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Key words: transmission performance, pressure and flow characteristics, hydraulic resistance load, clearance friction of landing gear. Text: 1. Research on the failure of automatic retraction of front landing gear of J-7 aircraft. Landing gear retraction system is an important part of aircraft. The working performance of the system directly affects the safety and maneuverability of the aircraft. The improved design of aircraft landing gear retraction system is mainly used to control the retraction of landing gear and the opening and closing of main landing gear door and front landing gear door. It is an important system of aircraft, and its normal operation will directly affect flight safety. Therefore, it is very important to maintain the system, analyze and study the faults, and carry out effective prevention. In preparation for the flight test of a new type of aircraft, when the crew connected the ground pressure source and power supply to adjust the parking and braking pressure of the aircraft, the nose of the aircraft began to retract slowly after the pressure supply was 65438±0.3min, which eventually led to the grounding of the nose, resulting in serious consequences such as tearing of the radome skin and nose fuselage 02, structural damage and deformation of the front landing gear. In this paper, the failure of automatic retraction of nose landing gear will be analyzed and studied, and preventive measures will be put forward on this basis. 1 Analysis diagram of landing gear retraction control principle 1 Schematic diagram of front landing gear retraction system The principle of front landing gear retraction system is shown in figure 1. When the landing gear retracting handle (hereinafter referred to as the handle) is in the retracting position during normal retracting clearance, the electro-hydraulic reversing valve L makes high-pressure oil enter the retracting pipeline, and the lowering pipeline B is communicated with the oil return pipeline. Under the action of high-pressure oil, the piston rod of the lower lock core retracts and the lower lock is opened. On the one hand, the hydraulic control check valve 13 of the other high-pressure oil is opened, so that the return oil of the door actuator 10 and 12 is slightly connected; On the other hand, oil enters the retraction actuator through the restriction valve 9, so that the piston rod extends and the landing gear retracts. Return oil from actuator 8 flows into the oil tank through pedal valve 7, emergency changeover valve 4, electro-hydraulic reversing valve 1 and emergency oil drain valve 2. When the landing gear is retracted, the coordination valve 1 1 is pressed down, and high-pressure oil enters the upper cavities of the door actuators lO and 12 to close the door. When the handle is in the lower position, the oil inlet is connected with the lower pipeline, the upper pipeline is connected with the oil return pipeline, and the landing gear is lowered. There is also a grounding interlock switch in the system. When the plane is parked, the interlock switch will automatically disconnect the circuit of the electro-hydraulic reversing valve. At this time, even if the handle is put in the stowed position, the electro-hydraulic reversing valve will not work, thus preventing the landing gear from being stowed on the ground by mistake. According to the principle of landing gear retraction control, the lowered position of the front landing gear is maintained by the rear strut with the lower lock, so the necessary condition for retracting the front landing gear is that the lower lock is unlocked. There are two situations when the lower lock is unlocked: the first is mechanical reasons, that is, the lower lock is in a false lock state when the landing gear is lowered, and it is disturbed by some external force during maintenance and use; The second reason is hydraulic pressure, that is, hydraulic oil enters the unlocking cylinder of the lower lock, which retracts the piston rod of the actuating cylinder and unlocks the lower lock. However, no false locking of the lower lock was found in the external inspection and post-release inspection. Therefore, the automatic retraction of the nose landing gear is caused by hydraulic pressure. There are many factors that cause the lower lock to be unlocked by hydraulic pressure. When the electro-hydraulic reversing valve works abnormally, the oil inlet and take-up pipeline are connected, or the interlock switch fails, and the ground mistakenly puts the handle in the take-up position. When the electro-hydraulic directional valve works, when the oil pressure is supplied to the aircraft, the lower lock will be unlocked. However, in both cases, the nose landing gear will retract at a faster speed rather than slowly, and the main landing gear will also retract at the same time. But this is inconsistent with the actual situation at the time of the accident, so it can be basically ruled out. Combined with the accident at that time, the reasons for the automatic retraction of the front landing gear are as follows. 2. 