescribed lift and thrust. The power is composed of three parts: useful thrust power, induced power, and proe le power. Here, the thrust and induced power are expressed in terms of the Kelvin impulse and kinetic energy associated with the sheet of trailing and shed vorticity left behind the e apping wing. The proe le power is computed using a quasisteady approximation of the two-dimensional viscous drag polar at each spanwise station of the wing. A variational principle is then formed to determine the necessary conditions for the circulation distribution to be optimal. Included in the variational principle is a constraint that the wing not stall. This variational principle, which is essentially the viscous extension of the well-known Betz criterion for optimal propellers, is discretized using a vortex-lattice model of the wake, and the optimum solution is computed numerically. The present method is used to analyze a conventional propeller as well as a rigid wing in forward-e ight e apping about a hinge point on the longitudinal axis.

A Theoretical Control Study of the Biologically Inspired Maneuvering of a Small Vehicle Under a Free Surface Wave.

Abstract: : This report considers a theoretical control study of low-speed maneuvering of small underwater vehicles in the dive plane using dorsal and caudal fin-based control surfaces. The two dorsal fins are long and are actually mounted in the horizontal plane. The caudal fin is also horizontal and is akin to the fluke of a whale. Dorsal-like fins mounted on a flow aligned vehicle produce a normal force when they are cambered. Using such a device, depth control can be accomplished. A flapping foil device mounted at the end of the tailcone of the vehicle produces vehicle motion that is somewhat similar to the motion produced by the caudal fins of fish. The moment produced by the flapping foils is used here for pitch angle control. A continuous adaptive sliding mode control law is derived for depth control via the dorsal fins in the presence of surface waves. The flapping foils have periodic motion and they can produce only periodic forces. A discrete adaptive predictive control law is designed for varying the maximum tip excursion of the foils in each cycle for the pitch angle control and for the attenuation of disturbance caused by waves. The derivation of control laws requires only imprecise knowledge of the hydrodynamic parameters and large uncertainty in system parameters is allowed. In the closed-loop system, depth trajectory tracking and pitch angle control are accomplished using caudal and dorsal fin-based control surfaces in the presence of system parameter uncertainty and surface waves.

Flapping flying apparatus

Abstract: A flying apparatus which generates a floating force by moving up and down a pair of wings, particularly by man power. Because the vertical motion of the wings can be easily attained by a limited force, floating can be made efficiently and the flapping operation can be made for a long period of time. This flapping flying apparatus includes unidirectional opening/closing means provided to both wings for supporting right and left two rings in such a manner that they can flap in the vertical direction, opening to allow air to pass when they move up and closing to inhibit air from passing when they move; mid-point foldable structure for connecting for pivotal supporting a main body side bar pivotally supported by a lower frame disposed below support portions of both wings with wing side bars pivotally supported on the support frame side of both wings; and a driving portion such as gripping means disposed near a connecting portion between the main body side bar and the wing side bar.

English Flapping and the feature [vibrant]

Abstract: In certain varieties of English, and most notably in the majority of North American dialects, alveolar oral and nasal stops undergo a process known as Flapping or Tapping in certain well-defined environments. At the present time, the resulting segments [ɾ] and [ɾ] cannot be satisfactorily captured by any known phonetic feature mainly because there is no way to distinguish a nasal stop from a nasal tap or flap. In this paper, I will argue that since vibration is the primary articulatory characteristic of taps, flaps and trills, the phonological rule of Flapping or Tapping constitutes evidence that [vibrant] should be added to the current inventory of features.

An Experimental Study on the Cross Coupling Effects of a Helicopter Rotor in Low Advance Ratio Using the Dynamic Running Test Facility.

Abstract: The cross coupling effects of a helicopter rotor in low advance ratio are experimentally studied using the Dynamic Running Test Facility which was built at National Defense Academy recently. Design features and basic performances of the facility are briefly described. Flapping motions of the model rotor blade are precisely measured together with its thrust and torque characteristics at various combinations of test parameters. It is clarified that nonlinear behavior of the cross coupling effects on the advance ratio are attributed to the phase shift properties of blade flapping motions due to nonuniformity of induced velocity distributions on the rotor disc. Usefulness of a dynamic running test is also ascertained for accurate measurements of aerodynamic and dynamic phenomena of a rotorcraft in low speed flight.

Flapping-wing aircraft

Abstract: An ornithopter has multiple flapping wings, which flap in turn to maintain blanced and smooth lifting force for flying. Changing the angles of wing surfaces when they flap can generate the pressure difference relative to air and in turn the lifting force.

An articulatory and acoustic analysis of English flaps: Evidence for attentional reduction in production

Abstract: Traditional models of American English phonology include a rule that coronal stops appear as flaps before unstressed vowels. Recent studies suggest that this flapping is better explained as the result of blending the articulatory specifications for the stops with neighboring vowels. This blending produces compromise articulatory positions for the stops, which cross over the perceptual boundary between stops and flaps. This paper tests this explanation against an x‐ray microbeam corpus of American English stops. The occurrence of flapping was determined via transcriptions. Regression analyses indicate that acoustic parameters such as VOT and voicing occurrence better predict transcriptions than articulatory parameters such as tongue position and motion during closure. In addition, articulatory analyses indicate a variation from [t] to flap which is generally consistent with a blending account. However, the contextual influence from the consonants on neighboring vowels and various aspects of articulatory postures in the consonants themselves is more consistent with a reduction of the stop, rather than a blending with neighboring vowels. These results suggest that flapping results from a linguistic convention which allows hypoarticulation of unstressed items and a secondary result of some of these items straying across an acoustic boundary. [Supported by the NSF and NIDCD.]