Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!decwrl!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: <1991Feb18.003645.28569@murdoch.acc.Virginia.EDU> Date: 18 Feb 91 00:36:45 GMT References: <2587.27BEAA64@ehsnet.fidonet.org> Sender: usenet@murdoch.acc.Virginia.EDU Organization: University of Virginia Lines: 83 Ireallyam: ds4a Reply to SKYDIVE@f15.n233.z1.fidonet.org (SKYDIVE) > You probably have never flown an airplane. Vector >forces and CG calculations are a basic part of ground school. Yes, but I seriously doubt they knew where those vector/CG equations came from (I know the guy I had for ground school was clueless). They are teaching applications, not derivations, which is what is needed to solve problems other than those few that fit in to the conditions for which the working equation was generated (i.e. you can learn physics without learning calculus, but don't count on being able to use it for anything). But no, I ain't no pilot. > For a 7,000 pound airplane (a Twin Beech fully loaded), that >puts the CG 5 to 6 feet BELOW and OUTSIDE the frigging airplane! >And, not fully loaded, or with a smaller plane, it makes it even >FARTHER below. Pilots usually calculate moment in pound-INCHES, >from the datum line, and here we have 60,000 pound-FEET! >so where is that going to put the CG for your C-182? >About 12 feet below it! Case in point. The plane and the jumper are connected by a bungee/static line -- not a rigid steel rod. They are NOT part of the same inertial body and calculation of moment is meaningless. The above calculation would suggest that the jumper would move in relation to the planes pitch. Sorry, won't happen. The system you describe above cannot move the aircraft's CG outside of the aircraft. > Again, not the same thing. Here you have 200 pounds hanging only >6 to 8 feet below the craft. And yes, I have TALKED to pilots >who have had student-in-tow. And the student swings wildly from >side to side of the plane, causing the pilot a HELL of a time to >maintain control. Have you actually TALKED to >a pilot who had a student-in-tow? Its damn hard to fly a >Cessna, even the larger 206, in such a condition. The jumpmaster >is so worried about the student, he is not paying attention >to the bricks that the pilot is excreting from his rectum. >(See, I am keeping my sense of humor here.:) Ah...had not thought much about side to side motion...might be important.... I would imagine the bricks would be pretty big (one of my buddies just had a 182 shoot out a piston -- jump doors are nice -- they let you open the door so the smoke clears out enough to see were you are going). > You are still not looking at it from the pilot's viewpoint. >Regardless of the PERIOD of oscilations, the pilot is going >to have to make severe adjustments in an unpredictable >direction, as a result of the "bouncing". The fact that the >plane "absorbs" it over a longer "period" is of little >comfort to the pilot. The only way to overcome this is >the relative MASS of the plane to the jumper, as Rob Prior >pointed out. Well, the PERIOD effects the severity in haw often the pilot has to reverse the direction of correction. Longer cord --> slower (less frequent) oscilation. But I can see your point. > Its obvious to me that little thought has been given >to the wonderful pilots who transport us skydivers >to altitude and dump us out. As a student pilot, I have I think about them all the time -- didn't we spend a couple K of bandwidth the last week or so making sure that they didn't get asked to do something stupid and unsafe?? Please don't insult me like this (or I won't play anymore :-( ). Closing remark: The problem with modeling a situation like this is that it has too many contributing factors. Dave made the mistake when he tried to make it simple weight and balance. I err'd trying to make it simple kinetic energy & projectile motion (but I do have a nifty set of mathematica definitions that would only require about 30 hours more work to better resemble reality -- there is a reason why they have cray y-mp programed to model the effect of impact on a dropped beer (pop?) can -- I kid you not). The basic problem is that we are trying to determine what the PILOT can handle. Obviously, that is impossible! NEXT TOPIC PLEASE. --> --> Dale UVa (ds4a@virginia.edu)