Aerodynamic Decelerators
Summary
- Overview
- How it Works
- More Physics
- Selected References
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The parachute is an aerodynamic system belonging to the class of aerodynamic decelerators. It is in fact used also for deceleration of aircraft and spacecraft on short runways, and delivery of payload into inaccessible areas. Some parachutes are known to be so precise that they can land on ... a dime. The photo at right was taken at a sport event in the Arizona desert (1997). The airmen were in fact able to land on the target with amazing precision. |
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HOW IT WORKS
The main idea behind the parachute is to create a bluff body of large aerodynamic drag. Drag coefficients higher than 4 can be produced.
Basically, all models require inflation of the canopy shortly after the parachute canopy is released from the deployment bag. Air starts flowing into the canopy, which has the effect of increasing the pressure over the air flowing around.
The canopy expands outward due to the radial component of the pressure. This makes
more air flowing in, which in turns causes a faster expansion. The process continues
until all the forces (radial, axial and inertia) are balanced. At this point the
parachute assumes a constant speed, said of terminal descent.
During the inflation process there are very strong dynamic forces, among which the
aerodynamic drag plays an important role. The amount of these forces and the size
and shape of the payload determines the speed of terminal descent. The payload is
ideally an axial-symmetric slender body.
Large non symmetric bodies create a wake flow that interferes with the air captured
by the canopy, either slowing the inflation process or creating an unsymmetric
situation wherein the operation is compromised.
The deformation of the canopy creates a strongly non linear interaction between the
aerodynamics and the structure. The aerodynamics is that of a bluff body, which is
one of the most difficult problems in terms of quantitative prediction.
The sinking speed (or rate of descent) U depends on the drag
coefficient, on the frontal area of the canopy and the total weight of the parachute. In a vertical descent it is simply a balance between the aerodynamic force (upwards) and the
wight (downwards).
MORE PHYSICS
Selected References
There is a number of publications dealing with paragliding and other aviation
sports. For specific references, please inquire.