EXPERIMENTAL DATABASE OF SAILS PERFORMANCE AND FLYING SHAPES IN UPWIND CONDITIONS

Transcription

1 EXPERIMENTAL DATABASE OF SAILS PERFORMANCE AND FLYING SHAPES IN UPWIND CONDITIONS F Fossati, S Muggiasca and F Martina, CIRIVE Wind Tunnel, Politecnico di Milano, Italy SUMMARY Aim of the present paper is to present some results concerning the relationship between upwind flying shapes geometry and sail plan aerodynamic performance and to provide an experimental database to the scientific community for numerical simulation benchmarking activities concerning upwind sails aerodynamics. In particular experimental data are available from wind tunnel tests performed by the authors in the Politecnico di Milano twisted flow wind tunnel using a typical IMS cruiser-racer : scaled model. Aerodynamic forces and three-dimensional sail shapes have been measured in upwind conditions at different apparent wind angles and sail trim settings. The measured shapes have been obtained using a computer based in house developed IR-camera system and processed in terms of global parameters (i.e. sail twist, camber and maximum draft position). Flying sails shapes at different height are provided at close hauled conditions together with aerodynamic coefficients and centre of effort position with reference to different sails trimming.. INTRODUCTION Improvements in yacht performance requirement demand for sails design improvement as well as sail materials behaviour. The design of sails, which until a few years ago was considered an art, today has been transformed into a science. From a strictly aerodynamic design point of view, a sail is a rather complex system to study. For example, a sail trimmed for upwind sailing behaves like a thin foil with a turbulent boundary layer, while the same sail trimmed for sailing further off the wind will have large areas with flow separation: this means that sails design methods must of necessity take into account the approaches and methods used in other fields of engineering, such as the behaviour of bluff-bodies (such as buildings or cars) and also of wing profiles to which the conventional methods used in the aeronautical field can be applied. A further cause of complexity lies in the fact that sails have a variable shape, both because the material they are made from is deformable and because of trimming by the crew while under way, and this makes the problem typically aeroelastic. In other words, the kind of forces a sail can develop depends on the kind of flow established around the sail, which is a function of the kind of shape assumed by the sail which, in turn, depends on the kind of flow. Finally, a reason that makes the study of a sail plan very different (and also more complex) with respect to other cases (such as those mentioned above) is the end use of the information on the aerodynamics of the sail plan. In other words, in many cases the design objective can be interpreted as a quite specific problem of aerodynamics. In the case of an aeroplane, for example, the specifics of the aerodynamic characterisation are very direct, and generally expressed in terms of aerodynamic coefficients (lift and resistance) at different angles of incidence of the airflow on the wing; the designer of a building is interested in establishing the peak pressure so as to design the structure in a suitable way, while the aerodynamic study of a vehicle generally aims to minimise resistance. In the case of sailing boats it is all much more difficult and besides, since a boat is a system that interacts with two different fluids, a good understanding of the aerodynamics of the boat demands an equally good understanding of the hydrodynamic behaviour of the hull. In the design of a racing yacht, the aim is to be able to sail over a given course faster than any other boat, so it s clear that the aerodynamic problem cannot be tackled separately from considerations concerning the hull, the winds likely to be encountered, class rules, the nature of the course and so on. For the very reason that the behaviour of a sailing yacht depends on both aerodynamic and hydrodynamic effects, the first computer programmes for performance prediction (known as Velocity Prediction Programs or VPP) used aerodynamic coefficients for the sail plan derived not from specific research on the sails but deduced by using the speeds the boat reached once it had actually been built, while the hydrodynamic coefficients of the hull were generally obtained by a series of measurements carried out on scale models in test tanks. The methods currently used to attempt to characterise a sail plan are tests either in wind tunnels on scale models or on real boats and the use of computational fluid dynamics. Up to date VPP in order to find the optimum sailing state try to include the effect on sail forces and moments due to the trim of the sail performed by the crew. Even though the design shape is essentially known it is still not easy predictable the final flying shape and relevant influences on the final yacht performance. As

