Theory Design Air Cushion Craft 2009 Part 8 ppsx

266 Design and analysis of ACV and SES skirts Fig. 7.31 Forces analysis for deformed fingers. F lwx + F 2cx 4Cx F = 0 F 2Cy + F 4Cy + F 5y + K cmg = 0 F 3x = 0 0 - fQmg = 0 F (7.27) where mg is the weight of the skirt on zfCDG per unit length (kg/m) and K c the coef- ficient of effective skirt weight. The value of K c is very difficult to estimate due to the fact that a part of the weight is supported by the water surface. In general, thanks to the small effect of skirt weight on geometry and part of the skirt weight supported on the water surface is far less, this component of the skirt weight can be neglected and does not cause large errors. Coefficient K c can be determined by the relation between the location of points C, D and the CG of the skirt. Now the cushion parameters p c , p { , the geometrical parameters of skirts d and length of curves AC, CD, DB, AC (diaphragm of D-shape bag), the location of points A, B, the parameters due to the emerged location of the skirt such as K w , d w , etc. are given, then the four variables R { , R 2 , R 3 , f} 2 (the angle between BD and the y axis) can be solved by the four equations (7.27). In the same way as the methods for static shaping of skirts, the locations of points C, D and the angle between the F 5 and the y axis may be determined and the tuck- under sensitivity of the skirt can then be estimated. The calculation mentioned above is rather complicated, but it can be solved with the aid of a computer. Comparison between the theoretical calculations and experimental results Experiments at MARIC are normally carried out in a circular water tunnel. The experiments for tuck-under of skirts with various configurations, emerged depth of the finger, and friction coefficients of the skirt are all carried out in this facility. Skirt bounce analysis 267 Comparing between the theoretical calculations and experimental results obtained, MARIC has derived the following conclusions: 1. It can be seen from data plotted in Fig. 7.32 that theoretical analysis agrees well with experimental results. 2. Larger K w (namely the larger skirt finger friction drag coefficient) gives deeper emerged length of finger d w , so that the fingers tuck under more easily. 3. Based on calculated results for the various skirt parameters, bow skirts are less resistant to tuck-under, so one has to take care to design a large deformable respon- sive skirt for this location. 4. The diaphragms of a D-shape bag with the tension of F gave the capability to con- trol skirt tuck-under, particularly in the case where the diaphragms were tightly mounted (rather than loose in the static hovering condition). 5. The calculation method mentioned above can be used in the design of bow skirts. In the case of selecting or designing the configuration of a bow skirt, the ability to prevent tuck-under had to be checked according to the K w and permitted emerged depth d. •;;7$ Skirt bounce analysis '> •; | ; ; ; ' : ' ; •, ; -', L ;jlvH; : 'i Bounce is a low frequency vibration of the skirts, which occasionally occurs in an ACV hovering static over a smooth surface or at low speed, both on Chinese ACVs such as models 711-IIA, 716A and 71611, and various ACVs from the UK and USA. During bounce, the skirts vibrate with a large amplitude, which will be several times Theoretical K W =OA Fig. 7.32 Comparison of tuck-under between analysis and experiment. 268 Design and analysis of ACV and SES skirts ground Fig. 7.33 Profile variation of skirt during bounce. 1: neutral position; 2: bounce up, air spilled; 3: bounce down, ground reaction. the static air gap under segments or fingers (see Fig. 7.33), thus causing a heavy vibra- tion of the hard structure in heave, which is harmful to equipment, engines and instru- ments and uncomfortable for the crew. In recent years, due to the appearance of low bag pressure responsive skirts, particularly the large deformation skirt, the probability of bounce motion, similarly to skirt tuck-under, has increased. Investigation of the causes of bounce and development of methods for preventing 0 4 8 12 16 20 24 o no bounce x bounce occurs Fig. 7.34 Bounce boundary. Skirt bounce analysis 269 it are an important requirement for the designer. Refs 50 and 66 introduced some experimental investigations on this subject and obtained encouraging results. Figures 7.34 and 7.35 show the vibration range for bag/finger skirts and indicate almost the same results, but show different experimental results for bounce motion of open loop skirts. In this case, ref. 50 noted that no bounce motion occurred, but it has occurred with this skirt during tests in a Chinese skirt test rig [66]. Strictly speaking, the theory for bounce has not been completely verified, but a simple explanation for bounce can be described as shown in Fig. 7.36 [4]. Lower boundary 50 100 PC Fig. 7.35 Bounce boundary. Unsteady flow under skirt Skirt moves up to regain equilibrium of tensions Skirt tip moves toward ground Flow reduced, local pressure increased under skirt Fig. 7.36 Brief rationale for bounce. 