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#11
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Help calculating Speed To Fly for headwind and tailwind
On May 29, 6:41*pm, Dave Nadler wrote:
On May 29, 12:22*pm, John Cochrane wrote: Thoughts? 9B Here's my 2 cents. If you're racing, not maximizing glide over the ground, and if you're far from a turnpoint -- meaning you will certainly have to thermal before you get to the turnpoint -- then as everybody notes, the wind speed is irrelevant. That assumes that thermals drift with the wind, and are as easy to core going upwind as downwind. Thermals actually drift a bit slower than the wind, and are anchored to ground sources. That means that going upwind is harder; you're effectively in a lower-performing glider, so in fact you have to fly more cautiously. *I seem to have an easier time centering when going downwind as well; that may be because I hit the obvious core first rather than be seduced by the driblets off downwind of the core. I also seem to stay in contact with streets better going downwind. (In general, better performing gliders use slightly higher Mc settings, because they are less likely to get in trouble) But back to theory which ignores all this stuff. The calculations in "upwind/downwind" assume you're *near the turnpoint. Here you're making the decision "do I climb at x before the turnpoint or do I wait, round the turnpoint and climb at y?" *It's only valid if the latter is an option before hitting the ground! In any decent wind, it's surprising how much difference there is between x and y. On the other hand, the graph quantifies common sense: if you are in an 8 knot thermal and all the other thermals are 3 knots, take it even if it's upwind! *The rule of thumb about turning upwind low isn't always right. I bug the clearnav team to put these numbers in about once a week. When you're above glideslope to the next turnpoint, it could show the equivalent Mc "after the turn" to your current Mc. So far no luck, but they may correctly perceive that there are about 3 of us who understand and care about this number. Many people make the mistake of thinking wind affects final glide. It does not (except for the above meteorological considerations). There does come a point, gliding in to the wind, that lowering your MacCready setting actually results in a worse glide. You'll see that -- you get low, turn down the Mc, and all of a sudden you're even lower! ouch! If that isn't enough, you need a thermal, and the thermal has to be stronger than this minimum Mc setting. If you're going downwind, a slightly negative Mc setting will result in a better glide. *I also encourage my favorite insturment makers to not allow the Mc setting to go below the value that gives the best glide over the ground, and allow it to go slightly negative downwind. Again, I think they rightly perceive this as unnecessary nerdiness. In both cases, there really is no valid reason at all for cruising at a lower Mc setting than the weakest (smooth, bottom to top average, including all centering etc) thermal you'd take. Equivalently, if you're cruising at Mc 2 and the weather gods grant you a smooth, guaranteed 3 knot thermal, you're better off taking it and then cruising at Mc 3 for a while. This is very hard to swallow, but it's true. John Cochrane A common thread in this discussion is the need to know the ACTUAL average climb over an entire thermal. Otherwise you're typically plugging too high a number into McCready theory, which doesn't work. For you SN10 pilots, this is why the instruments I've designed prominently display TAv - Thermal Average... Use it ! Don't use the 20 second averager peak... For John, perhaps time to switch back ;-) And if you don't, you can still use the SN10 in my plane to plan your task on the ramp ;-) Hope this helps, Best Regards, Dave "YO electric" And SeeYou has the VarA navbox (20 sec average) and the more useful VarT (average since circling) navbox and on the statistics page shows last four thermal averages, as well as rough graphical guide of the entry and exit heights of those last four thermals (sometimes handy to remind you what the climb was) and the average vario for all four last thermals. The stats pages also show global flight average vario, on task average vario, and last 60 minute average vario. Sorry but I think that beats the SN10 vario stats. But the stats pages don't show how long pilots spend starting at all this stuff trying to work out what is what :-) I fly with a VarT navbox. Darryl |
#12
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Help calculating Speed To Fly for headwind and tailwind
On 2010/05/28 03:02 PM, Andy wrote:
On May 27, 10:49 pm, Tim wrote: Anyone have a good set of equations or example of how to do this simply? The fastest speed through the air mass will give the fastest speed over the ground. The wind does not change the speed to fly. It only impacts best glide speed to a landing. So add 0xW for a headwind and subtract 0xW for a tailwind. Andy I am nnot very good at this kind of arithmetic, so - Let's do a little mental flying. Your mount is a first generation standard class. (assume 1:37, L/D max at about 85km/h, minimum sink at around 70km/h) You are thermal flying in a fairly strong wind - say 30kt at 5000 AGL. Thermals are consequently quite broken at lower height and quite strongly angled downwind once they get organised at 3000AGL. The turnpoint (an assigned area with a nn km radius) is upwind of you - around 20km. You are at cloudbase with minimum VFR clearance. There are cloud streets - producing an average 1kt climb. The best embedded thermals are around 4kt. If you fly Mc 1 into a 30kt head component you are flying ~95km/h into a headwind of ~55km/h - so you are covering ground at 40km/h while descending at around 125ft/m. To cover the 20km is going to take 30 minutes in this situation. With a height loss of 30*125 = 3750feet. This will get you into the turn area at 1250feet AGL. Maybe this is not such a good situation to be in - low down you are out of the working band on the thermals, and lift is broken and difficult to work. So you decide to thermal at the bottom of the working band - and end up going backwards at ~40km/hour every 2000/125feet = 16min. In those 16 minutes you have covered approximately 5km. Assuming you connect one of those 4kt thermals it is going to take you about 6 minutes to centre and climb back up. In those 6 minutes you have traversed back at least 4km. So at best you have a nett gain of 10km. Turn in a 2kt thermal and you have made 5km, turn in 1tk and you are back at start. This is an example of there being a cross over on the MC/wind speed. At some point your ground covering performance deteriorates to the point that you have to go faster than theoretically economical to make headway. Bad math? Cheers Bruce --- news://freenews.netfront.net/ - complaints: --- |
#13
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Help calculating Speed To Fly for headwind and tailwind
On May 29, 9:22*am, John Cochrane
wrote: Thoughts? 9B Here's my 2 cents. If you're racing, not maximizing glide over the ground, and if you're far from a turnpoint -- meaning you will certainly have to thermal before you get to the turnpoint -- then as everybody notes, the wind speed is irrelevant. That assumes that thermals drift with the wind, and are as easy to core going upwind as downwind. Thermals actually drift a bit slower than the wind, and are anchored to ground sources. That means that going upwind is harder; you're effectively in a lower-performing glider, so in fact you have to fly more cautiously. *I seem to have an easier time centering when going downwind as well; that may be because I hit the obvious core first rather than be seduced by the driblets off downwind of the core. I also seem to stay in contact with streets better going downwind. (In general, better performing gliders use slightly higher Mc settings, because they are less likely to get in trouble) But back to theory which ignores all this stuff. The calculations in "upwind/downwind" assume you're *near the turnpoint. Here you're making the decision "do I climb at x before the turnpoint or do I wait, round the turnpoint and climb at y?" *It's only valid if the latter is an option before hitting the ground! In any decent wind, it's surprising how much difference there is between x and y. On the other hand, the graph quantifies common sense: if you are in an 8 knot thermal and all the other thermals are 3 knots, take it even if it's upwind! *The rule of thumb about turning upwind low isn't always right. I bug the clearnav team to put these numbers in about once a week. When you're above glideslope to the next turnpoint, it could show the equivalent Mc "after the turn" to your current Mc. So far no luck, but they may correctly perceive that there are about 3 of us who understand and care about this number. Many people make the mistake of thinking wind affects final glide. It does not (except for the above meteorological considerations). There does come a point, gliding in to the wind, that lowering your MacCready setting actually results in a worse glide. You'll see that -- you get low, turn down the Mc, and all of a sudden you're even lower! ouch! If that isn't enough, you need a thermal, and the thermal has to be stronger than this minimum Mc setting. If you're going downwind, a slightly negative Mc setting will result in a better glide. *I also encourage my favorite insturment makers to not allow the Mc setting to go below the value that gives the best glide over the ground, and allow it to go slightly negative downwind. Again, I think they rightly perceive this as unnecessary nerdiness. In both cases, there really is no valid reason at all for cruising at a lower Mc setting than the weakest (smooth, bottom to top average, including all centering etc) thermal you'd take. Equivalently, if you're cruising at Mc 2 and the weather gods grant you a smooth, guaranteed 3 knot thermal, you're better off taking it and then cruising at Mc 3 for a while. This is very hard to swallow, but it's true. John Cochrane Okay I modelled this out just to see what the speed to fly implications