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Computational models for tethers, chains, and ropes are prevalent in mechanical and aerospace systems. Many different model structures, spanning multiple applications, appear in the literature; however, little work exists to show how these different models compare with respect to accuracy, computational expense, and numerical reliability. In this work three discrete tether models, a recursive multi-body model, a Kelvin-Voigt visco-elastic model, and a Standard Linear visco-elastic model, are all compared. Comparisons of each model are made with experimental data collected for tethers of varying material, physical properties, and loading conditions resulting in substantial nonlinear effects. Results show that each model, depending in the application and expected loading, has advantages and disadvantages in regard to computational burden, accuracy, and numerical stability. Guidelines based on the results provide suggestions to which tether models may be most appropriate for a variety of applications. [ABSTRACT FROM AUTHOR]

Autonomous precision placement of parafoils is challenging because of their limited control authority and sensitivity to winds. In particular, when wind speed is near the airspeed, guidance is further complicated by the parafoils inability to penetrate the wind. This article speci-fically addresses the terminal phase and develops an approach for generating optimal trajectories in real-time based on the inverse dynamics in the virtual domain. The method results in efficient solution of a two-point boundary-value problem using only a single optimization parameter allowing the trajectory to be generated at a high rate, mitigating effects of the unknown winds. It is shown through simulation and experimental results that the proposed algorithm works well even in strong winds and is robust to sensor errors and wind uncertainty. [ABSTRACT FROM AUTHOR]

During the shooting motion of a basketball, the path of the ball from the beginning of the lift to release follows an S-shaped trajectory. This study is the first to investigate how two distinctive features of the S-shaped ball path, the maximum curvature, κ max , and terminal curvature, σ, are associated with longitudinal accuracy. The sagittal plane ball path and curvature were found using Bezier curves for 31 professional male basketball athletes. Terminal release curvature had a very strong positive correlation to intra-individual release velocity standard deviation ( r  = 0.73, p  < 0.001) indicating that shooters with straighter terminal ball paths had better longitudinal shooting accuracy. It was also observed that κ max coincided with the transition to the forward shooting motion and that players with higher κ max tend to have a clear two-part shooting style rather than a single fluid motion. A comparison between groups of good and poor shooters identified a mean difference in κ max of 7.9 m -1 ( p  < 0.001, 95% CI: 11.8-4.0 m -1 ) suggesting that good shooters typically have a higher κ max than poor shooters.

Interaction between the shooting hand and ball at the moment a basketball is released generates a three-dimensional backspin of the ball. This study is the first to investigate how characteristics of the backspin alignment and variability contribute to lateral shooting accuracy. Spin axis (SA) direction and backspin magnitude were measured on 25 shot attempts for 26 collegiate basketball players (male: n = 16, female: n = 10). The mean SA alignment, as viewed from the shooting hand side, was found to be tipped down and towards the target ( p < 0.001). Standard deviations (SD) in the SA alignment were strong predictors of lateral accuracy (vertical SD: r = 0.80, p < 0.001, forward-backward SD: r = 0.51, p = 0.01), with variation in the vertical alignment being the best predictor. No significant correlation between mean SA misalignment and lateral accuracy was observed. However, intra-individual relationships between SA misalignment and lateral error revealed that individuals tended to have 0.17 degrees more misalignment for each cm of lateral error ( p < 0.001, 95% CI: 0.24-0.09). These indicate that while an individual's mean alignment may not predict lateral accuracy, improving one's SA alignment and reducing alignment variability may increase lateral accuracy.

The aim of this study was to investigate the role of release parameter changes within individuals (intra-individual) on basketball shooting performance across both free throws and three-point shots, and identify whether any velocity dependence exists. Twelve male basketball players were recorded shooting seventy-five three-point shots (6.75 m) and fifty free throws (4.19 m). Ball release parameters were estimated by combining an analytic trajectory model including drag, a least squares estimator, and gradient-based release distance compensation. Intra-individual release velocity standard deviations (SD) were found to be significantly smaller across all distances ([0.05-0.13 m/s] when compared to statistics reported by other studies [0.2-0.8 m/s]). Despite an increase in lower body motion and a 24% increase in release velocity (p < 0.001) as shooting distance increased, no increases in intra-individual release velocity or angle SD were observed indicating velocity-dependent changes in release parameters were absent. Shooting performance was found to be strongly correlated to the release velocity SD ( r = -0.96, p < 0.001, for three-point shots, and r = -0.88, p < 0.001, for free throws). Release angle SD (1.2 ± 0.24 deg, for three-point shots, and 1.3 ± 0.26 deg, for free throws) showed no increase with distance and unrelated to performance. These findings suggest that velocity-dependent factors have minimal contribution to shooting strategies and an individual's ability to control release velocity at any distance is a primary factor in determining their shooting performance., (© Journal of Sports Science and Medicine.)

It is hypothesized that butterfly wing scale geometry and surface patterning may function to improve aerodynamic efficiency. In order to investigate this hypothesis, a method to measure butterfly flapping kinematics optically over long uninhibited flapping sequences was developed. Statistical results for the climbing flight flapping kinematics of 11 butterflies, based on a total of 236 individual flights, both with and without their wing scales, are presented. Results show, that for each of the 11 butterflies, the mean climbing efficiency decreased after scales were removed. Data was reduced to a single set of differences of climbing efficiency using are paired t-test. Results show a mean decrease in climbing efficiency of 32.2% occurred with a 95% confidence interval of 45.6%-18.8%. Similar analysis showed that the flapping amplitude decreased by 7% while the flapping frequency did not show a significant difference. Results provide strong evidence that butterfly wing scale geometry and surface patterning improve butterfly climbing efficiency. The authors hypothesize that the wing scale's effect in measured climbing efficiency may be due to an improved aerodynamic efficiency of the butterfly and could similarly be used on flapping wing micro air vehicles to potentially achieve similar gains in efficiency.

Birds and insects naturally use passive flexing of their wings to augment their stability in uncertain aerodynamic environments. In a similar manner, micro air vehicle designers have been investigating using wing articulation to take advantage of this phenomenon. The result is a class of articulated micro air vehicles where artificial passive joints are designed into the lifting surfaces. In order to analyze how passive articulation affects performance of micro air vehicles in gusty environments, an efficient 8 degree-of-freedom model is developed. Experimental validation of the proposed mathematical model was accomplished using flight test data of an articulated micro air vehicle obtained from a high resolution indoor tracking facility. Analytical investigation of the gust alleviation properties of the articulated micro air vehicle model was carried out using simulations with varying crosswind gust magnitudes. Simulations show that passive articulation in micro air vehicles can increase their robustness to gusts within a range of joint compliance. It is also shown that if articulation joints are made too compliant that gust mitigation performance is degraded when compared to a rigid system.