2 previously said, both the crew trimming and the strain of the materials are responsible for geometry differences existing between the design shape and the flying shape of the sails under their operation. Full scale commercially available sail vision systems are routinely used for America s Cup yachts in order to collect relationship between sails setting i.e. sails flying shape and yacht performance: the drawback of this approach is that the use of these systems is practicable only for upwind conditions and sails geometry can be correlated only to the yacht performance and not directly to the sailplan aerodynamics only. In [] Krebber et al presented a numerical investigation of the effect of trim using numerical investigation. The base sail geometry was taken from a measured shape on the m cruiser racer dynamometer known as DYNA. The obtained shape was then parametrically varied and numerically analysed. In [2] a valuable research activity has been presented by Masuyama at al. concerning sails shapes and performance measurement using a sail dynamometer boat called Fujin. In this case load cells and CCD cameras were installed to measure the sail forces and shapes simultaneously. Authors approach is pretty similar to the MIT sailing Dynamometer developed by Milgram et al. []. twist, camber and maximum draft position) have been real time obtained during the tests by means of an in house developed computer based IR-camera system After a brief description of the experimental set-up and testing procedure used, measured flying sails shapes at different height are provided at close hauled conditions together with aerodynamic coefficients and centre of effort position with reference to different sails trimming. 2. MEASUREMENT OF AERODYNAMIC FORCES AND SAIL SHAPES 2. GENERAL Figs. and 2 show an overview of the Politecnico di Milano facility: it s a closed circuit facility in a vertical arrangement having two test sections, a 4 x 4m high speed low turbulence and a 4 x 4m low speed boundary layer test section. A peculiarity of the facility is the presence of two test sections of very different characteristics, offering a very wide spectrum of flow conditions, from very low turbulence and high speed in the contracted 4 x 4m section (I u <.5%, V max =55 m/s), to earth boundary layer simulation in the large wind engineering test section. The advantage of the abovementioned approaches is that they represent attempt to measure the aerodynamic coefficients of the sail plan directly in real working conditions. On the other side real life condition implies very high costs and potential results scattering due to environmental and meteorological effects. Tests in wind tunnels on scale models are decidedly more attractive, both for economic reasons and because they permit to work in controlled conditions ([7] [8] [9]). In [3] authors present results of a research activity performed at the Glenn Martin Wind Tunnel with the aim to develop computational tools to predict the flying shape of upwind and downwind sails. In particular some results concerning flying shapes measurements using photogrammetry and direct digitalization using coordinate measurements machine are reported. Aim of this paper is to present some experimental results concerning the relationship between upwind flying shapes geometry and sail plan aerodynamic performance available from wind tunnel tests performed in the Politecnico di Milano twisted flow wind tunnel using a typical IMS cruiser-racer : scaled model. Aerodynamic forces and three-dimensional sail shapes have been measured in upwind conditions at different apparent wind angles and sail trim settings. The measured shapes in terms of global parameters (i.e. sail Figure : Politecnico di Milano Wind Tunnel Focusing on the boundary layer test section, its overall size of 3m length, 4m width and 4m height allows for very large-scale wind engineering simulations, as well as for setting up scale models of very large structures including wide portions of the surrounding territory. The relevant height of the test section and its large total area (4m, 5m 2 ) allow for very low blockage effects even if large models are included. The flow quality in smooth flow shows 2% along-wind turbulence. A 3m diameter turntable lifted by air-film technology allows for fully automatic rotation of large and heavy models fitted over it (max load, N). The long boundary layer test section is designed in order to develop a stable boundary layer and the flow conditions are very stable also in terms of temperature due to the presence of a heat exchanger linked in the general control loop of the facility. The Wind Tunnel is operated through an array of 4 axial fans organised in

