kallend 2,027 #26 December 31, 2005 QuoteQuoteQuoteMore applicable if the dart and birdie weigh the same. True, but I think the result would be the same, if only less obvious. If the birdie were much heavier than the dart, it could go farther. But no need to split hairs further... So are you saying that exit order should depend on the weight of the skydivers? What matters is ballistic coefficient. Headdowners have a higher BC than bellyfliers and darts have higher BC than birdies, and my original example illustrated that quite well, no need for nitpicking.... The only sure way to survive a canopy collision is not to have one. Quote Share this post Link to post Share on other sites
sundevil777 102 #27 January 1, 2006 QuoteSo are you saying that exit order should depend on the weight of the skydivers? Of course not. Your original analogy had a flaw. As if you've never corrected anyone before. edit - I think Ming was better for you. People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am Quote Share this post Link to post Share on other sites
kallend 2,027 #28 January 1, 2006 QuoteQuoteSo are you saying that exit order should depend on the weight of the skydivers? Of course not. Your original analogy had a flaw. No, it didn't. Mass by itself predicts nothing. Ballistic coefficient is the relevant parameter. A 120 pound headdowner with TV of 160mph has a higher BC than a 240 pound belly flier with a TV of 120mph, and regardless of mass a dart has a higher BC than a badminton birdie. Your comment was incorrect and misleading. It's all just applied physics.... The only sure way to survive a canopy collision is not to have one. Quote Share this post Link to post Share on other sites
apoil 0 #29 January 1, 2006 QuoteLet's assume aircraft velocity on jump run is constant, so that at the moment of exit, all jumpers have the same velocity (speed and direction). Let's also assume level flight, and unchanging body positions. While these may serve to explain the theory, they are not exactly safe or realistic assumptions. Sure, maybe excellent skydivers present a perfect unchanging body position to the relative wind on exit, but in practice how often do you make corrections on exit? In practice how many RW launches funnel? A funneling RW launch and a squirrelly head down flower are probably identical in terms of surface area presented to the relative wind. These phenomena are COMMON and UNPREDICTABLE. Quote Share this post Link to post Share on other sites
apoil 0 #30 January 1, 2006 QuoteQuoteTry throwing a dart and a badminton birdie horizontally with the same initial speed. Which goes farther? That is a damn good answer While it may be a correct answer, I'm not convinced yet how good it is. The problem is that this concept is often applied intuitively by skydivers without a physics education leading to a lot of the "wrong science" we hear around the campfire. I've heard the theory that a horizontal versus vertical mode of flight will affect drift even when the relative wind is from below. I've heard a theory that if your heading is crosswind on opening you are more likely to have an off-heading opening. I've heard that on windy days it's harder to fly certain transitions relative to a ball. I've heard that you must hook lower on a windy day. I've heard that if you are setting up a 270 performance turn for a crosswind landing, that whether it is a left hand or right hand turn will affect your final groundspeed. These are all partially if not completely wrong, and I have heard them all from individuals with over 5000 jumps. -- And anyway, remember that a skilled skydiver is like a badminton birdie that can and will morph into a dart if he's falling too slow and vice versa. And beginning and intermediate skydivers effectively turn one type of formation into another unpredictably. I know from tons of ball jumps that a ball accelerates way faster on the hill. This is consistent with the physical theory. Head down skydivers should NOT be modeled like a skyball. So my issues with the throw theory are as I've stated before, that it is FAR more complex and difficult to model. Quote Share this post Link to post Share on other sites
sundevil777 102 #31 January 1, 2006 QuoteYour comment was incorrect and misleading. No it wasn't. Two items with the same shape that vary only in mass, given an initial velocity will slow down differently, because the acceleration = drag force/mass. So mass does matter, but I never said it 'predicts anything by itself'. It does not mean that I think it should be considered for exit order, just that your initial analogy was slightly misleading, because I would think that a badminton birdie is both lighter than a dart and has a higher coefficient of drag. So if you qualify your analogy by saying that both have the same mass, that is a better comparison to a belly vs freeflyer.People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am Quote Share this post Link to post Share on other sites