270 Design and analysis of ACV and SES skirts A large radius loop or bag membrane has very little damping, particularly for small movements of a segment, as the radius changes very little. Where a significant length of the skirt is over a flat surface and this section can move as a unit, a small disturbance of the cushion air flow can begin a motion, normally downwards, sealing the cushion. Local static pressure rises, causing the skirt geometry to change, lifting it. The static pressure reduces as air is again released under the skirt and the cycle starts again. The period of oscillation is the natural period in rotation for the total skirt section. The methods available for preventing bounce are: 1. Introduce anti-bounce diaphragms in the bag or loop to increase the damping for vibration, the shape of which is similar to the D-shape bag (Fig. 7.37). This method has given good results for solving the bounce problems which have arisen on Chi- nese ACV models 716-11 and 711-IId. 2. Change the length of the inner bag and the outer bag of the skirt in order to change the static deformation of the skirt and adjust the natural frequency of the skirt vertical vibration. 3. Strap weights (small sandbags or similar) to the skirt at the bag/segment outer attachment. These are generally only needed in the centre section of side skirts. The mass required is relatively small, since the lever arm about the skirt rotation point is large. This is normally optimized during craft trials. 7.8 Spray suppression skirts The high-velocity air escaping from an ACV or SES cushion will entrain water with it. This forms droplets which form a spray curtain. The higher the cushion pressure, Pi 150 100 50 o Test data, no bounce observed 50 100 PC Fig. 7.37 Protection function of D bag skirts against bounce. 1: bounce boundary; 2: open loop skirt; 3: test data, bounce restrained by diaphragm. Skirt dynamic response 271 the greater the quantity of water spray. Over land, the cushion air creates a dust or sand curtain over dry terrain. This spray curtain will normally be thrown back on to the structure of an ACV or SES and may then enter engine intakes if they are not protected by filters. In cold climates, water spray will become an ice rime on the structure after a while, adding to the effective mass of the craft. This ice can also be dangerous since vibration, either from the cushion or the engines, can break pieces off which are then drawn through air propulsors or possibly the lift fans, causing damage or failure. In both cases, a spray suppression apron around the bow and front part of the side skirt in the case of an ACV is found to be very effective in reducing spray to an acceptable level. There are several types of spray suppression apron. The simplest comprises a shaped piece of material which simply drapes over the bag or loop, reaching approx- imately halfway down the height of the segments or fingers (see Fig. 7.38(a)). A neater design, but rather more complex, comprises segment-like inflated teeth in the top half of each segment (Fig. 7.38(b)). Air pressure for inflation is provided from the cushion. Aprons These will flap, and they need to have an open top to avoid inflating. Flapping will be restrained by installing some weights (similar to domestic curtain hem weights) or heavier material for the lower section. Use tapes to restrain an apron and do not restrain at the segment top, just lay over the bag. Do not allow operation of a craft with a badly torn apron as this will affect performance and abrade the bag. Teeth These are shaped like double segments. Use cushion pressure (i.e. bleed from top of segment) to inflate. Use geometry shown in Fig. 7.38(b) approx. Attach to top of seg- ment. Use lightweight material. 7.9 Skirt dynamic response A skirt will respond by adjustment of geometry to two basic inputs, changes in pres- sure within the cushion and deformation against a solid boundary such as a wave. The deformation will change the volume of the cushion and the air distribution system and by doing so alter the balance of dynamic and static pressure within the system. A responsive skirt is one where the segments or fingers are able to follow an undu- lating surface by adjustments in the skirt section geometry without inducing large changes in the pressure field in the cushion. By definition such a skirt will be less stiff than a 'non-responsive' skirt. The optimum for a given craft is therefore likely to be a skirt with just sufficient responsiveness for its mission requirements and no more. In the next chapter we discuss ACV and SES motions in a seaway. To simplify an already complex analytical problem, skirt response is considered only as the cushion stiffness. The designer will therefore need first to assess the minimum stiffness required 272 Design and analysis of ACV and SES skirts for static stability from the skirt system (see Chapter 5) and once this is known, the stiffness characteristic giving most favourable motions in the design environment can be