3 two rows of seven 2 x 2m independent cells. 4 independent inverters drive the fans allowing for continuous and independent control of the rotation speed of each fan. This fully computer controlled facility can help in easily obtaining, in conjunction with the traditional spires & roughness technique, a very large range of wind profiles simulating very different flow conditions and different geometrical scales. All the typical various sets of spires have been developed in order to simulate the different wind profiles and an original facility has been recently installed allowing for active turbulence control in the low frequency range. With reference to yacht sail aerodynamic studies, the boundary layer test section allows for testing large scale models (typically : -:2 for IACC yacht model) with low blockage effects at maximum speed of 5 m/s. Due to the large dimensions of the boundary layer section it is possible to test two models at the same time to investigate blanketing effects for tactical purposes. Concerning the low-turbulence high-speed section, the large dimensions (4 x 4m) and the quite high wind speed (55m/s) enable quite high Reynolds numbers to be reached. In particular, with reference to yacht studies, the high-speed wind tunnel section allows development of specific appendage scale model tests typically on :2 scale model for IACC class keel and rudder models. with a 7 multi-turn control knobs that allow winch drum positions to be recorded and re-established if necessary. The sheet trims are controlled by the sail trimmer who operates from the wind tunnel control room. Fig. 3 shows a typical scaled yacht model in the wind tunnel boundary layer test section. Figure 3: Yacht model in the boundary layer test section A high performance strain gage dynamic conditioning system is used for balance signal conditioning purposes. The balance is placed inside the yacht hull in such a way that X axis is always aligned with the yacht longitudinal axis while the model can be heeled with respect to the balance. The wind tunnel is operated at a constant speed after the wind speed profile and wind twist have been properly tuned considering the desired targets, which are previously calculated considering the potential boat performance at different true wind speeds and yacht courses. Figure 2: Wind tunnel vertical section 2.2 SAILPLAN TESTING METHODOLOGY A complete model, consisting of yacht hull body (above the waterline) with deck, mast, rigging and sails is mounted on a six component force balance, which is fitted on the turntable of the wind tunnel (fig. 3). The turntable is automatically operated from the control room enabling a 3 range of headings. The large size of the low speed test section enables yacht models of quite large size to be used, so that the sails are large enough to be made using normal sail making techniques, the model can be rigged using standard model yacht fittings and small dinghy fittings without the work becoming too small to handle, commercially available model yacht sheet winches can be used and, most important, deck layout can be reproduced around the sheet winch, allowing all the sails to be trimmed as in real life. Moreover the model yacht drum type sheets are operated through a 7 channel proportional control system, Figure 4: Forces measurements reference system As previously said the velocity profile can be simulated by means of independent control of the rotation speed of Z Y X

4 each fan joined to the traditional spires & roughness technique, while the twist can be simulated by twisting the flexible vanes by different amounts over the height range. The wind tunnel speed is most usually limited by the strength of the model mast and rigging and the power of the sheet winches. Data acquisition can be performed in several ways: the usual procedure provides direct digital data acquisition by means of National Instruments Data Acquisition Boards (from 2 to bits, from 8 differential channels up to 4 single-ended) and suitably written programs according to Matlab standards. The data acquisition software calculates the forces and moments using the dynamometer calibration matrix. The forces are shown in the virtual panel designed on the computer screen in real time so that the sail trim can be optimised because the effects of trimming the sails on the driving and heeling forces can be directly appreciated. The model is set at an apparent wind angle with a fixed heel. After a sail trim has been explored, actual measurements are obtained by sampling the data over a period specified by the test manager (generally 3 seconds) with a sample frequency specified too. An important feature of wind testing procedure is that the model should be easily visible during the tests so that the sail tell-tales can be seen by the sail trimmer. For this purposes some cameras placed in the wind tunnel as well as onboard allow a view similar to the real life situation (fig.5). 2.2 (a) Sails trimming procedure For each apparent wind angle tested the first task was to reach the maximum driving force potentially achievable At the same time it was observed the influence of the sails trimming changes using the data acquisition program that visualizes the forces acting on yacht model in real time. Trimming the sails to obtain optimum sailing points proved to be the most challenging task of the testing process. Attempts were made to carry out the job as systematically as possible. Firstly, the maximum drive point was found by trimming the sails to the best using the cameras views, the tufts on the sails and the force measurements output data. From there, the heeling force would be reduced to simulate the trim of the sails for windier conditions. In fact in real life windy conditions, to keep the optimum heeling angle, heeling force has to be reduced by the crew. The sail trimming routine adopted was to choose the mainsail traveller position (initially quite high up to windward) and then to vary the incidence and the twist of the mainsail to power or de-power it, by over-trimming or easing the main traveller and main sheet. The genoa was initially trimmed in order to have the maximum driving force condition and was fixed varying the mainsail shape. Once a specific trimming condition is obtained using the real time force and moments values displayed by the data acquisition system, a 3 seconds acquisition sampling has been performed with Hz sample frequency, and both time histories and mean values of each measured quantity have been stored in a file. 2.2 (b) Sails aerodynamic properties Figure 5: wind tunnel top camera view during testing The usual way of analysing data is to compare non dimensional coefficients, allowing a comparison of the efficiency of sails of different total area at different conditions of dynamic pressure. The first analysis performed is the variation of driving force coefficient Cx with heeling force coefficient Cy. They are given by the expressions: Cx = Cy = 2 2 Fx ρsv 2 Fy ρsv 2 Figure : Wind tunnel aft and deck cameras view during testing where Fx is the driving force Fy is the heeling force S is the actual sail area