Ron 10 #32 January 1, 2006 QuoteThe problem is that this concept is often applied intuitively by skydivers without a physics education leading to a lot of the "wrong science" we hear around the campfire. Yes, but all of your examples have nothing to do with the topic here, and people have taken the "wrong science" and been wrong from both camps. QuoteI've heard the theory that a horizontal versus vertical mode of flight will affect drift even when the relative wind is from below. Thats true. Its based on time exposed to the winds aloft. A freeflier that has a freefall of 45 seconds will drift less than a flat flyer that is exposed for 1:20. QuoteAnd anyway, remember that a skilled skydiver is like a badminton birdie that can and will morph into a dart if he's falling too slow and vice versa. And beginning and intermediate skydivers effectively turn one type of formation into another unpredictably. I know from tons of ball jumps that a ball accelerates way faster on the hill. This is consistent with the physical theory. Head down skydivers should NOT be modeled like a skyball. So my issues with the throw theory are as I've stated before, that it is FAR more complex and difficult to model. It is the best model you have. You can claim that flat fliers will funnel...OK, but freefliers cork. You take a good freefly group and they penetrate much more into the hill than an expereinced 4way team that almost floats on exit."No free man shall ever be debarred the use of arms." -- Thomas Jefferson, Thomas Jefferson Papers, 334 Quote Share this post Link to post Share on other sites
kallend 2,027 #33 January 2, 2006 QuoteQuoteYour comment was incorrect and misleading. No it wasn't. Two items with the same shape that vary only in mass, given an initial velocity will slow down differently, because the acceleration = drag force/mass. So mass does matter, but I never said it 'predicts anything by itself'. It does not mean that I think it should be considered for exit order, just that your initial analogy was slightly misleading, because I would think that a badminton birdie is both lighter than a dart and has a higher coefficient of drag. So if you qualify your analogy by saying that both have the same mass, that is a better comparison to a belly vs freeflyer. The trajectory is determined by the ballistic coefficient. Two objects with the same ballistic coefficient have the same trajectory regardless of mass. An object with a high BC will go farther than one with a low BC. A dart has a high BC, a birdie has a low BC. A headdowner has a high BC, a belly flyer has a low BC. The analogy was fine. And that is that.... The only sure way to survive a canopy collision is not to have one. Quote Share this post Link to post Share on other sites
kallend 2,027 #34 January 2, 2006 QuoteQuoteQuoteTry throwing a dart and a badminton birdie horizontally with the same initial speed. Which goes farther? That is a damn good answer While it may be a correct answer, I'm not convinced yet how good it is. The problem is that this concept is often applied intuitively by skydivers without a physics education leading to a lot of the "wrong science" we hear around the campfire. I am a physics professorJust because skydivers do not behave perfectly is not a sufficient reason to change the exit order. It just adds to the margin of error that must be accommodated.... The only sure way to survive a canopy collision is not to have one. Quote Share this post Link to post Share on other sites
sundevil777 102 #35 January 2, 2006 QuoteQuoteQuoteYour comment was incorrect and misleading. No it wasn't. Two items with the same shape that vary only in mass, given an initial velocity will slow down differently, because the acceleration = drag force/mass. So mass does matter, but I never said it 'predicts anything by itself'. It does not mean that I think it should be considered for exit order, just that your initial analogy was slightly misleading, because I would think that a badminton birdie is both lighter than a dart and has a higher coefficient of drag. So if you qualify your analogy by saying that both have the same mass, that is a better comparison to a belly vs freeflyer. The trajectory is determined by the ballistic coefficient. Two objects with the same ballistic coefficient have the same trajectory regardless of mass. An object with a high BC will go farther than one with a low BC. A dart has a high BC, a birdie has a low BC. A headdowner has a high BC, a belly flyer has a low BC. The analogy was fine. And that is that. Regardless of mass? The ballistic coefficient depends on the mass. In order for the BC of an object to stay the same as you change the mass it would have to change shape. The ballistic coefficient (BC) is the mass of the object divided by the diameter squared that it presents to the airflow divided by a dimensionless constant that relates to the shape. The BC of a dart or a birdie depends on the mass, and your analogy was not so great because they have such a different mass. And that is that.People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am Quote Share this post Link to post Share on other sites
