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#21
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Article on glide strategy
Interesting approach.
I for myself use the MC for optimizing cruising, and I work with required L/D for safety - exclusively. The good thing about required L/D is that there are no assumptions whatsoever to it, it's plain geometry. In Alpine soaring (which I've been doing for the last couple of thousand hours), with my 47:1 ship I feel safe with a required L/D somewhere between 20 and 25, and unsafe above 30. That still depends on meterological conditions (end of day vs high winds...). I once had 20:1 in a 40:1 ship and I didn't make it :-( |
#22
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Article on glide strategy
At 15:23 07 March 2012, John Cochrane wrote:
On Mar 1, 9:36=A0am, Nine Bravo wrote: Thanks John. I tend to think about it slightly differently. On the one hand I think ab= out worst-case, inescapable sink for the conditions as measured by minutes = times sink rate (result - a fixed number of feet that I am at risk of losin= g - I don't generally consider a string of sink occurrences - I assume one = low probability sink patch is worse than multiple, higher probability sink = patches and that multiple low probability events aren't likely enough to wo= rry about). On the other hand I think about the probability of being able t= o find lift to recover after hitting a sink patch - which is a function of = altitude above ground. On very short final glides the constraint is the rate of sink (not much g= lide time left), on longer final glides the constraint is the probability o= f finding decent lift to get back up. The pinch point for me tends to be ar= ound 25 miles out - chances of finding lift are declining, odds of a long o= f stretch of sink still decent. This seems roughly consistent with your squ= are root rule, though the math is different. Your square root rule breaks down for me on very long final glides where = I tend to optimize more around trying to transition away from climb and gli= de to cruise-climb in an effort to avoid thermal centering losses. This tra= nslates to an altitude buffer of maybe -1000 feet on a 50 mile "final glide= " that you are trying to "bump-up" to +1000 feet by the time you get to 25 = miles out. Not sure if/how that figures in your analysis. 9B Good thoughts. But just to clarify, the article is not about final glides -- how to do it efficiently. The article is about safety margins -- how to do it with x percent chance of landing out. John Cochrane Very good thoughs both John and Andy, I tend to think of it more like a funnel. The further out the closer I can be to my actual glide ratio and the closer in i need way bigger margin. The funnel idea makes me put the "art of final glide" into a logrithmic equation in my head and is instantly scalable and movable to alternate landing sites. Especially flying out west where most of the airports that I fly at do not have any safe landing spots close by and the last few miles need to have extra high safety margins. Thanks for sharing CH |
#23
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Article on glide strategy
In addition to considering the probability of a landout, you have to take into consideration the consequences of one. If your final glide is over a landscape of neatly mowed large fields, the consequences of a landout are trivial and you can push closer to the theoretical limits. As Cliff says, many western sites have very poor options and carrying extra height, perhaps even more than mathematically indicated, might be wise.
I once got very very low over Zion National Park in Utah and have carried higher safety margins ever since! Mike |
#24
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Article on glide strategy
On Mar 7, 12:08*pm, Mike the Strike wrote:
In addition to considering the probability of a landout, you have to take into consideration the consequences of one. *If your final glide is over a landscape of neatly mowed large fields, the consequences of a landout are trivial and you can push closer to the theoretical limits. *As Cliff says, many western sites have very poor options and carrying extra height, perhaps even more than mathematically indicated, might be wise. I once got very very low over Zion National Park in Utah and have carried higher safety margins ever since! Mike Exactly. In the paper analysis, you adjust the "probability of not making it" parameter according to the consequences of a landout. The profile to follow over mowed fields, where the costs are inconvenience or contest points, is very different than the profile to follow over Zion. They're both square roots but the Zion profile is much higher up -- in the range of MacCready settings you may never have used before. That's one of the big points. We get used to Mc 3 or so glides and that they almost always work out. Over Zion, that experience is not good enough. To an earlier comment that experience trumps analysis, well, you don't want to be the guy that learns about how often Mc 1 glides work out by experience! John |
#25
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Article on glide strategy
On Wednesday, March 7, 2012 11:34:23 AM UTC-8, John Cochrane wrote:
On Mar 7, 12:08*pm, Mike the Strike wrote: In addition to considering the probability of a landout, you have to take into consideration the consequences of one. *If your final glide is over a landscape of neatly mowed large fields, the consequences of a landout are trivial and you can push closer to the theoretical limits. *As Cliff says, many western sites have very poor options and carrying extra height, perhaps even more than mathematically indicated, might be wise. I once got very very low over Zion National Park in Utah and have carried higher safety margins ever since! Mike Exactly. In the paper analysis, you adjust the "probability of not making it" parameter according to the consequences of a landout. The profile to follow over mowed fields, where the costs are inconvenience or contest points, is very different than the profile to follow over Zion. They're both square roots but the Zion profile is much higher up -- in the range of MacCready settings you may never have used before. That's one of the big points. We get used to Mc 3 or so glides and that they almost always work out. Over Zion, that experience is not good enough. To an earlier comment that experience trumps analysis, well, you don't want to be the guy that learns about how often Mc 1 glides work out by experience! John Excellent article and excellent thread. John, I am curious to hear your opinion about using higher MC vs degrading the polar (aka bug factor) for safety as some do instead or in addition. Ramy |
#26
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Article on glide strategy
Excellent article and excellent thread. John, I am curious to hear your opinion about using higher MC vs degrading the polar (aka bug factor) for safety as some do instead or in addition. Ramy I don't think using the bugs setting to calculate glides in real time is that helpful. I never know what percent bugs means. I do think it would be useful if our instrument makers could allow us to input lift or sink. I'd like to input, "MacCready 3, 500 foot reserve, and 100 fpm sink." Or when doing a final glide in Uvalde, "MacCready 3, 500 foot reserve, and 100 fpm lift." (Clearnav has a few emails from me on this!) This simply shifts the polar curve up and down by the given lift and sink, and would be easy for them to program. For contest final glides, by keeping track of average netto in the last few legs you could have an idea of lift/sink to be expected on final glide, and then bump up / be cautious accordingly. For safety reasons this would be very educational. You'd see directly just how disastrous small bits of extra sink can be on your glideslope. I also think many pilots would find it easier to take the advice "assume 200 fpm sink all the way to your safest landing" than they would to take the advice "input Mc 10 into your glide computer." The former sounds reasonable, the latter outlandish based on cross- country experience, yet they are the same thing. Good point -- I'll add this to the article. John |
#27
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Article on glide strategy
Super interesting. Thankyou!
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#28
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Article on glide strategy
Still important, but perhaps for a separate thread, is a strategy for escaping from a line of sink rather than just dealing with the mathematical consequences.
As I noted earlier, lift and sink lines are frequently aligned and knowledge of the relative heading of the glider along or across these lines would give the pilot useful information for an avoidance strategy. You can see the forecast predictions of any such lift/sink lines on the RASP Boundary Layer Up/Down Motion or the equivalent HRRR field "Mean Layer Vertical Velocity". Changing your final glide heading from a line of sink into a line of lift might have more benefit than accepting the inevitable and dialing up doom on your flight computer! Mike |
#29
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Article on glide strategy
On Mar 8, 9:55*am, Mike the Strike wrote:
Still important, but perhaps for a separate thread, is a strategy for escaping from a line of sink rather than just dealing with the mathematical consequences. As I noted earlier, lift and sink lines are frequently aligned and knowledge of the relative heading of the glider along or across these lines would give the pilot useful information for an avoidance strategy. You can see the forecast predictions of any such lift/sink lines on the RASP Boundary Layer Up/Down Motion or the equivalent HRRR field "Mean Layer Vertical Velocity". Changing your final glide heading from a line of sink into a line of lift might have more benefit than accepting the inevitable and dialing up doom on your flight computer! Mike Yes. In rivers of blue sink, I often just head 90 degrees off course and wait. Put another way, though, you have to use a much higher glide slope (MacCready value + reserve) for safety spots that are upwind/downwind or aligned with lift/sink streets than for safety spots that are crosswind or not so aligned. The airports are where they are, so going crosswind isn't always an option. John Cochrane |
#30
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Article on glide strategy
Great article, John, and another valuable service (as usual)! I like
to think it analyzes and explains what many pilots have been doing somewhat intuitively (based on experience) but I suspect we all have our own style. Often I will leave the MC set as speed to fly but keep climbing in what may be the last thermal. If the climb rate is above MC, I just keep climbing, of course. As the thermal weakens and they converge and I'm now above glide path, I bump the MC setting up enough to push the final glide margin down to zero (though I've still got my 300 or 500 goal margin set) to see how "fat" I am. I'm frequently surprised at how quickly the MC settings increase to what seem to be absurd levels in the East, which is another way of saying that using very high MC settings doesn't give you nearly the unassailable safety margin (in feet) you might think. I usually don't fly that absurdly high MC setting until I'm fairly close in (and/or over landable terrain or with good clouds and/or markers ahead to pick up a little extra). In fact, I'll switch back and forth and to see what the MC setting is that would take me all the way home and whether it's increasing or decreasing vs. what is my margin over final glide at cruise MC and whether that's increasing or decreasing. No wonder final glides are so confusing. This argues for decoupling final glide MC from speed to fly MC, as you suggest. Chip Bearden ASW 24 "JB" U.S.A. |
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