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Final Testing

The first goal of the final testing was to see if the shrink wrapped balsa wing would meet the performance requirements that I set. I also wanted to get data on what angle of attack and flap angle produced the best results at each apparent wind angle. This data could then be used by the person sailing the boat as a basic trimming guide.

Final Performance Requirements

• For apparent wind angles of 0-90deg (0 deg is in the direction of motion of the cart), the wing sail should create at least 50% more forward pulling force on the cart than the soft sail.

• The wing sail should be able to create forward pulling force on the cart at closer apparent wind angles than the soft sail.

Testing Set-up

     The set up was basically identical to the second prototype testing. One difference was that I wrapped the wheels of the cart with a strip of duct tape to prevent side slipping, which seemed to work better than the masking tape and rubber bands I tried before. I also used a different scale, which could not read negative values. Therefore I took the reading on the scale once the fans were on an subtracted it from the original mass of the weight. Additionally since I was only testing apparent wind angles of 90deg and less, I added an extra testing angle at 65 degrees (and got rid of the 120deg).

Procedure

For each set up I tried to keep the fans running as long as possible so that any fluctuations in voltage or motor temperature wouldn't skew my data.

Shrink wrap covered balsa wing

     For the balsa wing at each apparent wind angle I played around with the angle of attack and the flap angle to find the maximum lifting force. At each trim I would do a few trials to see how much weight the cart pulled. This was accomplished by pulling the cart back until the scale read 499g and then letting it go again to pull the weight up and reading the scale. I then took the average of these trials. I did this at each of the different angles of attack and flap angles that I tried. After I determined which one of those trims performed the best, I set the wing up that way again and attempted to recreate the results. This was somewhat difficult because setting the angle of attack to within a few degrees was not easy with the protractor I had. There were also other error factors that came into play, like the rolling friction of the cart and fluctuations in the fans. If I were to do more testing I might figure out a more accurate way to measure the angle of attack. However this is not going to be the most accurate testing ever because of the fact that the fans don't produce a uniform flow of air in one direction. The main goal of the testing was to prove the wing would perform better than the soft sail. Finding the trimming angles was only a secondary task.

Soft Sail

     For the soft sail I basically just played with the trim of the main and jib at each apparent wind angle until I found what created the most lift force. I didn't need to worry much about the effects that each change in the trim made because I'm not trying to write a trimming guide for the soft sail.

Results and Discussion

Table of results, foam wing data from previous testing added for comparison.

Plot of results

     As can be seen in the table above the shrink wrapped balsa wing sail met my first final requirement of creating at least 50% more forward pulling force than the soft sail. It just made the cut off for 90deg and far exceeded the requirement at 65 and 45 degrees. This trend in the data shows that the wing sail creates an increasingly greater amount of pulling force as the apparent wind angle decreases. 

     The data also meets my secondary requirement that the wing sail would create forward pulling force at an apparent wind angle closer than the soft sail. This is shown by the fact that the wing sail created about 56g of pulling force at 30deg while the soft sail produced none.

Trimming Guide

Below is a chart that gives the angle of attack and flap angle that produced the maximum forward force at each apparent wind angle. This information would be a good guide so that someone could trim the wing for the best performance at these angles. As discussed in the procedure this is not the best data because the angle of attack was not the easiest to measure just using a protractor. Also because of fluctuations in the fans, friction on the testing cart wheels, and other such errors fine tuning the wing was difficult. It is only intended to be a guide.

Videos

(View full screen for HD)

I took video of the max lift force for the wing and soft sail at each angle of attack, for documentation that I completed my requirements. The first two videos are the wing and soft sails at 90 deg angle of attack.

     In each video you can see the fans are running and the scale is showing the reduced weight of the 499g mass due to the forward pull of the cart. The scale reads 253g. So 499-253= 239g of lifting force as written above. The scale in the soft sail reads 344g, so 499-344= 155g of lifting force.

     In the wing sail test you can see a value of 443g on the scale which leads to 56g of lifting force. In the soft sail test you can see that the sail is mostly luffing in the wind from the fan. there is a decent amount of sideways force created, but not enough forward force to overcome the friction of the cart and register a weight on the scale.

Here is a PDF of the final testing data. FinalTestingData.pdf

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