Concrete Glider

The Myth:
Adam and Jamie take on an engineering myth that says it is impossible to create a functional glider made of concrete.

The Expert: Dr. Steve Smith - Aeronautical Engineer

Quotable/Memorable Moments:

Adam:(after flying one of his early models) If I had built a concrete glider this size, I mean, it would be known as a rock! 

Before any glider fabrication can begin, Adam and Jamie need to find the best mix of concrete for the job (i.e. the lightest mix of concrete). Since a main component of concrete is cement, they think that finding the lightest blend of cement will yield the best concrete. Jamie creates four different batches of concrete, each with different ingredients, to find the lightest formula for use in the glider. The four batches include a control batch with sand and gravel, along with a mix with foam bean bag beads, a mix with pumice, and a fourth mix with small glass beads.

To get some help with aerodynamics and glider construction, Adam and Jamie travel to a NASA research center and consult with Dr. Steve Smith. He informs them that they will be able to create a concrete glider so long as they can make a light enough mix of concrete and can reinforce it.

Back at M5, the sample mixes of concrete are removed from the mold and weighed. The control mix of gravel and sand is the heaviest, as expected, and the foam bead mix is the lightest. The next step is to create reinforced batches of the foam bead concrete to run some strength tests. They use different means of reinforcement, such as carbon fiber grids, wire mesh, and thin steel cables. The cables are shown to be the most effective additive in thin pieces of concrete.

Now that they have all of the information they need to begin building gliders, Adam and Jamie begin another build-off competition, much to Jamie’s chagrin. This time, Adam has insisted on predetermined rules so Jamie can’t bend them to his advantage. The winner will be determined by whoever can achieve the best glide ratio with their glider.

Jamie’s strategy is to buy a foam glider and add weight to it until it can barely glide, then to use that glider as a model for his concrete glider, which will be kept under the weight of the original weighted down glider. That method should ensure that his glider will attain a decent glide ratio at the very least. Jamie chooses two glider models, both of which have excellent lift and glide ratios. Through further testing, he decides that a hybrid model of the two models he chose to test with will yield the best glider. He also finds that the weight of his glider should not exceed 800 grams. Once one half of the concrete wing was made, he found that weight was becoming an issue, so he decides to use just that half of the wing as the whole wing, but since both pieces of the wing were needed to create an airfoil shape, he has to rig a string tensioning system that would bend the wing slightly and create the airfoil shape he needs.

Adam’s approach, however, is much more experimental. He decides to make his own glider out of a light material, which proves to be fairly successful. He then replicates his first glider using wood instead, which should test whether or not his design works under a heavier load. He throws the second wooden model off of the top of M5, and but it only achieves a 1:1 glide ratio and breaks apart when it hits the ground. He scraps his first two models and does some more research. Adam finds some information on the origins of the glider, and researches the works of Sir George Cayley, one of the first aeronautical experimentalists. He decides to use Cayley’s design for a glider, and makes a foam mold of his designs, onto which he applies the reinforced concrete. His glider ends up being much larger and heavier than Jamie’s.

Now that both Adam and Jamie have a finished glider, they have to start reducing the weight of their gliders to get better results. Once completed, they reveal their gliders to each other and weigh them. Jamie’s “wilting bird” weighs in at only 620 grams, while Adam’s glider is over 2000 grams. The next test uses a makeshift wind tunnel to find how much lift each glider could make. Jamie’s glider negates 38% of its weight, but Adam’s glider only negated 16% of its weight, so Jamie seems to have the upper hand in the build-off yet again.

Adam and Jamie, along with Dr. Smith, return to Moffet Field (of Helium Football fame) to conduct their final trials. Dr. Smith looks over the concrete gliders and recommends that both Adam and Jamie should make adjustments to their gliders so that the center of gravity is optimal. He also helps Adam set up a zip line which will help the gliders get up to higher speeds than those that could be achieved by simply throwing the gliders. The higher speeds should help improve the glide ratios of both gliders once released.

Once all final preparations have been completed, the gliders are ready to be launched. Adam goes first, and once his glider is attached to the zip line, it is released. Once free from the zip line, Adam’s glider travels 36 feet and falls nine feet, for a glide ratio of nearly 4:1. Jamie’s glider is next, but instead of using the zip line, he decides to throw it himself. His glider quickly drops straight down, but does manage a 1:1 glide ratio, thus giving Adam a Mythbusters Build-off victory.
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Myth B: Train Suction

The build team tackles the myth of lethal train suction, which claims that if you stand too close to the tracks as a train passes by at high speed, the low air pressure (suction) created is strong enough to pull you onto the tracks.

The team starts by setting up a small-scale wind tunnel to find whether or not a train will create any suction. Grant constructs a wind tunnel, fan housing, and smoke generating system which will help them observe the airflow patterns present around a moving train. To reduce turbulence from the fan, he uses cartons of drinking straws as a honeycomb filter to smooth out the airflow. With all 47,000 straws in place, the wind tunnel is complete.

Meanwhile, Kari and Tory travel to a model train shop to acquire different trains to test in the wind tunnel. While at the shop, Grant informs them that a freight train has a less aerodynamic profile compared to a passenger train, and thus should create stronger pressure differences. Armed with the new information, Kari and Tory buy one train of each type so they can compare them in their wind tunnel tests.

The team starts testing the trains, starting with the passenger train, then moving to the freight train. The source of the suction should be present in the form of a swirling air pocket directly behind the last car in the train. Their tests, along with the high-speed camera footage confirm that each train creates the air pocket. Next stop: suction strength testing.

To measure the amount of force needed to knock over a human, they resurrect a Mythbusters favorite, the chicken cannon. Instead of a plane fuselage as their target, they use Tory holding a newspaper. Grant sets up an anemometer to record the wind speed created when the cannon is fired. This will tell them how much force is needed to knock a person over. The first test, at 30 p.s.i., created a 24 m.p.h. wind gust, but barely moved Tory. The next test, at 50 p.s.i., created a 46 m.p.h. wind gust, which managed to push Tory backwards a few steps, but didn’t knock him over.

For the full scale tests, they create a ballistics gel dummy named Ted, whom they will place near the tracks as the train passes. If he is knocked over and pulled onto the tracks, the myth will be deemed confirmed. If he is left standing, it will be busted.

The team travels to Albuquerque, New Mexico, where they set up Ted, a small stroller, and Grant’s anemometer along the tracks. The stroller will serve as a demonstration of the forces at work when a train passes at high speed. The train speeds up to 79 m.p.h. and flies past the station, tipping Ted over and sending the stroller flying parallel to the train towards the other end of the station. However, the results seem to be caused not by suction, but by the horizontal rush of air. A second test is conducted, this time with the rear end of the train leading, but Ted is left standing, while the stroller is shot off of the platform. Grant’s instruments reveal that on the first test, the winds parallel to the train hit 50 m.p.h., but the second test only reached 26.8 m.p.h. In a third test, Ted is knocked over again, but it seems that any air pocket created behind the train is offset by the wake created by the train passing the platform.

Results:

Concrete Glider: PLAUSIBLE
Train Suction: BUSTED