[RC Wing Sailboat Project] First Prototype
Designing, Building and testing the first prototype of the wing sail
Originally Published Spring 2011 as part of my capstone engineering design project at Brown University
I went through several steps and processes to decide on my first prototype. I knew that it made sense to build a working wing first because I had a preexisting hull.
Initially I thought about designing and optimizing wing sections in software, then testing them in a wind tunnel of some sort. However, I realized that the goal of my project was not to build the best or fastest wing ever, but rather to build an accessible and easy to use, wing sailed boat. Thousands of man hours have already been spent optimizing airfoil and wing shapes and doing so wouldn’t help me achieve my goal. Thus, I decided I didn’t need the most optimized wing sail ever, but I did need one that would create a enough lift when the wind hit it to propel the boat through the water. At this point, the best strategy to determine if the wing would work is a dry land controlled experiment, but first I needed a wing design.
Through internet research I determined a shape and size for my wing. The following word document contains some of the information from experts and people that have tried similar designs that I used to make my decisions. Wingsailinfo.docx
The airfoil shape was determined by looking at what was currently used on C-class catamarans and what other people had tried on the small scale like me. The wing consists of two symmetrical airfoils, a thicker main element and a thinner flap attached to the trailing edge of the main element. The flap can be deflected to create an angle with respect to either side of the main element. This design is necessary so that the boat can sail on either tack i.e. with the wind coming over the left or right side of the boat when looking forward in the boat.
I settled on a 60% of total chord NACA0019 symmetric airfoil for the main section and a 40% of total chord NACA0009 symmetric airfoil for the trailing flap. NACA airfoils are standard shapes designed by the National Advisory Committee for Aeronautics. The 00 in the first two digits of the airfoil name indicate that the airfoil is symmetrical about the chord line. The last two digits of the 4 digit number are the maximum thickness as percent of the total chord of the airfoil. More information on NACA type airfoils can be found here:
The flap will have a pivot point at 85% of the main section’s chord ( .51 on the x-axis), while the main element will pivot on the boat at 30% of its own chord ( .18 on the x-axis, at its maximum thickness). . Below is an Excel plot of the
I chose the overall size of the wing based on the original sail area of the Victor Wildcat model catamaran. The original boat has 450in² of soft sail area. Since a hard sail is more efficient I reduced this number by about 15% to 390in² or .25m². I then chose a high aspect ratio of 6.25:1 . A higher aspect ratio typically means less induced drag in the three dimensional set up.
The prototype wing sections were cut using a CNC hot wire foam cutter. I decided to use this process of construction because it is relatively easy to get accurate wing sections quickly made. This will not be the final method of construction however because a CNC hot wire foam cutters are quite expensive and therefore wouldn’t meet my requirements. The wing cutting process involved imputing the data for each section into the FoamWorks 3.0 program then tweaking the wire voltage and cutting speed to obtain the desired shape.
While cutting the flap sections I realized that the NACA0009 airfoil shape was probably too thin to be easily built in the final method of construction that I envisioned (balsa “ribs” and plastic shrink wrap covering). The NACA0009 flap would be too flexible and fragile. Therefore I increased the thickness of the flap to a NACA0012 shape. The result was a flap that would be more feasible to build and stronger.
I also decided to test two different aspect ratio’s of the wing because there are significant trade offs for high/low aspect ratio wings on a sailboat. More specifically, high aspect ratio sails create less induced drag, but are detrimental to sail boats because the center of mass is higher and therefore produces a larger heeling moment. Heeling moment is what makes a boat tip over when the wind fills the sails . Low aspect ratio sails have more drag, but also have a lower center of gravity creating less heeling moment.
The higher aspect ratio foam wing sections that were constructed can be seen below.
To test my wing designs I planned on installing them on a rolling land “cart” that I could blow a fan on to see if the wing sail would move it across the ground. I used http://www.modellandyachts.com/ as a large influence in my testing rig design. The reason for this is that the sail area of their SL2 model (shown below) is very close to my design.
I soon realized that there wasn’t an easy quantitative way to measure the success of my wing design with this sort of test. Brainstorming with my professors led me to the idea of a static test using a high powered fan to blow on the sail cart, which would be attached to a force gauge. This would to effectively measure the force that the wing sail created. With this set up it would be easier to have a constant breeze speed and direction from the fan and thus obtain more quantitative data. Whereas with my other plan I would have to follow the moving cart with the fan. With the new test I could more easily compare the two different aspect ratio wing sails and the soft sail set up from the Victor Wildcat kit. Below is a picture of the force gauges I plan to try first in my testing set up. On the right is the completed high aspect ratio wing on the rolling cart.
Using a box fan I had readily available I conducted some preliminary testing of the first prototype wing sail on the cart to see if my design would work. I used a C-clamp to hold the force scale to the table and a piece of line to connect the scale to an threaded eye-hook on the cart. Below is a picture of the set up before testing.
Below is a short video of the preliminary testing scheme.
As you can see on the force scale in the video the wing sail does create a measurable amount of force. The scale reads about 60g or .6 N of force. It was great to see that the wing actually created lift. This seems like it could be a good set up for testing the different sail/wing set-ups after some refinement. Some of the problems I encountered are listed below.
-The main section of foam was very flexible, especially when the fan was pointed more toward the top of the wing.
-The flap was also very flexible and bent a lot during testing
-The control system for the flap and angle of attack was not very accurate.
For my 2nd prototype I plan to build the soft sail rig and the lower 4:1 aspect ratio foam wing sail so I can do a comparison test between the 3 setups. I can also use the testing rig to determine the optimal trim of the angle of attack of the main element and flap for each angle to the wind the boat would be sailing. To improve the testing I may reinforce the foam by extending the threaded rod through most of the entire wing to keep it from flexing too much. I will also improve the accuracy of the control system for the flap angle and angle of attack for the wing. Additionally the future testing will be performed using a more appropriate high powered fan, shown below.
Continue Reading about the Project here