Propulsion and Steering of an Autonomous Sailboat

Transcription

1 Propulsion and Steering of an Autonomous Sailboat Mert Urkmez (3 Credits) / Class of 2017 / Mechanical Engineer / mu63 Gabriel Zimmerman (4 Credits) / Class of 2017 / Mechanical Engineer / giz4 5/13/2016

2 Contents 1 Abstract 3 2 Introduction 3 3 Sail and Tail Design Dimensions Airfoil Selection and Analysis Sail and Tail Design Rationale Sail and Tail Manufacturing Sail and Tail Tests Actuation Test Actuation Test After Modification Cayuga Lake Testing Lessons From First Prototype 14 6 Future Design Work 15 7 Conclusion 16 8 Appendices Importing Airfoil Into CAD Joining Carbon Fiber Tubes Using Design Spreadsheet to Analyze Main Sail Axle Sail and Tail Parts List

3 1 Abstract This semester our goal was to build an autonomous sailboat that could successfully navigate GPS waypoints placed on Cayuga Lake in Ithaca New York. Based on previous semesters research, a directionally stable sailboat design involves both a rigid airfoil sail and tail. The airfoil tail acts as an air rudder and removes the need for a water rudder[2]. We aimed to increase durability while decreasing the overall weight of the sail and tail by using design and manufacturing techniques inspired by plane wings built by the Design Build Fly (DBF) project team. The sail and tail assembly met the set expectations. The predetermined weight limit of 750 grams, according to the mass budget calculations, was achieved with our completed sail and tail assembly weighing 700 grams. The sail was able to withstand the forces from wind up to 50 mph according to a 2-dimensional ANSYS finite element analysis discussed in detail below. Finally, and most importantly, the sail and tail assembly was able to propel and steer the entire sailboat on Cayuga Lake. 2 Introduction Each control surface, the sail for propulsion and the tail for steering, is contained in one larger assembly. Jesse Miller, a mechanical engineering student at Cornell University, created a MatLab simulation with an optimizing function which iterated through boat parameters including sail, tail, and keel dimensions to maximize the boat s velocity-made-good[1]. The sail and tail dimensions were optimized for 5 m/s winds. This sail is referred to as the low-wind-sail, geared towards generating maximum lift in low wind conditions. In harsher sailing environments, a high-wind-sail which is shorter in length should be used to reduce the total lift generated and therefore reduce the high stresses on the sail assembly. Generating less lift is also important in maintaining the boat s directional stability[1]. To dimension this high-wind-sail, the simulation and optimizing function should be run with the desired wind speed. 3 Sail and Tail Design 3.1 Dimensions The first step before manufacturing was finalizing the dimensions using the dynamic simulation previously mentioned. One parameter used in the optimization was Andy s Law[2]. Andy s Law dictates the sail and tail sizes and tail moment arm based on set parameters such as a desired aspect ratio of 4. As shown below, the final sail and tail dimensions provide this aspect ratio. AspectRatio = (SailLength)/(SailChord) = 1meter/0.24meters = 4.16 (1) 3

4 Figure 1: Optimized Sailboat Dimensions 3.2 Airfoil Selection and Analysis After acquiring the dimensions, we evaluated airfoils that best fulfilled our needs: a symmetric airfoil that would generate the most lift while minimizing the drag. Professor Ruina suggested using a fatter airfoil because increases in lift would likely offset increases in drag. The fatter airfoil would also stall more gently at higher angles allowing for extra lift generation. A stall is when a wing produces less lift and more drag. The increased drag causes the speed to decrease further so that the wing produces even less lift[3]. We therefore concluded that using a NACA 0021 instead of a NACA 0015 would benefit the overall propulsion. In addition to the added lift, a NACA 0021 affords for more room inside the sail for mounting the tail servo and trim arms (see sail and tail design for more details). Note: In the simulation the sail was modeled as a NACA 0015 because the experimental lift and drag coefficients at different angles were found over a 180 range only for a NACA After finalizing our design choices, we analyzed the behavior of a NACA 0021 under various conditions which were already tabulated[4]. We also created an ANSYS FLUENT workbench to acquire our own values for 2 dimensional lift and drag coefficients too. 4

5 Figure 2: Lift coefficient of NACA 0021 for regimes in which significant portions of the boundary layers can be laminar[5] Using coordinates found for a NACA 0021 airfoil, we created a SolidWorks model. As seen in figure 4, we added truss sections to our airfoil to decrease the overall weight of the sail while not compromising its strength. We simulated a wind force to show that the sail will not yield in wind speeds of 50 mph. One assumption while calculating maximum wind speed was that airfoil cross sections would break first under extreme conditions. Figure 3: FEA stress and strain analysis using SolidWorks on the balsa wood airfoil cross section for 50 mph winds 5

