Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!caen!uflorida!gatech!mcnc!uvaarpa!murdoch!dalton.acc.Virginia.EDU!ds4a From: ds4a@dalton.acc.Virginia.EDU (Dale Southard) Newsgroups: rec.skydiving Subject: Re: bungee jump from plane Message-ID: <1991Feb12.024253.21786@murdoch.acc.Virginia.EDU> Date: 12 Feb 91 02:42:53 GMT References: <38P3w3w161w@grafted.UUCP> Sender: usenet@murdoch.acc.Virginia.EDU Organization: University of Virginia Lines: 93 Ireallyam: ds4a to: Dave Appel Gee Dave, do a little research already... First off -- I am NOT saying that bungee jumping from aircraft is a possible or good idea, but... >First off, connecting the bungee cord to any point other than >the center of gravity is going to cause the plane to be unstable >at best, or tumble out of the sky at worst. Why Dave? I seem to remember hanging off the far tail of a skyvan with seveal others -- that's about as far from the CG as you can get. I assume that only one person is going to bungee about at a time. If you had a little experience in the sport, you would have seen pictures of people hanging beneath aircraft practicing the "slide down the static line and cut the student free" method of "saving" a student who is in tow after a failed static line jump. >SECOND, as each person leaves the plane, the center of gravity >changes. Thus the connecting point is going to move. Yea so? I don't move the connecting point for the static line in-between students, and I haven't lost any yet. >THIRD, it would be impossible to connect the bungee to the >"true" CG, but it would have to be connected to the frame >or fuselage at the nearest point. Thus, when a force is >applied on the connecting point, it will cause the plane >to rotate. Initially in a direction perpendicular to the >line segment between the connecting point and the true CG. Again, SO WHAT??? Bungee jumpers regularly connect the bungee to their ankles. The rate of deceleration that a bungee causes is obviously within that which the human knee can handle (or there are a lot of spare lower leg parts hanging around connected to a lot of used bungees :-) ). So, you are saying that the torque caused by this much force on a lever arm 1 meter long (we will assume that a sutable hardpoint can be found that close to the CG) is going to be more than the pilot can counter. Before you respond, remember that the force is not really perpindicular to the lever arm--if it does displace the plane via rotation, the lever arm is shortened and the torque reduced -- assuming that the attachment is located behind and below the actual CG, I would probably term the torque negligible. >FOURTH, the pendulum effect of a mass hanging at the end >of a long elastic cord is going to cause it to bounce around. >What effect do you think this will have on the pilot's efforts >to keep the plane stable? Well Dave, last I checked, the period of a harmonic oscilator becomes greater when the force constant is decreased (aka the less "stiff" the elastic band is, the slower the mass at the end will bounce up and down). Since the effective force constant of elastic is increased with its length, a body at the end of a long piece of elastic will bounce up and down more slowly than one on a short piece. For the proposed 100 meter elastics, the bounces would be very slow, probably on the order of one bounce every ten sec (but I am really guessing here) >How big a plane did you plan to do this from? I've noticed that >even when a skydiver shifts his position from sitting on his >(or her, excuse me) butt to a kneeling position in the back of >a Twin Beech, that the pilot immediately feels it and has >to make small adjustments. Now imagine the torque applied >by a 200 pound object at the end of a 300 foot elastic cord. I can imagine ... not much. (note that the length of cord has no effect on the torque as it is not rigid) >Maybe, just MAYBE, such a stunt would be possible from a C-130 >hercules, where the weight of one jumper would make only >a very small difference in the CG. But anything the size >of a DC-3 or smaller would be courting disaster. Well, I will be sure to keep that in mind the next time someone suggests square-dancing in the Mr. Douglas. Seriously, I don't mean to flame Mr. Apple for anything other that a failure to understand physics. There are lots of reasons not to bungee from a plane, amoung them is the sever spacial disorientation casued by spinning at the end of a bungee (you can't fly your body as well with your feet tied together, and they can't cut you away from the plane without risking a bungee/chute entanglement). Or how about getting tangled/hung in the bungee when you bounce back up into its slack? Or how about how stupid/trivial it makes skydiving look? But I would be of the opinion that aircraft stability/CG concerns would not preclude bungee jumping from something as small as even a 182. Flame away -- but don't bother unless you have HARD FIGURES, or at least a couple of equations to back you up. --> --> Dale UVa (ds4a@virginia.edu)