kallend 2,027 #36 January 2, 2006 QuoteQuoteQuoteQuoteYour comment was incorrect and misleading. No it wasn't. Two items with the same shape that vary only in mass, given an initial velocity will slow down differently, because the acceleration = drag force/mass. So mass does matter, but I never said it 'predicts anything by itself'. It does not mean that I think it should be considered for exit order, just that your initial analogy was slightly misleading, because I would think that a badminton birdie is both lighter than a dart and has a higher coefficient of drag. So if you qualify your analogy by saying that both have the same mass, that is a better comparison to a belly vs freeflyer. The trajectory is determined by the ballistic coefficient. Two objects with the same ballistic coefficient have the same trajectory regardless of mass. An object with a high BC will go farther than one with a low BC. A dart has a high BC, a birdie has a low BC. A headdowner has a high BC, a belly flyer has a low BC. The analogy was fine. And that is that. Regardless of mass? The ballistic coefficient depends on the mass. In order for the BC of an object to stay the same as you change the mass it would have to change shape. The ballistic coefficient (BC) is the mass of the object divided by the diameter squared that it presents to the airflow divided by a dimensionless constant that relates to the shape. The BC of a dart or a birdie depends on the mass, and is not such a great analogy because they have such a different mass. And that is that. Your argument is only relevant if comparing skydivers with the same exit weight. I know some very heavy belly fliers and some very light freefliers. The freefliers still fall faster and have more forward throw. It simply does not matter at all what the mass (weight) is, since we are comparing on the basis of terminal velocity. A spaceball weighing 1kg has the exact same trajectory as a skydiver weighing 90 kilos if they both have the same terminal velocity. I suppose next you'll be telling us that spaceballs can't work because they're not as heavy as actual skydivers.... The only sure way to survive a canopy collision is not to have one. Quote Share this post Link to post Share on other sites
sundevil777 102 #37 January 2, 2006 QuoteYour argument is only relevant if comparing skydivers with the same exit weight. I know some very heavy belly fliers and some very light freefliers. The freefliers still fall faster and have more forward throw. It simply does not matter at all what the mass (weight) is, since we are comparing on the basis of terminal velocity. A spaceball weighing 1kg has the exact same trajectory as a skydiver weighing 90 kilos if they both have the same terminal velocity. I suppose next you'll be telling us that spaceballs can't work because they're not as heavy as actual skydivers. I wouldn't argue any such thing about spaceballs. You are the one that offered an analogy that I said was slightly flawed and would be better if changed as I suggested. Instead of saying that my assertion was wrong, you change the subject. It was only slightly flawed. Engineers and physics teachers like us pick on this stuff all the time, to find the slight flaw in someone's assertion. A lot of people find that trait to be irritating. People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am Quote Share this post Link to post Share on other sites
kallend 2,027 #38 January 2, 2006 The object with higher BC will go farther, whether it has less, indentical, or greater mass. A dart has higher BC than a birdie. A headdown skydiver has a higher BC than a belly flyer. The analogy is fine.... The only sure way to survive a canopy collision is not to have one. Quote Share this post Link to post Share on other sites
sundevil777 102 #39 January 2, 2006 Quote The object with higher BC will go farther, whether it has less, indentical, or greater mass. Yes, but the BC depends on mass, so the shape would also have to change for your statement to be true. QuoteA dart has higher BC than a birdie. A dart has a lower coefficient of drag than a birdie. Its BC will also depend on the mass, so that a really heavy birdie could have a higher BC.People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am Quote Share this post Link to post Share on other sites
sundevil777 102 #40 January 2, 2006 QuoteThe analogy is fine. The point of the discussion, was to show that a freeflyer will travel farther along the direction of flight than a bellyflyer. The use of an analogy with objects of different drag and mass was not as useful as an analogy that only dealt with different drag.People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am Quote Share this post Link to post Share on other sites
kallend 2,027 #41 January 2, 2006 Kindly explain why you think it necessary to compare objects of identical mass as an analogy for comparing skydivers whose masses do not have to be indentical.... The only sure way to survive a canopy collision is not to have one. Quote Share this post Link to post Share on other sites
