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required LD versus required MC to make it home ??
Hi All,
I'm aware of and use the math formula to get my rental Cirrus back to home base, but I like to back it up with SeeYou mobile. I know it's recommended to use required LD to target making sure you have entered a correct polar and safety altitude. But this doesn't account for winds does it? If you are flying away from your target wondering how far you can safely fly, you can't depend on required LD because big headwinds can make this number useless. As an example, I notice I've got 25LD required to my home base. I turn around and because of the headwinds, I can only make 18LC. Outlanding anyone. I'm curious about MC required to target. Wouldn't that be better to use if you make sure all data is correct such as polar, winds, safety altitude and make sure the correct target is activated. This way, I can wander away from my home field and I know if my MC doesn't fall below about say 7 (which plays out to about 20 LD in no wind) I am fairly assured of making it and that this MC will be wind aware. Of course it can't know about hitting lots of sink, but it seems a better way for my type of non task, local soaring. Before I finish, I would like to note that the MC to target NavBox in SeeYouM doesn't always update very quickly if you change the winds aloft manually. For this problem, I scroll the MC value untill the little glide slope type indicater on the left side of SeeYou centers, then compare that MC to the required MC NavBox. akiley |
#2
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required LD versus required MC to make it home ??
On Aug 22, 7:10*pm, akiley wrote:
Hi All, I'm aware of and use the math formula to get my rental Cirrus back to home base, but I like to back it up with SeeYou mobile. *I know it's recommended to use required LD to target making sure you have entered a correct polar and safety altitude. *But this doesn't account for winds does it? *If you are flying away from your target wondering how far you can safely fly, you can't depend on required LD because big headwinds can make this number useless. *As an example, I notice I've got 25LD required to my home base. *I turn around and because of the headwinds, I can only make 18LC. *Outlanding anyone. I'm curious about MC required to target. *Wouldn't that be better to use if you make sure all data is correct such as polar, winds, safety altitude and make sure the correct target is activated. *This way, I can wander away from my home field and I know if my MC doesn't fall below about say 7 (which plays out to about 20 LD in no wind) I am fairly assured of making it and that this MC will be wind aware. *Of course it can't know about hitting lots of sink, but it seems a better way for my type of non task, local soaring. Before I finish, I would like to note that the MC to target NavBox in SeeYouM doesn't always update very quickly if you change the winds aloft manually. *For this problem, I scroll the MC value untill the little glide slope type indicater on the left side of SeeYou centers, then compare that MC to the required MC NavBox. akiley What is "the math formula". I am aware of many different math formulas, including many for calculating/estimating glider performance/ navigation. But what are you using? Required Mc is a kind of noisy number, especially if you think the difference between two large numbers helps you much. It is sensitive to high speed polar data and if you tried to fly it in a rental glider with an unknown actual polar without a lot of experience at pushing it is likely meaningless. For recreational flying, unless you are racing with lots of experience, I would focus less on twiddling Mc (or virtually twiddling with SeeYou Mobile telling you its Mc estimate to goal) and more on L/ D achieved and L/D required as one data pair and on arrival height as another. Arrival height factors in wind, uses the polar, bugs, Mc. Set some sane low Mc near what you actually fly at. Pad the polar with %bugs (start with max of 30% if new to XC) and have an arrival safety height (at least your usual pattern height, more when starting). You can try adjusting it at times and see what it does to your arrival height but if you are at the stage it sounds like mostly leave it set and don't go chasing large Mc numbers. Hide the navbox, there are better things to look at. In fact hide almost everything, except the two L/D boxes and arrival height and use the wind indicator on the main map to check it looks sane. And forget the rest, including the silly glideslope display, I cannot think of anybody who really uses that thing (oops now we'll hear from them...). Required L/D to target tells you what you need to achieve. It makes no sense to fold wind into that, its just the distance divided by the difference in height. Achieved L/D tells you what you are getting obviously with wind affects as well, all without any assumptions about polars, mass, bugs, or wind. That is the beauty of working with L/D required and achieved. But even better than asking on r.a.s. can you find a local accomplished XC pilot(s) who can mentor you on all this stuff? Darryl |
#3
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required LD versus required MC to make it home ??
