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[Phys-l] Free fall from helicopter




One of my students had a great inquiry about a problem in our text that treated a skydiver exiting from a helicopter and free falling for 3 seconds. In the text it was solved as a problem where you could neglect the effects of air resistance. He had worked around helicopters and thought that surely the down wash of the helicopter blades would put a significant force on the skydiver.

After some internet checking and watching a couple of u-tube videos of skydivers exiting from helicopters, it appeared that the effects of the down wash were not significant for helicopters at altitude (with no ground effect).

Happily a student I had last year is retired army and was a member of the army's Golden Knights parachute team. I posed the question to him and got the response that I have copied below. I thought you all would enjoy it. It gives insight in to this issue and a couple of others that could be of interest in an introductory physics class. Always nice to get some real-world feedback.

Here are his comments:

"1. There is no noticeable affect of the "rotor wash" on a parachutist exiting from a helicopter. Believe it or not, while hovering in a helicopter the rotor wash is very light close to the aircraft, but increases towards the tips of the blades.

2. Yes, if a person free falls from a hovering helicopter the acceleration from exit to terminal velocity is roughly the same as if jumping from a free standing object.

3. Yes, it takes about 3 to 5 seconds in free fall before you reach terminal velocity and begin to feel the resistance. That resistance is what allows a free fall jumper to have control of their bodies in the air just like the flaps and rudder on an aircraft. It's funny, but during those few seconds before terminal velocity you actually do feel like you are falling because you experience the feeling of "floundering", or lack of control, and can hear the wind noise increase.

4. All in all, jumpers do not take into consideration the effects of the down wash when jumping from helicopters. It is a rare treat to exit a helicopter from a hover at altitude though. Reason being that most pilots do not like to hover out of "ground effect" at altitude and because depending upon the altitude it requires a "high performance" helicopter.

5. Finally, the only thing jumpers typically concern themselves with upon exit is the speed of the aircraft. Experienced jumpers doing "relative work" jumps with multiple persons want a higher exit speed so that right out the door everyone is flying at terminal velocity already. It is very difficult and potentially hazardous for groups of jumpers to exit in close proximity to one another at low airspeeds.

I hope this answers all the questions.

Jason "

Cheers,

Rick Swanson




Richard E. Swanson, Ph.D.
Physics Professor
Sandhills Community College, Pinehurst, NC 28374
swansonr@sandhills.edu (910) 246-4951


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