RESEARCHING OF EMPIRICAL DISTRIBUTIONS OF RANDOM VALUES OF ROTOR VIBRATION OF A GAS TURBINE ENGINE OF A HELICOPTER IN THE CONDITIONS OF MOUNTAIN DESERT OF THE REPUBLIC OF UZBEKISTAN

ABSTRACT

The article presents the dynamic changes of the random vibration values of the front support of the gas turbine engine of the Mi-8MTV transport helicopter in the mountainous desert area of Uzbekistan. The subordination of empirical random vibration values to the normal Gaussian distribution is investigated. A comprehensive method for determining the technical condition of a helicopter GTE rotor during operation is presented, based on the measurement of vibration parameters and GTE oil temperature.

АННОТАЦИЯ

В статье представлены динамические изминения случайных величин вибрации передней опроры газотурбинного двигателя транспортного вертолёта Ми-8МТВ в условиях горно-пустынной местности Узбекистана. Исследовано подчинения эмпирических случайных значений вибрации нормальному закону распределению Гаусса. Представлена комплексная методика определения технического состояния ротора ГТД вертолета в процессе эксплуатации основанная на измерении параметров вибрации и температуры масла ГТД.

Keywords: parameters of a gas turbine engine, vibration, distribution laws, technical condition.

Ключевые слова: параметры газотурбинного двигателя, вибрация, законы распределения, техническое состояние

Introduction. A significant proportion of GTE failures, leading to serious consequences (touches on the rotor elements, to titanium cases) and, as a result, to early remove the power plant, is the destruction of the GTE shaft bearings [1,2]. In some cases, the diagnosis of malfunctions associated with gas turbine engines can be effectively solved using tribodiagnostics methods [3]. However, these methods are difficult to use and require highly qualified engineering staff.

Currently, helicopters use systems for monitoring the vibration level in the GTE rotor frequency range and issuing a light signal to the crew when its threshold value increases [4]. As the experience of GTE operation shows, this approach allows solving the problems of protecting the engine, but does not allow solving the problems of effective diagnostics of its rotor thrust bearings at an early stage of development of their defects [5, 6]. The statistics of early removal of gas turbine engines from helicopters showed that this is done due to defects in the rotor rolling bearings.

Thus, the issues of operational assessment of the technical condition of the gas turbine engines of helicopters, as well as forecasting the possibility of their further operation, occupy one of the most important places in the work of the aviation engineering service of the Republic of Uzbekistan.

Formulation of the problem

Using modern vibration measurement tools and information from standard helicopter control systems (GTE oil temperature, main rotor speed, etc.), develop a methodology for determining the technical condition of the system (free turbine shaft + bearing) of GTE in relation to the conditions of the mountainous desert terrain of Uzbekistan.

The solution of the problem

Studies have shown that the presence of a defect in the GTE rolling bearing can be diagnosed “by ear”, by the shape of the vibration signal, by the RMS of the signal spectrum, by the spectrum of the vibration signal envelope, using the “peak factor”, “kurtosis” and other methods [ 13, 14].

Figure 1. Wiev of the ET-1AO oscilloscope 1-hardware vibration indication device; 2-sensor seismic SW-230

To collect statistical information about the GTE vibration parameters, a digital oscilloscope of the ET-1AO type was used. The ET-1AO digital oscilloscope model allows you to receive signals via two or a separate channel with a bandwidth of 0 to 1000 Hz.

• The appearance of the device is shown in Fig.1.
• For the implementation of mathematical processing of flight data, the vibration characteristic of the TV3-117VM engine with an average time from the beginning of operation (BO) of 2000÷2500 hours was chosen.
• View of the experimentally obtained vibration signals of the Mi-8 helicopter, shown in Fig. 2. Their analysis made it possible to choose for the presentation of the diagnostic parameter - vibration velocity:

power transmission of transmission elements in the range of 30 ... 140 Hz - vibration velocity;

power transmission of transmission elements in the range of 30 ... 140 Hz - vibration velocity;

GTE shaft and gear transmissions of transmission elements from a frequency of 190 Hz to 400 Hz – vibration velocity;

main and tail propellers in the range from 0…19 Hz – vibration displacement; GTE compressor from frequencies above 500 Hz – vibration acceleration.

