Applications of jet tube servo valves in the aerospace industry

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Applications of Jet Tube Servo Valves in the Aerospace Field by Zhang Minying, Fang Qun, Jin Yaolan, Qu Lipeng, Wang Yahong (Servo Valve Division, No. 704 Research Institute of CSSC, Shanghai 200070) Abstract: Jet tube servo valves are named thus because their pre-stage consists of a jet tube amplifier; compared with conventional dual-orifice type servo valves, they exhibit excellent resistance to contamination. This article introduces the characteristics of jet tube servo valves, as well as the widespread use of such valves in the aerospace industry in developed countries such as the United States, Europe, and Russia. At the same time, based on a follow-up investigation by the United States regarding the jet tube servo valves used in civil airliners, the high reliability of these jet tube servo valves is demonstrated. Due to the difficulties involved in the design and manufacturing of jet tube servo valves, as well as foreign technological restrictions on China, currently only the 704th Research Institute of CSSC is capable of mass-producing such valves in China. Keywords: jet tube servo valve, aerospace, contamination resistance, reliability. Chinese Library Classification Number: TH137.0. Introduction *The jet tube servo valve gets its name from the jet tube amplifier used in its pre-stage; it possesses excellent contamination resistance compared to ordinary double-orifice type servo valves. The design principle of jet tube servo valves emphasizes reliability. The minimum opening of its pilot stage allows pollution particles to pass through at a size that is almost an order of magnitude larger than those that can pass through a flap valve. Generally, the oil cleanliness required when using a nozzle flap valve is at NAS6 level, and a filter with a filtration precision of less than 10µm must be installed before the oil inlet of the valve. In cases where jet tube valves are used, NAS8 grade is sufficient, and a 25µm filter is also adequate. In addition, it uses a large-diameter valve core to generate a strong driving force on the valve core, thereby enhancing its resistance to contamination ； A large stroke is used to minimize corrosion while improving zero-point characteristics. Therefore, due to its strong anti-pollution capacity and high reliability, it is widely used in the aerospace industry both domestically and internationally. 1 Principle of the jet tube servo valve: The jet tube electro-hydraulic servo valve is a force-feedback two-stage flow control valve; its structural principle is shown in Figure 1. The torque motor features a permanent magnet structure; a spring tube supports the armature jet tube assembly, thereby isolating the motor from the hydraulic section, so the torque motor is dry-type. The pre-stage is a jet amplifier, which consists of a jet tube and a receiver. When a control current is applied to the motor coil, the control magnetic flux generated on the armature interacts with the permanent magnet flux, resulting in a torque on the armature. This torque causes the armature, the spring tube, and the nozzle assembly to deflect by an angle that is proportional to the torque. The high-speed jet from the nozzle causes the pressure in one chamber of the receiver to increase while the pressure in the other chamber decreases. This creates a pressure difference across the valve element that connects the two chambers; the valve element moves until the torque generated by the feedback mechanism balances the torque exerted by the motor, causing the nozzle to return to its position between the two receivers. In this way, the displacement of the valve core is proportional to the magnitude of the control current, and thus the output flow rate of the valve is proportional to the control current. Figure 1: Schematic diagram of the structure of the CSDY type servo valve. Figure 2: Characteristics of the jet tube servo valve. 2.1 Pollution resistance: The jet tube servo valve operates by using a jet nozzle to eject the working fluid, converting pressure energy into kinetic energy; it controls the force by regulating the proportion of energy delivered to the two input ports. The valve jet nozzle in this design is large, suffers less damage from impurities such as dirt particles present in the working fluid, and has strong resistance to contamination. Moreover, the jet-tube type hydraulic amplifier features high pressure and volume efficiencies, generally exceeding 70%, and in some cases it can achieve efficiencies of over 90%. It also has a large output control force (spool driving force), which enhances its resistance to contamination. Secondly, since the pilot stage of the jet tube type has a greater control force than that of the nozzle plate type, the diameter and stroke of the spool in the jet tube type servo valve are larger and greater than those in the nozzle plate type, resulting in a higher driving force. Even in the presence of a small amount of debris or particles, the spool stage can still function properly, thereby further enhancing its resistance to contamination. 