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Getting Started Select Design Task Learn About Design
What To Observe : Surface Area & Shape : Canopy Layers : Weight : String : Vents : Frame               
MOVIE 1:   58 seconds
 

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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