Required L/D to target tells you what you need to achieve. It makes no sense to fold wind into that, its just the distance divided by the difference in height. Achieved L/D tells you what you are getting obviously with wind affects as well, all without any assumptions about polars, mass, bugs, or wind. That is the beauty of working with L/D required and achieved. But even better than asking on r.a.s. can you find a local accomplished XC pilot(s) who can mentor you on all this stuff? Darryl- Darry is spot on. I would go a little further and dispense with the Achieved L/D - I just use L/D required and watch for the trend: if it is getting better (lower L/D required) then you are gaining on the glide and can either speed up or relax more. If it's getting worse, or not changing and looks a bit high (say more than half your published L/D), then you need to stop and get some altitude. That takes care of the wind, bugs, etc. Totally agree with getting rid of all the navboxes that are "info only" - unless your PDA is hooked up to a 302 and getting air data, using GPS for fancy speed to fly info is a distraction. Use it as a digital sectional, with your task, airspace, and landable fields (with L/D required) on it, and in most cases turn off the terrain (exception is in ridge country where the terrain can be really useful). Less is more! Cheers, Kirk 66 |
#4
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required LD versus required MC to make it home ??
Well, just for a contrary opinion, I disagree with Darryl and Kirk. Mc
setting is the right set of units for everything in soaring. If you must think about glide angles, the right units are D/L not L/ D. L/D goes through infinity when you run in to lift. D/L (feet per mile, meters per kilometer) does not. If you gain 200 feet in lift vs. lose 200 feet in lift, L/D shows radically different changes, D/L does not. The "safety profile" for making it to a goal with constant (say 99%) probability follows a roughly square root function of distance. (Square root follows if lift/sink are independent over distance) Most of us approximate this with a relatively high Mc setting (3-4) plus a reserve altitude. Smoother conditions -- less lift or sink -- means less uncertainty about your glide. So, paradoxically, you can use more aggressive safety settings if there is no lift around, because then there is no sink around. Strong lift mans strong sink; half chance of escaping in 10 knots, half chance of hitting the dirt in 10 knot sink. Therefore, use a higher Mc setting and higher reserve altitude with stronger lift/ sink or general uncertainty. To fly a safety glide you want to have the glide computer at a high Mc setting, but fly slowly and accept weaker lift. Many pilots disconnect the glide computer from the vario for this reason. Well, I do. Instrument makers should recognize this difference and make it easier to have a different Mc for glide than vario. Wind is irrelevant here, with one exception. As you lower the Mc setting heading upwind, you will discover a point at which lower Mc setttings seem to make it worse. This is a featuer not a bug. The best glide in wind occurs at a higher Mc setting. don't fly slower than that, don't take weaker thermals than that, or you wont get home John Cochrane |
#5
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required LD versus required MC to make it home ??
On Aug 23, 6:40*am, "kirk.stant" wrote:
Required L/D to target tells you what you need to achieve. It makes no sense to fold wind into that, its just the distance divided by the difference in height. Achieved L/D tells you what you are getting obviously with wind affects as well, all without any assumptions about polars, mass, bugs, or wind. That is the beauty of working with L/D required and achieved. But even better than asking on r.a.s. can you find a local accomplished XC pilot(s) who can mentor you on all this stuff? Darryl- Darry is spot on. *I would go a little further and dispense with the Achieved L/D - I just use L/D required and watch for the trend: if it is getting better (lower L/D required) then you are gaining on the glide and can either speed up or relax more. *If it's getting worse, or not changing and looks a bit high (say more than half your published L/D), then *you need to stop and get some altitude. *That takes care of the wind, bugs, etc. Totally agree with getting rid of all the navboxes that are "info only" - unless your PDA is hooked up to a 302 and getting air data, using GPS for fancy speed to fly info is a distraction. *Use it as a digital sectional, with your task, airspace, and landable fields (with L/D required) on it, and in most cases turn off the terrain (exception is in ridge country where the terrain can be really useful). *Less is more! Cheers, Kirk 66 I generally use arrival altitude for everything, especially final glide. That way I know how much I need to climb to get to my goal and wind is accounted for automatically in the computer. I typically program in 1,000' for arrival altitude and speed up/slow down depending on whether the arrival height is building or declining. Typically I dial in 4 knots for the computation because it corresponds to a typical cruise speed. I try not to set below 3 knots unless it's a last resort. Except on very long glides low Mc settings just don't yield enough glide angle margin - a little sink and you're at best L/D or can't make it at all. 9B |
#6
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required LD versus required MC to make it home ??