2.2 Reliability: Since the jet tube is used for force control downstream of the nozzle, when the nozzle is completely blocked by debris, no energy is supplied to either of the two receiving ports, and there is also no hydraulic pressure acting on the two end faces of the spool. The bending force of the feedback spring causes the spool to return to its zero position. This allows the servo valve to prevent excessive flow output, possesses the capability of \"fail-safe centering,\" and avoids the so-called \"full-stroke\" phenomenon. Furthermore, the pilot stage experiences wear during operation; however, compared to the nozzle-plate type, the degree of wear and performance degradation is lower in the jet-tube type. Even if the sharp edge of the receiver becomes indented due to prolonged exposure to high-pressure oil, it still functions as a divide, and as long as its distance from the nozzle is no more than 3.5 times the nozzle diameter, its impact on the performance of the servo valve is minimal. In jet tube servo valves, all the torque motor components are integrated through press-fitting and welding, and strict aging treatment is applied to eliminate internal stresses. As a result, the structure is robust and stable, with minimal zero-point drift, and it can withstand strong impacts and vibrations. Based on the above, it demonstrates high stability and reliability. Higher than the dual-nozzle baffle valve. 2.3 Service Life: The service life of jet tube valves is specified by general standards as 5,000 hours or 107 operations; however, there are few statistics available on their actual service life. Abroad, there is a 7-year follow-up study conducted in 1971 by Boeing on 9,000 ABEX410 servo valves, and the results are detailed in Table 1. Table 1: Actual usage of the 410 jet tube servo valves
Duration of valve operation (hours), Total number of inspections, Number of valve failures:
1965: 376,000 hours, 310 inspections, 1 failure
1966: 663,000 hours, 275 inspections, 1 failure
1967: 1,155,000 hours, 203 inspections, 1 failure
1968: 1,641,312 hours, 602 inspections, 2 failures
1969: 2,297,908 hours, 481 inspections, 17 failures
1970: 2,454,598 hours, 692 inspections, 25 failures
1971: 1,475,554 hours, 471 inspections, 13 failures
Total: 9,687,372 hours, 2,718 failures

Note: Of the 84 failures, 83 were caused by oil leakage due to aging of the sealing rings, while the valves remained in good working condition. According to this table, the average detection time for the jet tube servo valve is 35,700 hours, and the average failure interval time is 115,000 hours. In industrial applications, accurate recording like that in airplanes is not common, but there are jet tube servo valves used in turbine governors that have operated without failure for about 63,000 hours over an 8-year period, and there are examples of them still in use. 3 Jet tube servo valves are used abroad. Jet tube servo valves are military products developed in the United States at the end of the 1950s; currently, in addition to being widely used in military aircraft and commercial airliners in the United States, jet tube electro-hydraulic servo valves are also extensively used in military aircraft and commercial airliners across various countries around the world, including Europe and Russia. The reason is that this product boasts unique advantages such as zero-position stability (under conditions of high-intensity impact, vibration, and shock), strong resistance to contamination, no drift in steering, safety and reliability, and a long service life. Since this valve can be used in military applications, foreign countries have imposed a technical blockade on domestic production, resulting in limited available information. Here, we present some of the information we have gathered. The main manufacturer of jet tube servo valves in the United States is ABEX Aerospace (now acquired by PARKER Company), with its main models being 405, 410, 415, 420, 425, 450, etc., which are widely used in various aircraft. By August 1971, over a hundred civil aviation companies around the world had adopted more than 9,000 ABEX410 servo valves; the main applications of these valves are detailed in Table 2. Table 2 Main Application Areas of 410 Jet Tube Servo Valves: Applications, Uses, and Usage Scenarios. Boeing 737: rudders, elevators, ailers; Boeing 747: rudders, elevators, center lateral control; McDonnell-Douglas DC-8: rudders; McDonnell-Douglas DC-8: rudders, elevators, ailers, direct takeoff control; After ABEX was acquired by PARKER, its jet tube servo valve models were changed to the PARKER 400 series. The main application areas are detailed in Table 3. Table 3: Main applications of PARK jet tube servo valves. Applications, uses, and scenarios: Primary flight control actuators for the Airbus A380; secondary flight control actuators for Boeing 737, 747, and 777 aircraft; primary drive and nose wheel control systems for the Boeing C-17 transport