5 V is the wind speed ρ is air density As an example fig. 7 shows a comparative plot of Cx versus Cy for the apparent wind angles tested. Each run (with its corresponding measured values) is shown for each AWA. For the purpose of the analysis, in the following only these points will be used. The centre of effort height, Ceh, is obtained by dividing the roll moment by the heeling force component in the yacht body reference system: Mx Ceh = Fy As an example, a plot of its variation with heeling force for all angles can be seen in fig 9. Both all the measured values and the envelope of the points corresponding to maximum driving force at each heeling force are reported. The results are given in terms of ratio between centre of effort height from boat deck and mast height (P+BAS). For the purpose of the analysis only envelope curves through the test points with the greatest driving force at a given heeling force will be used in the following (fig. ). Figure 7: Driving force coefficient versus heeling force coefficient It can be seen that there are some sails settings at the highest values of heeling force coefficients where the driving force is lower than the maximum value. These non optimum values were obtained by oversheeting the sails such that the mainsail generally had a tight leech and the airflow separated in the head of the sail. Therefore a selection was made to choose those points that formed the envelope curves (maximum Cx for a given Cy value) for each apparent wind angle (fig. 7). The centre of effort longitudinal position, Cea, is obtained by dividing the yaw moment by the heeling force component in the yacht body reference system: Mz Cea = Fy As an example, a plot of its variation with heeling force for all angles can be seen in fig. Both all the measured values and the envelope of the points corresponding to maximum driving force at each heeling force are reported. Cea is measured from the origin of the balance (positive to bow) which is placed behind the mast. Envelope curves have been drawn through the test points with the greatest driving force at a given heeling force. An example is reported in fig. 8. Figure 9: Centre of effort height versus heeling force coefficient Figure 8: Driving force coefficient envelope versus heeling force coefficient

6 The apparent wind speed V a and apparent wind angle are evaluated in the heeled plane perpendicular to the mast according to: ( cosγ ) ( sin γ cosφ) 2 2 a = t + t V V V Vt sinγ cosφ AWA = arctg Vt cosγ where γ represent the true wind angle (yaw angle), V t is the wind tunnel flow velocity corresponding to the mean dynamic pressure at each run and φ is the heel angle. Figure : Centre of effort height envelope versus heeling force coefficient As an example in figures 2-3 the C D and C L measured values at different AWA are reported. At each AWA, values corresponding to each run performed are reported and red full dots correspond to the maximum driving force condition trimming point. Figure : Centre of effort longitudinal position versus heeling force coefficient The results are given in terms of ratio between centre of effort longitudinal position from balance origin and yacht model waterline length. Figure 2: Drag coefficient versus apparent wind angle It can be seen that Cea moves forward as Cy reduces. This is explained by the way the sails are de-powered. Using the driving and heeling aerodynamic force Fx and Fy component in the yacht body reference system the corresponding drag and lift forces components can be obtained as follows: DRAG = Fxcos( AWA) + Fysin( AWA) LIFT = F sin( AWA) + F cos( AWA) x y Then the corresponding drag and lift coefficients C D and C L can be evaluated: 2 DRAG = ρvacd( AWA) S 2 2 LIFT = ρvacl( AWA) S 2 Figure 3: Lift coefficient versus apparent wind angle

