CHAPTER 2 LITERATURE REVIEW

Transcription

1 13 CHAPTER 2 LITERATURE REVIEW 2.1 INTRODUCTION The purpose of this chapter is to present a comprehensive literature review summarizing the previous published work relevant to research objectives, discussed in chapter 1. It starts with a detailed review of literatures relating to conventional (passive) landing gear system and followed by the active landing gear system. Then a review of simulation techniques to investigate the dynamic response of the passive and active landing gear system. It also covers the literature on application of PID controller used in the in the models for active landing gear system. Finally literature on random vibration analysis and ride comfort related to passive and active models are reviewed. 2.2 CONVENTIONAL (PASSIVE) LANDING GEAR Aircraft landing gears are crucial for safety and comfort both for passengers and pilots as the device are responsible for safely moving the aircraft on ground. Young (1986) presented the significant development of conventional landing gear from 1940 to the present, covering the base requirements, energy requirements, landing gear geometry and the types of landing gears. An idealized diagram of shock absorber reaction plotted against travel is shown in Figure 2.1. The reaction at any points during the shock absorber travel is the sum of the gas spring force and the oil damping force (which is usually a function of the square of shock absorber velocity).

2 14 The area under the curve represents the energy capacity of the shock absorber. On touchdown, the wheel spins up to ground speed under the influence of the ground reaction and tyre/ ground friction. The resulting drag force at the axle stores energy in the structure. When the tyre velocity matches that of the ground, the stored energy is released and the structure vibrates half a cycle to produce reverse loads at the axle. This is known as spring back. The shock absorber continues to close until all vertical energy has been absorbed, and the maximum stroke is defined by the polytrophic spring curve. It can be seen that the energy is absorbed in a relatively short time. The landing then enters its run-out phase with oscillation about the static position which will be dependent on the degree of airborneness at any point in time. Figure 2.1 Significant events during the landing cycle (Young 1986) Jenkins (1989) described a detailed presentation of design requirements and compatibility of shock absorbers. There are many different types of shock absorbers used for shock absorption effectively which depends

3 15 on the type of aircraft such as rigid axle, solid spring, levered bungee, oleo pneumatic shock strut, telescopic strut, articulating strut, semi articulating strut. In the rigid axle the shock absorption is done by the axle fitted with a cushioning rubber pad or axle with spring and the tyres. The solid spring arrangement is suitable for light aircraft where the shock absorption is not critical and having low sink speeds. The shock absorption of this strut is to take the vertical displacement through bending of the strut. In the levered bungee system utilizes a rubber, porous shock absorber in addition to metal strut. The energy absorption and damping is improved by rubber strands and the strut. The oleo pneumatic shock absorber system is commonly used in the medium to large aircraft. It provides shock absorption as well as effective damping. There are three common configurations of oleo shock absorber. These are telescopic strut, articulating strut and semi-articulating strut. The main difference between these three types of oleo shock absorbers is the positioning of the landing gear strut relative to the wheel and whether the shock absorber is structurally rigid with respect to the airframe. The oleo-pneumatic shock absorber has a high efficiency under dynamic conditions both in terms of energy dissipation and return to normal static conditions (Jenkins 1989). It can achieve in terms of energy dissipated efficiencies of per cent as a simple shock absorber and up to 90 per cent using orifice metering pins or orifice control devices to further improve the operational performance as shown in Figure 2.2. This research work focuses on oleo pneumatic shock absorber with active damping.