On Aug 23, 7:02*am, Andy wrote:
On Aug 23, 6:40*am, "kirk.stant" wrote: Required L/D to target tells you what you need to achieve. It makes no sense to fold wind into that, its just the distance divided by the difference in height. Achieved L/D tells you what you are getting obviously with wind affects as well, all without any assumptions about polars, mass, bugs, or wind. That is the beauty of working with L/D required and achieved. But even better than asking on r.a.s. can you find a local accomplished XC pilot(s) who can mentor you on all this stuff? Darryl- Darry is spot on. *I would go a little further and dispense with the Achieved L/D - I just use L/D required and watch for the trend: if it is getting better (lower L/D required) then you are gaining on the glide and can either speed up or relax more. *If it's getting worse, or not changing and looks a bit high (say more than half your published L/D), then *you need to stop and get some altitude. *That takes care of the wind, bugs, etc. Totally agree with getting rid of all the navboxes that are "info only" - unless your PDA is hooked up to a 302 and getting air data, using GPS for fancy speed to fly info is a distraction. *Use it as a digital sectional, with your task, airspace, and landable fields (with L/D required) on it, and in most cases turn off the terrain (exception is in ridge country where the terrain can be really useful). *Less is more! Cheers, Kirk 66 I generally use arrival altitude for everything, especially final glide. *That way I know how much I need to climb to get to my goal and wind is accounted for automatically in the computer. I typically program in 1,000' for arrival altitude and speed up/slow down depending on whether the arrival height is building or declining. Typically I dial in 4 knots for the computation because it corresponds to a typical cruise speed. I try not to set below 3 knots unless it's a last resort. Except on very long glides low Mc settings just don't yield enough glide angle margin - a little sink and you're at best L/D or can't make it at all. 9B I do the same with arrival altitude, only am more conservative than Andy. I increase or decrease MacCready accordingly. It seems the only rational piece of data you need - arrival altitudes of less than zero are likely to be less than useful. Mike |
#7
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required LD versus required MC to make it home ??
On Aug 23, 12:32*am, Darryl Ramm wrote:
On Aug 22, 7:10*pm, akiley wrote: Hi All, I'm aware of and use the math formula to get my rental Cirrus back to home base, but I like to back it up with SeeYou mobile. *I know it's recommended to use required LD to target making sure you have entered a correct polar and safety altitude. *But this doesn't account for winds does it? *If you are flying away from your target wondering how far you can safely fly, you can't depend on required LD because big headwinds can make this number useless. *As an example, I notice I've got 25LD required to my home base. *I turn around and because of the headwinds, I can only make 18LC. *Outlanding anyone. I'm curious about MC required to target. *Wouldn't that be better to use if you make sure all data is correct such as polar, winds, safety altitude and make sure the correct target is activated. *This way, I can wander away from my home field and I know if my MC doesn't fall below about say 7 (which plays out to about 20 LD in no wind) I am fairly assured of making it and that this MC will be wind aware. *Of course it can't know about hitting lots of sink, but it seems a better way for my type of non task, local soaring. Before I finish, I would like to note that the MC to target NavBox in SeeYouM doesn't always update very quickly if you change the winds aloft manually. *For this problem, I scroll the MC value untill the little glide slope type indicater on the left side of SeeYou centers, then compare that MC to the required MC NavBox. akiley What is "the math formula". I am aware of many different math formulas, including many for calculating/estimating glider performance/ navigation. But what are you using? Thanks for the good advice Darryl, My personal math formula is that, without winds, I want to be 15:1 L/D from my home