Theory Design Air Cushion Craft 2009 Part 7 ppsx

226 Manoeuvrability courses with worse yawing angles when the effect of not only the hull, but also the air propellers and fins affect the course stability, which can be seen in Fig. 6.14(b). Based on using the differential equations of motion for manoeuvrability (6.19) as the mathematic model, computer analysis can be carried out to analyse the course stabil- ity of an ACV with control surfaces and lateral profile as well as various operational environments, particularly when the ACV is operating on an ice surface. Professor Murao has taken advantage of the differential equations of motion to investigate the feasibility of handling the Japanese ACV model MV PP05A on an ice surface. Figure 6.15 shows the calculation results for this ACV at three different Test on calm water M. -0.10 (a) Fig. 6.14 Aerodynamic yawing moment acting on BH.7 at 50 knots: (a) hull, superstructure and stabilizers only; (b) hull, superstructure, stabilizers and air propellers. 1: yawing moment, hull only; 2: yawing moment, hul. oroos. fins. -1000 1000 Y(m) 7(m) -1000 Time(s) Rudder angle // v, 0_1 45° 1. 0.02 31.3kn >1 0° 2. 0.03 28.1kn -1 3. 0.05 20.8 kn 1000 4000 *(m) Of 1000 2000 3000 4000 (b) Fig. 6.15 Simulation of JapaneseACV model MV.PP5 turning manoeuvre: (a) no rudder control; (b) rudder control. ACV turning performance 227 speeds, 31.3, 28.1 and 20.8 knots, running on an ice surface. Three friction coefficients such as// = 0.02, 0.03, 0.05 were taken into account. Figure 6.15(a) shows the turn- ing track of the craft with no rudder control, which means that the pilot took a step of rudder with an angle of 45° in a time interval of 0-1 s, then kept the rudders in neu- tral condition (rudder angle a = 0°). Calculation results show that the craft would not keep a straight course in the case of high craft speed and small frictional coefficient (v = 31.3, 28.1 knots; JLL = 0.02-0.05). Too high speed and too small friction coefficient would allow the rudder action to lead to continued turning of craft when the rudder is returned to zero. In the case of small craft speed (v = 20.8 knots) and large frictional coefficient (ju = 0.05) continued turning would not occur and the ACV course would deviate, which would be the normal intent of a short application of the rudder. It is clear, therefore, that craft to be operated at high speeds in conditions of low surface interface drag need higher aerodynamic stability. Figure 6.15(b) shows that the ACV running on an ice surface could be kept on the required course and straight course line at various craft speeds and frictional coeffi- cients with the aid of automatic handling equipment of rudders. 6.5 ACV turning performance The turning track of an ACV can be obtained by means of solving the differential equations of motion. The position derivatives as well as the rotation derivatives, can be obtained in water tunnel, wind tunnel and towing tank tests. Some parameters can be considered based on the computer analysis in [60], [55] and [58]: 1. Figure 6.16 presents the effect of wind directions with a wind speed of 10 knots on the turning track of an amphibious hovercraft model landing craft LCAC weigh- ing 150 t, at an initial craft speed of 45 knots. Owing to its rotatable ducted air propellers, the ACV could be in steady turning even operating in wind. The longitudinal turning distance (on the x axis) would be maximum in quar- tering winds (angle of wind direction 0 = 135°) and the turning diameter would be maximum in the case of beam wind (9 = 90°) and minimum in the case of head wind (9 = 0°). This agrees well with experience in practice. It demonstrates that the craft can be turned quickly in a head wind because of large yawing angular velocity. 2. Figure 6.17 also shows the effect of wind directions with the wind speed of 25 knots on the turning track of the 150 t LCAC at the initial speed of 45 knots. It can be seen that the craft could be turned in the case of a head wind, while it could not be turned in the case of a tail wind (9 = 180°). Based on practical experience ACVs travelling at high speed downwind can be very difficult to turn. The normal approach to this problem is to slow down and possibly increase skirt friction by reducing cushion power, to reduce side-slip and then begin the manoeuvre. Rotating the craft into 'reverse' is an option for high powered craft, either mili- tary or small craft, but would be disconcerting for a passenger ferry. Based upon such computer analyses it is possible to evaluate the optimum profile of the craft 228 Manoeuvrability Fig. 6.16 Influence of wind direction with speed of 10 knots on the turning track of US ACV weighing 1501 at the initial speed of 45 knots. 