6 3.3 Sail and Tail Design Rationale Our sail and tail design started by a consultation with DBF members because our sail and tail are both very similar to their plane wings. By examining and evaluating DBF s wing assembly, we determined our main sail material would be balsa wood, and our main adhesive would be Great Planes Cyanoacrylate (CA). We also arrived at a feasible and structurally sound design. This involved adding the grooves in the airfoil ribs for the front spars as well as a long spine that runs along the length of the sail s leading edge. The grooves help with aligning the airfoils and ensuring they are parallel and the spine helps the process of applying MonoKote (see figure 13). Figure 4: Tail Cross Section with Grooves and Spine Figure 5: Sail Construction 6

7 We did stress analysis on our main axle (mast) using an excel spreadsheet that characterizes the maximum allowable stress on the carbon fiber support rods with a factor of safety (see Appendix C). Our analysis shows that our main axle is structurally sound in wind speeds of 5 m/s with a safety factor of at least 2. After speaking with Professor Petru Petrina, Professor Matt Ulinski, and FSAE composites member Alex Milde, we finalized a manufacturing process for joining carbon fiber tubes. This design involved drilling through holes in the main axle and inserting the smaller carbon fiber axle through those holes. These two rods form a t-shaped frame bound with carbon fiber tow, soaked in epoxy, and wrapped in a crisscross pattern (see Appendix B). 3.4 Sail and Tail Manufacturing We started manufacturing the sail and tail assembly by laser cutting the airfoils and spars from 1/8 balsa wood in the Rapid Prototyping Lab in Cornell s Rhodes Hall, Room 114. To laser cut, we oriented the SolidWorks part in the desired 2-dimensional orientation and saved the file as a DWG or DXF file. Figure 6: Sail Airfoil Cross Section Figure 7: Half of Front Spar for Sail The second step of the assembly was to construct the main carbon fiber frame for the sail where the cross sections slide (see Appendix C for processes) (Fig 5). Before the tail support arms are fixed to the main axle, the correct number of airfoil cross sections must be placed on the main axle. Once the carbon fiber tubes are joined, airfoil ribs can no longer be added. The airfoil ribs are equally spaced by placing each spar section into the airfoil grooves. All balsa wood connections are secured with CA glue which is a very strong adhesive that seeps 7

8 into the wood. Only a small drop or two is needed for a secure connection. This balsa wood cross section and spar assembly is then fixed to the carbon fiber axle using the same CA glue. Following the placement of cross sections, we glued 1/16 balsa wood sheets on the front and back of the airfoils which provided support for the MonoKote (front sheets not shown in Fig 5). Without these sheets, the MonoKote is prone to bow in between contact points. Holes were drilled in the back sheet of the sail to allow the trim arm, made of straight piano wire, to connect to the servo and tail. The trim arms slide through a brass tube mounted on the side of the sail. To help to secure the tube in place, balsa wood dust from sanding can be put on top of the tube and then CA glue can be applied. These wood shavings and CA create a strong connection and can be used to reinforce any connections (Fig 14). The brass tube helps to direct the wire and ensure the rotational motion of the servo is causing linear motion in the wire. The tubes also provided a minuscule opening making the system more waterproof. A viscous oil or lubricant can be placed in the brass tube to further waterproof this connection. Figure 8: Steel Wire in Brass Tube Connecting Servo and Tail Before MonoKoting, we finished sanding down the axle and all balsa wood components to achieve the desired airfoil shape. Upon finishing the wooden frame of the sail and tail, we finished the assembly with MonoKote, a self-adhering wrapping film covered with a layer of protecting transparent film on one side. Before applying MonoKote, this transparent film needs to be removed. The MonoKote side with the protective plastic should be on the exterior when laid upon the wood. Another way to know which way to lay the MonoKote is to ensure the shiny side is exposed. We laid out a generous amount of MonoKote, about 1-2 of extra material in all directions, over the sail and tail respectively. Iron the MonoKote at a low to medium heat setting onto the wooden ribs first, holding for about 1 second. Once the MonoKote is secured to the wood, iron it in areas where there are no wooden supports underneath. The wrinkles disappear when ironed, but if the iron is held for too long, the MonoKote will burn. Finally, we used a heat gun on a low setting at about 8-10 inches away 8

