andy2 0 #1 June 28, 2003 My understanding of the locking stows, is that you want to make sure they are symetrical, and about 3-4 finger "bites". If you have a canopy that opens notoriously hard, is there anything you can do with the locking stows that will make it open softer, i.e. shorter bites or longer bites? What about the size of the rubber bands on the D-bag? Is there one size that manufacturer's recomend for their containers, or will having just one size for all the rubberbands suffice, as long as they hold sufficient tension on the lines, and they are all the same size? What is the purpose of doubling the rubber band on the line stows? Is this just to keep the lines held with a little more tension, so they wont slip out/become messy? Or does this have an effect on how hard the parachute opens? If so, how? Ok, thanks for listening to the barrage of questions --------------------------------------------- let my inspiration flow, in token rhyme suggesting rhythm... Quote Share this post Link to post Share on other sites
quade 4 #2 June 28, 2003 Quote My understanding of the locking stows, is that you want to make sure they are symetrical, and about 3-4 finger "bites". If you have a canopy that opens notoriously hard, is there anything you can do with the locking stows that will make it open softer, i.e. shorter bites or longer bites? Not really. The reasoning behind the length of the bites has to do with the physics of acceleration when the bag is yanked out of the container by the pilot chute. Too short and the mass of the lines across the d-bag far exceeds that of the mass of the bites causing the bites to be pulled through the rubber bands, causing line dump. A really bad thing. Quote What about the size of the rubber bands on the D-bag? Is there one size that manufacturer's recommend for their containers, or will having just one size for all the rubber bands suffice, as long as they hold sufficient tension on the lines, and they are all the same size? As you go through your skydiving career you'll hear a lot of different theories about rubber bands and tube stows and other ways of doing things. When in doubt, follow the manufacturer's recommendations. Generally speaking, other than preventing line dump, there's not a lot you can or should attempt to do as far as slowing your canopy deployment with the line stows.quade - The World's Most Boring Skydiver Quote Share this post Link to post Share on other sites
alan 1 #3 June 28, 2003 QuoteThe reasoning behind the length of the bites has to do with the physics of accelleration when the bag is yanked out of the container by the pilot chute. I thought it was deceleration. You, the container, bag, etc. all falling at something around 176 ft/sec under reasonably normal circumstances. The pc is deployed and generates drag that is transmitted to the bag via the bridle. The bag pulls away from the pack tray because it falling slower than the rig and jumper. The locking stows are to keep the canopy in the bag until lines stretch. If, as you point out, the bag is stripped off the canopy before line stretch, the abrupt and unstaged deceleration can cause serious injury or death to the jumper and damage the equipment.alan Quote Share this post Link to post Share on other sites
Falko 0 #4 June 28, 2003 QuoteI thought it was deceleration. You, the container, bag, etc. all falling at something around 176 ft/sec under reasonably normal circumstances. The pc is deployed and generates drag that is transmitted to the bag via the bridle. The bag pulls away from the pack tray because it falling slower than the rig and jumper. "Alles ist relativ" (Albert Einstein) Since the bag is accelerated relative to the jumper, I'd say the use of either term is correct and just a "matter of taste". Quote Share this post Link to post Share on other sites
quade 4 #5 June 29, 2003 Exactly.quade - The World's Most Boring Skydiver Quote Share this post Link to post Share on other sites
Genn 0 #6 June 29, 2003 What is the purpose of doubling the rubber band on the line stows? Is this just to keep the lines held with a little more tension, so they wont slip out/become messy? Or does this have an effect on how hard the parachute opens? If so, how?Quote Doubling the rubberband isn't always necessary if you can downsize to a smaller rubberband. I've been taught that one of the causes of a line over malfunction is when there is insuffiicient tension on the line stows...causing them to unstow out of sequence..(bad thing) and there is a chance the when part of the canopy releases from the bag..the prematured line(s) can go over your canopy. You also want to avoid line dump at all costs. (I am NOT an expert...this is what I've been taught and read) Quote Share this post Link to post Share on other sites