25kn Wind 180 C — * No wind 0° \ 45kn Initial " ' craft speed Fxl0 3 (m) Fig. 6.17 Influence of wind direction with speed of 25 knots on the turning track of US ACV weighing 150 t at the initial speed of 45 knots. to maximize the control capability and improve the downwind turning perfor- mance of an ACV design. 3. Figure 6.18 [55] shows comparisons of the turning track of a 150 t ACV running at initial craft speed of 45 knots in zero wind velocity with three different propulsor layouts: (a) First project: single air propeller located on central longitudinal plane and at 12m aft of LCG; ACV turning performance 229 (b) Second project: two propellers located on both port/starboard at 6 m from central longitudinal plane, and 12 m aft of LCG; (c) Third project: two propellers located the same as the second project and another two propellers located on both sides of the craft at a distance of 6 m from central longitudinal plane and 12 m forward of the craft LCG. Owing to the arrangement of two bow rotatable ducted air propeller, the third craft could be operated at large yawing angle and it had a fine turning performance at high speed. 4. Figure 6.19 shows the effect of inward banked turning of 12° on the track of the craft; calculation shows that the turning diameter would be reduced by 23% in the case of inward banked turning of 12m, because the air leakage under the skirt of the outer side provides a centripetal force and in addition, contact of inner skirts with the water surface led to an increase of water drag of the skirt which increased the positive turning moment. All of this is known by experienced hovercraft pilots. 5. Figure 6.20, from [58], shows a typical turning track calculated by computer. Thus, Fig. 6.20(a) shows the turning track and the change of yawing angle at different locations of the craft, where v w is the wind velocity, v the craft speed and ju the fric- tional coefficient of the skirt on the terrain. Figure 6.20(b) shows the time history of rudder angles. Calculation shows that the pilot had to give a positive rudder angle, then a negative rudder angle in order to avoid continuous building up of the turn. After a while the pilot gave a small positive rudder angle again to correct and stabilize the yawing angle and maintain the final angular velocity in yaw of 0.0667 rad/s, thus forming the steady turn. These calculated results of the history of the rudder angle are very similar to the practical operation of pilots. 6. The computer analysis on experimental model MV PP05 (the scaled model of MV- PP5 and MV-PP15) was also carried out in [58]. It was shown that the turning per- formance of the craft model with puff ports and rudders would be better than that yxlO(m) Fig. 6.18 Comparison of turning tracks between various propulsion devices on the US projects' ACV weigh- ing 150 t at zero wind speed condition. 1: single central propulsor, 12m aft CG; 2: two propulsors offset 6m, 12m aft CG; 3: two propulsors offset 6m, 12m fwd CG. 230 Manoeuvrability yx!0 3 (m) Fig. 6.19 Comparison of turning tracks between bank and non bank turn on the US ACV weighing 1501 with initial craft speed of 45 knots at zero wind speed condition. 1: turn without banking; 2: banked turn with inward heel of 12°. -200 200 \ 300 X(m) <u ~c Ml ^ a >- ° 0 <U "0 U T3 T3 = 9S 50 \ \ 1) f\ 30 60 90 11 Time (s) (b) Fig. 6.20 A typical calculated turning track. Wind speed l/ w = 60 m/s; craft speed l/ s = 31 knots;// = 0.02. ACV turning performance 231 X(m) 100 200 Fig. 6.21 Influence of puff ports on turning track of an ACV: (a) air rudder plus puff port- control only. l/ w = 0 m/s, l/ s = 31.3 Kn,// = 0.02. air rudder with only single rudder (Fig. 6.21) due to the large positive angle to get the craft quickly into steady turning. The pilots used the puff port only at the initial phase of turning to build up the required yawing angle quickly, then they shut off the puff ports and used the air rudder only to reach steady