Динамический анализ деформирования и разрушения элементов конструкций из композита с межслоевыми дефектами

В работе проводятся исследования по оценке влияния внутренних дефектов типа расслоений различного размера и расположения на деформирование и разрушение слоистых элементов конструкций из полимерных композиционных материалов (ПКМ). Действующая нагрузка - динамическая. Для оценки прочности рассматриваемых элементов конструкций используются следующие критерии разрушения для ПКМ [1-5]: Hashin, Chang-Chang, Puck, LaRC03 (Langley Research Center), Fischer.

A review of variable-impedance acoustic liner concepts developed at NASA

This paper presents results attained in the NASA Langley Research Center test rigs using concepts for which the impedance varies over the surface of the liner. These liners are typically designed for significant sound absorption over a wide frequency range, but it is also possible to tune the design to achieve increased absorption at selected frequencies. A brief review is provided regarding a number of variable-impedance concepts. The first is a modified version of a conventional two-layer liner, in which the embedded septum location and acoustic properties are different for adjacent core chambers. Two concepts employ core chambers with different lengths, one with bent chambers to allow packaging within a limited volume, and the other with shared inlet ports to reduce the surface porosity. The last employs a perforated facesheet in which the hole diameter and porosity are varied over the surface of the liner. Data acquired in the NASA normal incidence and grazing flow impedance tubes are used to demonstrate the capabilities of these concepts. Impedance prediction models are also presented for comparison with these measured data.

Higher Harmonic Control: A Historical Perspective

Higher harmonic control (HHC) is an approach for achieving reduced helicopter vibration by controlling the vibratory rotor airloads in such a way that the fuselage excitation is minimized. This paper is a historical look at how a program aimed at helicopter vibration reduction started as an outgrowth of fixed wing flutter suppression at NASA Langley Research Center, proved the HHC concept on aeroelastically scaled wind tunnel models and went on to demonstrate viability in full-scale flight testing on the OH-6A helicopter in 1982. Following the OH-6A flight tests, the helicopter research community was stimulated to prove the effectiveness of HHC on different configurations through analysis, wind tunnel tests, and flight tests. All of these investigations have shown HHC to be effective in reducing vibration to levels not attainable with conventional vibration control methods and without any detrimental side effects. HHC development has progressed to the point that the technology provides one more option to address the ever-present vibration problem in helicopters. The literature demonstrates that helicopter ride quality equivalent to that of fixed wing aircraft is attainable with application of HHC.

Risk Assessment of Obstacle Collision for UAVs under off-nominal conditions

Enabling operations of unmanned aerial vehicles (UAVs) in low-altitude airspace, As widespread applications emerge, the need of risk assessment becomes increasingly important for UAV flights beyond visual line-of-sight, especially subjected to off-nominal conditions introduced by component failures, degraded controllability or environmental disturbances such as wind gusts in an urban canyon. From a safety perspective, collision with obstacles can be detrimental not only to the vehicle and payload, but also to the structure and people on ground. Although it is safe to assume that approved UAVs would be equipped with collision avoidance systems, . In this paper, a framework is presented for computing the risk of collision with obstacle based on a UAV's predicted trajectory, . The conditional probability of trajectory deviation is generated using a Bayesian Belief Network (BBN) based on on-board sensor measurements. Further, a kinematic 3-DOF model is implemented to compute deviation in UAV's trajectory subjected to one case study of off-nominal condition i.e. wind gusts. Finally, the integrated risk factor is demonstrated on real data from experimental flights of an octocopter at NASA Langley Research Center, in presence of simulated obstacles and wind conditions. The proposed approach would enable risk-informed decision making process for timely mitigation of current and future unsafe events.

Search and rescue under the forest canopy using multiple UAVs

We present a multi-robot system for GPS-denied search and rescue under the forest canopy. Forests are particularly challenging environments for collaborative exploration and mapping, in large part due to the existence of severe perceptual aliasing which hinders reliable loop closure detection for mutual localization and map fusion. Our proposed system features unmanned aerial vehicles (UAVs) that perform onboard sensing, estimation, and planning. When communication is available, each UAV transmits compressed tree-based submaps to a central ground station for collaborative simultaneous localization and mapping (CSLAM). To overcome high measurement noise and perceptual aliasing, we use the local configuration of a group of trees as a distinctive feature for robust loop closure detection. Furthermore, we propose a novel procedure based on cycle consistent multiway matching to recover from incorrect pairwise data associations. The returned global data association is guaranteed to be cycle consistent, and is shown to improve both precision and recall compared with the input pairwise associations. The proposed multi-UAV system is validated both in simulation and during real-world collaborative exploration missions at NASA Langley Research Center.