kallend 2,027 #42 January 2, 2006 QuoteQuoteThe analogy is fine. The point of the discussion, was to show that a freeflyer will travel farther along the direction of flight than a bellyflyer. The use of an analogy with objects of different drag and mass was not as useful as an analogy that only dealt with different drag. Why do you assume all skydivers have the same mass? Physics test: 1) Two skydivers, one with a mass of 60kg and the other with mass of 90kg. exit a jump plane. Which one goes farther down the line of flight? 2) Two skydivers, one with ballistic coefficient B1, and the other with ballistic coefficient B2, where B2 = 1.5*B1, exit a jump plane. Which one goes farther down the line of flight? 3) Does your answer to question 2 depend on which skydiver is heavier? 4) Two inanimate objects, one with mass = 60g and the other with mass 90g are dropped from a jump plane. Which one goes farther down the line of flight? 5) Two inanimate objects, one with ballistic coefficient C1, and the other with ballistic coefficient C2, where C2 = 1.5*C1, are dropped from a jump plane. Which one goes farther down the line of flight? 6) Does your answer to question 5 depend on which object is heavier?... The only sure way to survive a canopy collision is not to have one. Quote Share this post Link to post Share on other sites
sundevil777 102 #43 January 2, 2006 Quote Kindly explain why you think it necessary to compare objects of identical mass as an analogy for comparing skydivers whose masses do not have to be indentical. Because that is the most useful comparison for explaining how a free vs bellyflyer will travel after exiting. Their masses don't have to be identical, but it makes the best analogy. Kinda like saying, "Everything else being equal". A person not so knowledgable can relate to the comparison of the same person exiting in a different orientation, that the trajectory will be different. Why do you focus on ballistic coefficient, it muddles the comparison to make it useless.People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am Quote Share this post Link to post Share on other sites
sundevil777 102 #44 January 2, 2006 QuoteWhy do you assume all skydivers have the same mass? I don't, why do you assume I do? I do say that the comparison of the same person (or thing) exiting in different positions (different drag) is the most useful comparison, it most clearly gets across the point that must be understood for people to realize what you've been trying to get them to understand for so long about exit order. I'm on your side, you've done so much good in this regard. I don't understand the trouble. The comparison of a dart and birdie is not so useful, because someone can say, correctly, that the answer depends on their mass - an unusually heavy birdie could still go farther. QuotePhysics test: 1) Two skydivers, one with a mass of 60kg and the other with mass of 90kg. exit a jump plane. Which one goes farther down the line of flight? Indeterminate. Yes, it depends on the BC. Since you have fixed the mass, the answer can differ according to their Coefficient of drag. Quote2) Two skydivers, one with ballistic coefficient B1, and the other with ballistic coefficient B2, where B2 = 1.5*B1, exit a jump plane. Which one goes farther down the line of flight? The one with the higher BC Quote3) Does your answer to question 2 depend on which skydiver is heavier? Only to the extent that it affects their BC. Quote4) Two inanimate objects, one with mass = 60g and the other with mass 90g are dropped from a jump plane. Which one goes farther down the line of flight? 5) Two inanimate objects, one with ballistic coefficient C1, and the other with ballistic coefficient C2, where C2 = 1.5*C1, are dropped from a jump plane. Which one goes farther down the line of flight? 6) Does your answer to question 5 depend on which object is heavier? Same answers. That was an easy test, what does it prove?People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am Quote Share this post Link to post Share on other sites
kallend 2,027 #45 January 2, 2006 QuoteQuote Kindly explain why you think it necessary to compare objects of identical mass as an analogy for comparing skydivers whose masses do not have to be indentical. Because that is the most useful comparison for explaining how a free vs bellyflyer will travel after exiting. Their masses don't have to be identical, but it makes the best analogy. Kinda like saying, "Everything else being equal". A person not so knowledgable can relate to the comparison of the same person exiting in a different orientation, that the trajectory will be different. Why do you focus on ballistic coefficient, it muddles the comparison to make it useless. Easy. Knowing BC allows one to predict what will happen. Knowing mass does not. www.dropzone.com/cgi-bin/forum/gforum.cgi?post=2005164#2005164... The only sure way to survive a canopy collision is not to have one. Quote Share this post Link to post Share on other sites
