1 CHAPTER 1 INTRODUCTION 1.1 BACKGROUND Landing gear is an inevitable component of an aircraft. The main function of landing gear includes absorbing the energy during landing, supporting the whole body at all ground manoeuvres, such as take off, taxiing and landing roll-out. It plays a crucial role in preventing the airframe from vibration and excessive impact forces, improving passenger comfort characteristics and increasing aircraft flight safety. In the past, with paved runways and relatively short taxi distances, the simple shock absorber with spring and oil damping design was acceptable. Nowadays the airports for civil aircraft operations are increasing in size with a consequential increase in taxi distances. The aircraft are remaining in service for longer time periods and aircraft manufacturers are looking for longer fatigue lives of approximately around 90000 landings from the landing gear manufacturers. It is estimated that an airliner can taxi 500000 km (Jenkins 1989) during the life time. Certainly the need for the aircraft to handle more fatigue loads and the fatigue spectrum are now greater. Therefore good taxi performances are becoming more important in respect of passenger comfort and fatigue life of the both landing gear and the airframe. Operational roles of civil and military aircraft require them to operate from rough and damaged airfields which require even better performance of landing gears. For general taxing conditions, the ideal is for the airframe body to remain level under all conditions without heaving, pitching or sudden jerks not to be so soft that a motion sickness is induced. 2 However, the relative large weight and volume of the landing gear are no doubt adverse factors which affect not only the performance, but also the economy of the aircraft. For many years landing gear design was based on experienced certification requirements which could cover the most critical conditions which would occur during landing or taxiing. Firstly, the purpose of many certification requirements was to ensure the higher safety standard, but lack considerations about the best performance or low costs. Secondly, some of the certification requirements which were more or less based on empirical data could not represent the reality especially in large transport aircraft. Take the Airbus 380 for instance, in the static analysis the lateral loads of main landing gear during turning were exaggerated, whereas the torsion load was not covered (Kruger WR et.al 1997). In recent years, attentions have been drawn to the dynamic performance of landing gear, such as the response at the beginning of touching the ground, the taxiing performance, braking response, shimmy and vibrations. Among them, performance related to dynamic response not only influences the landing gear design, but also the relevant aircraft structure design, to account for that the most serious impact loads which the aircraft may experiences. Since design correction could not be made easily once the problem is noticed, the analysis and calculation in the design process became more significant. However, although the equations which describe the various parts of landing gear for a given situation could be established, solving these equations is relatively hard, slow, fallible, and laborious. 1.2 LANDING GEAR CONFIGURATION The landing gear is a critical component of the aircraft to operate safely on takeoff, landing and ground maneuvers. There are various types of landing gear configuration (Roskam J 1989) depending on the weight of the aircraft as shown in Figure 1.1. It is a very simple configuration, in which the wheel can be either Single Main Gear This gear has two main gears of similar size, fore and aft of the c.g. Bicycle Gear Small outrigger wheels are fitted on the wings to prevent the aircraft from 1.2.2 The disadvantage is instability and the large take off run. forward of the centre of gravity (c.g) or aft of the c.g. It is used in sail planes. 1.2.1 Figure 1.1 Landing gear configurations ( Roskam J 1989) 3 4 tipping sideways. It can be used on aircraft high wings and narrow fuselage. This type of landing gear is limited to high wing and low angle of attack. 1.2.3 Quadricycle Gear The main wheels are placed at nearly equal distances ahead and behind the c.g. The wheels are retracted inside the fuselage without interrupting the wing structure. This allows the cargo floor to be very low and close to the ground. 1.2.4 Tricycle Gear It is the most commonly used arrangement with two main wheels aft of the c.g and an auxiliary wheel forward of the c.g so that the aircraft is stable on the ground. The pilot has a good view while taxying on the runway. The brake force acts behind the c.g, so it is stabilizing and allowing the full use of brakes. Aircraft can be landed with large crab angle in a cross wind. Nose wheel must not be placed too far forward and also if it bears less than 8% of the weight the aircraft (Roskam J 1989) cannot be steered. The aircraft requires special structural provisions to accommodate the landing gear. When using a tricycle gear around 13% of the total weight of the airplane acts down on the nose landing gear and 87% acts down on the main landing gear and the weight distribution depends on the position of the centre of gravity. 1.2.5 Tail Dragger Tail dragger is a small light and simple to design. This landing gear has two main wheels forward of the c.g and auxiliary wheel at the tail. It is good for rough field operation. This aircraft lands in a flat attitude hence the drag is low. This has an inclined cabin floor which is uncomfortable for 5 passengers, inconvenient for loading and not good for pilot visibility during taxing. 