[RC Wing Sailboat Project] Second Prototype Testing
Testing the second prototype
Originally Published Spring 2011 as part of my capstone engineering design project at Brown University
Now that the problems with the foam wings were fixed and a soft sail rig was completed, their performance against each other could be tested. This was done using the rolling cart that I built for this purpose.
The sails are placed on the rolling cart and two stacked fans blow air onto the sail. I decided to use two fans so I could have the fans closer for a higher windspeed and still get breeze over the whole length of the different sails (especially the tall 6.25:1 AR wing). The fans are positioned .5m away from the pivot of the wing sail (mast of the soft sail) and at 4 different angles measured from front beam of the cart (see the white piece of paper). The angles I chose for this test were 30deg, 45 deg, 90deg, and 120deg. These correspond basically to 4 points of sail respectively: close hauled, close reach, beam reach, and broad reach. As the fans blow on the sails the cart moves forward slightly and pulls a line connected to a 500g weight on a scale. The scale is zeroed beforehand and the amount of weight the sail cart lifts can be read off the scale. This effectively measures how much forward force each sail set up creates, which is the combination of the lift and drag vector’s forward components.
Each of the sail’s trim was tweaked to find the maximum forward force at each wind angle. This included changing the angle of attack and flap angle for the wing’s and changing the trim of the main and jib for the soft sail. All of the sail’s were trimmed to create aerodynamic lift like airfoils, not just to catch the wind which only reflects the amount of sail area and not aerodynamic force. The testing set up took a little tweaking. At one point during testing the cart seemed to be sliding sideways which would skew the data because the scale wouldn’t only be showing the forward component of the force. This problem was fixed by placing a few rubber bands around the wheels to give them more traction.
The two wings performed about the same with a few differences. This was somewhat surprising as I expected the higher aspect ratio (6.25:1) wing to perform better, since a higher aspect ratio wing is supposed to create less induced drag. There could be a few reasons for this fact not showing up in the results. The fans did not quite reach all the way up the higher aspect ratio wing, so it may have lost some performance there. The wings were also not both perfectly constructed between the hot wire foam process and assembly, so there could have been differences there. Finally the test is only at one wind speed, so there could be more difference between the two at different wind velocities.
The soft sail did not create as much forward force as the two wing sails. At 30 degrees it failed to produce any forward force, the sails were only flapping in the breeze. This highlights the fact that a boat with a rigid wing can sail much closer to the wind than a traditional sail, which is very important when trying to race to get to a point that is upwind of you. At 45degrees the soft sail produced about half the forward force. It did better at 90degrees producing around 66% the amount of force of the wings. Then it actually out performed the wing sails at 120 degrees. This anomaly in the data can be explained by the fact that once the apparent wind is this far behind soft sail, it can’t really produce lift anymore. This data reflects the force of the fans pushing on the sails (not creating lift) which was not tested for the wing sails.
As a result of this test I decided that the best wing for the RC sailboat would be the 4:1 aspect ratio wing. It performed about the same as the higher aspect wing at this wind speed, and has some positive characteristics when placed on a boat. First it’s center of mass is lower so when the wind hits the wing sail it creates less heeling moment (moment that tips the boat over) than a higher aspect ratio wing. Additionally since the wing is shorter and wider it is less prone to bending and should be more robust and easier to construct at this small scale.
Continue Reading about the Project here
Originally published at http://engin1000.pbworks.com.