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String Length, Placement, Number and Type
String is an easy variable for students to change on a parachute. Some designs will place a single string
in the middle of the canopy. Those who chose to use 2 lines find that
they chutes will fold up rather than inflate, and then move rapidly
afterwards. Three or more is the typical number of strings
needed. More lines means more weight -- the minimum number of suspension
lines needed to help the canopy keep its fully inflated shape, and
strong enough to hold the payload are all that are needed. Also, extra
lines increase the likelihood that the strings will get tangled. They
are needed in real parachutes when larger canopies are used -- the
large drag forces generated can load fewer lines to failure.
Symmetry drives the placement of the lines. The load applied
to the canopy through the lines must be balanced -- rules of symmetry
help in this nicely. The type of string can in a small way determine
drag -- thinner are lighter but produce less drag. With wing-type
parachutes, a flattened cord is preferred to reduce the drag and enhance
the speed at which the parachute travels, which affects the amount
of lift the airfoil produces.
The length of the string should be such they do not have some
taut and some slack lines. Students need to fabricate
this aspect of their parachutes carefully. The strings
cannot be so short as to not allow the parachute to
inflate fully. Longer strings allow for full deployment
of the canopy. Strings that are too long, however, can
simply add to the weight, and are thus not preferred.
(The student who tied his long strings together as you
see at the end of the String movie lost the benefit
of full deployment.)
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