aircraft; automatic braking modules for the G5000 commercial airliner; automatic braking modules for the X-47 drone; Hawk 4000 business jets. In civilian applications, MOOG Corporation, the world’s largest manufacturer of servo valves, also offers its own jet tube servo valve products, with main models including 208A, 211A, 214, 215A, 218, 225A, 225B, 231, 240, 242, 261, etc. Since these products are intended for military use, they have not been promoted in domestic markets, and thus little is known about their application scenarios. In Europe, the company that currently produces jet tube servo valves is IN-LHC. It offers a wide range of models of such valves, which are widely used in aviation, aerospace, and industrial applications. Details of its use in aviation applications are shown in Table 4. Table 4 Main Application Areas of IN-LHC Jet Tube Servo Valves: Applications, Uses, and Contexts – Fighter jets such as the M88 turbofan engine; Bombardier CRJ-200 aircraft flight control systems; Apache helicopters and their auxiliary power units; Helicopters such as the SH-60H, EH101, and Ka-62R, as well as RTM322 engines for fuel control; Flight control systems for aircraft such as the CRJ-700, Dash-8, and BD-100; Front wheel control systems for CRJ-200 and CRJ-700 aircraft; EC225 and EC725 helicopters for flow and pressure regulation. In addition, Russia also has its own jet tube servo valves, which are used in aircraft and large rockets. 4 The application of jet tube servo valves in China is limited by the difficulties associated with their design and manufacture, as well as foreign restrictions on China’s jet tube technology; at present, only the 704 Research Institute in China is capable of mass-producing and developing such servo valves. The CSDY series of jet tube flow servo valves produced by me have been in development for over thirty years, and their manufacturing processes as well as performance parameters have reached or come close to those of similar foreign products. It is now widely used in ships and other industrial applications. It has now begun to be applied in various fields such as aviation and aerospace, including applications in the aviation industry for flap control systems, engine fuel systems, as well as determining the direction and speed of movement of the front-wheel steering actuators. The pressure servo valve for jet tubes that I have newly developed has shown initial success; it features a jet amplifier in its pre-stage and is primarily used in aircraft electronic anti-skid braking systems to generate braking pressure proportional to the control signal. Previously, all pressure servo valves used in China adopted a nozzle-throttle structure without exception; however, this has a fatal drawback, namely poor resistance to contamination, which represents a significant risk within the system. The successful development of the jet tube pressure valve has addressed the shortcomings of the nozzle flap pressure servo valve in terms of pressure control, thereby enhancing the reliability and stability of the system. 5 Conclusion Based on the above analysis, the jet tube servo valve outperforms the nozzle plate valve in terms of reliability characteristics such as resistance to contamination. The operational stability and durability of the jet tube servo valves are quite high; the servo valves I produce are currently being used in aviation and aerospace applications. It is believed that as jet tube servo valves become more widely used, they will have broad application prospects in the aerospace industry and various industrial control applications. References: Lei Tianjue, Editor-in-Chief. New Handbook of Hydraulic Engineering (Volume 1). Beijing Institute of Technology Press, 1998. Huang Zeng et al. Comparison between jet tube type electro-hydraulic servo valves and nozzle flap type electro-hydraulic servo valves. Fluid Power & Control, 2007(4): 43–45. Shimai Tatsuo. Structure and performance of jet tube type electro-hydraulic servo valves. IN-LHC Servo-valves Technical characteristics. He Youwen et al. Development of servo technology in Russia. APPLICATION OF JET PIPE SERVOVALVE IN AEROSPACE. Zhang Minying, Fang Qun, Jin Yao, Lan Qu, Li Peng, Wang Yahong (No. 704 Research Institute of China Shipbuilding Industry Corporation, Shanghai 200070). Abstract: Jet pipe servovalves utilize a positive jet amplifier, which gives them better contamination resistance compared to nozzle flap servovalves. This paper discusses the characteristics of jet pipe servovalves, and evidence from their use in civil aviation in the United States demonstrates their high reliability. Due to challenges in design and manufacturing as well as external technical limitations, only the No. 704 Research Institute is capable of producing these servovalves in bulk. Keywords: Jet pipe servovalve, Aerospace, Contamination resistance, Reliability. Author profile: Zhang Minying, female, born in 1973, engineer; her main research areas are electro-hydraulic servovalves and servo control systems. E-mail：clife_tang@hotmail.com.cn