9 to remove the final wrinkles from the MonoKote film. It should be taut around the entire assembly. It is important that the MonoKote fully wraps around the wing structure because this closed loop is a major source of torsional strength. The sail interfaces with the tail using bearing houses that are small metal cylinders with bored holes. Inside the bearing house is a nylon sleeve to reduce friction when the tail axle rotates. There is also another hole bored perpendicular to that hole for the support arms of the carbon fiber rod. 9

10 Figure 9: Final Assembly (trim arm and counter balance mass not shown 10

11 Figure 10: CAD Model of Bearing Housing We wanted the sail assembly to be balanced about its center of pressure, the quarter chord of the airfoil. To determine the counterbalance mass, we hung the sail from its main axle, and slowly applied a force using a digital force gauge until the sail was parallel with the ground. Placing a level on the sail surface can ensure the sail is close to parallel with the ground. This force was used to determine the necessary mass to have a balanced sail. 11

12 Figure 11: Final Assembly (trim arm and counter balance mass not shown 12

13 4 Sail and Tail Tests Upon finishing our assembly, we did a series of tests on Cayuga Lake and in Professor Ruina s Lab to check the actuation of our assembly and determine any necessary modifications. 4.1 Actuation Test We conducted a tail trimming test using the full sail and tail assembly with Arjan Singh, a member of the navigation team. The first challenge in our actuation test was determining the servo s zero position. When installing the servo, we did not run an electronics command to center the servo. Therefore, when the tail was sitting at an angle of zero with respect to the sail, the servo s position was not exactly zero. In the future, zeroing the servo before securing it inside the sail would help the interface with the electronics and navigation. Once the servo was centered, we experimented with the angular range of movement on either side of the neutral axis. (There is a known stall angle of 30 to either side of the neutral axis according to the dynamics simulation.) These tests allowed us to determine both whether the stiff piano wires would transmit the servo torque and also whether the desired range of angles was achievable. The results can be found in the Test Videos folder in the Sailboat Spring 2016 folder. The overall result of the testing was very positive. We were able to successfully trim the tail using the PCB. One potential issue was the trim wire interfering with the front of the tail at larger angles. 4.2 Actuation Test After Modification After the initial test, we decided to fix the trim arms controlling the tail as they were only allowing rotations of less than 15 degrees in both directions. We cut open the section of the sail containing the tail servo to replace the wires and brass tubes in a new formation which would allow for unhindered rotation. 13

14 Figure 12: Image of the steel wires entering from sail to trimming-arm from below After the modification, we tested the sail and tail while the sail was mounted to the deck. Both the sail and tail rotated without encountering a problem. 4.3 Cayuga Lake Testing We took our sailboat to The Merrill Family Sailing Center and Cornell Wellness on Cayuga Lake to test our final assembly. Our sail and tail assemblies were able to withstand wind gusts up to 30 mph with an average wind speed of 20 mph and steer the boat when controlled by an RC (see Test Videos in Autonomous Sailboat Folder). In winds speeds this high, however, the sailboat s behavior is sporadic. A high-wind-sail should be used in these wind conditions to reduce the lift and normalize the boat s movement. 5 Lessons From First Prototype The experience from the first assembly process dictated some changes we will be making to the assembly. Needed changes: 1. Switch the front and rear sheets to 1/32 balsa wood from 1/16 balsa or make sure to treat the 1/16 balsa wood before bending. We found that the balsa wood had uneven grain and would snap when attempting to bend. This limited our ability to math the front curvature of each airfoil, increasing aerodynamic drag. We think that using 1/32 balsa would afford the ease of bending, minimize weight, and any lost in strength would not be substantial. 2. When machining parts make sure to machine to the measured dimension, not the nominal dimension, of off-the-shelf-parts. For example, we ordered 14

15 two identical nylon sleeve bearings and each had a different outer diameter. Therefore, even though we machined both parts identically, they both did not work because of tolerances. This is a lesson to be applied to all machined parts on the boat. 3. When aligning spars, use purchased machining blocks to ensure 90 angles between each airfoil and the spar. Because the machining blocks have near perfect perpendicular faces, they can easily be used to help alignment. 6 Future Design Work 1. An adjustable camber for our cross sections would allow us to generate more lift compared to symmetrical airfoils we are using. After an initial research we found a patent for adjustable camber which uses an elastic material re-shaped by push rods[6]. Figure 13: Adjustable Camber in Sail Section Figure 14: Adjustable Camber in Sail Side View 15