airport, (not a cross country pilot yet) with a 1000 foot safety buffer. So this is 400 feet per NM. So for 5 miles, it's just 400x5=2000 feet AGL plus 1000 safety = 3000AGL. If there is a 5 knot headwind on the return home, I would divide the Cirrus 46kt best LD speed by 41 to get 0.9. That .9 can be divided by my 400 feet per NM to get 445 feet per NM for my new calculated L/D to target. I guess there are other factors, but this is ballpark for me. When I start to get 20 L/D from home I start to get nervous. But my formula is roughly based on one half my best LD plus a bit. I can't remember which book I got my math from, but it's airspeed plus or minus winds divided by airspeed which gives you a number to modify your L/D. Then divide 6000 by the modified L/D to get feet per NM required to reach your target, then add any safety altitude you desire to that agl altitude. Required Mc is a kind of noisy number, especially if you think the difference between two large numbers helps you much. It is sensitive to high speed polar data and if you tried to fly it in a rental glider with an unknown actual polar without a lot of experience at pushing it is likely meaningless. For recreational flying, unless you are racing with lots of experience, I would focus less on twiddling Mc (or virtually twiddling with SeeYou Mobile telling you its Mc estimate to goal) and more on L/ D achieved and L/D required as one data pair and on arrival height as another. Arrival height factors in wind, uses the polar, bugs, Mc. Set some sane low Mc near what you actually fly at. Pad the polar with %bugs (start with max of 30% if new to XC) and have an arrival safety height (at least your usual pattern height, more when starting). You can try adjusting it at times and see what it does to your arrival height but if you are at the stage it sounds like mostly leave it set and don't go chasing large Mc numbers. Hide the navbox, there are better things to look at. In fact hide almost everything, except the two L/D boxes and arrival height and use the wind indicator on the main map to check it looks sane. And forget the rest, including the silly glideslope display, I cannot think of anybody who really uses that thing (oops now we'll hear from them...). Well, I've flown long enough to know not to trust electronics. I have 600 power hours using all sorts of navigators. Funny, when you use a Garmin 396 on a computer, it sets magnetic variation to user set instead of auto. SeeYou has quite a few bugs and gochas too. So my primary is look at the down angle back to the airport. I do monitor arrival height, but it's really just another way of looking at MC as far as I can tell. The little glide slope indicator is also sort of another way of looking at your MC to target. I figure if I keep an eye on all of those, I'm less likely to trip over a bug in SeeYou which I've found several of. Required L/D to target tells you what you need to achieve. It makes no sense to fold wind into that, its just the distance divided by the difference in height. Achieved L/D tells you what you are getting obviously with wind affects as well, all without any assumptions about polars, mass, bugs, or wind. That is the beauty of working with L/D required and achieved. This is a good point, but since wind isn't factored into required L/D, you don't know what your achieved L/D is unless you turn around and head back to the airport. But it does seem to be safer and more straight forward. And I suppose since you are always aware of the winds, you can make a fairly accurate guess as to what you achieved L/ D is likely to be. If it's a straight headwind home at 5 knots, I could just mentally modify what I expect to achieve. But even better than asking on r.a.s. can you find a local accomplished XC pilot(s) who can mentor you on all this stuff? Yes, my club has several and I'm talking to them too. It's also funny about gadgets in aircraft. My feeling is learn to use the autopilot and whenever you can, learn navigators using simulators. Half learning electronics is the most dangerous in my opinion. I enjoy navigators, but I'm strict as to when and how to use them. Darryl |
#8
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required LD versus required MC to make it home ??