7 2.3 FLYING SHAPE DETERMINATION 2.3 (a) Flying shape measurement set up An in-house photogrammetric measuring system has been developed to recover flying shapes during tests. The photogrammetry based technique is relatively fast during the tunnel occupancy phase and in principle it requires only three digital images be recorded from useful points. In order to overcome difficulties arising from sails overlapping especially in downwind configurations and in order to be able to have at least three useful points in each part of the sails the system is equipped with eight cameras. For the present tests this system is composed of five cameras, filming reflective targets placed on sails in sync, and a PC equipped with acquiring and processing custom-made software. Cameras have resolution of 392 x 4 pixels, greyscale /2 CCD sensor, 7 fps (frames per second). Each of them mounts an optical zoom and a high intensity infrared (83 nm) LED illuminator, triggered to simultaneously flash with cameras frame rate. In order to reduce at the best cameras vibrations induced by the wind, it was decided to fix cameras on photographic heads constrained to the available stiffest points in the wind tunnel. Figure 5: Reflective markers on the main Then, custom-made software performs real time blob detection and stores images sourced from cameras on a hard disk. As a result of this routine a table with the 2D blob detected coordinates is available for post process. Cameras have been previously calibrated using a custom built calibration frame. The 3D marker points coordinate for each sail are then obtained by means of a DLT (Direct Linear Transformation) algorithm, reaching marker position with an uncertainty equal to.5 mm. Marker coordinates are obtained as mean of their position over a 2[sec] acquisition period with 7 Hz acquisition rate. Then this 3D points array is used for surface modelling as well as to extract the trim parameters as explained in the following paragraph. Figure 4: tunnel Yacht model and cameras in the wind High reflective markers are 8 mm in diameter, and are glued on 8 horizontal sections of each sail plus one on the top, on both windward and leeward side. On each of the mainsail sections there are six markers, and on the jib sections markers are 7: concerning targets on the jib at different heights, of them are equally spaced, and the seventh is halfway between the first and the second marker on entry angle at the luff, to acquire the most accurate changes in curvature. Figure : Flying shape measurement system layout

8 2.3 (b) Definition of sails parameter The flying shapes have been analysed using an in-house developed software, which get cloud of points from the measures obtained by multi-camera system, and fit data in order to evaluate maximum camber, maximum draft position and twist on four sections at fixed height for each sail. For each sail the 3D points array is fitted with a cubic spline over each horizontal section, with the aim of calculating a new pattern of points, dividing each curve in 5 equal parts. Therefore, cloud of points of two sails are fitted in vertical sense with cubic splines, building six curves for the mainsail and seven for the jib, starting from the top point, ending on the sail foot and passing through points in between, as shown in figure 7. For each section following parameters have been evaluated (fig. 9): maximum camber (% of chord length) maximum draft position (% of chord length) twist angle (referred to boat centreline) Figure 9: Measured sail shape parameters The twist angle of the boom is evaluated using two markers respectively placed at the boom gooseneck and at the aft end considering the yacht centreline as the reference line and the jib foot twist angle (from the centreline) has been also evaluated. Figure 7: Flying shape surface modelling Four different sections of sails have been considered at % 3% 5% and 7% height of P and I for mainsail and jib respectively as shown in figure 8. Figure 2: Sailplan tested Figure 8: Flying shape parameters extraction