16 2. Use a CNC foam cut leading edge to better match the NACA 0021 shape. The manufacturing process would be similar, still utilizing balsa wood cross sections, front spars, and MonoKote. The foam would be the only major change. The insulating foam used in the CNC wire cutter owned by the CU Air project team has a very similar density to balsa wood. Therefore implementing this design change should be fairly straight forward as it does not mandate any mass reduction. Figure 15: Updated Airofil Design Utilizing CNC Foam Leading Edge 7 Conclusion This semester, as an entire team, we made many improvements. Our sail and tail assembly functioned well, satisfying all the sailboat needs. The sail successfully generated propulsion and the tail successfully steered the boat. Future work should be catered around improving the sail and tail robustness and aerodynamics. Our current manufacturing process makes matching the airfoil shape precisely a challenge. Now that a functioning sail and tail are assembled, focus should be put on evaluating its performance when sailing on Cayuga Lake and determing the most immediate changes to be made. 8 Appendices 8.1 Importing Airfoil Into CAD 1. Find an airfoil whose size, lift, and drag coefficients meet the desired needs. Use airfoiltools.com as a reference. 2. Once the desired airfoil is found, copy and paste all of the x, y, z coordinates into a notepad document and save the document. 3. Open a new part in SolidWorks and go to Insert Curve Curve through XYZ points Browse for the file saved in part 2. Click OK. The full airfoil shape should now be inserted into the part. 4. Extrusions and modifications can be made to the inserted airfoil as needed. 16

17 8.2 Joining Carbon Fiber Tubes 1. A through hole matching the outer diameter of the smaller tube is drilled into the tube with bigger diameter. Use a fixture or rig to keep the tube still. Both holes must be drilled completely perpendicular and in the same plane. Using a fixture will ensure the hole placements are correct. 2. The smaller tube is placed through the drilled hole. The smaller tube can stick out from the hole if space permits in the general assembly. 3. Cut enough carbon fiber tow to wrap around the tubes at least 3 times. In our assembly, this was approximately 18, but the length needed will vary with tube diameters. Both ends are tied to prevent unraveling. One end is glued near the hole drilled on carbon fiber tube. 4. Paint epoxy onto the carbon fiber tow using a brush. 5. Wrap the carbon fiber tow around the tubes in a crisscross pattern. 6. Secure the loose end of the epoxy tow using a rubber band or a knotted thread to ensure the tow remains tightly bound around the joint. 8.3 Using Design Spreadsheet to Analyze Main Sail Axle 1. This design sheet is not a substitute for hand calculations and other forms of analysis. Its purpose is to give a preliminary design analysis. It can help determine a factor of safety before moving forth with manufacturing. Make sure additional analysis confirms design spreadsheet implications. 2. Load the Sail Tail Design Sheet.xlxs found in the Autonomous Sailboat Google Drive: Propulsion Block: Measurements and Calculations:. The purpose of this design sheet is to analyze the stresses on the main carbon fiber axle. Varying inputs and dimensions allow a general understanding of the structural integrity of the sail frame. 3. Be sure to read the Assumptions Tab. It outlines the simplifications made in the problem and explains how the forces and loads are applied. The image below shows the sail and tail assembly. Each of the two point loads contributing to the total stress state are shown. 17

18 Figure 16: Distrubuted Load Modeled as Two Point Loads on Sail Frame 4. Input all relevant sail and tail dimensions from the dynamics simulation. Iterative design can be used in determining the axle dimensions to ensure a desired factor of safety is achieved. 5. The design spreadsheet calculates a torsional stress and bending stress and then calculates the principle stress. It is this stress, when compared with the yield stress, that provides a factor of safety. 6. The principal stress is a combination of the maximum torsional and maximum bending stresses. Both the maximum torsional and bending stresses assume a point load at a different location, and therefore the magnitude of the principal stress is highly unlikely. 18

19 8.4 Sail and Tail Parts List Figure 17: Parts list of needed materials for assembling sail and tail. List excludes CA adhesive, drills, razor blades, and various items available in the lab. 19

20 References [1] Miller, J. Direcitonally Stable Robotic Sailing. Cornell University. (2016) [2] Augenstein, T. Cornell Autonmous Sailboat Team Fall 2015 Report. Cornell University (2015) [3] Shih, C., Lourenco, L., Van Dommelen, L. & Krothapalli, A. (1992) Unsteady flow past an airfoil pitching at a constant rate. AIAA Journal [4] NACA 0021 (naca0021-il) il [5] Menter,F.(2015) Taking Laminar-Turbulent Transition Modeling to the Next Level,ansys-blog [6] Patent No: US A1 20