Parachute Design Rules-of-Thumb

Students in the Learning By Design curriculum conduct experiments to generate their own design rules-of-thumb. They then share them as "design advice" with others in the class. Engineers and designers use rules-of-thumb to make an unfamiliar designer problems easier to solve. If you are making wheeled vehicle like a garden cart, or a child's racer, a good rule of thumb is to design it with pairs of bearings (not 1 or 3). Every day, designers create their own rules-of-thumb based on experiments they conduct on the fly.

A design rule-of-thumb connects the concrete world of practical testing to "intermediate abstractions" that link key design features to parachute performance. Rules-of-thumb can be posted in the classroom and then reviewed for accuracy and consistency with other rules-of-thumb. They can also get rewritten, just like any design prototype, until they make sense to most readers. In science classes, design rules-of-thumb justify students doing experiments, since a good rule-of-thumb can help make effective and test-based design decision-making easier for all students. In math classes, rules-of-thumb can be a context for doing estimations of experiments and reinforce qualitative reasoning about variables.

An informed designer of parachutes knows that design rules-of-thumb can sometimes conflict with one another. In describing how parachutes fall, a good rule-of-thumb should include both air resistance and gravity in the same breath.

Students will eventually have to reconcile one rule-of-thumb that says,

• More canopy results in more drag and longer drop times. A second might say,
  
• Heavy things fall faster than light things. A more inclusive rule-of-thumb would somehow indicate,
  
• A heavy thing with the same drag force on it as a lighter thing will fall faster.

Many students' design rules-of-thumb relate a parachute feature like surface area where the performance outcome is time of descent or average speed of descent. A team's design rule-of-thumb might read, "Using more coffee filters leads to slower rates of descent." Another might read "Canopy lines that are twice as long as the parachute is wide produce the slowest drop times." Getting students to relate design variables with Key Criteria as outcomes is an important aspect of informed designing of model parachutes.