9 3. RESULTS Wind tunnel tests were performed using a : scaled model of a 48 IMS cruiser-racer sailing yacht. Sailplan tested is a typical IMS type sailplan for upwind conditions: in particular a mainsail with the maximum IMS rule allowed roach and % non overlapping jib have been used (fig.2). deg AWA n test vs jib and main angles Boom Jib Foot Table shows the principal dimension of the yacht model sailplan. I 97 [mm] J [mm] P 7 [mm] E 58 [mm] Table : Principal sailplan dimensions Apparent wind angles were chosen to be 22, 27, 32 and 42 which cover the upwind range. The tests were conducted in upright condition. For each apparent wind angle, sail trimming during the wind tunnel tests were performed according to the abovementioned procedure (par. 2.2a). Experimental obtained results have been organised in a database in order to put in evidence the relationship between upwind flying shapes geometry and sail plan aerodynamic performance. In the following results relevant to close hauled tests at 22 and 27 apparent wind angles will be reported. In particular variation of sail performance will be provide with the change of mainsail twist and with the change of mainsail camber Apparent wind angle In the following the performance variation of the sailplan evaluated during the wind tunnel test will be shown with reference to the mainsail twist variation. Test was performed at 22 AWA and the mainsail camber was tuned by varying the back stay and check stay tension. Figure 2: Boom and jib foot angle (from yacht centreline) versus sail trim condition As can be seen during the experiment the jib was essentially fixed while the main boom angle changes by varying the mainsail sheet tension and the mainsail traveller position. In fig. 22 the measured twist angle relevant to different section chord lines (respectively at % 3% 5% and 7% of mast height) of both mainsail and jib are reported n test In fig. 2 the boom and jib foot twist angles (measured from the boat centreline) are reported versus test run number. Twist AWA 22 n test vs Twist Main 8 Main3 Main5 Main7 4 Jib Jib3 2 Jib5 Jib n test Figure 22: Mainsail and jib twist angle (for each sails section) versus sail trim condition Tests between n and n 8 are relevant to the trimming performed in order to reach the maximum performance (i.e. max driving force) and are not reported in figure. Starting from there figure 2 and 22 show that the sailplan has been progressively de-powered acting on the mainsail traveller position (initially quite high up to windward) to vary the incidence and the twist of the mainsail easing the main traveller and main sheet. The genoa was initially trimmed in order to have the maximum driving force condition and was fixed varying the mainsail shape. Figure 23 shows the camber variation measured in the same mainsail and jib sections during the wind tunnel test. For each section camber is expressed as a fraction of the chord length. Camber variation range is quite narrow because during the test the backstay and check stay were fixed.

10 8 AWA 22 n test vs Camber. AWA 22 Twist vs Cx,Cy Camber 4 2 Main Main3 Main5 Main7 Jib Jib3 Jib5 Jib7 Cx,Cy Cx Cy n test Figure 23: Mainsail and jib camber (for each sails section) versus sail trim condition Finally figure 24 shows the maximum draft position variation measured in the each mainsail and jib sections during the wind tunnel test. For each section it is expressed as a fraction of the relevant chord length. Maximum Draft Position AWA 22 n test vs Max Draft Position Main Main3 Main5 Main7 Jib Jib3 Jib5 Jib n test Figure 24: Mainsail and jib maximum draft position (for each sails section) versus sail trim condition A slight decreasing trend for the mainsail can be observed with the de-powering sequence. Figure 25 shows the variation of the driving force coefficient Cx and of the heeling force coefficient Cy with mainsail twist angle. The twist angle is the mean value of the four considered sections at different mast height mainsail. As can be seen during the de-powering the thrust force decreases of about 4% and heeling force reduction is 5% about Twist Figure 25: Driving and heeling force coefficients versus twist angle Figure 2 shows the corresponding variation of drag and lift coefficients with mainsail twist angle. Cd,Cl AWA 22 Twist vs Cd,Cl Twist Figure 2: Lift and drag coefficients versus twist angle Cea,Ceh AWA 22 Twist vs Cea,Ceh Cea Ceh Cd Cl Twist Figure 27: Centre of effort height and centre of effort longitudinal position versus twist angle

11 Figure 27 shows sailplan centre of effort (CE) positions varying mainsail twist. CE height is evaluated from boat deck and results are given in terms of fraction of mast height (P+BAS) AWA 22 Camber vs Cea,Ceh CE longitudinal position is given from the mast foreside and is expressed as a fraction of yacht water length. Minus value means directing afterward the mast. Figures 28-3 show the variation of the same quantities with mainsail camber. The main camber is the mean value of the maximum draft of four considered sections of the mainsail at different mast height mainsail. Finally figures 3-33 show the variation of the same quantities with mainsail maximum draft position i.e. the mean value of the four considered mainsail sections at different mast height. Cea,Ceh Camber 4 Cea Ceh Figure 3: Centre of effort height and longitudinal position versus camber Cx,Cy AWA 22 Camber vs Cx,Cy Cx Cy Cx,Cy AWA 22 Max Draft Position vs Cx,Cy Cx Cy Camber Figure 28: Driving and heeling force coefficients versus camber Max Draft Position Figure 3: Drive and heeling force coefficients versus maximum draft position Cd,Cl AWA 22 Camber vs Cd,Cl Cd Cl Cd,Cl AWA 22 Max Draft Position vs Cd,Cl Cd Cl Camber Figure 29: Lift and drag coefficients versus camber Max Draft Position Figure 32: Lift and drag coefficients versus maximum draft position