RichM 0 #7 June 29, 2003 Quote Doubling the rubberband isn't always necessary if you can downsize to a smaller rubberband. I've been taught that one of the causes of a line over malfunction is when there is insuffiicient tension on the line stows...causing them to unstow out of sequence..(bad thing) and there is a chance the when part of the canopy releases from the bag..the prematured line(s) can go over your canopy. You also want to avoid line dump at all costs. Doubling stows carries a bag lock risk if the stow is too tight. Stow tension and materials are a compromise between these two undesirable extremes afaik.Rich M Quote Share this post Link to post Share on other sites
alan 1 #8 June 30, 2003 QuoteSince the bag is accelerated relative to the jumper, I'd say the use of either term is correct and just a "matter of taste". No, the bag is decelerated relative to the jumper. You'd be incorrect in your matter of taste. But then, it is only skydiving, why be precise and accurate when "well, we all know what you mean" will suffice.alan Quote Share this post Link to post Share on other sites
alan 1 #9 June 30, 2003 QuoteExactly. Exactly wrong. Perceptions and illusions have always been a good substitute for reality.alan Quote Share this post Link to post Share on other sites
quade 4 #10 June 30, 2003 Alan -- It's not an illusion at all. An acceleration and a deceleration is exactly the same thing. At least as far as physics is concerned.quade - The World's Most Boring Skydiver Quote Share this post Link to post Share on other sites
alan 1 #11 July 1, 2003 Energy is equal to the mass times the velocity squared, I think I understand that part of physics. The value of energy changes exponentially as the velocity changes and in one case energy is increasing and in the other it is decreasing. I'm trying to think of a reason why Poynter refers to his Parachute Manual as " A Technical Treatise on Aerodynamic Decelerators" instead of Accelerators. I mean, hey, as far as pysics is concerned, it is the same thing, right. Einstein not withstanding and for all reasonable purposes, gravity is a part of the environment we live in, as is the earth. As far as an illusion, how many times have we heard a whuffo ask "why are they going up so fast" after viewing a video of a tandem opening as the videographer falls away? We know they are not going up. They are still going down. They are decelerating. The resultant energy is being reduced. It only appears as if the tandem pair is accelerating away from the camera because the camera is still travelling at a relatively constant velocity at that point in time.alan Quote Share this post Link to post Share on other sites
darkvapor 0 #12 July 1, 2003 It's not entirely accurate to link energy and acceleration. Acceleration is the time derivative of velocity, ie change of velocity / change in time. Change being the key word. Acceleration is acceleration, regardless if it is a positive or negative value. 'Deceleration' is just a term to note negative acceleration, but there is no difference otherwise. No one uses the term deceleration in engineering and sciences, it is a layman's term. Quote Share this post Link to post Share on other sites
freeflydrew 0 #13 July 1, 2003 QuoteQuote Doubling the rubberband isn't always necessary if you can downsize to a smaller rubberband. I've been taught that one of the causes of a line over malfunction is when there is insuffiicient tension on the line stows...causing them to unstow out of sequence..(bad thing) and there is a chance the when part of the canopy releases from the bag..the prematured line(s) can go over your canopy. You also want to avoid line dump at all costs. Doubling stows carries a bag lock risk if the stow is too tight. Stow tension and materials are a compromise between these two undesirable extremes afaik. Scott Miller from PD was out at the Ranch, and he says, that you should use small rubber bands, and if you do use big rubber bands, you must double stow them. (Maybe I was missing something like line size or something, but it seems to make sense to me)... If you choose not to double stow them, your lines are not going to be held secure enough, they may shift during the closing of the container, they may pull away from the rubber bands during the deployment, and your proper deployment sequence may be interrupted. BAg Lock becomes a whole other discussion... "Doubling stows carries a bag lock risk if the stow is too tight. Stow tension and materials are a compromise between these two undesirable extremes afaik" and I don't think that that statement can really be backed up within the context of this discussion. Wouldn't it have to also do with the stows being far too big along with some other factors? -drew Quote Share this post Link to post Share on other sites
