Rotor Egg Drop
Rotor Egg Drop is a Division B event from the 2012-2013 in which teams must build and test an auto-rotation device to safely land an egg on the ground from a variable height. Previously called Helicopter Egg Drop, the event's name was changed to avoid confusion with Helicopters. Helicopter Egg Drop was run as a Division B trial event for 2012 in New York and at the National Tournament in 2013 at Wright State University and 2014 at the University of Central Florida.
In this event, teams must construct a helicopter rotor device prior to competition. The event proctor will provide each team with a raw egg, a sandwich bag, tape, and a cup. The team must seal the egg in a sandwich bag and the put it inside a cup. The cup must mount or suspend from the bottom of the device. It is permitted use masking tape to keep the egg in the cup, but not for cushioning. The entire device including the cup must fit in a 51 cm x 51 cm x 51 cm cube. The rotor can fit in this cube in any orientation, but only if the device is dropped in the same orientation. Teams may request a new egg if they break theirs before the drop, but they will be penalized two seconds. Teams have one drop during the competition, and that one drop determines the team's score for the entire event, so it is encouraged to be extremely careful! The device must be impounded before the competition. The proctor will weigh and measure the device and make sure it passes all the event parameters.
The score is determined in several tiers. If the egg does not crack at all and met all the construction parameters, the team is placed in Tier 1, which means that the team is automatically scored above anyone whose egg cracked. If the egg didn't survive, but the device met the construction parameters, the team is placed in Tier 2. If the egg survived but the device did not meet the construction parameters, the team is ranked in Tier 3. Lastly, if the egg cracked and the device did not meet the construction parameters, the team is ranked in Tier 4. Within each tier, teams are scored based on how long their device was in the air before making contact with the ground. The longer the flight time, the higher the device will be ranked. Tie-breakers are determined by the weight of the device without the cup and the egg. In the 2013 season, the lighter device would win tiebreakers. However, in the 2014 season, the heavier device would win the tiebreaker. Because it is very unlikely to get the exact same time as another team, most competitors generally focus on maximizing time and stability without an emphasis on the weight tiebreaker.
Stability is a very important component of a rotor egg drop device. Even if the device scores a good flight time, if the rotor egg drop wobbles a lot during the flight and touches the floor before the cup, the team will get Tier 3 or 4. This has happened to many competitive teams, so it is imperative that one focuses on making sure the device is stable, especially at high drop heights. Historically, nationals has a drop height of 10-20 meters. The high drop height means that there is more time for the rotor egg drop device to get too unstable and ultimately tiered. Some competitors try to increase the stability of the device by placing weights at the bottom of their device, such as below the cup that they use to hold the egg. Another way to increase stability is to make sure the device is balanced. This is especially important for lighter devices. This can be done by weighing individual sticks of wood (or whatever building material you use and adding small amounts of clay to the device.
Building the Helicopter Rotor Device
In the competition, the goal is to have the longest air time while still rotating. A design that was seen often at competition, was to make a square shape and cut a silt on the edge so that specific part tilted making it a rotor and it spins. This design uses more area so it can drop slower, but if teams are not careful the membrane will act like a parachute.
When building the rotor device, teams must keep in mind aerodynamic principles, since energy-producing mechanisms are not allowed at all. This includes rubber bands. This means that a design is going to be based solely on classical physics (such as Newton's Laws and Bernoulli's principle) and how much lift and air resistance the device can make. The rotation of the rotors as it falls through the air should cause a difference in air pressure above and under the blades, if the use of "auto-rotation" of the rotor is used. These principles are the ones that are used for the construction of windmills, airplanes, and propellers. Remember that a rotor at a flat angle will create no spin. The rotors have to be at an angle to produce maximum lift and a spinning force.
