Gravity Defying System Design Student Materials

STUDENT

WHY DOES THIS PROBLEM MATTER ?

Figure 1: Glide test of the shuttle Enterprise as it lifts away from a 747.

Gravity is an inescapable part of life, sometimes essential to achieving our goals, sometimes a serious obstacle, sometimes helpful, and sometimes just a nuisance. Engineers are continually seeking ways to take advantage of gravity (the space shuttle uses gravity to return to earth and glides to landing sites, figure 1) and to avoid the problems which it causes. For example, how handy and efficient it would be to fly from one place to another if we could simply cancel gravity (which no one has figured out how to do). This project gives you a chance to think about and create solutions to a problem related to those of practical air transportation, the problem of minimizing the unwanted effects of gravity.

WHAT ARE WE SUPPOSED TO DO?

Your basic task is to design, fabricate, and test a simple system that defies gravity in two particular ways, using only the materials indicated below. Specifically, you are asked to design and fabricate a Gravity Defying System (GDS) which you can evaluate as follows:

1. be launched by hand (by a member of your team) from a location specified by your instructor,

2. travel the greatest horizontal distance from the launch point,

3. take the greatest time in flight,

4. be undamaged by the experience.

WHAT EQUIPMENT AND MATERIALS CAN WE USE?

Your team's GDS may contain no materials other than the following:

• cellophane (Scotch-type) tape (3 ft. limit),

• plastic (Saran-type) film (5 sq. ft. limit),

• string (10 ft. limit),

• aluminum foil (4 sq. ft. limit),

• spaghetti (20 piece limit),

• cardboard (2 sq. ft. limit),

• metal paper clips (10 piece limit),

• plastic drinking straws (5 each limit),

• chewing gum (new or used, 1 piece limit).

Your team can use the following tools: a ruler, scissors, and wire cutters.

HOW WILL WE KNOW HOW WELL WE HAVE SUCCEEDED?

The Basic Performance Requirement for your GDS is that it be launched, travel, and land without damage. A GDS meeting this minimum requirement will be judged satisfactory.

An Extra Performance Index (EPI) will be calculated for your GDS using the formula:

In order for your team to get a high EPI, your system must stay aloft as long as possible and travel as far as possible. These goals may require you to trade some distance for flight time; alternatively, you might need to trade flight time for distance. Regardless, your team needs to take this into account during the design phase.

WHAT ELSE MIGHT BE USEFUL TO KNOW?

If you look around you will see many examples of real-world systems, both natural and man-made, which do indeed fly through the air. Ordinarily, these systems rely on some combination of initial velocity, additional thrust from a source of power, aerodynamic lift from surfaces, light weight, and maximization of the ratio of aerodynamic lift to drag forces. Figure 2 shows the four forces of thrust, weight, drag, and lift and how they act on an airplane. Objects that move through the air must take into account each of these forces. According to Newton's second law, if we want an object in air to move forward and up, then the lifting force must be larger than the weight, and the thrust force must be larger than the drag.

Figure 2: Four forces of flight

Thrust is usually provided by some type of engine or launching assemblage. The launching apparatus is not necessarily part of the flying system. Gravity force is simply the weight of the flying system. Gravity forces are proportional to the amount of material needed to construct an object. The more materials and heavier those materials, the more a flying system will weigh. Drag and lift force result from the flow of air over the object.

Figure 3: Lifting force induced by air flow over arched wing section.

Aerodynamic lift is the force that enables aircraft to stay in the air. It is possible due to several principles of physics. When air moves over an arched surface, the air pressure above the surface drops. Wings on aircraft generally have an arched cross-section shape which causes air to speed up as it flows over the upper surface. As the air speeds up, the air pressure drops above the wing. The difference in air pressure between the upper and lower surface produces a force in the direction of low pressure. This is the aerodynamic lifting force as shown in figure 3. Aerodynamic lifting forces are generally proportional to the area of the lifting surface (large "wing" area will provide more lift than a smaller wing area).

Drag force is the resistance of the movement of an object through air. There are three primary types of drag that occur when an object travels at low subsonic speeds. These are induced drag, skin friction drag, and pressure drag. Induced drag is a by-product created by the production of lift and is greatest at the lowest speeds. Pressure drag is due to the disruption of the smooth flow of air over the aerodynamic surface. It is minimized by "streamlining" the object. Skin friction drag results from friction between the air and the surface over which it flows. At low speeds, the effects of skin friction drag are small compared to the effects of pressure drag.

You probably can find a number of different and promising concepts for your GDS by exploring the application of various combinations of the above factors. Don't be afraid to test your team's different concepts.

In summary, your design should consider the following:

• How will your design be launched?

• How will your object's weight affect its performance?

• How will your design harness lifting forces?

• Will your design survive landing?

• How will drag forces affect your design?

• Is the launch area windy and how will wind affect your design's flight performance?

Each member of your team should have a clear idea how your design answers these questions. Depending on your team's design, there may be additional questions that are important, but this gives you and your team a good start.

SAFETY CONSIDERATIONS UNIQUE TO THIS PROJECT

Be particularly careful that you don't fall off of your launch platform. In addition, watch where your GDS might end up after launch and don't hit anyone or anything (eyes are particularly vulnerable). Finally, be careful with tools, and be sure you are adequately checked out before using power tools.

Have fun and think ahead, so you don't end up like Spiff the Spaceman!