1 The performance of electro-hydraulic reversing valve is poor. Landing gear electro-hydraulic reversing valve is used to control landing gear retraction pipeline. It is a three-position and four-position electro-hydraulic valve. When the handle is in neutral position (no power supply), the electro-hydraulic reversing valve is in neutral position (power off). At this time, the oil supply pipeline is blocked, and the landing gear retraction pipeline is connected with the oil return circuit, as shown in Figure 2. Because there is a radial gap 6 between the slide valve and the valve sleeve, two identical rectangular throttling gaps are formed by 6, and the throttling area of this gap is A=W8. Because of the shape 6, and the flow through this throttle valve is very small, Reynolds number M is also very small, and the flow state belongs to laminar flow, so the flow q through this throttle valve is: in the formula:-the pressure difference between the two sides of the throttle valve; —— Dynamic viscosity coefficient; -Throttle area gradient. Then the flow through the two throttle valves at this time is: where:-the oil supply pressure of the main hydraulic system; —— Pressure of oil return pipeline. As can be seen from the above formula, the leakage is mainly determined by the gradient shape of the throttle area and the radial gap 6. The larger the gap 6, the greater the leakage. The shape is mainly related to the diameter of the spool. The larger the diameter, the greater the gradient. The size of 6 is mainly related to the shape of the valve port, manufacturing technology and processing quality. When the design is reasonable, the process level and processing quality are high, and there is no eccentricity between the slide valve and the valve sleeve, the 6 is small. If it is a new valve, the radial clearance is small, so the leakage is also small; If it is an old valve, because the control edge is worn, the leakage area increases and the leakage amount also increases. In order to measure the leakage, remove the electro-hydraulic reversing valve, plug the two connectors leading to the actuator, apply 20.59MPa hydraulic pressure to the pressure supply connector, and connect the measuring cup to the oil return connector. After 3 minutes, the oil leakage at the oil return joint is 45mL, which is far greater than the specified requirement of no more than 20mL. The leakage schematic diagram of electro-hydraulic reversing valve is shown in Figure 3. 2.2 The system is incomplete and the oil return circuit is blocked. In order to improve the reliability of the landing gear retraction system, redundancy technology is adopted in the system design. That is, when the landing gear fails to retract normally, the pilot can use cold air to lower the landing gear urgently to ensure safe landing, as shown in figure 1. In order to prevent a large amount of hydraulic oil from flowing back to the closed pressurized oil tank during emergency landing gear release, so that the oil tank will burst due to too much oil return, an emergency oil drain valve is installed on the oil return path of the electro-hydraulic reversing valve. When the emergency landing gear is released, the oil collected in the pipeline is directly discharged to the outside of the aircraft. Usually, when the main hydraulic system supplies pressure and the electro-hydraulic reversing valve does not work, the oil leaked from the electro-hydraulic reversing valve into the retraction pipeline can directly flow into the oil return pipeline through the emergency oil drain valve, so the pressure of the retraction system will not increase; If the oil return pipeline is blocked and cannot return oil, the leaked oil will enter the collection and discharge system (see Figure L and Figure 2), which will increase the pressure of the system. When the pressure rises to a certain value, it will cause system failure. It is understood that before the accident, the emergency oil drain valve was removed for maintenance due to failure, and the oil return circuit was blocked by the plug, making it impossible for the landing gear retraction system to return oil. In this way, the pressure oil leaked from the electro-hydraulic reversing valve to the retracting pipeline cannot be released, and the oil pressure of the retracting system will gradually increase. Because the unlocking pressure of the lower lock of the front landing gear is lower than that of the main landing gear, when the pressure reaches a certain value, the lower lock of the front landing gear will be unlocked first, so that the aircraft will automatically stow the front landing gear under its own weight. 