Experience of virtual simulation of aircraft flight accidences related to strength problems

摘要

The strength investigation tasks, concerning to aircraft flight accidences investigation, includes: 1. Why has the accident happened - took place the strength deficiency (static? fatigue?) of the structure or not? 2. Investigation of accident conditions, namely - flight conditions identification using flight parameters recorders, analysis of technical condition of the vehicle under investigation, crew and witnesses evidences and other dates, related to the accident; 3. Residual strength of damaged aircraft analysis - analysis of possibility of exploring of the vehicle after accident, concretization of check zones for nondestructive inspection (NDI) implementation to find hidden flaws. Estimation of the satisfactory of repair scheme for permanent exploitation of the aircraft, estimation of safety of one-time flight having damaged structure or some degree field repair is needed; 4. More exact analysis of strength of aircraft structure - obtaining of true values of margins of safety of significant structure items, using accident loads. The significant difference between fixed wing and rotary wing aircrafts is what helicopters are safer -having more possibility of crew surviving and keeping safe structure. As result, it is possible, to estimate true strength of structure using different extreme load conditions. The list of tasks is not complete enough, but they are these, where a virtual simulation can be effectively used, namely using finite element method (FEM) modeling. But these numerical models should meet stronger requirement comparing to usual FEM-models of design phase of aircraft. It stems from physical nature of accident process. It is dynamic, nonlinear and the structure may be heavily damaged, with, as result, varying during the process the stiffness, mass and other properties. So the main feature of these models is what these models should be multidisciplinary. Namely - analyst should have possibility to solve static, dynamic, fracture, post buckling behavior fluid-structure interaction problems. Also it may such situation, when behavior of system 'structure - aerodynamic loads - control' should be investigated. The situation may be more difficult due to time and schedule limitations. These problems may be solved using a single (universal) multidisciplinary model or having a set of different models, in the case effective interface between these models should be exist. In present time there are several programs (sets of software products), which meet the requirements, numbered above - for example 1. Nastran (NastranMD) + Dytran + Marc + EASY5 + ADAMS (MSC. Software); 2. ANSYS; 3. LMS.Virtual Lab, LMS AMESIM, NX (Siemens). Approach of virtual modeling may be used only when analysis models are verified due comparing with experimental (flight or ground) measurements. Mil Moscow Helicopter plant had developed a set of such models, what meet the requirements'. These models correspond to all manufactured now aircraft types - Mi-28, Mi-38, Mi-28, Mi-8 (Mi-171). Of course, these models were developed not only with aim to investigate flight accidences, but they were used in design process of aircrafts structures. In the article unique examples of successful employment of the approach were discussed: 1. Residual strength estimation of fuselage of Mi-26 helicopter having one frame damaged. 2. Analysis of emergency landing process of Mi-8 helicopter. 3. Investigation of strength and fuel keeping capacity of fuel tank of Mi-8 after it's over keeling. 4. Analysis of strength of structure of Mi-26 after lightning strike. 5. Investigation of flight accidence case and analysis of fatigue resistance of Mi-26 body structure. Results obtained helped to find reasonable solutions in any the cases, to verify a possibility of safe exploitations of the aircrafts and to develop recommendation to future structures modifications.