## Hovercraft – science olympiad student center wiki gas buddy

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Hovercraft is a dual lab, consisting of a test and a build portion. The test portion should take at least 20 minutes and competitors are allowed only 8 minutes to run their device. A 3-ring binder of any size is allowed, provided that all materials are hole punched and secure. Calculators of any type are also allowed and need not be impounded.

Hovercrafts have a few essential parts – the base of the hovercraft itself, the fans, the batteries, the wiring/switch, and the skirt. The best teams will experiment with all these parts to find a successful design. The speed control methods will vary from device to device, ranging from fine-tuned potentiometers to simply covering up the thrust fan with tape to reduce airflow. Unlike some events, there is no single "best" method for all teams; instead, the best method is the one which has been tested most.

The base is likely the simplest thing to construct on the hovercraft. However, it still merits consideration in designing the most efficient device possible. A good base will make it easy to test and swap out components without destroying any structural support, among other considerations. It should also be strong enough to support all components without pushing the weight above 2000 g.

There are many choices of materials when constructing a base. Common selections include foam core and basswood sheets, though others are certainly possible. Materials should be light enough to allow weight adjustment to change while maximizing the mass score.

Fans are some of the most important components to a functional, efficient hovercraft. Fans have entirely different requirements for lift and thrust, and should be selected accordingly. It can be very helpful to enable swapping of fans, which facilitates testing.

Centrifugal blower fans are considered the most efficient type of lift fan, both in real-world designs and Science Olympiad devices. To fully support a 2 kg hovercraft, a fairly high pressure is required, making blower fans more efficient than traditional axial fans.

Thrust fans should instead prioritize CFM to provide maximum acceleration. Although it is possible to successfully use a powerful computer fan, hobby motors have the potential to be more effective and more adjustable. Devices need to be able to work on all surfaces, not just those they have been tested on.

Although voltage is an important specification for any batteries used in hovercraft, so is capacity. Many teams use 3000 mAh batteries, which are more than enough for the amount of testing done in the Hovercraft event. 5000 mAh batteries are also available, and allow for longer testing periods but are more expensive. Some companies offer 1600 mAh batteries, but higher capacity is preferable.

Caution is needed when using either LiPo or NiMH batteries, which can be extremely dangerous if used improperly. NiMH batteries are typically less likely to catch fire than LiPOs, which can go up in flames should they be improperly charged. However, should a NiMH battery be damaged, they explode rather than catch fire. However, this is only the case if the battery is very compromised.

Wiring should be fairly cut-and-dry. Wires should be selected based on the power specifications of fans and batteries. There are many options available for switches, and very simple designs will work. Many household switches have two sides, perfect for the two circuits needed to power two fans. JST connectors or similar can be used to enable easy swapping of components, especially batteries and fans.

The skirt is likely the most difficult part of a hovercraft to perfect. Beyond hardware like fans and batteries, the skirt is the largest determining factor of a device’s success. Skirts need to have as low of friction as possible to have accurate results and to enable low speed runs. Many hovercraft failed at the MI and PA state competitions when the track was created out of plywood, as many designs were unable to compensate for such high friction.

The bag skirt is exactly what it sounds like – a completely contained cushion of air, possibly with small holes poked in it to decrease friction. This is one of the most common designs in Science Olympiad, seeing as it is easy to construct and fairly stable. However, its shortcoming lies in friction, as bag skirts are generally one of the most high friction skirt designs.

The wall skirt is the other variation likely to be found in Science Olympiad competitions. The skirt itself is only a wall around the pocket of air, but all surfaces must be flat for the purpose of Science Olympiad, so a wall skirt can still contain a pocket of air. Unfortunately, the wall skirt is more prone to air leakage if constructed improperly and is difficult to construct for most competitors. Unorthodox techniques are often needed to construct an effective wall skirt. This is the most common design implemented among top teams, and variations of this skirt can be found on devices from teams such as LASA, Troy, Boca Raton, and others.

Other innovations can be added to these types of skirts. One design used in many real-world hovercraft is the momentum curtain, which directs airflow to the edges of the hovercraft before bending down to propel the hovercraft up. This has the benefit of placing the highest pressure region near to the ground as opposed to in the center of a cushion of air. Also, many Science Olympiad hovercraft use a flat piece of wood or foam core at the bottom of a bag skirt to create a flat surface and decrease friction.

The hovercraft itself can weigh no more than 2000 grams. Since the mass score counts for half of the entire device score, it is optimal that the hovercraft weigh as close to the 2000 g limit as possible, to achieve as high a score as is possible. However, teams should not abandon functionality for the sake of a perfect score – if a hovercraft is too heavy, it may not complete a successful run, meaning an overall build score of 0. Also, heavier hovercraft will often experience more friction and be harder to control, so there is a balance between run score and mass score.

Part 1 of the event consists of a written test which draws from the AP Physics 1, or algebra-based mechanics, curriculum. This test must contain at least five questions from each of the following topics: Newton’s laws of motion, kinematics, kinetic energy, air cushioned vehicles and applications, and fluid mechanics for Div. C only. 20 – 30 minutes is suggested to complete this test. All answers must be provided in metric units along with the appropriate significant figures.

Viscosity is the internal friction within a fluid, or the resistance to flow. A viscous liquid tends to cling to the solid surface of its container – this is why there is always a thin boundary layer near the outside of a pipe where fluid velocity is almost 0.

The pressure difference required to maintain a given flow rate is proportional to $\frac {L}{R^4}$, where L is the length of the pipe and R is the radius. Even a small reduction in radius can drastically increase the required pressure difference.

Previous equations assume a laminar flow with zero viscosity. However, this is actually impossible as a small amount of viscosity is required to ensure laminar flow. Above a certain critical speed, fluid flow will change from even, laminar patterns to irregular and chaotic turbulence.

While these sections may no longer be tested under the 2018 rules, a grasp on the history of the Hovercraft may yield useful insights into the device building process. Many important ideas for hovercraft construction are historically documented, and are most certainly still applicable.