determined and the skirt geometry adjusted, so long as the dynamic requirements are greater than those from static analysis. Fig. 7.38 Spray suppression skirt types: (a) spray suppression apron; (b) spray suppression 'teeth'. s Motions in waves 8.1 Introduction The purpose of this chapter is to introduce the reader to ACV and SES vehicle dynamics. In a seaway (or over rough terrain for amphibious ACVs), the craft will respond to the undulating surface by pitching, heaving and rolling as it moves forward along its track. Dynamic sway and yaw motions may also be significant for ACVs over undulating terrain. These motions are generally considered together with design of the craft control systems, rather than as analysis of the motions themselves, (see Chapter 7). We will concentrate in this chapter on the heave, roll and pitch, which gov- ern ride quality and speed loss in a seaway. The cushion system responds as a damped spring system, while movement of the skirt and SES sidehulls into and out of the water induce varying lift and drag forces. The forces do not vary linearly. Initial models for ACV and SES motion attempted to linearize the response, to make prediction simpler. More recently, nonlinear solutions have been proposed and are now being further developed. We present these theories later in this chapter, after reviewing the main parameters which affect ACV and SES motions. The objectives in carrying out such analyses are to identify the motion characteris- tics themselves as an input into defining: • instability boundaries, e.g. plough-in, heave bounce and cobblestoning; • criteria for dynamic stability, to compare with static stability requirements from Chapter 2; • passenger and operating personnel motion response and so assessment of ride quality; • assessment of externally excited vibration. Hovercraft seaworthiness An ACV is able to display the special characteristics for which it is best known while running at high speed over shallow water, rapids, ice and swamp - places no other craft can go. While these 'special abilities' interest many military and civil users with particular mission requirements, such environments do not include the wind-driven waves found in an open seaway. Generally, a craft's capabilities in an open seaway will 274 Motions in waves control its transit capabilities between locations where a special mission may be required. Meanwhile, the SES can best demonstrate its own high speed and work capacity rel- ative to displacement vessels in light weather conditions in a seaway. Where the envi- ronmental conditions are favourable, the SES is capable of demonstrating significantly higher transport efficiency than other vehicles. In conditions typical of an open seaway, the seaworthiness of current ACV/SES still leaves a lot to be desired, especially in comparison with deep submerged hydrofoil craft, or larger high speed catamaran ferries. Part of the problem is that an ACV or SES has a higher work capacity than the competing craft, so that the comparison is almost always with a larger vessel. In a seaway, once vessel length is significantly less than L s , the mean length of waves of height H s , it will follow the wave surface profile, with much increased motions. A smaller ACV or SES travelling at higher speed than a more conventional vessel therefore needs a cushion system which can reduce motions by being 'responsive'. Other key points distinguishing ACV and SES response from conventional vessels are as follows: 1. Amphibious hovercraft hydrodynamic drag is very small in calm conditions. In the case where the craft runs in waves, there will be a rapid build-up of skirt drag and speed degradation, unless the skirt and cushion system is very responsive to the waves. Such skirt systems are only now reaching the point where safe craft can be designed for high-speed operation. 2. While there is a small area in contact with the water surface on an ACV, it has a large frontal area (mostly the skirt area actually), leading to a large air profile drag when running into head winds, in a similar way to the build-up of hydrodynamic resistance of a conventional ship running in head seas. Moreover, the air propeller thrust will be reduced, leading to additional speed degradation. Further, the manoeuvrability also deteriorates as described in Chapter 6. Ducted propulsors can be designed to have lower thrust degradation, while remaining as quiet as a large open propeller and so minimize these problems. SES powered by water jets or high speed propellers have the same design problem as any high-speed ship (see Chapter 15). 