Parks College Parachute Research Group
Modeling the Interaction of Sub-scale Parachutes With Atmospheric Turbulent Eddies
Saint Louis University
St. Louis, MO 63103, USA
Planning Systems Inc.
Reston, VA 20191, USA
Presented at the 18th AIAA Aerodynamic Decelerator Systems Conference and Seminar,
Munich, Germany, May 23-26, 2005, Paper AIAA 2005-1675
By virtue of being light and featuring small canopy diameters
(i.e. < 10 lbs & < 3ft), sub-scale parachute systems such as
hand-thrown wind-sensing sondes are most susceptible to the
turbulent "gusts" that populate the atmosphere, even when the
latter is characterized by low or moderate turbulence levels.
Depending on parachute and payload design, atmospheric turbulence
will generate descending flight trajectories that include small
and large random transverse (i.e. tilting) motions of the parachute
and/or of the payload. Gusts may even trigger undesirable flight
modes such as parachute coning. As a result, these unsteady flight
characteristics may reduce stability, which for small atmospheric-sampling
probes may translate into data-collection performance degradation.
This paper discusses a simple calculation of the time scales associated
with the sometimes random, sometimes cyclical transverse displacements
that are caused by atmospheric turbulence. The calculation will be based
a simple picture of turbulence, namely as islands of turbulent eddies,
gusts or vortices distributed randomly in a vast expense of air moving
in a "laminar" fashion. The interaction between the system and atmospheric
turbulence will be assumed to arise whenever the parachute-payload is
entering an atmospheric eddy and is exchanging energy with portions of
the eddy. By using Kolmogorov's turbulent energy density equation, this
theoretical framework will be used to assess to atmospheric-gustiness
sensitivity of several parachute systems, including a sub-scale system
and a full-scale, military cargo airdrop system.