turning. Using computer analysis for predicting the manoeuvrability of craft, the time and cost of model and full-scale ship tests on manoeuvrability may be reduced significantly. In addition, a large number of control surface arrangements can also be investigated. It can be seen that computer simulation provides a time history of turning track and rudder angle which is close to the practical operation of pilots; therefore, the calcula- tion results also coincide with the practical use of control surfaces by pilots. Owing to the practical difficulties of obtaining the various derivatives and many assumptions for deriving the differential equations of motion, solution of the differ- ential equations is not accurate. Thus computer analysis is a tool which can be used by designers to analyse the sensitivity of changes to control surfaces on manoeuvra- bility at the initial design phase of craft. In addition, tests of radio controlled free- flying models can be carried out to improve the predictions from analytical solutions. Design and analysis of ACV and SES skirts 7.1 Introduction Early in the development of ACVs, before the flexible skirt had been thought neces- sary, powerful lift engines were used to obtain a hovering gap of 50-150 mm under the hull hard structure. High-pressure peripheral air jets were used at that time to pro- vide this vertical obstacle clearance over land and water. These craft had sufficient amphibious capability and vertical obstacle clearance to prove the air cushion concept, but they often encountered terrain with variation in surface elevation larger than 100 mm (for example hollows in the ground, rocks and stones, tall grass). The craft hard structure then collided with the ground. For this reason, they could only operate on fairly smooth or prepared terrain, or smooth water. These early air jet craft looked most impressive, reminiscent of a flying saucer - they literally appeared to 'hover'. The cushion demanded high power levels to achieve this clearance and they were very 'sensitive' to control. The logical way to increase the clearance height of the hull, while also reducing the height of the air jet to increase stability, was to design a flexible membrane which extended the jets. This was indeed the approach taken initially at Saunders Roe (later BHC), in the UK. In 1958 C. H. Latimer Needham invented the flexible skirt concept and interested Saunders Roe in the idea for SR.N1. In 1960 flexible extensions to the peripheral jets of the SR.N1 were installed and this enabled a large rigid hull clearance and gave the ACV real amphibious and obstacle clearance capability. A little later this concept was extended by adding an inflated bag around the craft to act as an efficient air distribu- tion duct, from which the flexible jets were extended (Fig. 7.1). The advantages of flexible skirts can be outlined as follows: 1. significant reduction of lift power; 2. practical obstacle clearance; 3. true amphibious capabilities; 4. decreased calm water resistance, particularly at hump speed; 5. improved manoeuvrability by use of skirt lifting and shifting; 7 Introduction 233 Fig. 7.1 Evolution of BHC skirt section designs from peripheral jet to segment and bag. 6. improved seaworthiness through wave-following capability of third-generation designs such as low bag pressure responsive skirts; 7. improved maintainability of ACVs and SES, since flexible skirts can be easily detached/ attached and replaced. The appearance of flexible skirts accelerated hovercraft development at the beginning of the 1960s. The function of skirts for hovercraft has proved to be as important to the ACV as that of pneumatic tyres for an automobile. Skirt configuration greatly affects the performance of an ACV or SES. With respect to air cushion performance, stability and seaworthiness, designers in the 1960s initially concentrated on investigation of internal air flow, the velocity distribution of jet flows from the cushion peripheral jets and the relation between air jet pressure and air gap of the craft. Investigation focused on optimization of the peripheral jet flow models by various theories, such as thin or thick nozzle theory, exponential theory etc., as dis- cussed in Chapter 2. The flexible skirt developments gradually improved the obstacle clearance and amphibious capabilities of hovercraft. A