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
Students will eventually have to reconcile one rule-of-thumb
- 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.