# Differences

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 parachute_testing [2015/07/22 17:38]ana parachute_testing [2016/05/22 14:10] (current)sam 2016/05/22 14:10 sam 2015/07/22 17:38 ana 2015/05/11 15:58 ana 2015/05/06 22:24 ana 2015/05/06 22:23 ana 2015/05/06 22:23 ana 2015/05/06 22:22 ana 2015/05/06 22:22 ana 2015/05/06 22:20 ana 2015/04/29 22:23 ana 2015/04/29 22:23 ana 2015/04/29 22:17 ana 2015/04/29 22:15 ana 2015/04/29 22:15 ana 2015/04/29 22:14 ana 2015/04/29 22:13 ana 2015/04/29 21:36 ana 2015/04/29 21:28 ana 2015/04/29 21:26 ana created 2016/05/22 14:10 sam 2015/07/22 17:38 ana 2015/05/11 15:58 ana 2015/05/06 22:24 ana 2015/05/06 22:23 ana 2015/05/06 22:23 ana 2015/05/06 22:22 ana 2015/05/06 22:22 ana 2015/05/06 22:20 ana 2015/04/29 22:23 ana 2015/04/29 22:23 ana 2015/04/29 22:17 ana 2015/04/29 22:15 ana 2015/04/29 22:15 ana 2015/04/29 22:14 ana 2015/04/29 22:13 ana 2015/04/29 21:36 ana 2015/04/29 21:28 ana 2015/04/29 21:26 ana created Line 1: Line 1: - ====== Parachute Testing ====== + this page has moved to [[orb:parachute_testing|Parachute ​Testing]] - + - ====== Oscillation of a Parachute ====== + - + - {{ :parachute.png ​|}} + - Pvs stands for parachute vortex shedding. The pvs is the cause of resonance of the system. At high altitudes,​ + - the free-stream density is sufficiently low to result in a relatively small ratio of apparent mass to payload mass, + - causing the system center of gravity to be located very close to the payload. This result in a long momentum arm + - between the canopy and the system center of gravity, such that the pvs can dominate the dynamics of the complete + - system. + - + - $f_{pvs}= \frac{S_t . V_{term}}{d_{ratio} . d_o}$ + - + - + - $S_t$ stands for Strouhal number and $d_o$ is the nominal diameter of the parachute. + - + - + - ====== Constraints ====== + - * In our modelisation we won't consider the PVS oscillation due to the mass of the rover 2Kg and the altitude. + - * The parachute that we'll implement will reduce the terminal velocity in 90%. + - * For an altitude smaller than 1,6 Km we can apply the general dragging formula $2.W=c.\rho.V_{term}^2.A$ where W stands for the weight, $\rho$ is the air density, c the dragging coefficient,​ $V_{term}$ the terminal velocity and A the area of the parachute. + - * The recomending $V_{term}$ or landing velocity is between $3.5{m}{s}$ to $4.5{m}{s}$. + - * The dragging coefficient for a half-sphere $c$ is 0.47. + - * The density of the air increase with low temperatures;​ at -10°C ​ $\rho=1.34 {Kg}{m^3}$ + - * The area of the parachute can be obtained: $A=\frac{2.g.M_{rover}}{c.\rho.V_{term}^2}$ + - * For a round parachute the diameter is obtained: $d=\sqrt{\frac{4.A}{\pi}}$ + - * The spill hole to gain stability should have a diameter of 20% of the total diameter of the canopy. + - * If the area and/or the diameter of the canopy are fixed, we can change the diameter of the spill hole in function of the terminal velocity, reducing the total area in the formula above. + - * Dynamics of free fall with air resistance, says that a body will arrive to the terminal velocity after 5 seconds, with out depending on the altitude. + - + - + - + - + - **References:​** + - *  F. M. WHITE and D. F. WOLF.  "A theory of three-dimensional parachute dynamic stability."​ Journal of Aircraft, ​ Vol. 5, No. 1 (1968), pp. 86-92. doi: 10.2514/​3.43912 + - * A Method to Characterize ​Parachute ​System Instability Modes by Michael Bernatovich Dr. Ian Clark 5/6/2013 +