Vol 14, No 3-1 (2015): Special Issue

The role of Central Institute of Aviation Motors (CIAM) as the leading scientific-research institute in the system of provision of the required dynamic and strength parameters of aircraft engines developed in the country is shown in the paper. Principles and traditions of the CIAM’s scientific strength school as applied to solving practical problems, establishing the causes of defects and working out specific certain recommendations on the basis of fundamental and engineering approaches are discussed. The fact that CIAM brings together engineers and researchers, as well as employees of industrial enterprises, universities, academic and branch institutes, research institutions of the Ministry of Defense and civil aviation, working in close contact with them, is a specific feature of the CIAM’s scientific strength school. The role of the outstanding scientists I. Sh. Neiman, S. V. Serencen, R. S. Kinasoshvili, I.A. Birger in the forming and development of the CIAM’s scientific strength school is also shown. A wide range of subjects dealing with the study of aircraft engine dynamics with an appropriate combination of theoretical, computational and experimental investigations in the area of creating and expanding the unique research-and-trial facilities for strength testing of engines and their assemblies and systems is noted. Much attention is given to conversion programs on ground-based application of gas-turbine engines including the development of Russian and International regulatory documents. Special emphasis is placed on close ties between the CIAM’s strength specialists with Samara’s aircraft engine manufacturers and the scientific strength school of CIAM and Samara’s aerospace cluster, as well as the outstanding role of academician N.D. Kuznetsov in the development and strengthening of these ties.

Requirements of the technical regulations for the order and procedure of estimating structural strength of metal alloys used in critical and main parts of aviation engines in their certification are outlined in the paper. The scope of work to be carried out by the engine designer, the developer of the material/semi-finished product and the engine manufacturer to confirm the compliance with the mentioned requirements is described. The tasks to be solved at different stages of general and special qualification of the material are discussed. The spectrum of the main mechanical characteristics of alloys, including mono-crystal alloys, to be determined during the qualification tests is presented. The standards in accordance to which the tests are to be performed are mentioned. Relationships for the determination of the material characteristics to be used in estimating the strength and service life of engine parts are given. The requirements for the statistical processing of the results of investigating the structural strength of alloys are discussed, as well as the requirements for setting the values of the mechanical properties guaranteed in the delivery of materials/semi-finished products. The necessity to have data on possible presence of different defects in the main (critical for safe operation) engine parts is mentioned. The reference list includes principal technical regulations the requirements of which are to be met during special qualification of alloys used for the manufacture of the main and critical aviation engine parts.

The paper is related to gas turbine engine starter-generator blade containment tests. The starter turbine case is expected to contain a fragment resulting from rotor fracture with the maximal level of kinetic energy. It has been proved that one third of a rotor is a fragment with the maximal level of kinetic energy. It is possible to provide this type of fracture for rotors with central holes by special trimming. However, if the disc and the shaft are a one-piece structure formation of such a fragment seems to be a problem. This type of fracture is assumed to be impossible. The aim of this work is to specify the most energy-consuming fragment in the case of possible fracture and choose a way of conducting certification tests. The method presented in the paper is based on introducing a “weak link” –local thinning- in the area under the rim. It allows separating the rim and the blades with an admissible level of kinetic energy. Some proposals for certification tests are presented.

The main tasks of using spin rigs for carrying out certification, engineering and technological tests of gas turbine engine parts and assemblies are formulated. Specific design features of spin rigs are described in the paper. The main characteristics of the spin rigs used at CIAM are presented. The peculiarities of carrying out different tests using spin rigs and the main requirements for rigs are discussed. Examples of using spin rigs for carrying out tests are given. Examples of using the rigs are given, among them are tests intended to confirm the rotor load capacity or technological hardening of the rotor material; equivalent-cyclic tests to confirm the rotor life; investigation of the  vibration properties of rotating parts (optimization of structural damping of vibrations and specifying high-cycle fatigue of rotating blades, etc.); tests carried out to confirm the containment of rotor fragments in the engine casing and to determine the  rotor integrity under the impact of bird or other foreign objects sucked in the engine gas-air flow duct. The necessity of combining spin rig tests with calculations and physical investigations is shown. The main directions of updating spin rigs are considered. In particular, the necessity of developing rigs for different tests of engine fans with high bypass ratios; perfecting rigs and testing procedures for high-cycle fatigue investigations of rotating blades; producing the equipment for thermal cycling tests of rotor parts, especially parts made of composite or ceramic materials; investigation of vibrations in the conditions of rotor-stator contact; improvement of hardware and methods. 

