The111 0 #26 November 11, 2003 QuoteBut if chipping is only caused by vortex shedding, how are we controlling it by relaxing? I mean, am I really subconsciously countering each wobble, or are the vortices not being shed as unevenly, or what? I was thinking that same thing. Maybe relaxing allows the perturbations in the air to adjust our body dynamically (rather than us "subconsciously" adjusting it ourselves). Just a guess.www.WingsuitPhotos.com Quote Share this post Link to post Share on other sites
champu 1 #27 November 11, 2003 based on some fun physics and control theory... really the only two things that matter are center of pressure and center of gravity. if they are vertically aligned the system is at an equalibrium, technically it won't go anywhere unless you add some outside force. if the center of pressure is below the center of gravity (ie de-arching while belly flying to slow down) this is an unstable equalibrium, meaning a little poke one way or the other and the system will try and flip over so that... if the center of pressure is above the center of gravity it is a stable equalibrium, and if poked one way or the other, the system will oscillate or rock back and forth, eventually settling back to this same position. what decides how quickly the system will return to it's stable equalibrium is how far apart the center of pressure and center of gravity are. In the case of a shuttlecock or a dart, the center of pressure is way behind the center of gravity, so if you tap the tail it will very quickly return to it's stable orientation. if you think of a potato-chipping student, the center of pressure might be millimeters above the center of gravity, so if you tap them and let them be, the rocking motion may not recover before the dive is over Quote Share this post Link to post Share on other sites
sammer 0 #28 November 11, 2003 Yes, I think you are right about the vortex shedding. I am not really sure if it matters what drives the object to instability. If it does not tend to damp out with time...I usually call it dynamic unstability. I guess the point I was trying to make is that the rigid body might tend to be statically stable, but still might go for an exciting ride when actually put into the airflow. Quote Share this post Link to post Share on other sites
sammer 0 #29 November 11, 2003 I think that your discription is correct when decsribing the movement of a simple object that is not subject to any external aerodynamic forces (like the vortex shedding Kallend was speeking about). It is actually very easy to design an aircraft that has its center of mass forward of it's center of pressure but will never recover from a disturbance. If the aerodynamic forces that are trying to return the aircraft to it's original state are too strong, the aircraft will overshoot it's original position. Depending on the damping of the system...the oscillations can continue and get larger or may dampen over time (or stay the same in the case of neutral damping). It gets even more complex when you start to consider flexible structures (aeroelasticity) and/or automatic control systems. I am starting to get a little geeked out here I think I will go back to lurking the talkback forum. Quote Share this post Link to post Share on other sites
sducoach 0 #30 November 12, 2003 FWIW, I don't even work as an aerospace engineer, I'm current working on a shoulder mounted missile system for a major defense contractor. If you think you might be using this system when it hits the field, let me know, and I'll warn you so you can stay away from it and protect yourself. Can I say..............I'm sorry????????? Or should I just DUCK!!!!!! Blues, J.E.James 4:8 Quote Share this post Link to post Share on other sites
Ang 0 #31 November 12, 2003 I did my first AFF jump out of Mike Mullins King Air. Upon exit, my main-side jump master got whipped beneath me, which sent all three of us into a roll. My only thought was I have to arch to get out of this. I arched as much as humanly possible and the video is amazing. The roll lasted 5 seconds. And you can CLEARLY see that as soon as I went into my arch I came out of the roll. So it is true, your arch is your stability. Of course, relaxation doesn't hurt either. Quote Share this post Link to post Share on other sites
elfanie 0 #32 November 12, 2003 Quote And you can CLEARLY see that as soon as I went into my arch I came out of the roll. So it is true, your arch is your stability. Of course, relaxation doesn't hurt either. When I had my "unstable exit" for my AFF7....I got video of that as well. Got turned the wrong way into the relative wind which flipped me over onto my back into a dearched position. Like you...I ARCHED. As soon as I arched...FLIP back into belly-down. Here is the video... My graduation jump Now...that being said...I don't think that if someone were able to maintain a frozen/fixed position that they would maintain a frozen/fixed freefall. Lets say that the frozen position is one in which one shoulder and/or one leg is dipped down as if in a turn. Like the windmill or a pinwheel, the skydiver would turn...quickly falling into a spin. As the spin became stronger, their body (in the same 'frozen' position) would rotate onto their side somewhat..right? due to the centripital force of the spin. So a stable position becomes a dynamic one..one which responds one way to begin with (turning/spinning) and yet changes with the speed of the spin. that's just an example of what I can see as the position, although frozen, still becoming dynamic in the manner in which it falls. Of course...I'm still just a baby in the sport, so I fully expect someone to tell me that I'm way off base (although hopefully they will also explain to me WHY I'm full of it, so I can learn...) Also waiting for someone to tell me what cool pictures/video I have for such a newbie. heh. (give me a break...I haven't had the experience or time in the sport to get some of the awesomely cool pictures/video that some of the people here have earned!) Elfanie My Skydiving Page -------------------------------------------- Elfanie My Skydiving Page Fly Safe - Soft Landings Quote Share this post Link to post Share on other sites
The111 0 #33 November 12, 2003 QuoteCan I say..............I'm sorry????????? Or should I just DUCK!!!!!! Wasn't threatening, just saying since you apparently doubt my competence you might not want to use this system (if you plan on using any shoulder mounted missiles in your life - I sure don't). Hehe.www.WingsuitPhotos.com Quote Share this post Link to post Share on other sites
Aviatrr 0 #34 November 12, 2003 QuoteWasn't threatening, just saying since you apparently doubt my competence you might not want to use this system (if you plan on using any shoulder mounted missiles in your life - I sure don't). Hehe You have to test your work! Oh, and you have to invite me along to help! If you don't want to fire it, I will! Just be sure you aren't near any nearby vehicles....never know which one I might choose to test it on! Mike Quote Share this post Link to post Share on other sites
champu 1 #35 November 12, 2003 haha, oh right... damping... sorry, I'm too mired up in matricies and block diagrams to actually think about what the hell is going on. It's about that time in the semester where I need a week off. Quote Share this post Link to post Share on other sites
kallend 1,623 #36 November 12, 2003 QuoteQuoteWasn't threatening, just saying since you apparently doubt my competence you might not want to use this system (if you plan on using any shoulder mounted missiles in your life - I sure don't). Hehe You have to test your work! Oh, and you have to invite me along to help! If you don't want to fire it, I will! Just be sure you aren't near any nearby vehicles....never know which one I might choose to test it on! Mike I hate to repeat myself, but aerodynamics is not an exact predictive science. That's why tests are needed despite all the fancy computer modeling. So - make cardboard cut outs and see what happens.... 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
sducoach 0 #37 November 13, 2003 I'm not going to use one......... But I will remain a "moving target"!!!!!!James 4:8 Quote Share this post Link to post Share on other sites