12 Cea,Ceh AWA 22 Max Draft Position vs Cea,Ceh Cea Ceh Twist Main Main3 Main5 Main7 Jib Jib3 Jib5 Jib7 AWA 27 n test vs Twist Max Draft Position Figure 33: Centre of effort height and longitudinal position versus maximum draft position Apparent wind angle In the following the performance variation of the sailplan evaluated at 27 AWA are reported. In Figure.34 the boom and jib foot twist angles (measured from the boat centreline) are reported versus test run number. In Figure 35 the measured twist angle relevant to different section chord lines (respectively at % 3% 5% and 7% of mast height) of both mainsail and jib are reported. Figure 3 shows the camber variation measured in the same mainsail and jib sections during the wind tunnel test. For each section camber is expressed as a fraction of the chord length. Camber variation range is quite narrow because during the test the backstay and check stay were fixed. Finally figure 37 shows the maximum draft position variation measured in the each mainsail and jib sections during the wind tunnel test. For each section it is expressed as a fraction of the relevant chord length. deg n test 5 27 AWA n test vs Jib and Main Angles 4 Jib Foot Boom Figure 34: Boom and jib foot angle (from yacht centreline) versus sail trim condition n test Figure 35: Mainsail and jib twist angle (for each sails section) versus sail trim condition Camber Main Main3 Main5 Main7 Jib Jib3 Jib5 Jib7 AWA 27 n test vs Camber n test Figure 3: Mainsail and jib camber (for each sails section) versus sail trim condition Maximum Draft Position Main Main3 Main5 Main7 Jib Jib3 Jib5 Jib7 AWA 27 n test vs Max Draft Position n test Figure 37: Mainsail and jib maximum draft position (for each sails section) versus sail trim condition

13 Driving and side force coefficients as well as centre of effort position and drag and lift coefficients variation with the mainsail twist are reported in figures Tests up to n 5 (not reported here) correspond to the maximum performance (i.e. max driving force) search. In particular tests within n and n were performed tuning the main traveller position and main sheeting in order to achieve the maximum thrust. AWA 27 Twist vs Cx,Cy..4 7 Cx Cy 2 Starting from there figures 34 and 35 show that the sailplan has been progressively de-powered acting on the mainsail traveller position to vary the incidence and the twist of the mainsail easing the main traveller and main sheet. As can be seen during the experiment the jib twist was essentially fixed until to test n. Then in the test n the jib sheet has been slightly eased increasing the jib camber and reducing the jib twist in the highest part. This is also put in evidence by the corresponding pictures of figures Cx,Cy Twist Figure 38: Driving and heeling force coefficients versus twist angle AWA 27 Twist vs Cea,Ceh.5 Figure 4: Top view during test n Cea,Ceh.3 Cea Ceh Twist Figure 39: Centre of effort height and centre of effort longitudinal position versus twist angle AWA 27 Twist vs Cd,Cl. 7.4 Cd Cl 2 3 Figure 42: Top view during test n Cd,Cl Proceeding up to test n 7 sailplan has been progressively de-powered easing the mainsail traveller to reduce the flow angle of incidence (fig. 34). Figure shows the corresponding variation of the mainsail twist and corresponding changing of the mainsail camber and of the maximum draft position Twist Figure 4: Lift and drag coefficients versus twist angle A aftward shift trend of the max draft position can be seen in the lower part of the main and an opposite trend in the top of the main. Finally during the test n 8 the genoa car was moved