An original CIAM know-how for the experimental determination of thermal-fatigue life of parts of the gas-turbine engine hot section, including parts with heat-resistant and ceramic thermal barrier coatings, in the conditions of superficial uneven heating by high-frequency currents with continuous cooling of the parts at the preset air consumption is outlined in the paper.  The possibility of using high-frequency heating of the parts with coatings (including thermal protection coatings) for modeling superficial heating of full-scale parts of gas-turbine engines is shown on the basis of the tests of model samples with coatings controlling the temperature of the surface by means of a thermal imager. The method used makes it possible to simulate the thermal state of the part in the operating conditions. Temperature differences observed both over the surface and the thickness of a part create thermal stresses in the material, similar to those arising during the operation of the engine. If necessary, mechanical loads synchronized with changes in temperature, to simulate for example, the centrifugal force may be applied to the part. The article presents the results of experimental studies of thermal-fatigue life of rotor blades of the 1st stage of the turbine made of an advanced monocrystal alloy VZhM-5 without coating and with various protective coatings, conducted according in the regime Tmin↔Tmax = 350↔1050˚C.

The traditional approach to designing turbine blisks based on the application of disk-blade pawl-coupling does not allow improving the design from the point of view of increasing the service life, reducing the wheel mass and improving the engine efficiency on the whole. This problem can be solved by using dissimilar materials in the integrated bimetallic wheel (blisk). Bimetallic blisk prototype models of a high-temperature turbine have been produced for strength durability research. The bimetallic blisk prototype model consists of one monocrystal cooled blade and a granulated Ni-based disk part. These elements are connected by the hot isostatic pressing (HIP) method. Capsular equipment is designed for producing the prototypes. It ensures the air tightness of the zone of joining of the blisk elements and preservation of the cooling lines in the blades during the HIP process.  Investigations of micro- and macrostructures of prototype joining zones have been carried out. High-cycle fatigue tests of bimetallic blisk prototype models have been carried out. It has been found that fatigue cracks originate in the blade material, whereas the zone of diffusion bonding of the two alloys in test conditions proved to be stronger than the blade material.

Technological and experimental research in support of creation of a high-temperature turbine nozzle unit made of diamond dust dispersion-hardened silicon carbide and the flame tube of a high-temperature combustion chamber made of carbon-fiber-reinforced-ray of ceramic composite material based on a ceramic polymer has been conducted. The studies demonstrated:

• technological capability of creating a composite structure of the nozzle made of super-hard diamond dust dispersion-hardened silicon carbide compatible with the mating metal turbine units;

• the results of testing the non-cooled flame tube made of carbon-fiber reinforced ceramic composite material based on a ceramic polymer.

The results will be used in the design, manufacture and tests of non-metallic components of combustion chambers and turbines of auxiliary power units, small-sized gas turbine engines and helicopter engines.

Surge is a dangerous aero-elastic phenomenon. When surge occurs a shock wave originates in the gas path of the compressor, which damages individual elements of the compressor, as well as the test beds for measuring the parameters of the workflow. Therefore, early diagnostics of pre-surge states of multistage axial compressors of modern aircraft engines characterized by extremely high aerodynamic loading and small radial clearances between the rotor and the stator is an urgent task. The paper presents the results of the analysis of dynamic processes that take place during throttling of aircraft engine multistage compressors in test bed conditions.  Diagnostic features of the pre-surge state of the compressor were revealed in the signals from strain gauges mounted on the blades, and the signals from sensors of static pressure pulsations installed in the compressor housing. The research was carried out using up-to-date methods of processing and analysis of the signals of dynamic processes based on the fast Fourier transform and wavelet analysis using a 3D representation of the results of the study. The results of the study indicate the possibility of early diagnosis of surge, and also show that several forms of axial vibrations and their respective acoustic frequencies depending on the size of connected masses may occur in the case of loss of gas dynamic stability. The developed methods and means are proposed to be used for the creation of automated systems for preventing surge in compressor rig testing to determine the gas dynamic stability margin.