3. Wave pumping, motion pumping and the rapid changes of skirt air leakage area of hovercraft running in waves all lead to significant vertical acceleration. This can affect the operation of machinery, engines, equipment and crews etc. Use of a responsive skirt system and/or a cushion air damping system ('ride control system') can improve this enormously. This technology has been developed during the 1980s and needs further work to give really smooth ride quality at high speed. Developments over the last decade have improved responses through the low-pressure amphibious ACV skirt and SES cushion venting systems. The design basis is now available; it is a matter of extending application to larger vessels. Historical review While the performance of early ACVs on calm water was very impressive, this was not the case in rough seas. Hovercraft dynamic motions were uncomfortable due to high Introduction 275 vertical accelerations. Speed loss and reduced manoeuvrability in a seaway of some early craft were significant, resulting in a marginal ability to stay above hump speed. Much research effort has therefore been applied to find ways to improve the sea- worthiness of ACV/SES, with the aim to reduce craft motions and accelerations and allow higher speeds in a given sea state, in order to improve transport efficiency. Initial work was concentrated on improving stability. Once ACV designers had found ways to provide acceptable dynamic stability with low-pressure ratio skirts, their attention turned to improving ride quality by improving its responsiveness to waves. SES designers initially experimented with sidewall displacement ratio and geometry, also in the search for optimum balance between dynamic stability and drag, before turning to ride quality, in this case by use of cushion air venting systems. In the 1960s Sir Christopher Cockerell in the UK together with the research staff at Hovercraft Development Ltd studied the motion of hovercraft in waves, considering the wave effects on the cushion as a piston moving in a cylinder as an adiabatic process. This showed hovercraft motions to generate high vertical acceleration. Benya [9] considered hovercraft as a rigid body, similar to conventional ships and derived dif- ferential equations of motion with multiple degrees of freedom from an analytical basis. Unfortunately, he did not make clear how to determine the various derivatives (static force and rotary moment derivatives) in his differential equations, and their physical meaning. In the late 1960s Beardsley [16] began introducing the wave pumping concept for hovercraft running in waves and predicted that the wave pumping motion would strongly affect the seaworthiness and vertical acceleration of hovercraft. He main- tained that a hovercraft had to be designed with enough reserve lift power to reduce the vertical acceleration of hovercraft in the case where craft were travelling in waves. At that time, most researchers were interested in the static hovering theory of hover- craft and so they did not realize the significance of Beardsley's work. In 1972 Reynolds of the UK [67] first derived the linear equations of motion for hovercraft based on the condition that skirts do not physically come into contact with the water surface. The Froude-Krilov hypothesis was assumed to be valid and he did not consider the response of an air duct-fan-skirt system to waves. Reynolds then derived the coupled heaving equations of motion for an ACV and obtained a mathe- matical solution of the equations. In the 1970s, Doctors [68], and Zhou, Yun and Hua [69, 70], developed nonlinear equations of motion for hovercraft and obtained a numerical solution by iteration. Although the equations and their numerical solution were more complicated, solution by time-step iteration for several wave frequencies of a hovercraft in regular waves could be obtained with the aid of a computer. The solution gave the researchers insight into the full process of craft motions in waves. In the equations, not only the nonlinear system of fan-air duct-skirt, but also the compressibility of cushion pres- sure of hovercraft moving in waves was introduced. Lavis of the USA first pointed out the effect of compressibility of cushion air on the seaworthiness of hovercraft moving in waves and predicted that great distortion would occur to the prediction of seaworthiness quality of craft from model experi- ments in a towing tank [71], caused by air cushion compressibility not being able to be scaled correctly. However, researchers and designers were still interested in experimental investigations using scale model tests and real ship trials to determine [...]... density of air cushion, i.e the sum of flow rate with respect to wave pumping, motion pumping of the craft and compressibility of the air cushion The flow rate of air leakage Qe Ge = Gel + Ge2 + Gab (8- 54) where Qel is the flow rate of air leakage under the bow skirt, Ge, = . fans, air ducts and the cushion chamber. The hovercraft can thus be considered as a more complicated vibration system with multiple degrees of freedom, i.e. fan /air ducts/skirt /air cushion/ hull, . sim- plified as a rectangle. 3. The change of air density and pressure inside the air cushion are considered adia- batic, but the air flow of cushion air into the atmosphere is considered. written Therefore fie, = 4,^ (8. 18) We assume the change of air density in the cushion depends upon the law of adiabatic change, and the air density in the cushion at the static hovering