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Entry 22
 
 
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Comments

Our design was tested over several months-we have a MESA class so one advantage is that we didn't wait until the last moment. The CMHS team tested 4 to 5 hours a week for months. I spoke with Richard who said JPL engineers tested for -oh, about 2 to 3 hours- total. "Does it show?" We laughed about that one! Almost all of September was spent just on the launcher. We stressed the measurement of everything we could get our hands on. We used a magnetic "protractor" to measure the 5 degree angle and practiced on the blacktop-we were used to the glider landing on concrete and not grass. We used a force gauge to measure the spring in the launcher and found out some interesting things. Launcher springs get tired after use and lose their force. At first we thought the launcher had so much force we wouldn't have to worry about drag and we could launch a brick, but it wasn't as strong as we thought.

The first few gliders would fly off the launcher easily when they went straight, but as soon as they would turn, they would eat the ground. One Trigonometry student suggested it was a vector problem-it needed a force at right angles to the launcher, but it didn't have one. The Costa Mesa team has won first place at Engineering Day in the distance balsa glider contest for three years in a row, and we learned a few things from that. We have a design for the fuselage (we still aren't sharing!) that works well. We discovered that through trial and error a couple years ago, and we are still not sure how it works, but it does. During the distance trials, we needed a big flat wing with a high aspect ratio. To get distance, you need to "float" and any airfoil will just cause a stall. Other teams actually laughed at our design as too big and ungainly, until we landed 50 feet ahead of them. They didn't even know what a glider was supposed to look like. At MESA Day last year conditions were awful, wind, rain, and trees in the way. Our first shot was high and to one side, and we had to repair the glider and change the balance between shots. The team did that and the second shot stayed flat and went straight. We knew that we had to have a design that could be adjusted between shots.

For the Wright Turn, a flat wing wouldn't have enough lift to get it to the target. After building a few wings from scratch, we found one at a hobby store that seemed to work. It has a foam core and a smooth outer surface. There were several choices, but we used the one with the longest wingspan we could find, and the wingtips were gradual, not stubby. Like R/C airplanes, it was attached with rubber bands for two reasons. You could adjust the wing angle between trials, and when the glider hit the pavement it wouldn't break off. The worst that could happen is that you'd need a new rubber band. As a teacher though, rubber bands are bad news because the students always shoot them at you.

Practice, practice, practice. The students found that you could get it to turn just by cranking the wing, and we used a balsa tail mounted with a rubber band too, so we could try different angles. We found that the tail surfaces weren't effective right off the launcher and they only worked near the target, so we threw out ailerons and rudders. A tail cranked to the left and a wing cranked to the right would turn both ends of the glider. Positioning the glider on the right of the launcher helped too-measurements again-and placing it 1 and 3/4 inches from the outside of the launcher was the way to go. Balance and CG were important. A physics teacher at our school used to fly in the US Air Force and taught us about balance. You want the wing to be close to the center of gravity, and we knew from distance trials that you usually need weight in the nose. You can balance the glider just with your fingers-those fancy balancing devices from the hobby store aren't necessary. If distance is needed, you want the glider to land flat. If the nose or tail hits first it is out of balance.

We did use lead weights in quarter ounce increments. We'd just wrap tape around them but they'd break off. Eventually the weights were stuck on with double stick tape, wrapped on the outside, and mounted high enough so they didn't scrape the ground. With our design (which weighed 200 grams) we used 1 and 1/4 to 1 and a half ounces.

For the prelims we had a fairly crude design that seemed to work. The launcher at the prelims was putting out too much force-35 N as opposed to the 27.6 N it was supposed to put out. Our design was very efficient. We were afraid it would overshoot the target and it did. Before the last shot Joe Powers said, "watch this, I'm going to make it stall." Fantastic! Instead of overshooting it came down near the target, and was the closest distance from both USC and the CSUF prelims combined.

We knew that with many, many trials we'd get a lucky shot and nail the X every now and then by accident. One day we launched two gliders and they both landed on top of each other, both on the X. Yeah, right! The key is REPEATABILITY. We didn't want to win by accident, we wanted to win by design. The balsa fuselage was good because we could record wing angles on it with pencil. If you hit the X it was fun to yell and scream, but if you didn't record what you were doing it was worthless. Sometimes hitting the ground would throw the wing out of whack and you'd have to start all over again.

The tail was breaking on impact sometimes and if it had too much mass the balance was out. It was replaced by carbon fiber-the strongest, lightest substance available. Carbon fiber is used in the body of an Indy car, and in the wheels of a solar car. It never breaks, but we had to figure out how to work with it. If it is thin enough, you can still cut it with a hacksaw. We didn't use it for the horizontal stabilizer, only the rudder. The horizontal stabilizer needed some thickness. We used about $40 worth for the two planes. Use it carefully, as it can have sharp edges and can cut you. Ted went home one day with blood all over his shirt.

Going into the finals we had two planes that were almost the same. We screwed up and should have entered three teams-we probably would have qualified a third team and had a couple more shots at it. Read the rules! Winning an engineering contest is always about how closely you read the rules. If other teams are complaining about you it is because you read the rules more closely than they did. Often it is about finding something the rules DIDN'T say, and that's what you do!

We were very relieved on the day of the contest to find the launcher at the finals was putting out 27 N of force. Just right, just what we practiced at. Very little wind, also good. We were reading weather reports with bated breath for a few days. We tried different fuselage designs, including a clear plastic tube, but they didn't resist wind as well. The hollow tube went all screwy when there was a gust. I was hoping that the winner would be determined by their design, not by a fluke. We practiced in the grass at JPL and glider #2 was more accurate than #1. The launch order put #1 at the top and #2 at the bottom. On the first round Joe launched #1 and it went straight, into the crowd. Made some dramatic video, but it told us we needed more wing angle. When it goes straight you get way too much distance! Joe told Ted about the wing angle and Ted's first shot with #2 was only 6 feet off. Decent, but we were in 3rd behind some Jr High kids. I told my team I didn't mind losing to JPL, but never to Jr High kids! They practiced quickly between rounds and worked together as a team. In round two Joe's second shot with #1 was much better, about 11 feet off. The angle was just right, but it was undershooting. He told Ted what the angle was, but they moved the wing up to get more distance (range). Our last shot with Ted launching #2 was right on- only 1 foot 9 inches from the X. First place. Overall.

We learned alot of the old fashioned lessons. Success comes from teamwork, practice, and hard work. Nobody wins because they are so smart they can just slap something together at the last moment and go home with a trophy. Engineering is a process of inspiration, design, testing, redesign, testing, and so on. Smart students are everywhere you look. The ones that win are the students who test their designs!
 
 
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Jet Propulsion Laboratory National Aeronautics and Space Administration California Institute of Technology