A technique for equivalence assessment of operational loads reproduction while heavy transport helicopter bench tests

DOI: 10.34759/vst-2022-4-116-124

Аuthors

Osipov D. N.^*, Yuskin S. A.^**

,

*e-mail: osipovdn@ncplg.ru
**e-mail: yuskin_sa@ncplg.ru

Abstract

With all the variety of the existing programs for the of aircraft structures behavior mathematical modeling in real operating conditions, bench tests of fuselages and individual structural elements still remain the basic method, substantiating the confirmation (increase) of the resources. One of the tests elements is the reproduction equivalence assessment of operational loads on the bench. As of today, the basic method for equivalence assessment is the tested sample strain-gauging with the bench prior to the testing commence, the assessing criterion herewith is a measure of the discrepancy between the stresses at the structure critical points on the bench and those measured in previous flight tests. This criterion is sufficient for testing aircraft fuselages and, all the more, for individual elements of the aircraft structure. However, for the helicopter fuselage, particularly for the single rotor scheme, subjected to asymmetrical loads in a wide range of frequencies, availability of the technique allowing assessing the structure behavior in total even indirectly would be extremely useful.

The authors propose a new method for the reproduction equivalence assessing of the operational loads during bench tests of the Mi-26(T) helicopter fuselage.

The Mi-26(T) heavy transport helicopter was released with a declared assigned resource of 12,000 hours. However, the assigned resource up to date confirmed by tests is 4,200 hours with the possibility of a phased increase to 4,800–6,000 hours for helicopter instances, depending on the year of manufacture and technical condition. To achieve the designated resource declared by the Developer, it is necessary to continue bench tests of the fuselage. Up to the present day, two sets of such tests have been conducted.

A great number of cracks (up to the 1000 items per a single item, and about 10000 over the whole fleet) of a stringer from the 01420 aluminum-lithium alloy is being detected from the very beginning of the Mi-26 (T) helicopter operation. This alloy has not been applied since 1992, but more than 90% of the Mi-26T helicopter fleet in the civil aviation of the Russian Federation consists of helicopters produced in 1987-1992. Thus, their airworthiness maintenance is an urgent need.

Since 2002, the Federal State Unitary Enterprise GosNIIGA has been keeping records of these cracks, namely the location, the helicopter operation time at the moment of detection, etc. are being recorded, and the generalized map of stringers cracks for the entire fleet and crack maps for the separate samples of helicopters have been created and constantly updated. With all the negative impact of this defect on the operation, its mass character (if the crack occurrence is considered as an event from the viewpoint of the probability theory) allows full application of the mathematical statistics methods for its description. It should be noted particularly that distribution of the number of stringers cracks along the fuselage and in individual compartments qualitatively reflects the stresses distribution in specific zones.

The presented technique is based on a periodic comparison of distribution of the number of stringers cracks on the tested sample with the distribution of the number of cracks on the fleet of helicopters operated or previously operated in the civil aviation of the Russian Federation. The said technique suggests employing the Kolmogorov hypothesis likelihood estimation method for this comparison. This technique application allows assessing the structure behavior in total while the testing process, bringing it as close as possible to real operating conditions. Timely correction of the loading program allows increasing the sample durability on the bench. The said technique herewith does not require costly equipment and great time consumption.

The article demonstrates the technique approbation on the example of technical condition assessing of a specific helicopter fuselage (RA-06015) and by the example of a sample on a test bench.

Keywords:

operational loads reproduction, bench tests, distribution of the number of stringers cracks, Kolmogorov criterion, test sample, of the loading program correction, sample durability

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