Free Oscillation Technique Research Articles

Changes in the lower leg moment of inertia due to child's growth

During growth the size and shape of the child's body changes. It is not clear whether the shape of a body segment changes proportionally in children between the age 5 and 18 years. The aim of this study is to describe these changes for the lower leg moment of inertia in a population of children. The segment moment of inertia describes the mass distribution along the body segment axis. The moment of inertia of the lower leg (including the foot) was measured by the free oscillation technique in 90 healthy children (61 boys and 29 girls) between 5 and 18 years of age. The period of free oscillation was measured with and without external mass loading. The moment of inertia was calculated using a relation between the mass and the period of oscillation. A two-cylinder model of constant body density was used to prediet the moment of inertia. Anthropometric measurements of length of the lower leg and foot, the circumference of the knee, ankle and foot were made. Experimental and model data of the lower leg of inertia were described by a fifth power function of body height. The experimental and model data showed high degree of convergence, confirming that the segment growth of the human body can be treated like the volume growth of a cylindrical object of constant body density. Thus it was experimentally confirmed that the lower leg segment growth between age 5 and 18 years may be considered as proportional.

Measurements of Dynamic Stability Derivatives of Hyperballistic and Conic Shapes at M = 6.85

SummaryExperimental measurement of the pitching stability derivatives and of AGARD hyperballistic standard models HB1 and HB2, a hemisphere-cylinder double flared configuration designated HBS and three 10° semi angle cones of bluntness ratios 0.0, 0.2 and 0.4 are reported. Data were obtained at a Mach number of 6.85 in a short duration light piston tunnel using the small amplitude free oscillation technique. The tests were conducted at Reynolds numbers from 0.2 × 106 to 0.85 × 106, based on centre-body diameter, for the hyperballistic shapes and from 1.45 × 1016 to 2.17 × 106, based on base diameter, for the conical shapes. The mean angle of attack was varied from 0 to 17.0 degrees and the frequency parameter from 0.0020 to 0.0043 for the hyperballistic shapes and from 0.0018 to 0.0092 for the cones. All models tested were found to be dynamically stable for all positions of the axis of oscillation considered. The significant observed non linear variations of the damping derivative with angle of attack for the 0.2 and 0.4 bluntness cones and the HB2 shape could be partially accounted for qualitatively by boundary layer transition induced effects. The variation of the derivatives with angle of attack for the pointed cone were consistent with a fully turbulent boundary layer, whereas it was probable that for the HB1 and HBS models the boundary layer remained laminar throughout. Variation of the frequency parameter resulted in no significant effect on the measured stability derivatives.

Comparisons of Aerodynamic Data Obtained by Static and Dynamic Techniques

Introduction T HE conventional internal strain gage balance system has been a standard tool for wind tunnel aerodynamics for many years. Although widely used to obtain static aerodynamic data, experimentalists have been well aware of the difficulties in obtaining accurate force and moment data at small angles of attack and of the inaccuracies which result from differentiating experimental moment data to obtain static stability derivatives. Free oscillation techniques have likewise been widely used to obtain dynamic stability data. Although persons familar with dynamic testing techniques are certainly cognizant, the technical community in general may not be aware that static aerodynamic data can readily be obtained from dynamic tests. The purpose of this paper is to make a comparison of static aerodynamic data obtained with an internal strain gage balance and a small amplitude, free oscillation technique. Normal force coefficients, pitching moment coefficients, static stability derivatives, and center of pressure locations are compared for a biconic configuration with a 15% spherically blunted nose. Also included are the results of numerical calculations, using a shock capturing code developed by Solomon.

Aerodynamics of a Slender Cone with Asymmetric Nose Bluntness at Mach 14

T effect of nose shape asymmetries on the aerodynamics of a 10° cone (d= 5 in) was studied in ARL's 20-in.-hypersonic wind tunnel (M^ = 14.2, pt = 1500 psi, Tt = 2000°R, Re^d = 2.3 x 10, TJTt = 0.3, y = 1.4). Surface pressures were measured at various angles of attack and sideslip. Schlieren pictures were taken with a special nose model at several roll attitudes. Stability derivatives were measured by the free oscillation technique at the reduced frequency cod/IV^ « 0.003. The results are summarized and some conclusions relating to flight dynamics are drawn. Figure 1 shows a sketch of model A and the pressure distribution over the afterbody. The asymmetry of the nose tip is the result of a planar cut 40° off the x-z plane. The projected frontal area of the cut is approximately ^% of the base area. The schlieren picture on the left-hand side of Fig. 2 shows the shock contour in the x-y plane near the nose of model A. The plot below is a cross section of model and shock at the axial station l/L = 0.2. In the neighborhood of $ = 270°, the observed shock nearly coincides with the sharp cone shock, as given by the expression

動安定風胴試験の一方法について

Descriptions about dynamic wind tunnel tests, using a free oscillation technique at low speed, are given together with the obtained results.Two kinds of in-put signals are taken. One of them is the step signal, and the other is the random signal. And data of expriments in the former are analyzed by Fourier integral method, and the ones in the latter by computing the power spectrums of signals.Obtained results show;1) By a free oscillation technique, which is one of the simplest techiques, we can get the dynamic stability derivatives of the model airplanes; Cmα, Cmq+Cmα, Clp, Cnβ and Cnr-Cnβ, and these are able to simulate the one of the true airplanes.2) Especially paying attention to the accuracy of the analysis, we can get the derivatives; Clβ, Clr and Cnp, at any rate.