4 16 Figure 2.2 Shock absorber efficiency-different spring types (Jenkins 1989) Jenkins (1989) also addressed different testing methods conducted on landing gears such as stress analysis test, dynamic drop test, flight by flight fatigue testing and mentioned the materials used for landing gears. The trends of major landing gear design parameters had been presented by Greenbank (1991) reflecting the developments in aircraft performance over the last two decades. These trends illustrate the likely requirements for the future and identify the differences between civil and military landing gears. The aircraft manufacturer will inevitably demand a lower mass landing gear, increased service life and lower support costs of the product. A number of likely changes required to meet future aircraft demands are studied and how they will test the innovation and ingenuity of the landing gear designer. The books of Currey (1998), Roskam (1989) are the standard text books which cover the whole conventional landing gear design process landing gear location, suspension layout, landing gear operation, kinematic analysis, and the selection of wheels and tires. The oleo pneumatic shock absorber comprises

5 17 several key parts that dynamically interact to absorb the landing energy of aircraft. Typical Oleo pneumatic shock absorbers contain an outer cylinder, inner cylinder, piston, orifice plate and working fluids. The outer cylinder lower chamber is filled with a hydraulic fluid in which a piston is accommodated which absorbs the excess energy. The piston is fully submerged in hydraulic oil, and the upper chamber contains air/nitrogen. The piston, or inner cylinder, has a series of holes in the top which act like an orifice plate. When not loaded the inner cylinder is fully extended with respect to the outer cylinder, due to the excess pressure of the air inside the outer cylinder. When the landing load is applied to the bottom of the inner cylinder it forces it to move in a longitudinal direction, into the outer cylinder, as seen in the middle diagram of Figure 2.3. Figure 2.3 Kinematic operation of landing gear (Roskam 1989) This places pressure on the hydraulic fluid within the inner cylinder, and forces it through the holes in the piston head. This is effectively the dampening of the system where the majority of the landing energy is absorbed, as the vertical kinetic energy is converted to heat energy within the hydraulic fluid. As the hydraulic fluid passes through the piston head it

6 18 reduces the volume of the air within the outer cylinder. This provides substantial resistive force, and forces the hydraulic fluid back through the piston head, which removes more energy, in a recoil motion, thus acting effectively like a spring. The three stages of operation can be seen in Figure 2.3. Load deflection curves are critical in the design and testing of landing gear. These work diagrams are a plot of the resistive force of a given shock absorber against its stroke displacement. The plots are easily determined during testing, by attaching an accelerometer to the shock absorber, and a simple displacement sensor to the inner cylinder. These plots as shown in Figure 2.4 are useful for the selection of shock absorber and determining the shock absorbers performance such as maximum loading, maximum deflection and efficiency. The efficiency of the shock absorber is critical as a more efficient shock absorber is inherently lighter and more compact, and thus reduces the weight and maximizes the cargo volume of the aircraft. However the efficiency, size and weight of each shock absorber must all be considered simultaneously during landing gear design. Figure 2.4 Load deflection curves (Roskam 1989)

7 ACTIVE LANDING GEAR Irwin Ross & Edson (1982, 1983) investigated first to consider an actively controlled landing gear to reduce the effect of landing loads. This report presented the design of an active control landing gear system for a supersonic aircraft, the purpose of which is to minimize the forces to which the aircraft subjected to a landing impact and rollout, take off and taxi operations. It included the design of an electronic controller and modifications of the existing landing gear. The electronic controller compared the kinetic energy of the aircraft with the work potential of the gear until the work potential exceeded the kinetic energy. The wing gear interface force present at this condition becomes the command force to a servo loop which maintains the wing gear interface force at this level by providing a signal to an electro hydraulic servo valve to port flow of hydraulic fluid into or out of the landing gear shock strut piston. The work demonstrated the benefits of using an actively controlled landing gear in reducing loads and vibration under various runway profiles. As an extension of the above work, Horta et al (1999) developed a facility to test various active landing gear control concepts and their performance. The facility used a Navy A6-intruder landing gear fitted with an auxiliary hydraulic supply electronically controlled by servo valves. An analytical model of the gear was presented including modifications to actuate the gear externally and test data were used to validate the model. The work described the control design, closed loop test and analysis. He developed a mathematical model and the non linear equation of motion for a telescopic main gear modified with an external hydraulic system for actuation and control of gear. Sheperd et al (1992) investigated the active landing gear control scheme that can provide some improvement in ride quality along the passenger cabin as determined by fuselage normal acceleration during takeoff and landing rollout and dynamic response to