On Aug 22, 10:10*pm, akiley wrote:
Hi All, I'm aware of and use the math formula to get my rental Cirrus back to home base, but I like to back it up with SeeYou mobile. *I know it's recommended to use required LD to target making sure you have entered a correct polar and safety altitude. *But this doesn't account for winds does it? *If you are flying away from your target wondering how far you can safely fly, you can't depend on required LD because big headwinds can make this number useless. *As an example, I notice I've got 25LD required to my home base. *I turn around and because of the headwinds, I can only make 18LC. *Outlanding anyone. I'm curious about MC required to target. *Wouldn't that be better to use if you make sure all data is correct such as polar, winds, safety altitude and make sure the correct target is activated. *This way, I can wander away from my home field and I know if my MC doesn't fall below about say 7 (which plays out to about 20 LD in no wind) I am fairly assured of making it and that this MC will be wind aware. *Of course it can't know about hitting lots of sink, but it seems a better way for my type of non task, local soaring. Before I finish, I would like to note that the MC to target NavBox in SeeYouM doesn't always update very quickly if you change the winds aloft manually. *For this problem, I scroll the MC value untill the little glide slope type indicater on the left side of SeeYou centers, then compare that MC to the required MC NavBox. akiley It's probably also worth noting here that the Std Cirrus had some of the least accurate static sources (under the wing) ever put on a glider, so the accuracy of the ASI and any computer attempting to calculate wind info using ASI will be compromised. Symptoms would be significant changes in reported wind when you change heading or ground track. At cruise speeds, Std Cirrus system reads higher than calibrated airspeed, this should show as a headwind component that's always there. There's a technical solution (tailcone statics), but it will be at odds with your certification paperwork. I'll leave the teaching of final glides via r.a.s. to the others :-). -T8 |
#9
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required LD versus required MC to make it home ??
On Aug 23, 10:00*am, John Cochrane
wrote: Well, just for a contrary opinion, I disagree with Darryl and Kirk. Mc setting is the right set of units for everything in soaring. If you must think about glide angles, the right units are D/L not L/ D. *L/D goes through infinity when you run in to lift. D/L (feet per mile, meters per kilometer) does not. If you gain 200 feet in lift vs. lose 200 feet in lift, L/D shows radically different changes, D/L does not. The "safety profile" for making it to a goal with constant (say 99%) probability follows a roughly square root function of distance. (Square root follows if lift/sink are independent over distance) *Most of us approximate this with a relatively high Mc setting (3-4) plus a reserve altitude. Smoother conditions -- less lift or sink -- means less uncertainty about your glide. So, paradoxically, you can use more aggressive safety settings if there is no lift around, because then there is no sink around. Strong lift mans strong sink; half chance of escaping in 10 knots, half chance of hitting the dirt in 10 knot sink. Therefore, use a higher Mc setting and higher reserve altitude with stronger lift/ sink or general uncertainty. To fly a safety glide you want to have the glide computer at a high Mc setting, but fly slowly and accept weaker lift. Many pilots disconnect the glide computer from the vario for this reason. Well, I do. Instrument makers should recognize this difference and make it easier to have a different Mc for glide than vario. Wind is irrelevant here, with one exception. As you lower the Mc setting heading upwind, you will discover a point at which lower Mc setttings seem to make it worse. This is a featuer not a bug. The best glide in wind occurs at a higher Mc setting. don't fly slower than that, don't take weaker thermals than that, or you wont get home John Cochrane I did a little calculation for a standard Cirrus with no wind. I did this by using SeeYou mobile in simulator mode. I did manual math for angle and feet per NM. Here are a few numbers. They might not format correctly. Required L/D 38, 28, 20, 15, 10 Required MC 0, 3.5, 7.4, 12, 22 glide angle deg 1.7, 2.2, 2.9, 3.8, 5.7 feet per NM 158, 215, 300, 400, 600 Then I added a big headwind. Required L/D stays the same, but MC corrects for winds. ... akiley |
#10
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required LD versus required MC to make it home ??
On Aug 23, 7:00*am, John Cochrane
wrote: If you must think about glide angles, the right units are D/L not L/ D. *L/D goes through infinity when you run in to lift. D/L (feet per mile, meters per kilometer) does not. No expression using the terms lift and drag is appropriate for defining a ground referenced flight path angle. The proper term is, surprise, flight path angle. It has been used in the aerospace industry for many, many, years. It has a range +/- 90 degrees. Required FPA describes the required geometry to reach the goal and instantaneous FPA will descibe the current glider flight path accurately whether climbing or descending. Required and instantaneous FPA are just as easily calculated as the incorrect term L/D or the misleadingly named term "efficiency". Where did the use of L/D (the ratio of lift to drag) to describe a ground referenced flight path angle originate? I know it has been perpetuated by SeeYou, but did they start it? Andy (GY) |
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