large air gap under the skirts was no longer necessary because their static and dynamic deformation over rough surfaces or obstacles maintained a sufficient air gap to minimize drag as the hovercraft travelled over it and so attention began to turn away from the air jet theories. In 1962 Dennis Bliss of Hovercraft Development Ltd led by Sir Christopher Cockerell, invented the convoluted or segmented skirt (see Figs 7.13 (d) and (e)). This concept significantly reduced drag forces by the segments' ability to individually deform to an irregular surface. This allowed the air gap under the skirt to be reduced and save lift power. It was no longer necessary to use peripheral air jets to maintain a visible air gap under the cushion and so high pressures were not required in the bag- like upper skirt. 234 Design and analysis of ACV and SES skirts It should be noted here that two names are used for these convoluted skirt compo- nents. The segment generally refers to a geometry where the outer and inner 'faces' of the segment are at 90°. This is stable as an inflated structure. BHC and some others have used a variation where this tip angle is less than 90° and there is reinforcement of the cloth used in the 'unstable' lower area (see Figs 7.1 and 7.9) or alternatively where the base is trimmed horizontally to reduce air leakage through the delta area between the fingers. These variations can prove useful to fit fingers to the underside geometry of a medium to high-pressure bag skirt. Over a number of years, designers experimented with lower and lower overpres- sures in the bag compared with the main cushion, to further save installed lift power. Below a pjp c ratio of about 1.2 it was found that the upper part of the skirt also responded better to surface undulations as the craft travelled forward, giving a softer ride. This was the beginning of 'responsive skirt' technology, which has since been continuously developed in China, as well as in the UK. As skirts evolved, the bag/finger type, particularly the responsive skirt, led to a great change in air cushion efficiency, stability, seaworthiness and ride quality. Skirts have evolved with steady improvements to the ability to deform, while still maintaining overall vehicle stability. Early skirts had a tendency to buckle beyond a certain deformation limit, causing plough-in or overturning due to sudden build-up of drag. The evolution of flexible skirts led designers to be more interested in the investigation of skirt shaping and quasi static analysis of forces acting on the skirt membrane components. This affects craft static stability and the dynamic response, which in turn affects seaworthiness and obstacle clearance. The design issues concerned with materials, attachment, joints, damage, lifting mechanism design and internal force analysis of skirts will be discussed in Chapter 13. In this chapter we plan to introduce the geometry and theory of skirts, while in Chapter 13 component selection and design aspects will be discussed, based on these fundamentals. We will discuss three main issues: 1. the main skirt configurations and their development, leading to the current state of the art for hovercraft and SES; 2. the static forces acting on skirts from the terrain and analysis of forces acting on different skirts, leading to determination of skirt section geometry; 3. forces leading to skirt section instability, for example skirt tuck-under over a water surface and skirt bounce, as these phenomena particularly affect the design of bow skirts and bag geometry/pressure ratio. The study sequence, including static hovering performance, longitudinal and trans- verse stability, vertical stability and also seaworthiness qualities is as follows: 1. Static hovering theories, including the various jet nozzle theories, in which the rela- tion between the total pressure of the peripheral jet, the air gap and cushion pres- sure, as well as flow rate, are discussed. 2. Analysis of the jet nozzle, air duct and fan characteristic as an integrated system. This means the characteristics of the air duct and fan have to be studied as well as the jet flow of nozzles. 3. Considering the jet nozzle, air ducts, fan and skirts, as an integrated system and then studying the hovering performance of such systems. This means, based upon Development and state of the art skirt configuration 235 (2) above, that the forces acting on skirts and the deformation characteristics of skirts should be included as part of the system analysis. 