8 20 typical random and discrete runway profiles. In this work, the active control system studied here uses feedback from airframe mounted sensors to modify rigid body and structural response. The system is based primarily on modifying the damping characteristics in the nose gear oleo. This is achieved by reducing the damping orifice area with active control area about this new datum value. In addition the benefits of a fully active nose gear using a separate supply of hydraulic fluid are evaluated. Freymann & Johnson (1985), Freymann (1987, 1991) demonstrated analytically and experimentally the benefits of actively controlled landing gears in reducing landing loads and vibrations under various profiles. An active control undercarriage for the alleviation of aircraft landing gear and structural loads during operation on rough runway surface is described. For quantitative determination of the improvements obtained with an active control undercarriage are compared with conventional landing gear systems. Aircraft testing is realistically simulated by means of a laboratory test set-up especially designed for this kind of testing. Bender & Beiber (1971) have done a feasibility study of the design of an active control landing gear system and the design of an electronic controller which sends a signal to an electro hydraulic servo valve to control the flow of hydraulic fluid into or out of the landing gear shock absorber. The development of landing gear mathematical model and non linear equations for a telescoping main gear with an external hydraulic system for actuation and control of the gear was done by Daniels (1996). The passive and active control mode of operation of landing gear of F-106B fighter interceptor has been studied by Howel et al (1991). In this study, a potential method for improving the operational characteristics of aircraft on the ground by the application of active control technology to the landing gears to reduce ground loads applied to the airframe has been investigated. An experimental investigation was conducted on a series-hydraulic active control nose gear. The experiments involved testing the gear in both passive and active control

9 21 modes. Results of this investigation show that a series hydraulic active control gear is feasible and that such a gear is effective in reducing the loads transmitted by the gear to the airframe during ground operations. The study of semi active suspensions by Wentscher et al (1995) for advanced landing gears and optimization of their associated design parameters to achieve minimum weight, maximum comfort under strict requirements with respect to safety and even increased lifetime by reducing the loads during landing impact and taxiing. Jayarami Reddy et al (1984) explained a set of non linear differential equations describing the response of a semi levered suspension type of landing gear with a single stage oleo-pneumatic shock strut. This included the kinematics of the articulation of the gear, oil compressibility effect, wheel spin up as a function of slip ratio, and the hydraulic, pneumatic, and friction forces of the shock strut. A parametric study on a gear of a helicopter has been conducted and the effect of variations in the main orifice diameter, coefficient of discharge of main orifice, initial air volume and pressure, polytrophic exponent for air compression process, coefficient of friction at lower and upper bearings and the horizontal velocity of the aircraft during landing on the behavior of the landing gear have been studied. A special adaptive shock strut of landing gear with double cavity and double damping is studied in the work done by Shuhua Zhu et al (2008). Its mathematical model and virtual prototype, which is established based on the software ADAMS are described. In this paper has emphatically studied the sensitivities of its different configurations and main parameters to the taxing performance of landing gear. The analysis shows that all of the side orifice, variable orifice and the high pressure cavity can reduce the overload and stroke of the adaptive shock absorber. The results also show that

