The density of helium is less than 1/7 of the density of nitrogen, which is a good approximation for air.
Two ways that this is helpful come to mind:
1. Aerodynamic effects like drag and vorticity are largely governed by the Reynolds Number, which holding all else constant, varies linearly with density. Changing the density of the fluid can drastically change the aerodynamic situation. See
http://www.grc.nasa.gov/WWW/k-12/airplane/dragsphere.html for an interesting and accessible example.
2. "Flow-induced vibration" is caused by the swirling gases crashing like stormy seas, making platters vibrate and heads flutter. Switching to helium cuts the momentum in these turbulent flows to a seventh of what they would be with air.
Two ways that this is helpful come to mind:
1. Aerodynamic effects like drag and vorticity are largely governed by the Reynolds Number, which holding all else constant, varies linearly with density. Changing the density of the fluid can drastically change the aerodynamic situation. See http://www.grc.nasa.gov/WWW/k-12/airplane/dragsphere.html for an interesting and accessible example.
2. "Flow-induced vibration" is caused by the swirling gases crashing like stormy seas, making platters vibrate and heads flutter. Switching to helium cuts the momentum in these turbulent flows to a seventh of what they would be with air.