amir1967 0 #26 November 30, 2003 Quote>Ahhhhh...so if this happnds (change of air speed and canopy >deflates), what do you do????? Pump the toggles? It depends where you are. At altitude (i.e. >1000 feet) generally doing nothing is the best approach. Your canopy is remarkably stable, and it will recover its airspeed on its own if you let it. Just keep it going straight. Down lower (say, below 100 feet) in general the best plan is to stay at full flight unless you feel a serious loss of airspeed or canopy collapse - at that point going to 1/4 to 1/2 brakes will prevent your canopy from diving so hard and/or deflating. Finally, if it happens below 20 feet or so, generally your best approach is going to be to flare and get ready to PLF. If you've got some airspeed left, the flare will slow your descent. If you have no airspeed left, then adding toggle will help your canopy reinflate. (That's why canopies are packed with their brakes stowed.) It may reinflate in time, it may not - in which case getting ready for that PLF may save the day. as always good advice from Billvon (why not all instructor teach this to all the students, not on the first jump but there after ) amirAM67 Quote Share this post Link to post Share on other sites
genoyamamoto 0 #27 November 30, 2003 I've noticed that I pick up some ground speed at about 20 ft above the ground. Has anyone else noticed this? I don't know if i'm imagining this or not but it seems like it could be possible because there should be a boundary layer above ground where the airspeed decreases logarithmically to zero at ground level. The nerd in me needs to know. Thanks! Gotta go... plaything needs to spank me Feel the hate... Photos here Quote Share this post Link to post Share on other sites
kallend 1,623 #28 November 30, 2003 QuoteI've noticed that I pick up some ground speed at about 20 ft above the ground. Has anyone else noticed this? I don't know if i'm imagining this or not but it seems like it could be possible because there should be a boundary layer above ground where the airspeed decreases logarithmically to zero at ground level. The nerd in me needs to know. Thanks! There is indeed a layer of wind gradient, and it extends quite high (just take a look at "winds aloft" forecasts). Is the decrease logarithmic? Beats me.... 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 1,623 #29 November 30, 2003 Quote ... And conversely, if I was flying along in a 15 mph headwind, with a 5 mph groundspeed and a 20 mph airpseed and that person with a 50 mph fan flew in front of me and blew it at me, I would suddenly have a significantly higher airspeed, right? I don't want to guess how much higher because I am sure there are too many factors involved. But it would be an increase of at least 35-50 mph in airpseed, initially. Is this conceptually correct? If so, I think I get it. If I am way off base, then, well back to the drawing board and more research about airspeed and inertia. No dude with fan is required, the atmosphere quite capable of producing this effect all by itself.... 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
genoyamamoto 0 #30 November 30, 2003 I was referring to the boundary layer "close" to the ground, since there is a boundary condition at z=0 that the air velocity goes to zero from whatever it is before the boundary layer. I don't know how high this layer goes, but am thinking it depends on how rough the surface is. Maybe i'm just getting excited when i'm close to the ground and think I feel my decent rate and ground speed increase. Am I on crack or does this happen to others? Thanks! Gotta go... plaything needs to spank me Feel the hate... Photos here Quote Share this post Link to post Share on other sites
scottjaco 0 #31 November 30, 2003 When I'm landing over dirt or semi dirt on a hot day I actually feel myself slow down a bit, it's almost a hovering effect about 50 feet off the ground. I'm told the heat bounces air off the ground and pushes upward, where as grass is cooler, causing you to get sucked in. Quote Share this post Link to post Share on other sites