1.2.6 Multi Bogey In this shock absorber strut, multiple wheels are fitted in tandem fashion in the structural element called Bogey. The number of wheels depends on the weight of the aircraft. A single main wheel per strut is used in an aircraft weighing below 50,000 lb. Two wheels per strut are fitted between 50000 lb and 150000 lb. Two wheels per strut are used for aircraft weighing up to about 2,50000 lb. Between aircraft weights about 200000 lb and 400000 lb the four wheel bogey is usually employed. For aircraft over 400000 lb from each bogey with four or six wheels, spread the total aircraft load across the runway pavement (Roskam J 1989). 1.2.7 Releasable Rail This is a special type of landing gear which is designed for the aircraft to take-off while airborne and is not expected to land on the ground or sea. Rockets and missiles are in the same category in terms of landing gear configuration. The main function of this attachment is to hold the vehicle while launched. 1.2.8 Skid It is a beam type structure and not a regular landing gear fitted in the helicopters and vertical take-off and landing aircraft. The cantilever beams play the role of shock absorber deflected outward during landing and take the entire load of the aircraft. The skids are not as efficient as oleo shock absorbers. 6 1.3 Classification Based on the Controllability The design of landing gear based on the compromise between ride comfort and vehicle stability has been the driving force for advancements in the suspensions as shown in Figure 1.2 (Chalasani RM 1986). Figure 1.2 Passive suspension design Ride comfort can be quantified by the amount of energy transmitted through the suspension into the passenger compartment (sprung mass). Ride quality is closely tied to the acceleration of the sprung mass. A typical vehicle suspension is made up of two components: a spring and a damper. The spring is chosen based solely on the weight of the vehicle, while the damper is the component that defines the suspension’s placement on the compromise curve. Depending on the type of vehicle, a damper is chosen to make the vehicle perform best in its application. Ideally, the damper should isolate passengers from low-frequency road disturbances and absorb high-frequency road disturbances. Passengers are best isolated from low-frequency disturbances when the damping is high. However, high damping provides poor high frequency absorption. Conversely, when the damping is low, the damper offers sufficient high-frequency absorption, at the expense of low-frequency isolation. Three types of suspensions as shown in Figure 1.3 that will be reviewed here are passive, fully active, and semi-active suspensions. 7 Figure 1.3 Classification based on controllability 1.3.1 Passive The passive system composed of springs and dampers has fixed suspension characteristics which lacks any adjustability and controllability. The choice of the damping coefficient is a compromise between ride comfort and vehicle stability. A low damping coefficient will result in a more comfortable ride, but will reduce the stability of the vehicle. A high damping coefficient results in better road holding capacity but also transfers more energy in to the aircraft body which is perceived as uncomfortable by the passengers. This has inherent limitations of spring rate and damper characteristics. But this type is widely used due to their simplicity, reliability and cost effectiveness where wide variations in operations are not encountered. 1.3.2 Active The inherent limitations of the passive system have necessitated the investigation of the active landing gear system. The active landing gear generating active control force can provide higher performance then its passive counterpart. The active landing gear is to change the hydraulic 8 damping as per the operating conditions. The advantage is that it generates active control forces depending on the load requirements and also easily reduces the bounce, pitch and roll of the aircraft on ground. A good control scheme can result in a much better compromise between ride comfort and vehicle stability. The disadvantages are heaviness, size, complexity and high power consumption to the aircraft. 1.3.3 Semi Active Semi active suspensions also use springs and dampers like passive system but with adjustability and controllability of these elements in real time. An external power is supplied to them for the purpose of changing the damping level. The expense of semi active is more than passive and less than active system. The reliability of the semi active system is much higher than that of active systems due to their inherent nature of operating at a pre set characteristics in case of failure controls. 1.4 REQUIREMENTS OF LANDING GEARS 1.4.1 Basic Requirements The basic requirement of landing gear is to have stable base and track relative to the c.g. This will prevent the aircraft sideways over turn, tail tipping back during ground manoeuring on the runway or taxi track. The next requirement is to absorb energies during landing impact and ground rolling (Young 1986). The impact energy is absorbed by the shock strut and the horizontal energy is absorbed by the wheel and tires. The retraction mechanism design and the volume of space are also important to enable the aircraft attain the critical speed. It should have ease of maintenance and operation. 9 1.4.2 Life Requirements The life requirement is the most important design requirement for both the civil and military aircraft in terms of number of flights. The modern day military aircraft is being designed for 6000 flights which is equivalent to 6000 flying hours. Whereas the civil aircraft is designed for 60000 flights or more with a significantly greater number of flying hours. In case of landing gears, the life requirement is equal to the number of landing cycles. 