Homiletics

Homiletics

Effects of Sweep Angle on the Boundary-Layer Stability Characteristics of an Untapered Wing at Low Speeds

An investigation was conducted in the Ames 12-Foot Low-Turbulence Pressure Tunnel to determine the effects of sweep on the boundary-layer stability characteristics of an untapered variable-sweep wing having an NACA 64(2)A015 section normal to the leading edge. Pressure distribution and transition were measured on the wing at low speeds at sweep angles of 0, 10, 20, 30, 40, and 50 deg. and at angles of attack from -3 to 3 deg. The investigation also included flow-visualization studies on the surface at sweep angles from 0 to 50 deg. and total pressure surveys in the boundary layer at a sweep angle of 30 deg. for angles of attack from -12 to 0 deg. It was found that sweep caused premature transition on the wing under certain conditions. This effect resulted from the formation of vortices in the boundary layer when a critical combination of sweep angle, pressure gradient, and stream Reynolds number was attained. A useful parameter in indicating the combined effect of these flow variables on vortex formation and on beginning transition is the crossflow Reynolds number. The critical values of crossflow Reynolds number for vortex formation found in this investigation range from about 135 to 190 and are in good agreement with those reported in previous investigations. The values of crossflow Reynolds number for beginning transitions were found to be between 190 and 260. For each condition (i.e., development of vortices and initiation of transition at a given location) the lower values in the specified ranges were obtained with a light coating of flow-visualization material on the surface. A method is presented for the rapid computation of crossflow Reynolds number on any swept surface for which the pressure distribution is known. From calculations based on this method, it was found that the maximum values of crossflow Reynolds number are attained under conditions of a strong pressure gradient and at a sweep angle of about 50 deg. Due to the primary dependence on pressure gradient, effects of sweep in causing premature transition are generally first encountered on the lower surfaces of wings operating at positive angles of attack

Elementary School Size and Differences in What Principals Emphasize and How They Train Their Teachers: A National Analysis

In this investigation, differences in what principals emphasized, in how they spent their work time, and how they trained their teachers were examined as a function of student enrollment numbers.  Data were acquired from the Early Childhood Longitudinal Study Kindergarten Class of 2010-2011 principal survey.  Three school categories were generated with student enrolment data: Small-size schools, Moderate-size schools, and Large-size schools.  Inferential statistical analyses revealed the presence of statistically significant differences in the way principals reporting spending their time and the training areas they emphasized.  Principals of Large-size schools spent more hours at work, invested more time working with teachers, and emphasized more training their teachers than principals of Small-size schools and Moderate-size schools.  Suggestions for future research and implications for policy and practice were made