Jimbo 0 #32 December 1, 2003 QuoteI now favor "no wind" landings because I know I have that foward speed to generate lift, I don't trust the headwind to slow down my canopy, it's too risky to depend on somthing so inconsistant. Forward speed does nothing to generate lift, airspeed generates lift. This has been discussed many times here, even within the last week. See the following links for more info: Wind and Canopies Canopy control Downwind turning tendencies Here's the most recent thread that I can remember. Losing altitude in your carves in wind versus no wind There's plenty more available, do a search for airspeed and groundspeed. - Jim"Like" - The modern day comma Good bye, my friends. You are missed. Quote Share this post Link to post Share on other sites
The111 0 #33 December 1, 2003 QuoteAnd conversely, if I was flying along in a 15 mph headwind, with a 5 mph groundspeed and a 20 mph airpseed and that person with a 50 mph fan flew in front of me and blew it at me, I would suddenly have a significantly higher airspeed, right? I don't want to guess how much higher because I am sure there are too many factors involved. But it would be an increase of at least 35-50 mph in airpseed, initially. Correct, your airspeed would temporarily increase by 50mph (if your headwind inreased by 50mph) and during that brief amount of time you would have amazing amounts of lift available, but you'd also have a lot of drag, which would quickly slow you back down to your equilibrium airspeed.www.WingsuitPhotos.com Quote Share this post Link to post Share on other sites
kallend 1,623 #34 December 1, 2003 QuoteI was referring to the boundary layer "close" to the ground, since there is a boundary condition at z=0 that the air velocity goes to zero from whatever it is before the boundary layer. I don't know how high this layer goes, but am thinking it depends on how rough the surface is. Maybe i'm just getting excited when i'm close to the ground and think I feel my decent rate and ground speed increase. Am I on crack or does this happen to others? Thanks! Here are some data from yesterday (11/30) in N. Illinois. After making 3 skydives at SDC I flew to Beloit (Wisconsin) and back to Lansing, IL, in my Mooney and used my GPS to determine the wind profile. At 6000ft the wind was over 50kt. At 3000ft the wind was 40kt. At 1000ft the wind was 30kt. At 250ft the wind was 25kt, and on the ground the wind was 13kt at the airport AWOS. Now these were perfectly jumpable conditions for an experienced skydiver, and illustrate that the concept of a uniformly moving airmass is a gross oversimplification. There was a serious wind change between 250ft and ground level.... 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 #35 December 1, 2003 Quote If you start your turn, and suddenly lose that tailwind (i.e. winds become zero) your canopy still has that groundspeed of 20 kts, so it's now seeing a _faster_ airspeed than it saw before. If you're halfway through the turn, there should be little effect. If you've almost completed the turn, and are more-or-less facing into the wind, you will see a loss of airspeed similar to what a jumper coming straight in will feel. Help me here Bill. The canopy flies due to "relative wind" creating positive pressure in the cells and to some point creating a "lift factor". It makes no difference which direction the wind is blowing if it is constant. Only the ground speed is effected. However, in turbulent wind conditions the "sudden" decrease in the velocity of the "relative wind" can and will cause the canopy to deflate and/or stall. The canopy does in fact know what the wind is doing in relation to the airfoil. It does not know what the ground speed is doing. Relative wind and effective wind. I believe you are a pilot Bill, think turns around a point or figure 8's. Constant airspeed, varying ground speed. Blues, J.E.James 4:8 Quote Share this post Link to post Share on other sites
Ron 7 #36 December 1, 2003 Not directed at any one person. QuoteAccording to Monique, the wind which James was in "disappeared" immediately when he dropped below the treeline Which is NOT windshear...Its wind being blocked by the trees. Happens all the time if you land right infront of or around trees. Yes, windshear will drop you on your ass. But so will landing in a turbulent area. And clearly landing in a turn will also do damage. So add in landing downwind or around trees, the simple fact that wind is not always constant (And in fact if I had to land into a closed area like a stadium I would rather have it be a no wind day). And add in a low turn (that will always in itself be a bad thing) you have what we call skydiving. If you find yourself landing in a tight area, you need to know how the objects around you