The results of investigations in experimental and computational estimation of the efficiency of engine part vibration damping using different ways of damping carried out at CIAM are presented. Vibration tests have been carried out on a dynamic simulator for a turbine stage with dampers of various masses. The damper with the optimal mass and stiffness is specified according to the results of blade strain-gaging. The possibility of efficient damping of compressor blade vibration using thin damping coatings is shown. A method of damping a fan blade without a root platform is presented. The efficiency of the method proposed has been analyzed and experimentally confirmed using a plate imitating the fan blade. The possibility of active damping of gas turbine engine part vibrations using piezoelectric elements has been considered, and the potential efficiency of the method has been experimentally confirmed. Reducing resonant stresses in a thin titanium plate is an example of applying the method.

The results of modeling nonlinear parametric vibrations in aircraft tooth gears are presented in the paper.  Resonance vibration with loss of tooth contact is analyzed.  A lumped-parameter dynamic model of cylindrical gears is investigated using the results of gear stiffness modeling obtained from a finite element model for simulating elastic-damping connection for each step during one phase of mesh. Parametric vibrations of a system with base excitation due to variable mesh stiffness and non-linear effects determined by modeling the conditions of loss of tooth contact are studied. It is shown that the maximal level of dynamic loads does not exceed a limit value determined by the presence of discontinuous vibrations in the system. A number of parametric studies are presented to demonstrate the influence of major gear contact ratio on the resultant dynamic behavior. The results obtained by the numerical simulation of steady-state responses of two different pairs of spur gears are in satisfactory agreement with the experimental results. 

The helicopter ground-airborne health and usage monitoring system (HUMS) presented in the paper illustrates the basic methods of determining the technical condition of aircraft drives by a set of signals received in operation. These algorithms can be realized using the kinematic scheme of a rotor unit with information about the number of teeth in gears, the number of splines in spline connections, the design parameters of the bearings as well as the presence of synchronization signals that are recorded in parallel with vibration signals. The presented methodology for the estimation of vibration parameters of multi-shaft units takes into account the fluctuation of specific frequencies in the real operating conditions of the gearbox. Hardware requirements are formed on the basis of vibration models of gears and gearboxes. The operating frequency envelope of vibration sensors is to include specific frequencies of all gears, spline connections and rolling bearings. The dynamic range is determined based on the vibration activity of the gearbox and units in the places where the sensors are installed. The requirements to the equipment of data accumulation and pretreatment are presented and validated. A method of balancing helicopter classical and coaxial rotors is presented.  The method is based on the analysis of the effect of varying the values of the adjustment parameters of rotor blades (balance weight, assembly swash plate, tab angle) on the values of amplitudes of vibration harmonic components. The main goals of the support center concerning the technical condition of helicopter drives and its cooperation with the operator are stated.

Problems connected with ensuring reliability and service life of bearing assemblies of helicopter gas-turbine engines and accessory gearboxes have a significant place within the framework of ensuring reliability of the latter. The experience of operating the existing aviation gas-turbines and gearboxes, as well as the experience of creating new types of structures, shows that determination and establishment of the service life of the main and most loaded parts that include bearing assemblies is one of the first-priority problems facing producers of aviation gas-turbine engines and gears. The paper demonstrates an attempt to analyze the main problems which are at the bottom of shorter service life and lower reliability of domestic aviation bearings as compared to bearings made by foreign manufacturers. The scope of work to ensure the reliability of bearing assemblies in aerospace products is specified, proposals for solving the existing problems are put forward. Experimental methods of confirming the service life of bearings are outlined.

A lubrication system with supply and scavenge oil gear pumps with electric drives is the subject of research. The features of the workflow in lubrication system units are analyzed and the possibility of applying a homogeneous model for the description of a fine-dispersed air-oil mixture is evaluated. The principle of construction of a mathematical model of a lubrication system based on dynamic equations of the typical processes in the nodes – the flow of the two-phase mixture, mixing of the air and liquid flows, filling cavities of gear pairs in the suction zone of the pump is outlined. The developed mathematical model is verified by comparing the calculated and experimental processes.