14 backward increasing the twist of each section as shown in figure 35 and in the corresponding pictures of figures Figures 4-48 show the variation of the same quantities with mainsail camber. The main camber is the mean value of the maximum draft of four considered sections of the mainsail at different mast height mainsail. AWA 27 Camber vs Cx,Cy Cx,Cy Cx Cy Figure 43: Genoa view during test n Camber Figure 4: Driving and heeling force coefficients versus camber AWA 27 Camber vs Cea,Ceh Cea,Ceh.3 Cea Ceh.2. Figure 44: Genoa view during test n 8 Then during the last test the jib sheet was sheeted giving the twist reduction shown in figure 45 and the variation of camber and maximum draft position shown in figures 3-37 and in the corresponding picture of figure Camber Figure 47: Centre of effort height and longitudinal position versus camber AWA 27 Camber vs Cd,Cl Cd,Cl Cd Cl Figure 45: Genoa view during test n Camber Figure 48: Lift and drag coefficients versus camber 2

15 Finally figures 49-5 show the variation of the same quantities with mainsail maximum draft position i.e. the mean value of the four considered mainsail sections at different mast height. Cx,Cy AWA 27 Max Draft Position vs Cx,Cy Cx Cy Max Draft Position Figure 49: Drive and heeling force coefficients versus maximum draft position Cea,Ceh AWA 27 Max Draft Position vs Cea,Ceh Cea Ceh Max Draft Position Figure 5: Centre of effort height and longitudinal position versus maximum draft position Cd,Cl AWA 27 Max Draft Position vs Cd,Cl Max Draft Position Figure 5: Lift and drag coefficients versus maximum draft position 4 3 Cd Cl CONCLUSIONS In the present paper experimental measurements system developed for sails flying shape detection has been presented. The system can be used during wind tunnel testing and sails actual flying shapes resulting from wind loads and trimming can be real time detected and related to the sailplan aerodynamic behaviour. After a brief description of the experimental set-up and testing procedure used, some results concerning measured flying sails shapes at different height are provided at close hauled conditions together with aerodynamic coefficients and centre of effort position with reference to different sails trimming. 5. REFERENCES. Krebber B. Hochkirch K., Numerical investigation on the effects of trim for a yacht rig, High Performance Yacht Design Conference Auckland, 4--Feb Masuyama Y. et al., Database of sail shapes vs sail performance and validation of numerical calculation for upwind condition, 8 th Chesapeake Sailing Yacht Symposium- Annapolis, March Razenbach R., Keene J., Utility of Flying shapes in the development of offwind sail design databases, High Performance Yacht Design Conference Auckland, 4--Dec Hochkirch K. Brandt H., Full scale Hydrodynamic Force Measurement in the Berlin Sailing Dynamometer, 4 th Chesapeake Sailing Yacht Symposium- Annapolis, March Masuyama Y. Fukasawa T., Full scale measurements of sail force and validation of numerical calculation method, 3 th Chesapeake Sailing Yacht Symposium- Annapolis, March 997. Milgram J. et al., Modelling IACC sail forces by combining measurements with CFD, th Chesapeake Sailing Yacht Symposium- Annapolis, March J. M. C. Campbell, & A. R. Claughton, Wind Tunnel Testing of Sailing Yacht Rigs, 3 th HISVA symposium Amsterdam R. G. J. Flay, A twisted flow wind tunnel for testing yacht sails, Journal of Wind Engineering and industrial Aerodynamics, 3 (99) Fossati F. et al., Wind Tunnel Techniques for Investigation and Optimization of Sailing Yachts

16 Aerodynamics, High Performance Yacht Design Conference Auckland, 4--Feb. 2. AUTHORS BIOGRAPHIES Fabio Fossati holds the current position of full professor of Applied Mechanics. He is the scientific co-ordinator of wind tunnel testing of sailing yachts at the Wind Tunnel of the Milan Polytechnic with special reference to researches on sail plans and hull appendages. He was in charge of testing carried out in the Wind Tunnel for the PRADA Challenge America s Cup team in 23 and the Luna Rossa team in 27. Currently he is Research Associate of the International Technical Committee of the Offshore Racing Congress. Sara Muggiasca holds the current position of researcher at Politecnico di Milano Department of Mechanics. Her researches are in wind engineering field with particular reference to aeroelasticity. Fabrizio Martina is a PhD student at Politecnico di Milano Department of Mechanics. His research mainly concerns photogrammetry and 3D geometry detection techniques.