Jonsmann 0 #14 July 1, 2003 They not only appear to be accelerating away, they DO accelerate away. Your energy considerations are wrong. You are confused about the frame of reference. Stop thinking about the jumpers speed relative to the ground, it has no relevance to the inertial forces involved in the deployment. - Jacques > It only appears as if the tandem pair is accelerating away from the camera because the camera is still travelling at a relatively constant velocity at that point in time. Quote Share this post Link to post Share on other sites
alan 1 #15 July 9, 2003 Sorry for the late reply, I've been away for a few days. QuoteYour energy considerations are wrong. I stated that energy is equal to mass times velocity. How is that wrong? QuoteYou are confused about the frame of reference. Stop thinking about the jumpers speed relative to the ground I don't think so. I'm thinking of the pc and freebag relative to the ground. QuoteStop thinking about the jumpers speed relative to the ground, it has no relevance to the inertial forces involved in the deployment. So then there really is no such thing as a hard opening?alan Quote Share this post Link to post Share on other sites
sundevil777 102 #16 July 9, 2003 Momentum is mass times velocity, not energy.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
alan 1 #17 July 9, 2003 Please accept my apology for the untimely nature of my reply, I've been away for several days. QuoteIt's not entirely accurate to link energy and acceleration. Yes, I believe my exact words were, "Energy is equal to the mass times the velocity squared, ". What is unclear about that? QuoteAcceleration is the time derivative of velocity, ie change of velocity / change in time. Change being the key word. Once again, I think I said the same thing. Here is what I said, "The value of energy changes exponentially as the velocity changes and in one case energy is increasing and in the other it is decreasing." Maybe I didn't put enough emphasis on the word change for you. QuoteAcceleration is acceleration, regardless if it is a positive or negative value. OK, if you say so. But, if you are in a car travelling at a brick wall at 10 mph, would you rather be accelerating or decelerating? I think I understand though, you would not mind acelerating as long as it had a negative value. Quote'Deceleration' is just a term to note negative acceleration, but there is no difference otherwise. I think most of us laymen understand the difference between accelerating towards a wall and decelerating towards it. Our world consists of a reality that extends beyond numbers and formulas. QuoteNo one uses the term deceleration in engineering and sciences, it is a layman's term. I am a layman. I would venture to guess that half or more of the people reading this forum are laymen. I think it was appropriate to use the term in view of the context. Once again......"I'm trying to think of a reason why Poynter refers to his Parachute Manual as " A Technical Treatise on Aerodynamic Decelerators" instead of Accelerators. " alan Quote Share this post Link to post Share on other sites
alan 1 #18 July 9, 2003 QuoteMomentum is mass times velocity, not energy. I must have been exposed to some faulty science books. E=mc2 is what we learned. But if you say Momentum is mass times velocity, I'll take your word for it with a degree of scepticism.alan Quote Share this post Link to post Share on other sites
indyz 1 #19 July 9, 2003 QuoteOur world consists of a reality that extends beyond numbers and formulas. We are talking physics here, so actually it doesn't. QuoteI think most of us laymen understand the difference between accelerating towards a wall and decelerating towards it. The definition of deceleration as most people use the word is simply "acceleration opposite the direction of motion." Deceleration towards a wall is exactly the same thing as acceleration away from the wall. Quote Share this post Link to post Share on other sites