3 Fault Verification In order to verify the correctness of the above analysis, the following tests were carried out on the original machine: (1) Provide pressure and power to the main hydraulic system and put the handle in neutral position. After holding for 30 minutes, the nose landing gear will not lock. This shows that under the condition of complete system, the pressure oil entering the retraction system due to the leakage of electro-hydraulic reversing valve can be discharged from the emergency oil drain valve to the system oil tank in time. (2) Simulate the system environment when an accident occurs, remove the emergency oil drain valve and block the oil return circuit with a plug. After 5 minutes of gas supply from the main hydraulic system, the lower lock of the front landing gear began to work, and the lower lock was completely unlocked after 6 minutes. This test is enough to prove that the oil leaking from the landing gear electro-hydraulic reversing valve into the landing gear retraction system can really unlock the front landing gear, which shows that the above analysis is completely correct. 4 Maintenance Countermeasures According to the above analysis and verification, there are two reasons for this accident: First, the leakage of the landing gear electro-hydraulic reversing valve exceeds the regulations; Second, the landing gear retraction system is incomplete, which makes the system lose the ability of "self-digestion" of bad factors. In order to effectively prevent such accidents, the following measures are suggested. (1) Improved design of landing gear retraction pipeline After careful analysis, it is not difficult to find that there are some shortcomings in the system design of this type of aircraft. The function of the emergency oil drain valve is to discharge the oil collected in the pipeline to the outside of the aircraft when the emergency landing gear is lowered. Because the emergency oil drain valve is installed on the oil return pipeline of the system, on the one hand, when the emergency oil drain valve fails, it will affect the oil return of the whole system, and then affect the work of the system; On the other hand, when the electro-hydraulic reversing valve fails and the landing gear is released urgently, the oil in the oil collection pipeline cannot be discharged from the emergency oil release valve to the outside of the aircraft, so that the landing gear cannot be released urgently, that is, the emergency landing gear is also affected by the operation of the electro-hydraulic reversing valve. During the test flight, the emergency landing gear failed to reach the designated position because of the failure of the electro-hydraulic reversing valve. Therefore, this installation is unscientific and reduces the reliability and security of the system. However, if the emergency oil drain valve is installed in the tension pipeline, that is, at the outlet of the tension joint of the electro-hydraulic reversing valve, the function of the emergency oil drain valve will not be affected, the reliability of the system will be improved, and the above accidents will not occur. Therefore, it is suggested that the relevant departments should install an emergency oil drain valve at the outlet of the tension joint of the electro-hydraulic reversing valve after full demonstration. (2) Improve product quality and strengthen pre-installation inspection. Electro-hydraulic reversing valve is the core accessory of landing gear retraction control system, which has specific requirements for its manufacturing quality and performance index. However, in the actual production and use process, people often pay attention to its function, but seldom pay attention to its leakage and other indicators, and always think that leakage has no effect on the work and performance of the system. Therefore, it is suggested that on the one hand, efforts should be made to improve the process level and processing quality, maintain the concentricity of the slide valve and the valve sleeve, and reduce the radial clearance between the slide valve and the valve sleeve as much as possible; On the other hand, it is necessary to strengthen the determination of its performance indicators before installation and use, and it is not allowed to install and use electro-hydraulic reversing valves with leakage exceeding the specified amount. 2. The data meets the requirements. Why can't the nose landing gear come down? On April 3rd, 1995, when our fighter fighter plane fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters fighters At this time, the front landing gear automatically falls off and the crew pushes the front landing gear onto the lock. After a preliminary inspection, the plane was towed to the inspection squadron. After the second overhaul on February 9 1992 19, the plane flew for 236 hours and landed 446 times. Prior to this, there was no abnormality in the rise and fall of 445. 1, ground inspection and simulation test In order to find out the cause of the failure, the inspection team conducted a special inspection on the relevant parts that may lead to improper placement of the nose landing gear. 1. 1 After the plane landed, the main hydraulic system of the plane still had a residual pressure of 60 kgf/cm2, the oil quantity was normal, and the fuel tank was well pressurized. Landing gear was retracted *** 10 times in the routine inspection squadron, and no abnormality was found. Landing gear retraction time is 8 seconds (not more than 15 seconds according to regulations), and the time difference between left and right landing gears retraction is 1 second (not more than 1.5 seconds according to regulations). 