are - that is, what's the optimal STF going into an upwind/downwind turnpoint depending on whether you thermal before or after the turn and what kind of penalty do you pay flying off the optimal speed. First, it confirms John's theory on how picky you should be about thermals and the consequences of going into a turnpoint low versus high depending on the wind. If the there is strong wind it matters A LOT how you do this. Just by way of a real world example - a couple of years ago I stretched a glide into an upwind turnpoint on a contest day when the winds were 30 knots on the nose, then caught a strong thermal on the following downwind leg. The math shows that this cut my total task time by more than 12 minutes! If you assume 20 miles of glide into the turn, the time savings vary from 3 minutes for 15 knots of wind up to 12 minutes for the aforementioned 30 knots of wind. The other surprising thing is how much the optimal speed to fly varies depending on whether you are planning to take one more thermal before the turn or wait until after - the speed differences can be more than 20 knots, though the penalties for flying a bit too slow are only a couple of mph. Note to BB: when you did the theory for optimal MAT strategies did you include the high/low turnpoint strategy for the up/downwind versus crosswind courses?. If you assume even a few thousand feet of high/low strategy I think the up/downwind approach may dominate the crosswind one. I think it may also pay to up the number of legs to maximize the number of times you can play the high/low turnpoint game per mile of task. Try programming THAT into a ClearNav. :-) 9B |
#14
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Help calculating Speed To Fly for headwind and tailwind
Note to BB: when you did the theory for optimal MAT strategies did you include the high/low turnpoint strategy for the up/downwind versus crosswind courses?. If you assume even a few thousand feet of high/low strategy I think the up/downwind approach may dominate the crosswind one. I think it may also pay to up the number of legs to maximize the number of times you can play the high/low turnpoint game per mile of task. Try programming THAT into a ClearNav. *:-) 9B Yeah, the theory that says crosswind is better ignores streeting (or sink streeting!) and the option to turn upwind low and downwind high. The best MAT is probably to find three turnpoints in an up/downwind line, and do all your thermaling going downwind. Also final glide into the wind. Another good reason to hope CDs don't call unrestricted MATs. That's not a lot of fun. John Cochrane |
#15
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Help calculating Speed To Fly for headwind and tailwind
On May 29, 9:22*am, John Cochrane
wrote: Many people make the mistake of thinking wind affects final glide. It does not (except for the above meteorological considerations). There does come a point, gliding in to the wind, that lowering your MacCready setting actually results in a worse glide. You'll see that -- you get low, turn down the Mc, and all of a sudden you're even lower! ouch! I was involved in an animated discussion about this just last weekend when a group of us was dining out after a fine day of soaring. I was assured that at least one popular glide computer does take wind into account for final glide and that a setting MC=0 would always give the best range glide solution for the known headwind or tailwind. In other words the glide computer is finding the MC setting that gives max range (more than zero for a headwind and less that zero for a tail wind) and displaying that as the zero MC setting. I maintained that I never had a glide computer that did that, but admitted it was possible that someone had implemented it that way. So designers, or users, of popular glider computers - Does your instrument give max range at MC=0 regardless of wind or do you have to adjust MC for the minimum vaue of required altitude? |
#16
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Help calculating Speed To Fly for headwind and tailwind
On Jun 1, 11:44*am, Andy wrote:
On May 29, 9:22*am, John Cochrane wrote: Many people make the mistake of thinking wind affects final glide. It does not (except for the above meteorological considerations). There does come a point, gliding in to the wind, that lowering your MacCready setting actually results in a worse glide. You'll see that -- you get low, turn down the Mc, and all of a sudden you're even lower! ouch! I was involved in an animated discussion about this just last weekend when a group of us was dining out after a fine day of soaring. *I was assured that at least one popular glide computer does take wind into account for final glide and that a setting MC=0 would always give the best range glide solution for the known headwind or tailwind. *In other words the glide computer is finding the MC setting that gives max range (more than zero for a headwind and less that zero for a tail wind) and displaying that as the zero MC setting. *I maintained that I never had a glide computer that did that, but admitted it was possible that someone had implemented it that way. So designers, or users, of popular glider computers - Does your instrument give max range at MC=0 regardless of wind or do you have to adjust MC for the minimum vaue of required altitude? That's a poor solution. A sticky knob that won't let you go below max range would be better. If it shows Mc 0 but it's really set at (say) Mc 2 because you're in a howling headwind, you need to know to cruise at Mc 2 and not to take any 1.9 kt thermals. If the indicator shows Mc 0 you don't know that John Cochrane |