10 22 adjusting the pressure of the double cavities can reduce more overload than adjusting these volumes. Javad Marzbanrad et al (2002) designed an optimum preview control of vehicle suspension system traveling on rough roads. A three dimensional seven degree-of-freedom car riding model and several descriptions of the road surface roughness heights, including hole/bump and stochastic filtered white noise models, are used in the analysis. The performance and power demand of active, active and delay, active and preview systems are evaluated and which compared with those for the passive system. Somieski (1997) analysed the shimmy oscillations are still a problem in the design and operation of landing gears. A linear and nonlinear mathematical method is applied to the shimmy analysis of a simple model of a nose gear. Numerical simulation is used as a valuable tool in the shimmy analysis. Esmailzadeh & Farzaneh (1999) investigated shimmy vibration which is a very important type of motion in the landing gear system during either takeoff or landing of an aircraft. Dynamic model is developed for the aircraft nose gear to investigate its transient response for the lateral deviations and shimmy angles. Variations of different design parameters namely the energy absorption coefficient of the shimmy damper and the location of the gravity center of the landing gear are analysed. Xiao-Hui Wei & Hong Nie (2005) introduced a new concept named the landing region, to attenuate landing impact loads during touchdown. A landing region is actually a mechanism that is built on a runway. The main contribution of this study is to demonstrate a new way to attenuate landing impact loads. The performance of simulated landing gear systems landing on this landing region is theoretically evaluated during touchdown. Allen & O Massey (1981) presented a nonlinear mathematical model to investigate the

11 23 dynamics of this divergence in a braked, dual tire landing gear and the effect of design parameters on longitudinal stability.caijun Xue et al (2012) proposed a method of optimizing the cushion properties for a certain amphibious aircraft landing gear, which is establishing an accurate simulation model in least mean square by adjusting parameters. He also verified the reliability of the landing gear with the optimal parameters through a drop test PROPORTIONAL INTEGRAL DERIVATIVE(PID) CONTROLLER Shinners (1964) explained the basics and design of various control systems in the book. The Proportional Integral Derivative (PID) controller principles, tuning methods and the application of PID controller in the industry has been studied in the Datta et al (2000). Haitao Wang et al (2008) had developed a two degree of freedom system mathematical model and the derived equations to describe the active landing gear system controlled by PID controller. The active control model landing gear system used the Proportional Integral Derivative (PID) strategy and the developed model included the nonlinear characteristics of the system. The performance of passive system was compared with the active system using numerical simulations. The work demonstrated the reduction in the impact loads and the vertical displacement of the aircraft s centre of gravity caused by landing and runway excitations greatly reduced using the active system, which result in improvements to the performance of the landing gear system, benefit the aircraft fatigue life, taxing performance, crew/passenger comfort and reduces requirements on the unevenness of runways. Rahmi Guclu (2004) analyzed the dynamic behavior of eight degree of freedom vehicle model having active suspensions. A PID controlled passenger seat was examined and

12 24 demonstrated three cases of control strategies. First, only the passenger seat was controlled. Second, only the vehicle body was controlled. Third, both the vehicle body and the passenger seat were controlled at the same time when the PID controller theory is applied. The development of an active suspension for the quarter car model of a passenger car to improve its performance by using a integral derivative (PID) controller addressed by Mouleeswaran Senthilkumar (2008). Jinzhi Feng et al (2008) designed the process of a controller for bandwidth limited active hydro pneumatic suspension for an off road vehicle co simulation technology. First, a detailed multi body dynamic model of the vehicle is established by using the ADAMS software package, which is followed by validation using a vehicle field test. Second a combined PID and fuzzy controller is designed for the band width limited active suspension system and then programmed by means of S-functions in Matlab/Simulink. Simulation results show that the proposed active suspension considerably improves both the ride and handling performance of the vehicle and therefore increases the maximum traveling speeds even on rough roads. 2.4 RIDE COMFORT The study of research on human vibration, concerned with the response of human body to the amounts and exposure times of exerted forces are done in this work. The typical examples are ISO (1997) and BS 6841 (1987), which is related to whole body vibration and ISO related to hand transmitted vibration. Griffin et al (1986) studied the performance of air crew members concerned to the whole body vibration. Kim et al (2011) studied two passenger cars driven at several speeds over several road profiles to evaluate the subjective rating of ride comfort. To measure the acceleration signals that are transmitted to the feet, hip, back and hand, four tri-axial translational accelerometers and one tri-axial gyro sensor

