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:
- C. W. Peterson; "High Performance Parachutes"; Scientific American, May 1990.
- D. Poynter; "Parachuting, The Skydiver's Handbook" ; 5th Edition; Para
Publishing (Santa Barbara, CA 1990).
- J. H. Strickland and H. Higuchi; "Parachute Aerodynamics: An Assessment
of Prediction Capability"; Journal of Aircraft, Vol. 33, pp 241-252 (1996).
- J. Potvin; "Parachute Inflation"; to appear in the McGraw-Hill 1998 Yearbook of Science and Technolgy.
See also Jim Bates' "Rigger's Notebook", a monthly column on parachutes which appears in the Atlantic Flyer web site.