apoil 0 #46 January 2, 2006 Quote Yes, but all of your examples have nothing to do with the topic here, and people have taken the "wrong science" and been wrong from both camps. That's why my point conceded that the answer was correct, but questioned the "goodness" of it because it can lead to some wrong thinking. QuoteI've heard the theory that a horizontal versus vertical mode of flight will affect drift even when the relative wind is from below. >Thats true. Its based on time exposed to the winds aloft. A freeflier that has a freefall of 45 >seconds will drift less than a flat flyer that is exposed for 1:20. In editing that item for brevity I suppose I deleted the "wrong science" part of it. The belief was that drift was affected due to surface presented to the winds aloft - misunderstanding that only presentation to the relative wind matters. I also based my statement on somewhat of an ambiguity about the term drift. Not how far you drift but how fast. The statement I was referencing would have it go the other way. Vertical flyers present a greater surface to the winds aloft and hence drift further. >It is the best model you have. You can claim that flat fliers will funnel...OK, but freefliers cork. I said that both happen, both are common, and both increase the complexity of the model. I'm not arguing for a different exit order, I'm arguing for a better understanding that things may not not always as simple as our models make it seem. >You take a good freefly group and they penetrate much more into the hill than an expereinced 4way team that almost floats on exit. fine. but lets be honest. Who is on most of the loads at most of the dropzones? Not kickass freefly groups penetrating with maximum effectiveness into the relative wind, and not flat groups that are launching funnel free. Who is the rule of thumb for? The experienced guys know how to keep themselves safe in any condition. Quote Share this post Link to post Share on other sites
kallend 2,027 #47 January 2, 2006 QuoteQuoteWhy do you assume all skydivers have the same mass? I don't, why do you assume I do? I do say that the comparison of the same person exiting in different positions is the most useful comparison. QuotePhysics test: 1) Two skydivers, one with a mass of 60kg and the other with mass of 90kg. exit a jump plane. Which one goes farther down the line of flight? Indeterminate. Yes, it depends on the BC. Since you have fixed the mass, the answer can differ according to their Coefficient of drag. Quote2) Two skydivers, one with ballistic coefficient B1, and the other with ballistic coefficient B2, where B2 = 1.5*B1, exit a jump plane. Which one goes farther down the line of flight? The one with the higher BC Quote3) Does your answer to question 2 depend on which skydiver is heavier? Only to the extent that it affects their BC. Quote4) Two inanimate objects, one with mass = 60g and the other with mass 90g are dropped from a jump plane. Which one goes farther down the line of flight? 5) Two inanimate objects, one with ballistic coefficient C1, and the other with ballistic coefficient C2, where C2 = 1.5*C1, are dropped from a jump plane. Which one goes farther down the line of flight? 6) Does your answer to question 5 depend on which object is heavier? Same answers. That was an easy test, what does it prove? ... You didn't answer question 3 or 6 correctly. The answer is "NO" to each, since the relative BCs have already been given in questions 2 and 4. You got 4 out of 6 Since the answers are the same for skydivers as for inanimate objects as you so kindly confirmed, it proves that when using inanimate objects as an analogy for skydivers, their relative masses do not matter. Hence using a dart and birdie is a perfectly good analogy for a freeflyer and a belly flyer.... The only sure way to survive a canopy collision is not to have one. Quote Share this post Link to post Share on other sites
sundevil777 102 #48 January 2, 2006 I don't understand why you went with Goldfinger? edited for wrong Bond character People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am Quote Share this post Link to post Share on other sites
sundevil777 102 #49 January 2, 2006 Now you're splitting hairs, my answer was implicitly no. Come on now... Your analogy is lousy because you don't know which BC is higher, it depends on both the mass and Cd. Did you consider going with Ming's counterpart from the '74 movie Flesh Gordon, Emperor Wang? People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am Quote Share this post Link to post Share on other sites
NelKel 0 #50 January 2, 2006 Now you're splitting hairs, my answer was implicitly no. Come on now... Your analogy is lousy because you don't know which BC is higher, it depends on both the mass and Cd. Did you consider going with Ming's counterpart from the '74 movie Flesh Gordon, Emperor Wang?<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< If the coefficient X were to slip past the effeciency rating of B, could the CG of X equal the remainder of Z?_________________________________________ Someone dies, someone says how stupid, someone says it was avoidable, someone says how to avoid it, someone calls them an idiot, someone proposes rule chan Quote Share this post Link to post Share on other sites