1.4.3 Ground Clearance Requirements The clearance between the aircraft and ground is an important requirement which is provided by landing gear to protect the aircraft structure from the ground. The clearance is measured from the lowest point of the aircraft to the ground. In some aircraft, the lowest component is the wing (e.g. low wing); while in some aircraft is the fuselage (e.g. high wing), and in some other aircraft, the jet engine has the lowest height from the ground (e.g. a transport aircraft with engines hang underneath the low wing). In the case of an aircraft with prop-driven engine(s), the prop tip is often the lowest point. In any case, a clearance needs to be provided via the landing gear height. The magnitude of the clearance is a function of several design parameters including cost, safety, performance, weight, stability, engine inlet, loading, and operational requirements. 1.4.4 Take-off Rotation Ground Clearance Requirements During takeoff time, the aircraft rotates at high speed to increase the lift and to gain high angle of attack. The propeller is rotating about the main gear so that the height of the landing gear is important for the ground clearance to prevent the tail or fuselage striking the ground during takeoff rotation or landing with a high angle of attack. 10 1.4.5 Energy Requirements The energy requirement depends on vertical descent velocity of the aircraft. The vertical descent velocity range from 3.05 m/sec for civil aircraft through 3.66 m/sec for fighters and 4.00 m/sec for trainers to above 6 m/sec for deck landing aircraft (Jenkins 1989). The shock absorbers, retractable landing gears, the multi wheel bogie landing gear, the wheel brakes, disk brakes, high pressure tubeless tires, antiskid system carbon brakes and the radial tire need to be designed and developed to be structurally strong to absorb the energy due to impact during landing. 1.5 OBJECTIVES AND APPROACH The aim of the thesis is to develop a mathematical model of the aircraft with active and passive landing gears and to investigate the dynamic response of the aircraft to bring out the effectiveness of the active landing gear in absorbing severe landing impact and vibrations while taxying on uneven runway surfaces during landing roll out, take off and ground maneuvers. This study focuses on the following primary objectives. To evaluate analytically the aircraft with active landing gears dynamic response while travelling on the runway with irregularities. To investigate the effectiveness of the active landing gear when encountering the runway irregularities such as bumps, step, ramp, pothole and different grades of random runways with different taxying speeds. 11 To compare the performance of aircraft with active landing gears and with that of passive landing gears. To evaluate the benefits of the active landing gear system. The first step in accomplishing the objectives of this research was to develop the aircraft models used in this research, along with the passive damping and active damping control models. Two aircraft models are used for this research: a two-degrees-of-freedom model. It is also called as a “quarteraircraft” model and a six-degrees-of-freedom full aircraft model. The two models use passive representations and the active modeling with PID controllers. Using a quarter aircraft model provides the opportunity to compute dynamic responses to bump input, step input and ramp input. The parametric analysis is also done with the use of quarter aircraft model. therefore, provides a good understanding of how each model parameter affects the behavior of the aircraft. Numerical simulations as well as parametric studies have been performed using the quarter model. However, the bounce, the pitch and the roll responses can only be studied with a full aircraft model. The six degrees -of -freedom full aircraft mathematical model has been developed. The numerical simulations have been done to study the full aircraft dynamic response to both bump inputs and random road disturbances. 1.6 ORGANIZATION OF THESIS The objective and approach of the thesis has been explained and a general introduction to the problems considered in this thesis has been outlined in the previous section of this chapter. A detailed review of literatures concerning conventional landing gear system and active landing gear system is presented in this chapter 2. 12 In chapter 3, the quarter aircraft mathematical model is formulated for passive landing gear system. The derived equations are simulated with a runway bump input for parametric analysis. Then the mathematical model of active landing gear system with PID controller is developed .The PID control theory and the hydraulic power supply system has been studied. In chapter 4, the full aircraft mathematical model has been developed to investigate the dynamic response and to compare the passive and active landing gear system. The effect of aircraft body bounce, pitch, and roll are studied for independent runway excitations. The various discrete runway inputs are generated for numerical simulations. In chapter 5, the method of generating random profile is studied. The different grades of runway profiles are generated in the Mat-lab/Simulink. The generated random profiles are used for finding the dynamic response of aircraft. In chapter 6, Simulation results of the two degree of freedom system model and full aircraft six degree of freedom model are discussed. Series of simulations have been done in the MATLAB environment. The obtained results of acceleration, displacement and strut travel are used to compare the passive and active landing gear response on different runway irregularities. In chapter 7, the main contributions of the thesis are summarized. Important conclusions and suggestions for future work along with the research carried out in this thesis are also presented.
