Aerodynamic Constraints Research Articles

The automatic vowel of Cr clusters

Two phenomena are reported in the phonemic descriptions of various languages: Some languages tend to create /Cr/ or /rC/ clusters by reducing an intervening unstressed vowel. Other languages tend to dissolve such clusters by lengthening the vocalic transition that occurs between the two segments, this vocalic transition being originally automatic, i.e., due to articulatory and/or aerodynamic constraints. Oscillograms were made to measure the duration of such automatic vowels in stressed and unstressed syllables in Spanish. Pressure records were also made to show the variation in air‐flow during articulation. Perception tests were then performed to assess how clusters form and dissolve when duration of the automatic vowel is systematically shortened or lengthened. Edited utterances were played to native speakers of Spanish who were asked to judge whether they heard a version with a cluster or with no cluster. The likelihood of a stimulus being perceived as a cluster was inversely proportional to the duration of the automatic vowel. However, the perceived cluster was not so readily dissolved before following full vowels /a/ or /o/.

Wind tunnel measurements of blade/vane ratio and spacing effects on fan noise

Introduction R turbofan engine designs aimed at increasing the structural rigidity and reducing the weight of the engine frame lead to a reduced number of stator vanes and an abandonment of vane-blade ratios greater than required for fundamental tone cutoff.' These designs, which consider stator solidity aerodynamic constraints, consist of a relatively few longer chord, larger thickness stator vanes which are load-carrying members of an integrated vane-frame. Basic acoustic data on the effects of vane-blade ratio and spacing under conditions which simulate flight are needed to determine the acoustic consequences of adopting alternate designs such as the integrated vane-frame. Expected noise consequences of varying fan stage design parameters, such as vane-blade ratio and rotor-stator spacing have often been masked in static testing. This is largely due to the fact that the fundamental blade passing tone level and, to a lesser extent, the tone harmonics levels are controlled by rotor-inflow interaction mechanisms,' which mask the rotor-stator interaction noise. The blade passing tone level of a fan designed for fundamental tone cutoff is greatly reduced with forward velocity. Noise studies performed in an anechoic wind tunnel' have shown results similar to those obtained in flight tests. In the present study the noise of a 50.8-cm-diam research turbofan simulator was measured in the anechoic wind tunnel described in Refs. 6 and 7. Both vane-blade ratio and rotorstator spacing were varied. Specifically, two stator vane numbers were chosen to achieve a cuton and cutoff condition with respect to propagation of the fundamental rotor-stator interaction tone as predicted by the theory of Ref. 8. The noise associated with each of these stator configurations was measured at three rotor-stator spacings ranging from 0.5 to 2.0 rotor chords.

3-D Analysis of the Flight Trajectories of Flies (Drosophila melanogaster)

Abstract We have developed a computer system for reconstructing and analyzing three dimensional flight trajectories of flies. Its application to the study of the free flight behaviour of the fruitfly Drosophila melanogaster is described. The main results are: a) Drosophila males only occasionally track other flies; b) in such cases the fly’s angular velocity is a function of the error angle under which the leading fly is seen; c) body saccades can be demonstrated during cruising flights; d) high angular velocities are strongly correlated with low forward velocities, probably reflecting an aerodynamic constraint of flight. The 3-D technique described may provide an adequate tool for studying the organization of the systems present in flies and for relating the free flight behaviour to previous analyses of tethered flies.

Temporal control of voicing during occlusion in plosives

A study was made of duration of vocal-fold vibration prior to release of occlusion (prevoicing during initial voiced bilabial stops and of duration of voice continuation after occlusion (glottal leak) in intervocalic voiceless bilabial and velar stops. There were six female and five male English-speaking subjects. Prevoicing duration in CVC syllables spoken in isolation was greater before front vowels than before back vowels and greater preceding high vowels than preceding low vowels. Females showed longer prevoicing durations than males. Glottal leak duration in voiceless intervocalic plosives in bisyllabic English words spoken in isolation was longer for bilabials than for velars, longer following than preceding stressed vowels, and longer for bilabials in the environment of [ɪ] than in the environment of [a]. Duration in velars was similarly affected by quality of the preceding but not of the following vowel. Glottal leak durations were less for females than males. Results are discussed in terms of the relative roles of learned adjustments and obligatory aerodynamic constraints arising from upper cavity configurations.

Orbiter Abort Guidance

Abort is defined as a premature or abnormal termination of a mission as a result of either a system or human failure. The abort guidance during the ascent phase, among all flight phases of the shuttle vehicle, is most critical; because it involves guiding a heavily loaded shuttle vehicle that is passing through severe environmental conditions. This paper describes a predictive guidance algorithm for the abort mode initiated during the orbiter ascent flight phase. Contrary to the concept of the minimization1 of fuel consumption during a nominal ascent flight phase, the Space Shuttle orbiter is required to deplete its fuel tank in order to be separated from the vehicle during the abort mode. The guidance objective during the abort mode is to attain a proper terminal energy state in relation to the landing site, consistent with the aerodynamic heating and loading constraints. In the absence of a well-defined payoff function for optimization during this mode, the abort guidance algorithm presented in this paper is formulated to minimize aerodynamic forces. Minimization of aerodynamic forces was considered because of the possibility that the failure that caused the initiation of the abort mode may have weakened the orbiter structure. Immediately after the abort mode initiation, consistent with the philosophy of the minimization of the aerodynamic forces, the orbiter is forced to fly a lifting trajectory to attain higher altitude, thereby reaching a low dynamic pressure environment. The predictive guidance algorithm uses the knowledge of the current orbiter state and the desired terminal state in relation to the landing site, and determines the time sequence of the powered maneuver and the orientation of the thrust vector throughout the abort mode. For example, the magnitude of the down-range component of the acceleration, the time at which to begin to return toward the desired landing site, and the radial acceleration necessary to attain the desired terminal altitude (while remaining in the low dynamic pressure environment) are computed. The key formulations in the predictive guidance algorithm are analytical expressions for the terminal state that the vehicle will reach for a given sequence of thrust orientations, and the associated Jacobian matrix. Therefore, these have been carefully developed in the paper. In the terminal phase of the abort mode, the predictive guidance reduces to a velocity to be gained scheme which has the desirable effect of reducing the deviations in the end-state prediction that results from variations and uncertainties in the vehicle characteristics. Detailed results for representative abort cases and estimates of the computer requirements are also included in the paper.