Author: Steve Nagle
Having read Brian’s post on “glide bar” and Gareth’s examples on “speed to fly” I was inspired to get back to some calculations on this and produce a ready reckoner. Here is how I put it together, what it looks like, and why I did it.
This is the standard “text book” picture of a polar showing the best glide as the line from the chart origin that is a tangent to the polar. Speed is in km/h and sink in m/s. This is my guess at what an M6 polar looks like. I’d be keen to hear any views on whether this is accurate or not.
All of this is very pretty, but not new, and most glide computers will do all of this for you if you input your polar. Also when we are racing in summer we can often expect a next climb of 2.5m/s to 3.0m/s, so unless we are low or have a great tailwind then we fly at or close to full bar unless we are climbing. So what’s the point of doing this analysis? I like being a nerd for its own sake so I don’t need any other reason, but I made some up anyway.
My first reason to have a more manual speed to fly rule of thumb is that unless I know what my flight computer is calculating it is not possible to critique its inputs, and I could be using the output on a false premise. That might be as simple as not putting in the correct polar for my glider. More subtle though is the “expected next lift” question. If you have plenty of height and reliable triggers and approaching the strongest part of the day then you can probably confidently predict the next lift (and fly through weak lift to get to it). For example if you know there are 2.5 to 3 m/s climbs to be had and your glide is though air sinking at up to 0.5m/s then (assuming no tailwind) you are in the red zone and that says full bar. That’s the easy case. If however you are on glide over a damp valley, in neutral air needing a glide of 9 to arrive above the next ridge to look for lift then it’s not that simple, even if that climb is a guaranteed boomer. On full bar, unless the next climb is before the ridge, then you may not reach it. So your judgement of “next climb strength” needs to include where you can get to for that next climb and you might need to slow down to arrive high enough to make it a reality. I don’t think even XCSoar is working that out for us yet.
My second (made up) reason for looking behind the speed to fly theory is that the reality of our situation is slightly more “fuzzy” than knowing exact airmass movements and translating that into a speed to slavishly fly at. The visualisation provides an easy view of a range of speeds that provide close to optimal speed to fly, across a range of inputs. For instance if I have tailwind of 8km/h, net airmass sink of 0.5m/s and expect to pick up a 2m/s climb then I am in the middle of the orange zone, and the “third step” conclusion is robust to +/- 0.5m/s or +/-4km/h. So I’ll be on the third step, maybe increasing and decreasing to manage the wing.
I’d be interested to hear how others think about this and whether the theoretical speeds to fly are in line with intuition.
Author: Steve Nagle
My first reason to have a more manual speed to fly rule of thumb is that unless I know what my flight computer is calculating it is not possible to critique its inputs, and I could be using the output on a false premise. That might be as simple as not putting in the correct polar for my glider. More subtle though is the “expected next lift” question. If you have plenty of height and reliable triggers and approaching the strongest part of the day then you can probably confidently predict the next lift (and fly through weak lift to get to it). For example if you know there are 2.5 to 3 m/s climbs to be had and your glide is though air sinking at up to 0.5m/s then (assuming no tailwind) you are in the red zone and that says full bar. That’s the easy case. If however you are on glide over a damp valley, in neutral air needing a glide of 9 to arrive above the next ridge to look for lift then it’s not that simple, even if that climb is a guaranteed boomer. On full bar, unless the next climb is before the ridge, then you may not reach it. So your judgement of “next climb strength” needs to include where you can get to for that next climb and you might need to slow down to arrive high enough to make it a reality. I don’t think even XCSoar is working that out for us yet.
My second (made up) reason for looking behind the speed to fly theory is that the reality of our situation is slightly more “fuzzy” than knowing exact airmass movements and translating that into a speed to slavishly fly at. The visualisation provides an easy view of a range of speeds that provide close to optimal speed to fly, across a range of inputs. For instance if I have tailwind of 8km/h, net airmass sink of 0.5m/s and expect to pick up a 2m/s climb then I am in the middle of the orange zone, and the “third step” conclusion is robust to +/- 0.5m/s or +/-4km/h. So I’ll be on the third step, maybe increasing and decreasing to manage the wing.
I’d be interested to hear how others think about this and whether the theoretical speeds to fly are in line with intuition.
Author: Steve Nagle