#17
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Help calculating Speed To Fly for headwind and tailwind
On Jun 1, 9:52*am, John Cochrane
wrote: On Jun 1, 11:44*am, Andy wrote: On May 29, 9:22*am, John Cochrane wrote: Many people make the mistake of thinking wind affects final glide. It does not (except for the above meteorological considerations). There does come a point, gliding in to the wind, that lowering your MacCready setting actually results in a worse glide. You'll see that -- you get low, turn down the Mc, and all of a sudden you're even lower! ouch! I was involved in an animated discussion about this just last weekend when a group of us was dining out after a fine day of soaring. *I was assured that at least one popular glide computer does take wind into account for final glide and that a setting MC=0 would always give the best range glide solution for the known headwind or tailwind. *In other words the glide computer is finding the MC setting that gives max range (more than zero for a headwind and less that zero for a tail wind) and displaying that as the zero MC setting. *I maintained that I never had a glide computer that did that, but admitted it was possible that someone had implemented it that way. So designers, or users, of popular glider computers - Does your instrument give max range at MC=0 regardless of wind or do you have to adjust MC for the minimum vaue of required altitude? That's a poor solution. A sticky knob that won't let you go below max range would be better. If it shows Mc 0 but it's really set at (say) Mc 2 because you're in a howling headwind, you need to know to cruise at Mc 2 and not to take any 1.9 kt thermals. If the indicator shows Mc 0 you don't know that John Cochrane- Hide quoted text - - Show quoted text - |
#18
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Help calculating Speed To Fly for headwind and tailwind
On Jun 1, 9:52*am, John Cochrane
wrote: That's a poor solution. A sticky knob that won't let you go below max range would be better. If it shows Mc 0 but it's really set at (say) Mc 2 because you're in a howling headwind, you need to know to cruise at Mc 2 and not to take any 1.9 kt thermals. If the indicator shows Mc 0 you don't know that Agree, but he seemed very sure that he had been told by his instrument designer that MC zero would alway yield max range glide since the computer knew the wind and would take account of it. Maybe I should ask Dave directly since the SN10 was the instrument in question. Andy |
#19
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Help calculating Speed To Fly for headwind and tailwind
On Jun 1, 2:01*pm, Andy wrote:
On Jun 1, 9:52*am, John Cochrane wrote: That's a poor solution. A sticky knob that won't let you go below max range would be better. If it shows Mc 0 but it's really set at (say) Mc 2 because you're in a howling headwind, you need to know to cruise at Mc 2 and not to take any 1.9 kt thermals. If the indicator shows Mc 0 you don't know that Agree, but he seemed very sure that he had been told by his instrument designer that MC zero would alway yield max range glide since the computer knew the wind and would take account of it. Maybe I should ask Dave directly since the SN10 was the instrument in question. Andy Hmm.... I wonder what my PDA program does in this case? Anyway, final glide is in table form for me (Soarpilot), which shows me glide slope numbers compared with different airspeeds. When there's a non-zero MC value for best glide it's apparent on that table. Example screen shown he http://www.soaringpilot.org/dokuwiki...l_glide_screen |
#20
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Help calculating Speed To Fly for headwind and tailwind
Agree, but he seemed very sure that he had been told by his instrument designer that MC zero would alway yield max range glide since the computer knew the wind and would take account of it. Maybe I should ask Dave directly since the SN10 was the instrument in question. Andy I pity our poor instrument designers. If they do nothing, mad pilots will write in "something's wrong with your instrument, as I lower Mc setting it shows me a worse glide angle!" If they make Mc=0 always the glide extending setting, mad pilots write in with "something's wrong, I select Mc 0 and it's telling me to fly 70 knots." If they set a sticky knob that won't go below the glide-maximizing Mc value, mad pilots will write with "something's wrong, I can't lower the Mc setting." John Cochrane |
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