The analysis of the experimental data of the vibration signals of the helicopter revealed the following features that were taken into account in the method of synthesizing the system for extracting the diagnostic parameter, namely, diagnosing the RMS spectrum of the vibration signal in the informative range of rotation of the FT GTE shaft - 190-340 Hz:

‒ instability of maintaining the specified speed of the helicopter gas turbine engine during the measurement due to various reasons (gusts of wind, gas-dynamic variability of the combustion process in the gas turbine engine, low throttle response, etc.);

‒ for serviceable helicopters, errors in the operation of the automatic control equipment of the gas turbine engine lead to a spread in the rotational speeds of mechanical units from their nominal value. As an example, Fig. 3 shows the spectra of vibration accelerations of three helicopters with a spread in the rotational speeds of mechanical units from their nominal value.

Figure 2. The received vibration signal of the helicopter, presented as: a - vibration displacement, b - vibration velocity, c - vibration acceleration

Figure 3. Spectra of vibration accelerations of the FT shafts of two helicopters with different degrees of non-stationarity of their GTE revolutions: a - 0.5%, b - 1%

The found distribution law of the diagnostic parameter has the form, Fig.4. The density function of the distribution of the diagnostic parameter is shown in Fig.5.As an example, Fig. 3 shows the spectra of vibration accelerations of three helicopters with a spread in the rotational speeds of mechanical units from their nominal value.

The found distribution law of the diagnostic parameter has the form, Fig.4. The density function of the distribution of the diagnostic parameter is shown in Fig.5.

Figure 4. Type of the law of distribution of the value of the vibration velocity of the FT GTE shaft of the Mi-8MT helicopter, for the period of operation 2000-2500 h

Figure 5. Empirical distribution function F*(x) value of the vibration velocity of the FT GTE shaft of the Mi-8MT helicopter

The diagnostic parameter was the RMS value of the vibration velocity of the GTE shaft, measured at the installation site of the front thrust bearing.

Confidence interval of vibration of the FT GTE shaft with reliability , was found by the formula (1):

                                                                   (2)

Interval Estimation Accuracy:

Figure 6. Dependence of the change in the dynamics of the degradation stochastic characteristics of the rotor vibration on the knowledge of the engine oil temperature – a and its approximation - b

The analysis of the conducted literature showed that when diagnosing complex technical systems, none of the considered diagnostic tools is universal and does not exclude the need to use the others. Thus, to analyze the technical condition of the helicopter GTE rotor, as an additional parameter, we will use the knowledge of the engine oil temperature Tm. This diagnostic parameter showed its high information content when monitoring the technical condition of the helicopter gas turbine engine [15].

The found dependence of the change in the dynamics of the degradation stochastic characteristics of the rotor vibration on the knowledge of the engine oil temperature Tm is shown in Fig.6. Approximation confidence coefficients - R² of the regression dependence (Fig. 6, b) have the following values:

 Т_м = a₁∙a + a₂,                                                                       (3)

where a1 = 3,16; a2 = 55,4 – regression dependence coefficients (3). Approximation confidence factor - R² > R2crit = 0.7, where R2crit is the critical value of the coefficient R² = 0.91. Based on the calculated values and (3), the proposed method for determining the technical condition of the helicopter GTE rotor during operation, based on measuring vibration parameters and GTE oil temperature, has the form (Fig. 6).

Figure 7. Method for determining the technical condition of the helicopter GTE rotor during operation in the conditions of the mountain desert area of Uzbekistan

To integrate diagnostic information, it is proposed to use a variant of the form [2]:

                                                               (4)

where, are the degrees of belonging of signs a and (see Fig. 7) to the D-th class of the technical condition of the GTE rotor (determined by the method of expert survey);

 − a clear decision on the technical condition of the GTE rotor ( > 0,7) – неисправность ГТД, при < 0,7) – no malfunction; 0,7 − decision threshold..

Methodology. The engine mode on the ground is cruising (continuous) according to the TV3-117VM operation manual.

1. Within 15-20 seconds, the parameters of random values of vibration and temperature of the gas turbine engine oil are taken and stored in the memory of the ET-1AO device.

2. The RMS of the vibration velocity of the ST GTE of the helicopter is determined in a given frequency range.

3. The current RMS of the vibration velocity and – a is compared with its interval reference value, see (1).

4. The current engine oil temperature Тm is compared with the reference one calculated at the current value of the vibration velocity - a see (2).

5. To issue a decision on the technical condition (free turbine shaft + bearing) of the gas turbine engine, the obtained information is combined according to formula (4).

Conclusion

The results of a statistical analysis of the parameters of vibration of the GTE rotor of a helicopter in the conditions of the mountainous desert area of Uzbekistan are presented. A multi-parameter regression model of the dependence of the FT GTE shaft vibration on the oil temperature has been obtained. A comprehensive method for determining the technical condition of the system (free turbine shaft + bearing) of the helicopter gas turbine engine during operation based on measuring the parameters of vibration and oil temperature of the gas turbine engine is presented.

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