What is Speeed To Fly

basicaly it's a theory how fast -how much speed bar- should you fly in order to get
  • furthest(highest) possible given the wind/sink you're facing
  • fastest possible to the destination given the amount of lift you expect to get when you get there (McCready)
Ever found yourself leaning forward in your harness figuring out will you make it over those trees to the landing field or not? How about giving yourself the best chance possible? First thing you should do is lean back and make as little drag with your body as possible. Next is push that speed bar as much as needed to get the best glide angle possible. But how much bar is this? Trim, one third, half, full? That's what S2F tries to answer.

1. Glide angle

Glide angle is relation between lost height and flown distance. That is how far can we fly with certain height. Glide ration of 8 means we fly 8000m from 1000m of height. Glide angle is the relation between the speed you fly and the sink(vario) you're facing. Different sink rates at different speeds are drawn in a graph called POLAR function.
  • Glide = speed(m/s) / sink(m/s)
  • Speed(m/s) = Speed(km/h) / 3.6;
  • Glide = speed(km/h) / 3.6 / sink(m/s)
Starting from minimum speed, sink is decreasing until certain point after which it's increasing again. The most upper point of the curve where the sink is the smallest is called MINIMUM SINK speed. At the graph it's 33km/h. Flying with this speed you will stay the longest in the air. But will you get the furthest? No! The furthest you will get when the relation between speed and sink gives you the best possible glide. If you compute glide angle for all speeds you will see that this is at 39km/h with glide of 8.88!
  • Yellow is flying with min sink speed and will land last
  • Violet is flying with best glide speed will land fist but will get further
When do you want to fly min and when with best glide speed?
When on transition you want to get as far as possible and be there as high as possible. So you want to fly with best glide. But what about thermaling? When you're thermaling you care only about finding the best lift. In this case you're trying to minimize your sink rate. That's when you should fly with min sink speed!

2. How does wind affect Glide angle

We see from the previous example that the best glide angle is at 39km/h. This measurements are always reffering to speed of glider through the air. When there is no wind, speed of glider moving through the air is the same as the speed of glider moving over ground. But when there is some wind component the wing is still moving through the air with same speed but the speed of moving over ground is different. If we're facing 10km/h head wind the glider is moving 39km/h through the air but only 29km/h over ground.
We said before that the best glide is at 39km/h in no wind. Let's make now an assumption that we're facing a head wind of 39km/h. We will be standing still over ground. Our glide ration would be 0! If we push just a little bar to go faster we would move forward and would improve our glide ration. So we see that in head wind we should fly faster to obtain the maximum glide. But how fast is this? That's what we're trying to figure out with our analysis!

3. How does sinking air affect glide angle

The air we're flying through is never still and this affects the speed we should fly at. Similiar as with the head wind we should fly faster if flying through the sinking air. The more it goes down, faster we should fly. Again we will not go into details of mathematics but you can check it in the analysis!

4. McCready

McCready gave a theory that we should fly faster than optimal glide speed if we know we will face lift when we arrive at the destination(next thermal, ridge...). How much faster should we fly depends on the average of lift we'll face when we get there. Average of lift should include time to center thermal and average thermal lift once in it.
  • Blue is flying with best glide and will get higher into the next thermal
  • but Red is flying faster with optimal McCready speed and will gain more height thermaling in mean time
Again we're not interested in mathematics how this is computed but it's important we're aware of this if we wan't to optimize our flying. Average Joe should not give to much attention to this but if you're doing competitions or planning to make the best out of your flying day than you should dig a bit into it.

Check out in the analysis how expected lift dictates our speed.





Hope this helps you and feel free to comment or contact me at s2f@adrenalinco.si
Miha Slamič 2019-02-22 10:25:25
Zgleda super, je treba klapu navadit da je treba pa peljat pod gasom :D
Uroš Komel 2019-03-27 12:08:55
Top razlaga z matematičnim prikazom!
Björn 2019-03-27 16:56:54
I often fly on speed bar when I have a sure lift and in the next strong therminal and out of heavy sink. But less or not in a weak situation. When on glide I use most often the glide ratio on gps or instrument when I push the bar.