opening canopy

Parachute Inflation

By Dr. Jean Potvin

Parks College Parachute Research Group

Extracted from the McGraw-Hill 1998 Yearbook on Science and Technology

Parachutes continue to be the simplest and cheapest devices used for the deceleration of loads, people and vehicles ever since their first recorded use by Jacques Garnerin who jumped from a balloon over Paris in 1797. Being made of cloth and suspension lines, their construction is far simpler than that of aircrafts. It is rather ironic, however, that their very simplicity makes their aerodynamics so much more complicated. Indeed, unlike aircrafts which are solid structures that deflect air around them, inflating parachutes are devices which not only deflect surrounding air, but also adopt shapes which are dictated by the airflow that they generate. such shapes can very complex and, as they change continuously, the process is intrinsically unsteady. Finally, given the lack of streamlining during inflation, turbulent rather than laminar flow dominates the aerodynamics.

Parachute Types

Since their first conception in the 11th Century, parachutes have come in many shapes and fabric. Modern parachutes are either "circular" or "square", and most are made of nylon fabric. When fully inflated, circular parachutes have the shape of a hemispherical cap. In some cases the cap is slightly conical, and in others slots or concentric gaps are cut to enhance stability (more details below). Square parachutes are shaped like an aircraft wing. They are able to maintain such a shape because the wing is built out of cells which are inflated by ram-air action, that is, by the relative wind created by the parachute's motion which enters through inlets cut on the leading edge of the parachute.

Deployment Stages

The initial deployment stages of square and circular parachutes are similar. Both begin with the extraction from the harness (or vehicle) of the bag containing the folded parachute, usually by a drogue chute. As the bag separates from the harness, the suspension lines unfold first. It is only when the lines are fully stretched that the parachute is allowed to unfurl from the bag and begin inflating. Inflation is typically characterized by several stages which will vary in duration according to the design of the parachute. For sport square parachutes, the deployment of the suspension lines typically lasts for a second or so; inflation, on the other hand, may last anywhere between two to five seconds.

Parachute Inflation

The inflation stages of a circular parachute are easy to visualize. After unfurling out of the bag, the parachute first adopts a rather elongated shape, not dissimilar to that of a vertical tube opened at its lower end. Because of the system's rapid descent, air rushes in through the tube's opening and accumulates at the apex of the canopy to create a high pressure air "bubble". Steady inflow continues to build up internal pressure, thus allowing the bubble's volume to expand horizontally as well as vertically. However, there is more expansion along the vertical than the horizontal. This process goes on until the bubble is large enough to occupy the entire design volume of the parachute.

The shape of the expanding air bubble being trapped inside the canopy is dictated by the balance of aerodynamic forces that act in opposite directions along the boundary defined by the parachute's fabric The bubble's expansion rate first depends on the pressure differential between the outside and inside of the parachute. Like for any blunt objects moving through air, wake turbulence generated on the downwind side of the parachute causes the external pressure to be lower than the internal pressure near the apex. The faster the parachute, the larger the pressure differential, the faster the inflow into the parachute and, consequently, the faster the bubble expansion. On the other hand, rapid expansion generates a large external pressure which squeezes the bubble on its upwind side, thus slowing down the expansion. This pressure arises from the fact that the bubble deflects outside air outwards and increases air resistance in the process. Again, this squeezing effect increases with the parachute's descent speed. The balance between these factors (i.e. rapid inflow, low external pressures near the apex and squeezing force at the lower end) is achieved by the bubble adopting the "optimal" shape and leads to faster expansion along the vertical than the horizontal. Because the parachute/load system decelerates during inflation, this balance of pressure inside and outside the parachute is continuously readjusted as the inflow becomes slower.

The inflation of square parachutes evolves similarly, at least during the first seconds of the inflation process. Here the difference resides in the fact that inflation takes place in the cells of the parachute, which later define the shape of the canopy. Initially, these cells are shaped like horizontal tubes with inlets at one end which scoops up the air coming from below. The walls between the tubes have holes which allow the internal pressure to spread more evenly over the entire parachute. Here again the inflation process is regulated by the pressure differentials between the outside and inside of the cells in a manner similar to the processes described above.

Reefing Systems

If left to their own devices, opening forces in some designs may be so large to cause the destruction of the parachute itself and damage to its load. Typically, if a parachute inflates too quickly, little or no deceleration has occurred: the parachute and its load still travel at very large speeds by the time the parachute has opened, thus generating a very large amount of drag and consequently a large force on the parachute structures. Reefing devices are used to limit for a time the rate of canopy expansion during the early phase of inflation. With some circular parachutes for example, a line routed around the skirt limits the inflow and permits early deceleration, until a pyrotechnic device cuts the latter at a preset time or altitude. On sport square parachutes on the other hand, a slider is used to achieve the same result: here a square of nylon fabric is allowed to slide freely and slowly down the suspension lines, at a rate which is controlled by the slider's own drag and the tension created by the suspension lines which are fanning out of the slider.

To learn more about parachutes:

See also Jim Bates' "Rigger's Notebook", a monthly column on parachutes which appears in the Atlantic Flyer web site. log file