7.2 Development and state of the art skirt configuration AmphibiousA^ We will start with the evolution of Chinese and British skirt configurations, because most of the skirt types applied world-wide to ACVs or SES to date are similar to these (see Figs 7.1 and 7.2). Initially the flexible skirt appeared as a type of extension jet nozzle. The skirt of SR.N1 is shown in Fig. 7.1 and its plan configuration in Fig. 7.2. Designers attempted to gain the benefit of the peripheral jet air curtain to seal the air cushion and enhance the hovering efficiency, while improving the amphibious and obstacle clearance capa- bilities through skirt flexibility and drag reduction. The drag hump was found unsatisfactory with early extended jet skirts, and the inflated geometry was not completely stable. At this time the flexible components were considered as fabric ducts, rather than as 'inflated' structures. This type of skirt was also applied to the Chinese ACV 711-1 in 1965. Figure 7.4 shows the original jetted bag type skirt, which was developed from the foundation of the extended jet nozzle and had increased pressure in the diffusion bag compared to the cushion pressure, to form a stable geometry. This type of skirt was applied successfully to various types of British ACVs; the SR.N2, SR.N5, SR.N3, Fig. 7.2 Evolution of BHC skirt plan configuration, including stanility trunks. [...]... this method However, the assumption implicit in the calculation above, that the cushion pressure acting on the stern skirt is uniformly distributed, may not be accurate, particularly in the case where craft are running on a water surface Cushion air is blown from the air cushion just like a venturi tube as shown in Fig 7. 26 According to [65], the pressure acting on the base of the skirt bag is not... advantages of the captured air bubble principle, and thus the lift power, in order to be more suitable for development of larger-sized craft At the end of the 1960s MARIC began to design a balanced seal (Fig 7. 15) mounted on the first Fig 7. 15 The balanced rigid stern seal on Chinese passenger SES 71 3 Development and state of the art skirt configuration Chinese passenger SES model 71 3 The principle is the... the craft Stern seal with air bag This type, i.e the inflatable bag, as shown in Fig 7. 16, has been mounted on both Chinese and US craft [61] Unfortunately, most of these seals, as shown in Fig 7. 16, have not been developed due to their poor practicability and complicated structure Three-dimensional bag-finger bow seal and two-dimensional double bag type stem seal As shown in Figs 1.14, 1.16, 1. 17, 7. 17. .. Fig 7. 21 (a), the following relation holds: n r p — 2 n r\ (7. 1) where p is the cushion pressure (N/m ), r{ the radius of the inflatable ring (m) and t\ the tension of the membrane (N/m) For any given membrane width and unit length, the membrane tension can be obtained through the following equation (Fig 7. 21(b)): (7. 2) 2 *, sin (0/2) = 2pr2 sin (9/2) thus (7. 2a) where 9 is the angle as shown in Fig 7. 21(b)(°)... = - Rptcosty + ft- a,) F]y= (l-/? ct )cos(y?- a,) F 2y = Pi dpci cos (a - ft + a,) mgv = mg (7. 17) Forces acting on skirts 2 57 %(mm) 300 - 250 - A calculation o measured data 200 y £ (mm) 250 200 0.5 0.6 0 .7 0.8 0.9 1.0 Pc/p, Fig 7. 23 The variation of location for finger tip of modified skirt on craft type 71 1-11 with high responsiveness Then the supporting forces acting on joints A, B can be calculated:... mgv) (7. 18) where r\ denotes the angle between tangent of curve AC at point A and the x axis as in Fig 7. 19 25S Design and analysis of ACV and SES skirts xE(mm) 200 o test data, loose diaphragm . Figs 7. 1 and 7. 2). Initially the flexible skirt appeared as a type of extension jet nozzle. The skirt of SR.N1 is shown in Fig. 7. 1 and its plan configuration in Fig. 7. 2. Designers. cushion power, to reduce side-slip and then begin the manoeuvre. Rotating the craft into 'reverse' is an option for high powered craft, either mili- tary or small craft, . the craft at a distance of 6 m from central longitudinal plane and 12 m forward of the craft LCG. Owing to the arrangement of two bow rotatable ducted air propeller, the third craft