[RC Wing Sailboat Project] Second Prototype
Designing and building the second prototype of the wing sail
Originally Published Spring 2011 as part of my capstone engineering design project at Brown University
For the second stage of my project I sought to fix the problems of the first prototype foam wing I built, then construct a 4:1 aspect ratio wing and a soft sail rig. Once these tasks were completed, I could test the two different aspect ratio wing sails and the soft sail on the rolling cart to qualitatively measure their performance against each other. After testing I could choose the best wing sail design and build it in a technique that is more accessible than the foam (hot wire foam cutters are not cheap).
Improving the First Prototype
Wing Bending
One of the problems I experienced with the 6.25:1 Aspect ratio Foam wing was that it would bend a lot when struck by the wind. This was not desirable in that the force that was going into bending the wing was not being used to propel the cart forward. I was also concerned that the foam might snap if it bent too much. I brainstormed a few ways to fix this problem. I thought about adding a threaded rod inside the foam that would extend through both wing sections, but thought that it might be too heavy and still not solve the bending problem fully. I also thought about covering the foam with paint or some other material to stiffen it. (I eventually used this technique to stiffen the flaps later). Finally I realized that the easiest and most effective way to stop the bending was to copy what most other sailboats use to keep their mast’s and sails in the air, standing rigging.
As can been seen in the pictures below I added fishing line rigging to the 6.25:1 Aspect ratio foam wing. The fishing line is attached at the top of the foam by a small length of acrylic that is screwed into the foam. This piece of acrylic lets the stays keep the same tension as the wing rotates. The other end of the fishing line stays are connected to the cart by eye bolts, and can be easily untied so the rigs can be swapped off the cart.
After more testing of the high aspect ratio wing with only one set of stays, and seeing that there was still bending in the middle of the wing, a second set of stays that attach to the wing where the two sections of foam meet were added. After the 4:1 aspect ratio wing was built I added a similar stay system to stop its bending, however it only needed the one set of stays that attached to the top of the wing because of its lower aspect ratio. The lower aspect ratio wing is also thicker than the higher AR wing which keeps it from bending as much.
Flap Stiffening
Another problem I was having with the foam wings was that the narrow flaps of foam would flex a lot when the wind flowed around the wing, a picture of this bending can be seen below.
This was definitely not desired because it affects the gap between the main element and the flap, changing the way the flow behaves. I brainstormed and researched a few techniques to fix this bending. There turned out to be many things that you could coat the foam with. Fiberglass , Elmers glue, wood glue, polyacrylic, epoxy, and even packaging tape. I eventually tried out epoxy, wood glue, and packaging tape. The application of the epoxy can be seen below. It took a long time to dry and while it didn’t seem to damage the foam, it was hard to apply and didn’t seem to dry very “hard” which would mean the flap could still flex.
I also tried the wood glue, which was also difficult and messy to apply evenly over the entire surface of the flap. My test ended up having a lot of drips and bubbles because the glue ran before it dried. The wood glue also took a long time to fully dry ( 24+ hrs). Once it did dry however it created a very stiff flap that would have probably done a good job on the wing if you could spend the time to get a perfect even coat (which I didn’t have).
I ended up settling on clear packaging tape because of the ease of uniform application and its ability to stop the flex of the flap sections. I built a testing rig that measured the amount a 12" span flap would bend when a 50g and 100g mass was placed on the end. I tested an uncovered flap and then flaps that had 1/4in, 1/2in and 15/16in (half of the tape width) overlap of packaging tape. (Add table if you have time). Pictures of the testing set up with a taped flap with no mass, a 100g mass on an uncovered flap, and the same mass on 15/16in overlapped taped flap can be seen below.
As you can see in the pictures the overlapped packaging tape does a really good job at reducing the flex in the foam flap. After performing all the testing I decided to use the 15/16in overlapped tape because its performance was the best and I wasn’t concerned with the weight penalties associated with it, since this was just a prototype for doing accurate testing and not intended to be the lightest possible wing. I then disassembled both of the foam wings and applied the packaging tape to the flaps. A picture of the same wing as shown above with the bad the flap flexing is shown below after packaging tape was applied. It is obvious the flap deformation is much much less, and will provide more accurate testing of the wing shape.
Building another wing and a Soft Sail Rig
4:1 Aspect Ratio Foam Wing
As explained above I planned to build 2 different aspect ratio wings to test the difference in their performance. This wing has a .25m total chord and 1m span, giving it a Sail area of .25m². The chord of the main element is .15m and the chord of the flap is .1m . The 4:1 aspect ratio wing went together much the same way as the 6.25:1 wing. I used a threaded rod screwed into the main section (at 30% of Chord length) as a pivot point and as a way to attach it to the cart. Small rectangular sections of acrylic were screwed to the top and bottom of the main section and top and bottom of the flaps. These hold the flaps on to the main section and let them pivot. The pivot point is at 85% of the main element’s chord. (Pictured Below)
Since the hot wire foam cutter could only cut 24in wing spans out of foam, both wings had to use multiple sections to obtain the desired height for the wingspan. To attach the two pieces of the main wing section two small threaded rods were placed inside the top of one and the bottom of the other. For the flaps small wooden skewers were placed into the top of the bottom flap, through another piece of acrylic (that controls the flap) and into the bottom of the top portion of the flap. (Pictured Below)
Soft Sail Rig
The pieces of the soft sail rig that came with the Victor Wildcat Model needed a lot of assembly and included gluing, drilling and tedious line tying. Due to time constraints I decided to build my own rig out of available materials in prince lab. Building my own set up would also enable me to make it easily interchangeable with the rolling cart for testing. I used a square section of acrylic for the mast which holds up the main sail. I drilled and tapped the bottom of the mast and screwed in the same threaded rod used in the foam wings so it would fit into the same hole on the rolling cart. A piece of rectangular section acrylic was used as the “boom” which hold the bottom part of the mainsail for trimming. The boom was allowed to pivot on the threaded rod that was placed into the bottom of the mast. The main sail was attached to the mast and boom by drilling holes into the acrylic sections and using some line to attach the two together. The completed mainsail can be seen below, attached to the rolling cart. The small sail in front, called the jib also needed to be attached. This was accomplished by adding a threaded post at the front of the cart that a jib boom (acrylic piece attached to bottom of jib used to trim it) could be connected to the cart. (Pictured Below)
The top of the jib was connected to the mast using more line. Finally “sheets” (lines used to trim the sails) were added by attaching one end of a piece of line to the end of each sail’s boom and placing the other end through horizontal holes drilled in the center beam of the cart. The line was then clamped using aligator clips on the other side of the hole so that the sheets could be easily adjusted. Pictures of the completed soft sail rig on the testing cart can be seen below.
Continue Reading about the Project here
Originally published at http://engin1000.pbworks.com.