13 25 were mounted on the steering wheel and on the passenger seat and the floor, respectively. Correlations were performed to determine the relationship between the measured accelerations and the subjective of expert drivers using a psychophysical power law. Ju Seok Kang et al (2011) analyzed the dynamic behavior of the body exposed to vibration by a railway vehicle investigated with ride quality and the three dimensional human biodynamic model is used for a whole body vibration analysis. The acceleration of the body is calculated by summing the accelerations in three directions. Body behavior when exposed to random acceleration inputs measured at the floor of the railway vehicle, investigated across a frequency domain. Root mean square values of the acceleration magnitudes in the body regions are calculated from the Power Spectral Density (PSD) values of the acceleration responses. The absolute magnitude of the accelerations of the body is compared with the ride index, which is calculated using the input accelerations and standard methods. It is shown that the absolute magnitudes in the body regions are proportional to the standard ride index and that the ride index accurately reflects vibration damage to the body. Karen et al (2012) analyzed a simulation based model of a full car suspension system to predict the ride comfort. This simulation uses seat-back, seat surface, and feet acceleration values collected from four different road vehicles which were run on six different roads. Using this simulation model, the best ride comfort values were computed without a need for physical prototypes. The developed algorithm can be very helpful as an assistant tool for engineers during vehicle design and manufacturing process. The application of multidisciplinary multibody modeling to the analysis of a particular aircraft landing gear induced stability, known as gear walk. This low frequency fore and aft oscillation of the Landing Gear (LG) is primarily due to the coupling of the LG deflection with the brake anti skid control system characteristics studied by Stefania Gualdi et al (2008). Semi active

14 26 landing gear performance of both landing impact and taxi situation is studied by Wu Dong et al (2007). A kind of Non Linear Predictive Control (NMPC) algorithm for semi active landing gear has been studied. The NMPC algorithm uses genetic algorithm as the optimization technique and chooses damping performance of landing gear at touchdown as the optimization object. 2.5 RANDOM ROAD PROFILES Feng Tyan et al (2008) reviewed two of the most commonly adopted methods namely shape filter and sinusoidal approximation for generating one dimensional random road profiles, that are used in the simulation of a quarter car or half car vehicle suspension system control. For the shaping filter method, it is found that the time constant of the first order generating the road profile is independent of the grade of road. While for the sinusoidal approximation method, a detail derivation of the amplitude of each sinusoidal function is rederived for completeness. Jai-Hyuk Hwang & Jung- Soo Kim (2000) studied the motion of aircraft landing gear over a random runway can be modeled by a nonclassically damped system subject to nonstationary random excitations. In this work, the approximate analysis methods based on either the real or complex normal modes for computation of non stationary response covariances are proposed. Dynamic displacement and acceleration response of cars with uncertain parameters under random road input excitations are investigated by Jun Dai et al (2011) using a quarter car model. Based on the theory of random vibrations, the vehicle s random displacement and acceleration responses are developed in time domain and frequency domain. Uys et al (2007) investigated the spring and damper settings for the optimal ride comfort of an off road vehicle, on different road profiles and at different speeds. Optimisation is performed with dynamic Q algorithm on a