will cause problems with the wind, and they will effect the way your canopy flies. All of these arguments are stupid really....A canopy does not fly differently in a steady wind than it does in a no wind day EXCEPT in relation to the ground. Yes winds will effect the canopy whenever there is a change in them. DUH!!! is this a really hard concept to grasp? If I come in to land and the wind just drops....Yes, my canopy is going to dive to try and get back to its flying speed. And if I come in for a swoop and the winds pick up I am going to have to adjust to the new winds. If I ask what happens if I pull the strings down. All I need for an answer is that it will flare the canopy. I don't need to get into a damn debate on Newtonian Physics. Most don't care, and will not learn it anyway. edited for PA's"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
billvon 2,400 #37 December 1, 2003 >Which is NOT windshear...Its wind being blocked by the trees. >Happens all the time if you land right infront of or around trees. Windshear is any rapid change in wind that happens at a physical location, usually at a specific altitude. Wind being blocked by trees qualifies as windshear. Quote Share this post Link to post Share on other sites
billvon 2,400 #38 December 1, 2003 >It makes no difference which direction the wind is blowing if it is > constant. Only the ground speed is effected. Right. > However, in turbulent wind conditions the "sudden" decrease in the > velocity of the "relative wind" can and will cause the canopy to > deflate and/or stall. Agreed. That's really all your canopy cares about. If you have a headwind, for example, and you suddenly go below the treeline and lose that wind, the canopy will drop like a rock. If you're heading downwind in the same conditions, the canopy will balloon upwards; as it loses the tailwind it will see a net increase in airspeed, and it will translate that (usually) into lift. Quote Share this post Link to post Share on other sites
sducoach 0 #39 December 1, 2003 Quote If you start your turn, and suddenly lose that tailwind (i.e. winds become zero) your canopy still has that groundspeed of 20 kts, so it's now seeing a _faster_ airspeed than it saw before. Bill, This part is what's confusing. Please explain the relationship to the 20 kts groundspeed and a faster airspeed? Blues, J.E.James 4:8 Quote Share this post Link to post Share on other sites
The111 0 #40 December 1, 2003 It really isn't a result of the groundspeed, that's just an easy way to illustrate it. Picture it this way. When your tailwind disappears, the moving mass of air you were flying in stopped moving, but you still have momentum, so your airspeed will increase.www.WingsuitPhotos.com Quote Share this post Link to post Share on other sites
billvon 2,400 #41 December 1, 2003 >This part is what's confusing. Please explain the relationship to the >20 kts groundspeed and a faster airspeed? Your canopy cannot change speeds instantaneously. Groundspeed has no effect in a steady wind; but once the wind changes, your canopy's speed does not instantly change with it, and it sees a temporary change in airspeed. If the winds go to zero, your airspeed is suddenly set equal to your groundspeed. That might be higher or lower than your previous airspeed, which can cause problems. Quote Share this post Link to post Share on other sites
winsor 186 #42 December 1, 2003 QuoteQuote If you start your turn, and suddenly lose that tailwind (i.e. winds become zero) your canopy still has that groundspeed of 20 kts, so it's now seeing a _faster_ airspeed than it saw before. Bill, This part is what's confusing. Please explain the relationship to the 20 kts groundspeed and a faster airspeed? Blues, J.E. Physics 101 - we're talking VELOCITY here, not speed. The difference between the two is that velocity is a vector quantity, where it has both magnitude and direction. When you have a tailwind and descend into dead air, the airspeed your canopy sees is your initial airspeed PLUS the difference between the tailwind and the dead air. If you are flying into a headwind and descend into dead air, the airspeed your canopy sees is your initial airspeed MINUS the difference between the headwind and the dead air. Thus, if you have a canopy airspeed of 20 mph and a tailwind to clear the tall trees of 20 mph (and no turbulence, of course, to keep the model simple), when you drop below the trees and hit air that