Fundamentals of calculation of random vibration acceleration signals during high-speed track tests of new aircraft samples

A key feature of track testing of aircraft and rocketry is the acceleration of the test object, which is comprised of a carriage sliding along the supporting surface of rail guides to designated application speeds, using solid rocket motors. These rocket sled include engine cradle supports rigidly connected to sliding bearings (shoes), a mounting unit for the cantilevered test object, and automatic control and measurement systems for the recorded parameters. Vibration acceleration sensors are housed in the shoes and in the test object mounting bracket. These sensors are designed to measure vibration and shock loads on track equipment components. However, the size and design of the sensors are such that armor protection is required for their reliable operation under high-speed monorail testing conditions. Therefore, the sensors are mounted in locations that provide this protection, rather than at the centers of mass of the carriage's structural components. Consequently, the problem of recalculating experimental data for actual accelerations of the rocket carriage components and the test object arises. When a tracked sled is accelerated by rocket engines, the motion dynamics are characterized by the following modes: the cannon launch, due to the inertia of the overall mass of the payload, is perceived as an impulse force in the direction of motion in the moving coordinate system. Subsequently, the difference in engine thrust and aerodynamic drag forces increases the carriage's velocity. Sliding friction forces are low and subsequently decrease as the aerodynamic lift component increases. The existing track – the track – has irregularities and deviations from straightness along the vertical and lateral axes in the fixed coordinate system. To ensure the so-called “passage condition”, the shoes are manufactured and installed with minimal but sufficient lateral and vertical clearances between the contact surfaces. Consequently, the rocket sled experiences random impact forces from the irregularities and rail joints, which are transmitted through the structure to the test object. The problem of describing the dynamics of the sled motion over the entire period of the experiment is nonlinear due to the presence of lateral and vertical gaps between the shoes and the rail, therefore this article examines the vibration accelerations measured by sensors placed on the structural elements during the acceleration of the experimental setup on a limited section of the track up to 600 m long, i.e., before the appearance of nonlinear effects. In addition to vibrations from the shoes, the test object is subject to variable aerodynamic drag forces and moments from these forces, creating a random spatial variable field of vibration-impact effects applied to the test object. The structural response at the sensor locations at ultra-high speeds reflects a complex, integrated resultant pattern. The primary objective of this study is to analyze the response of the rocket carriage's structural elements as it accelerates to designated application speeds at various track sections and at various points in flight, in terms of structural strength and stability against extreme force effects. This paper presents a methodology for calculating the structural response to random vibration-impact effects during the unsteady acceleration of an experimental monorail installation, and includes an example of calculations based on experimental launch data. The probability density distribution of the recorded vibration acceleration signals is shown to conform to a normal law. Amplitude-frequency spectra of the maximum impacts on the test object simulator structure are obtained. Dynamic transfer function coefficients for vibration acceleration signals along the vertical axis from the front shoe to the sensor located in the test object model were determined. An analysis of the instantaneous amplitude-frequency characteristics of the vertical vibration-impact signals during the transition from transonic to supersonic speed of the rocket carriage was performed. It was determined that individual maximum response amplitude values of the test object simulator, calculated from the instantaneous amplitude-frequency characteristics, exceed similar values obtained using Fourier transforms by an order of magnitude.