15 27 land rover defender 110 modelled in MSC. ADAMS software for speeds ranging from10 to 50 Km/h. Fedrica Giubilato & Nicola Petrone (2012) developed a method for measuring and computing the vibrational response of different racing rear wheels to the excitation caused by riding on irregular road surfaces. Four different wheels were selected for the study. Vertical accelerations at rear wheel axis and at the seat post were measured during field tests performed while cruising on different road surfaces at different constant speeds. The results show that the ranking between comfort properties of different wheels varies with the road surface roughness and the cruising speed considered. Lak et al (2011) studied the relation between road unevenness, the dynamic vehicle response and ground borne vibrations. The dynamic vehicle response for six roads with different types of pavement is supplemented by numerical predictions of ground vibrations. The predictions are performed in two stages. In the first stage, the dynamic vehicle response is computed based on the road unevenness. In the second stage, the dynamic road soil interaction problem is considered and transfer functions between the road and the soil are computed. Finally the results are used to investigate the relation between the indicators of road unevenness such as the ISO 8608 road class. Most researchers assumed the road surface inputs as stationary random process in time domain when studying vehicles with variable speed. This assumption was made to avoid the complexity of the non stationary random process. Xinfeng et al (2010) presented a new method of analyzing the non stationary random response of bridges. Using the covariance equivalence technique, the non stationary random response of bridges was developed. Liang lei et al (2011) did the simulation analysis by means of ADAMS /Aircraft software, used the standard model of the prototype aircraft and generated different levels of pavement with white noise filtering method. Prabakar et al (2013) presented a control of the stationary response of a quarter car model to random road

16 28 excitation with a magnetorhelogical damper as a semi active suspension analyzed using multi-objective optimization technique. 2.6 ABOUT SIMULINK Simulink is a software package for modeling, simulating, and analyzing dynamical systems. It supports linear and nonlinear systems, modeled in continuous time, sampled time, or a hybrid of the two. Systems can also be multi rate, i.e., have different parts that are sampled or updated at different rates. For modeling, Simulink provides a Graphical User Interface (GUI) for building models (SIMULINK/MATLAB) as block diagrams, using click-and-drag mouse operations. With this interface, we can draw the models just as we would with pencil and paper. This is a far cry from previous simulation packages that require us to formulate differential equations and difference equations in a language or program. Simulink includes a comprehensive block library of sinks, sources, linear and nonlinear components, and connectors. It helps to customize and create our own blocks. Models are hierarchical, so the models can be built using both top-down and bottom-up approaches. The system can be viewed at a high level, and then double-click on blocks to go down through the levels to see increasing levels of model detail. This approach provides insight into how a model is organized and how its parts interact. After defining a model, simulation can be done using a choice of integration methods, either from the Simulink menus or by entering commands in MATLAB s command window. The menus are particularly convenient for interactive work, while the command-line approach is very useful for running a batch of simulations. Using scopes and other display blocks, the simulation results can be seen while the simulation is running. In addition, it is easy to change parameters and immediately see what happens, for what if exploration. The simulation results can be put in

17 29 the MATLAB workspace for post processing and visualization. Model analysis tools include linearization and trimming tools, which can be accessed from the MATLAB command line, plus the many tools in MATLAB and its application tool boxes and because MATLAB and Simulink are integrated, we can simulate, analyze, and revise our models in either environment at any point. The models can be built from scratch, or take an existing model and add to it. Simulations are interactive, so the parameters can be changed and immediately see what happens. At any instant, it is easy to access to all of the analysis tools in MATLAB, and take the results to analyze and visualize them. With Simulink, it can be moved beyond idealized linear models to explore more realistic nonlinear models, factoring in friction, air resistance, gear slippage, hard stops, and the other things that describe real-world phenomena. It turns our computer into a lab for modeling and analyzing systems that simply wouldn t be possible or practical otherwise, whether the behavior of an automotive clutch system, the flutter of an airplane wing, the dynamics of a predator-prey model. 2.7 SUMMARY The majority of the papers presented in the literature review, have reported analytical studies. Model simulations and analytical studies have dominated the studies in this area. Most of the studies have thoroughly investigated with the dynamic response of the aircraft with the two degreesof freedom system single landing gear model. Two degree of freedom mass model does not represent the geometric effects of the aircraft and it does not include the pitch and roll degrees of freedom hence the effects of longitudinal and lateral interconnections cannot be studied. In the other literatures experimental investigation was done to realize the benefits of the active landing gear system than the conventional system. This was lacking the theoretical studies. Hence the new approach in this research intends to contribute to the formulation and use of full aircraft mathematical model for investigation of dynamic response of aircraft with active landing gears.