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Skydiver

Main concept

A parachute jump from an aircraft can be modeled using the basic principles of fluid dynamics in gravitational field. A skydiver with mass m, altitude h and vertical velocity v is subject to gravitational force Fg and the drag force Fd.

m ⅆⅆ t v=Fg+Fd

The gravitational force can be evaluated as Fg=mg, where g=9.81 m/s2 and remains relatively unchanged for typical skydiving altitudes. The drag on an object with a drag coefficient Cd which presents orthographic projection A on a plane perpendicular to the motion in a fluid of density ρ can be modeled as

Fd=12ρCdAv2

This model is accurate for large values of Reynolds number Re=ρdvμ103 (where d  is a characteristic length, and μ is the fluid viscosity) which is typically true for both free fall and with parachute.

 

The air density in Earth's atmosphere ρ drops exponentially with the altitude and can be calculated using reference values of href=7500 m and ρ0=1.2 kg/m3 according to

ρ=ρ0ehhref

In our model, the jump consists of three phases (depending on the initial conditions, one or two of them might be missing):

1. Skydiver is in free fall, both his drag coefficient Cd  and his cross-sectional area  A  is that of a cylinder or a flat strip, depending on the body position.

2. Skydiver pulls his ripcord at the opening altitude and his parachute starts to open. The drag coefficient Cd=1.33 (inside of a hemisphere) and the cross-sectional area  A increases linearly over the period of 10 seconds to the full size of 45 m2 (corresponding to a typical military-grade round parachute).

3. The parachute is fully inflated and descents to the ground.

 

Adjust the sliders to set the initial and opening altitudes, as well as the skydiver's mass and free fall position. Observe how the fall rate is affected in all three phases.

By adjusting the position and ground speed of the plane and speed of the wind, you can try to land inside the red target (50 m across, a typical landing area).  Speed of the animation can be controlled on the fly.

Jump altitude  (km)

 

Parachute opening altitude  km

Aircraft ground velocity ms

Wind velocity  ms

Aircraft position (m)

 

Skydiver's mass      kg

Free fall position    

Jump altitude (m)

 

Opening altitude (m)

Aircraft velocity (m/s)

Wind velocity (m/s)

Peak velocity (m/s)

Free fall time (s)

 

Animation Speed:

 

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