Unconventional skydives: balloon jump!

Jun 23 2008 Published by under Physics, Sports

A couple of weeks ago I attended Skyfest 2008, a large skydiving convention ('boogie') which had attendees from all over the country. They also had a number of unconventional aircraft, namely helicopters and hot air balloons. I decided to make a hot air balloon jump, my second, but this time I got video of the jump, which appears after the fold!

This jump was quite successful, not only for the video but for the painlessness of the process: we managed to land right at the drop-zone.  Though the balloon pilots try and account for the wind conditions and take off from a location which will pass near the DZ, just as likely as not a skydiver will have to find an improvised landing zone in some farmer's field, wait for the chase vehicle to find him, and possibly drive around looking for the balloon to help pack it back up!  After my first jump, several years ago, my fellow jumpers and I were stuck in the back of a pick-up truck for about 1 1/2 hours while the driver tried to figure out exactly where the balloon had landed.

So why bother with a balloon jump at all?  Because falling out of a hot air balloon is a completely different experience than jumping out of any other aircraft, and it is the only skydive one can do in which one truly feels weightless.  (I'm not referring to 'base jumping' as 'skydiving'; we'll come back to that in another post.)  I've never heard this explained by anyone before, but I thought I'd give it a try using some rough physics.

Keep in mind that an ordinary airplane is moving at probably 90-100 mph when the jumper exits.  The skydiver experiences wind resistance at every part of the skydive.  This is illustrated below; the red arrows indicate the direction of wind resistance at each point of the jump:

True 'weightlessness' is only experienced in what a general relativity expert would call a free-float frame, in which one's entire body is accelerating freely with gravity.  To put it somewhat grotesquely, you only truly feel weightless when your insides are accelerating at the same rate as the exterior of your body, and are free to 'float around' inside you.  When a skydiver leaves an airplane, he/she immediately experiences a 'breaking' force in the form of wind resistance: the exterior of his/her body is being slowed by the wind while his/her interior is attempting to keep moving according to inertia.  Once the skydiver has reached terminal velocity, he feels his full weight and the sensation is no different than laying on one's belly on top of a very big fan!

The sensation is completely different upon leaving a hot air balloon.  Because the balloon is essentially a stationary object in the air, there is no wind resistance.  The jumper is very nearly in a true free-float, weightless motion, and the experience can be quite shocking.  On my first balloon jump, I shouted, "Holy crap!" as I left; unfortunately, everyone on the balloon heard me and mocked me later!

3 responses so far

  • flamendialis says:

    How long did you feel weightless? The terminal velocity for the human body being quite low, the drag would be significant by the time you reached half the terminal velocity. I think much better times for weightlessness are achieved in aircraft following parabolic trajectories.

  • "I think much better times for weightlessness are achieved in aircraft following parabolic trajectories."

    Oh, indeed; to experience a true extended period of weightlessness, you need to take a ride on the vomit comet - or take a trip away from Earth altogether! These aren't financially viable options for most people, however...

    The weightlessness is, as you more or less note, continually decreased as one's velocity increases and drag increases. You've probably got about 3 seconds of acceleration in effectively 'still air' before you really start to notice drag effects. That's not terribly long, but it's still a really unique experience for a skydiver and one that's not easy to come by.

  • flamendialis says:

    Three seconds is quite a long time, and the aircraft times were not too high either, according to the wikipedia article (about 30 seconds).

    I was quite skeptical of all the Hollywood movies depicting people jumping out of jet aircraft anyway. I remember seeing a program on discovery about a pilot who broke every blood vessel in his face by ejecting while supersonic.

    Another point which came to mind is that flight simulators might actually work (approximately). When a system is accelerated by a, the effective g forces will be $sqrt{a^2+g^2}$, and the simulator can give the actual direction, but not the magnitude by tilting the platform. I am not sure how effective it is really going to be as far as training goes, if your arm weighs 50 kg during a maneuver!