1.2 Start to check the working conditions of the hydraulic pump and hydraulic system. The system works normally, and the pressure reaches 140 kgf/cm2 from startup to idle speed. , in line with the regulations (specified as 140-5 kgf/cm2). 1.3 Pull this plane and another intact J -7 plane to the take-off line at the same time, jack them up, and compare the landing gear retraction in the idle working state, and retract the landing gear 10 times, and no abnormality is found; Measure the clearance of all parts of the nose landing gear, which meets the requirements of 1.4. Check the lock arm and lock groove of the front landing gear. The surface is smooth without burr, and the rocker arm rotates flexibly. The measured clearance H between the piston rod and the unlocking arm of the front landing gear unlocking cylinder is 3.5mm, which is within the upper limit, but within the allowable range of the specified value. 1.5 simulate the landing state of the aircraft. When the engine is at the end of unloading the low-speed hydraulic pump, first put down the flap speed brake, then put down the landing gear, and conduct the landing gear retraction test again (adjust the pressure of the ground oil pump truck to 80 kgf/cm2. )。 This test has been done 12 times, and the main landing gear has been unlocked in place for 3 times. After the main landing gear down indicator light is on, the front landing gear is not unlocked. When the system pressure returns to the adjusted pressure value, the nose landing gear will be unlocked and put in place, but it will make a loud noise when it is unlocked. 2. Cause analysis In view of the fact that the front landing gear of the aircraft was difficult to unlock and put down late for three times in the simulated retraction test, the inspection team concentrated on analyzing the possibility that the front landing gear could not be put down due to the abnormal cylinder of the aircraft unlocking action. As shown in Figure (4), under normal circumstances, the work of the front landing gear unlocking cylinder can be divided into three stages: in the first stage, the extension length h of the piston rod is 2-3.5 mm to eliminate the gap between the piston rod and the unlocking arm; In the second stage, the extension length L of the piston rod is 20-2 1mm, the locking hook mechanism is unlocked, and the upper (right) end surface of the piston is at the edge of the oil hole of the "B" nozzle; In the third stage, when the extension length s of the piston rod is 29 ~ 3 1 mm, the "B" nozzle is opened and the front landing gear retraction cylinder is oiled. Under normal circumstances, as long as the above sequence conditions can be met, it can be guaranteed that the lock will be unlocked before the landing gear is released. After measurement, the H value, L value and S value of this machine are 3.5mm, 20.5mm and 30.5mm respectively. From the measurement, the machine is normal except that the H value is in the upper limit position. According to the working principle of the unlocking action cylinder, when the H value is in the upper limit position (3.5mm) and the limit position (2mm) respectively, the value reaches 1.5 mm. For a given unlocking action cylinder, if the H value is 2 mm, the locking hook mechanism is unlocked after the piston rod extends out of L, and the upper (right) end face of the piston is just at the edge of the "B" nozzle, then when it is. The oil passage of the "B" nozzle has been opened, and the upper cavity of the retraction cylinder of the front landing gear has been opened in advance, so that the front landing gear generates a downward torque, which acts on the lock block through the lock groove of the raised part on the strut, increasing mutual friction and making the pressure of the hydraulic system less than 80 kgf/cm2. The sum of this friction force and the tension of the lock spring is likely to be greater than the unlocking force of the piston rod of the front landing gear unlocking action cylinder, resulting in that the front landing gear cannot be unlocked and lowered. In order to further determine whether this failure of the aircraft conforms to the above analysis, the inspection team conducted the following tests on the ground: pressurizing the "A" nozzle of the unlocking action cylinder with a hand pump, and disassembling the "B" nozzle joint (to check the fuel supply timing of the "B" nozzle). It was found that the landing gear hook mechanism with the piston rod extending length of 2 1mm had not been unlocked, and the "B" nozzle began to be oiled. The test results are completely consistent with the above analysis. Why did the plane work normally during the modified 445 flight and landing, but the nose landing gear was not put in place during the 446th landing? Why did the landing gear retract 102 times after the problem occurred? The inspection team analyzed that this may be because the pressure in the hydraulic system is high (80 ~ 100 gf/cm2. ), although there is also a problem that the nozzle of unlocking cylinder "B" is turned on, because the unlocking action is continuous (without stopping