As parachute-like devices are specifically prohibited in the rules, teams will be disqualified if their device includes anything like a parachute. While this may close some methods of longest suspension time, this also forces teams to create rotors. Many designs include three or four rotors with a membrane that covers these rotors. Other possible designs include the "flower petal" design or the "corkscrew" design. The pitch should be fairly low to gain maximum time and air resistance. Don't include blades that are very close together because it increases turbulence between the blades, making the device less efficient and more unstable (increasing the chance of getting tiered). Most importantly, due to the danger of disqualification, if in doubt, don't use a design that can be mistaken for being a parachute. This MAY include loose rotor coverings. (Depending on judge)
Many competitive designs are very lightweight (about 4-5 grams) and are built using very thin, low-density balsa wood that is bent into curves.
For this event, some basic tools are needed. The first, and most important is the X-Acto or craft knife. This is used to cut the wood and is the heart of rotor egg drop building. Some other tools are needed as well, such as sandpaper, scissors, pins, a foam board, a ruler, tape, and other things found around the house.
This event is one in which teams must find the balance between super-light, and strong. A device should be as light as possible so that it will descend more slowly through the air; however, it also must be strong enough to hold the egg and keep the device stable in the air. Keep in mind that the variation in egg weight renders small weight changes in the device futile. If the device is too light it may sway like a piece of paper as it is falling through the air, causing a blade or another part of the device to hit the ground first, rather than the cup that contains the egg. A very common material for the frame of the device is balsa wood. Other commonly used materials are basswood and carbon fiber rods. Most devices use a frame made out of one of the materials listed above and a covering that coats it. This maximizes efficiency and produces the least amount of weight possible. The membrane is usually condenser paper, Japanese tissue, or Mylar, but shopping bags can be used as a cheaper alternative.
There are many types of glue out there, but the best for this project are superglue, CA glue, and liquid cement (Ambriod). Superglue and CA glue are extremely similar and dry very quickly. Be very careful, as they set in about 5 seconds and it is easy to get your fingers glued to the device. Liquid cement is a bit easier to work with, but it requires more time to dry and different methods of applying it to the pieces. Be careful while using glue, and do not use excessive amounts of it. The glue adds up and can easily add an extra few grams to a device. A good way to secure a bond after it is done drying is to put a drop of thin CA glue on the joint. It soaks into the wood to create a better bond. And whatever you do, DO NOT use gorilla glue. It is extremely heavy and expands several times its size while drying and can warp delicate pieces. The best way to glue the membrane to the frame of the rotor is to use spray glue. It applies an even covering of glue to the frame, creating it a lightweight and strong bond.
Testing The Device Before the Competition
Testing is vital to the competition since it is crucial should know whether or not the device works. This way teams can modify and improve the design and know which designs are more efficient.
Grade A large eggs are defined as at least 56 grams or over. To test, teams could drop eggs to see if the device works, but it could get very messy if the device isn't successful. Another idea is to create a weight that won't explode so that the way the device twirls and floats through the air can be observed. Anything will do--even a bunched cloth tied with rubberbands--if it is the same weight as an average egg. It is a good idea to make a fake egg out of some household materials before dropping an actual egg.
However, teams also have to test on a real egg in order to test if the egg will break or not. Make sure that the plastic bag is sealed tight. One major mistake that many people do is leave some air trapped inside the bag. This is against the competition parameters. Be sure that there is as little air as possible inside the bag before dropping it.
Some good testing locations include malls, stairwells, balconies (make sure there is no grass underneath, as it cushions the egg), and basically any high place. Make sure there is room to the sides, as some models tend to go off to the sides and hit the walls and other objects.
The event supervisor will provide teams with scissors, a sandwich bag, a cup, masking tape, and an egg. The egg must go in the sandwich bag, which must go in the cup. The cup should be attached to the device in such a way that the cup will be the first thing to contact the floor. Remember that there should be no air or any other materials such as a wadded up sandwich bag or any masking tape in the bag, as it may cushion the egg and result in the device being tiered.
Remember that in competition, you only have one try, so make it count!
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