masher 1 #20 July 9, 2003 E=mc^2 is the energy equivelance equation. It has nothing at all to do with this discussion. What you're interested in, and have discussed above are these: Momentum = mass * velocity Energy = 0.5 * mass * velocity^2 The energy doesn't change exponentially as velocity increases; it's only a quadratic increase. That's much slower. . At deployment, the tandem _does_ accelerate away from the camera. In freefall, they are falling at a constant velocity, the TM then deploys the canopy _slowing the tandem down_ (the TM has decelerated (or more properly -vely accelerated)) relative to the camera. Relative to the tandem, the camera accelerates away from them. ahhh the fun of relative motion.... Negative aceleration is the same as deceleration. There is no difference what so ever.-- Arching is overrated - Marlies Quote Share this post Link to post Share on other sites
fundgh 0 #21 July 9, 2003 Physics guy eh? I like that...Physics is cool, and I want to go back to school to learn more of it. So it is true then, that in a vacuum, a guy with a wingloading of 1.1 would have the same problem as a guy with a wingloading of 2.1? and a guy in freefall?...FUN FOR ALL! Quote Share this post Link to post Share on other sites
bch7773 0 #22 July 9, 2003 QuotePhysics guy eh? I like that...Physics is cool, and I want to go back to school to learn more of it. So it is true then, that in a vacuum, a guy with a wingloading of 1.1 would have the same problem as a guy with a wingloading of 2.1? and a guy in freefall? well, since technically in a vacuum, all three people would be dead, since they can't breathe. MB 3528, RB 1182 Quote Share this post Link to post Share on other sites
alan 1 #23 July 9, 2003 QuoteThe energy doesn't change exponentially as velocity increases; it's only a quadratic increase. That's much slower. Again, bad schooling/learning on my part. We were taught that in velocity^2, that the 2 was called an exponent. Hence I derived the increases exponentially. By the way, what is the 2 in this case called? QuoteNegative aceleration is the same as deceleration. No arguement. But that would mean that aceleration is not the same as deceleration......."negative" aceleration is. QuoteThere is no difference what so ever. OK. I'll get in a car and acelerate from 60 mph to 120 mph away from a brick wall. You get in a car and decelerate from 120 mph to 60 mph towards the brick wall. Or try this one, I'll travel towards it to. We both start at 20mph. I'll decelerate 15 mph and you acelerate 15 mph. Funny, out of all my comments people choose to reply to, none has addressed why Poynter titled his book THE PARACHUTE MANUAL A Technical Treatise on Areodynamic Decelerators. Why didn't he call them Areodynamic Acelerators?alan Quote Share this post Link to post Share on other sites
billvon 3,076 #24 July 9, 2003 >No, the bag is decelerated relative to the jumper. Not really. An observer on your back would see the bag accelerate away from the jumper. It is decelerating from the point of view of a ground-based observer, but if einstein taught us one thing, it's that you have to define your frame of reference and stick to it. Quote Share this post Link to post Share on other sites
alan 1 #25 July 9, 2003 QuoteThe definition of deceleration as most people use the word is simply "acceleration opposite the direction of motion." Please accept my apology, I refered to an apparently obsolete 1992 Webster's Dictionary of the English Language, which I thought would be close enough to "as most people" would use the word. It defines it as negative acceleration. Referencing acceleration with repect to Physics it says; The rate at which the velocity of a body increases per unit of time: used also of decrease of velocity, which is expressed as a negative acceleration. Going in circles but it seems as if acceleration involves an increase in velocity and deceleration (negative acceleration) involves a decrease of velocity. Can you please explain why Poynter would have been so misguided as to title his book, THE PARACHUTE MANUAL A Technical Treatise on Aerodynamic Decelerators? QuoteDeceleration towards a wall is exactly the same thing as acceleration away from the wall. See, here is where the world of Physics seems to have some contrast with the real world. We'll both drive at a wall at say 20 mph. As well approach the wall, I'll decelerate 15 mph so I impact the wall at 5 mph. You on the other hand, can accelerate the same 15 mph and impact the wall at 35 mph. Or, better yet, I'll accelerate away from it from 60 mph to 120 mph, while you decelerate into it. You'll soon discover the difference.alan Quote Share this post Link to post Share on other sites