in the middle) and the dynamic friction is small, the failure that the nose landing gear cannot be released cannot be exposed. The failure that the nose landing gear can't be put down will only happen when several factors such as low pressure, continuous retraction and unlocking pause exist at the same time. According to the pilot's report, this flight was a small route landing, and the pilot may have used speed brake in front of the landing gear. So the situation at that time may be: when the pilot used speed brake, the hydraulic system was at the end of unloading, and the system pressure was very small. After the speed brake is released, the pressure is further reduced, and then the landing gear is released, and the pressure is even smaller (according to the ground test, the pressure can be reduced to 0), which makes the extension process of the piston rod of the unlocking cylinder suspended and the unlocking action cannot be continuously completed. When the pressure of the hydraulic system rises, the "B" pipe is just filled with oil, so the retraction cylinder exerts pressure on the locking hook mechanism to increase the unlocking friction. Therefore, during this landing, several factors, such as low pressure, continuous take-up and unlocking pause, just appeared at the right time, which caused the front landing gear to be unable to unlock and release. In addition, this time the plane landed on a small route, and the time from the pilot's release of the landing gear to the landing of the plane was shortened, and the nose was grounded before the hydraulic system pressure rose enough to unlock and release the front landing gear. 3. Conclusion According to the above analysis, the large gap between the piston rod of the unlocking cylinder and the unlocking arm (although within the specified range, it is at the upper limit) is the direct reason for the improper placement of the front landing gear of this aircraft during this landing. 3. Summary: Through the above analysis, it is shown that the faults of the J-7 aircraft, such as inaccurate take-off and landing, inaccurate lowering and failure of the action cylinder, are mainly caused by oil pollution, insufficient oil supply performance of the oil pump and some design defects. After theoretical calculation, maintenance or experiment, the problem can be made transparent, and it is possible to solve the problem better, which can ensure the improvement of flight quality and reliability and improve flight safety factor. Finally, it may also be provided to aviation maintenance enterprises. 4. Thanks: My graduation project and thesis have been helped by many classmates and teachers, and I would like to express my sincere thanks to them. After nearly three months, my thesis was finally successfully completed, which not only completed the task assigned by the teacher, but also sublimated the whole professional knowledge of my university! In the process of writing my thesis, I deeply feel that my professional knowledge needs to be further improved and my basic knowledge needs to be further consolidated! The narrowness of knowledge is the most prominent problem when I finish this paper. After fully realizing my own shortcomings, I worked harder to collect a large number of professional materials in my spare time, and tried to absorb the essence of them, and finally turned them into my own things through my own independent thinking, and put forward my own opinions to a certain extent, successfully realizing the ultimate goal of transforming theory into practice! Of course, the successful completion of the thesis can not be separated from the guidance of the tutor, especially in the internship of one semester, you have been instilling in us the consciousness of "thinking more and doing more", which has played a very good enlightening role in actively thinking independently and solving some practical problems when I conceive the thesis! Here, I want to say to you and all the instructors: you have worked hard! In the future work, I will continue to uphold your teaching, win honor for teachers with the actions of an excellent employee, and add luster to the Institute of Aeronautics and Astronautics! There is no opportunity for professional practice during the completion of graduation thesis. Although I work hard to write my thesis, due to my narrow knowledge and lack of internship experience, there will inevitably be some loopholes or deficiencies in this paper which is urgent in time. Please forgive me! Thank you teacher! At the same time, I want to thank my classmates. Three years of college life, they helped me learn a lot, made me understand a lot of truth and laid a good foundation. Only in this way can I successfully complete this graduation thesis design and make continuous progress in my future work and life. Thanks again, thank you! V. Reference 1. Stone. Fault diagnosis and maintenance technology of hydraulic system. Beijing: Machinery Industry Press, 1990.7.2. A certain type of aircraft ground training textbook Volume II. Xi 'an: 603rd Institute of Aviation Industry Corporation, 1995. 10.3. Huang Shuzhi. 1986:276-287