isn't moving with regard to the ground you're going through it at 40 mph, and will likely balloon like a bastard. If you're going into a headwind of 20 mph with your 20 mph canopy and come down behind the same trees, you go from a vertical descent with 20 mph airspeed to zip for airspeed in dead air. If you're hanging on the brakes, you'll likely stall and arrive in whatever configuration the canopy sees fit to deliver you (a broken coccyx is cheap tuition for that lesson). If you're in full flight, the canopy will likely surge forward and slap you into the ground in that direction. Don't worry, even Einstein screwed up the distinction between scalars and vectors upon occasion (what's the value of V^2/C^?). Blue skies, Winsor Quote Share this post Link to post Share on other sites
sducoach 0 #43 December 1, 2003 "WinDsor", And you too Bill. Winsor, Everything you state is correct however, Scalars, Vectors, and velocity aside. Your statements come almost directly from wind shear training programs and are correct. That is the reason for increased power commands upon rapid performance increase on approach/final. All this is based upon relative wind and effective wind in relation to the airfoil i.e. canopy. Again, going from a 20 kts tailwind and turning into a 20 kts headwind your canopy will not see an increase in airspeed nor ground speed. Your airspeed will remain constant and ground speed will be reduced, UNLESS the velcity and vector changes suddenly. Correct Winsor & Bill? What's the value of V^2/c^?. Absoutly nothing to me at this moment.......... I'm sitting at my computer! Blues, J.E.James 4:8 Quote Share this post Link to post Share on other sites
billvon 2,400 #44 December 2, 2003 >Again, going from a 20 kts tailwind and turning into a 20 kts > headwind your canopy will not see an increase in airspeed nor > ground speed. Your airspeed will remain constant and ground speed > will be reduced, UNLESS the velcity and vector changes suddenly. Oh, I see what you're saying. Correct, if that 20kts doesn't change (i.e. there's no low level change from trees, the direction doesn't change) then your canopy will not really care about what direction the wind's coming from. I was talking more about what happens to a canopy that's turning as it goes through that change in wind you see at treetop level. Quote Share this post Link to post Share on other sites
kallend 1,623 #45 December 2, 2003 Quote Don't worry, even Einstein screwed up the distinction between scalars and vectors upon occasion (what's the value of V^2/C^?). Blue skies, Winsor Relativistic corrections now? That will confuse the issue.... 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 #46 December 2, 2003 Bill, Yes however, that is not a wind shear. I think we are on the same page now. Thanks. Blues, J.E.James 4:8 Quote Share this post Link to post Share on other sites
sducoach 0 #47 December 2, 2003 Hey Prof. I'm going to go scatter a few photons for Winsor. Say Hi to Dave up there at SDC for me. Blues, J.E.James 4:8 Quote Share this post Link to post Share on other sites
winsor 186 #49 December 2, 2003 QuoteAgain, going from a 20 kts tailwind and turning into a 20 kts headwind your canopy will not see an increase in airspeed nor ground speed. Your airspeed will remain constant and ground speed will be reduced, UNLESS the velcity and vector changes suddenly. Correct Winsor & Bill? What's the value of V^2/c^?. Absoutly nothing to me at this moment.......... First off, that should have been vee squared over cee squared (typo on my part), which is a vector operation (dot or cross product?) over a scalar operation. It's a mismatch that most people miss, and gives rise to much comic-book fizix. As far as turning in the headwind vs. tailwind scenario, it sounds like you're describing a turn in a continuous block of air, e.g., no wind shear. It is the wind shear that makes things interesting - and sometimes painful. If wind shear wasn't enough, dead air behind obstacles can result in rotors, which suck REGARDLESS of your direction of entry. Blue skies, Winsor Quote Share this post Link to post Share on other sites
darkwing 4 #50 December 2, 2003 Winsor, sorry, but v^2/c^2 is not a mismatch of vector and scalar. the v^2 is v.v, the dot product, which is a scalar quantity. Einstein had it under control. Physics guy is out. -- Jeff My Skydiving History Quote Share this post Link to post Share on other sites
