How fast do you fly in thermals?
One of the observations I've made recently about my flying is that I don't turn tight enough when thermalling. It's really obvious when I compare my tracklogs with those of other XC pilots.
Here is a beautiful example from one of the local SoCal skygods Dimitri Soloviev. 1550m in a strong thermal, without looking like he fell out of either the front or the back once.
Here is another beautiful example of totally different type of thermals - some of the worlds best pilots thermalling wide and smooth in Governador Valadares. Notice how all the pilots fly around the same size large circle. I'm guessing this is because these are fast, high aspect wings and all very similar.
Hard to say how fast these guys are flying, the circles are very wide and flat because of the humid Brazillian air. I've flown once in GV, and the thermals were the biggest, softest I've ever flown.
Here is one of my own sloppy thermals...lots of room for improvement! This was a while ago now from Piedrahita, Spain. It was a very rough thermal with quite strong wind, and my technique was a mess from the chaotic start, falling out the back, a couple of direction changes and drifting towards a giant row of windmills the whole time.
Interestingly, I could not find any real information on how fast to fly in thermals. There are a lot of very useful articles out there, with tips on turning tighter or widening circles, and many deal with relative brake position of either hand while in the thermal, controlling bank with the outside wing etc, but very few deal with how fast to actually fly.
One of the best articles was from Cross Country Magazine
The first is: Turn tighter in strong lift and flatter in weak lift. The first part of this rule aims to keep us in the strongest lift for as long as possible: by turning tightly hopefully we won’t lose the core. This is a technique to help you hold onto a core once you’ve found it.
But, the second part of this rule tells us to widen our circles within the thermal to cover more ground, and increases the chances of bumping into a core. This rule is the rule of choice if we’re either in a strong climb or hunting around within a thermal looking for one.
However, there is a second rule of thermalling that appears to contradict the first. It says: Turn as wide and flat in strong lift as you can and turn as tight as you can if you fall out of it. This second rule appears to contradict the first until we look closer.
By turning as flat and wide as possible we maximise the performance and climb rate of our glider, because ultimately a glider climbs best in a straight line, without any angle of bank. But if we lose the core and fall into weaker lift, we should turn quickly to get back into it – even at the momentary expense of our sink rate. In reality it’s far more important to be climbing in the core, than to suffer a lower sink rate for a few seconds.
Still no mention of how fast to fly. Perhaps that's why in 14 years of flying (off and on...I've had a very slow learning curve), I've never really thought about speed to fly in thermals much!
A little bit of basic math and some assumptions shows how big the difference of flying speed could be. Flying slowly could mean seven seconds greater duration to complete one 360 assuming a 50m thermal radius.
So the question becomes is it better to fly slower and flatter in the same circle radius or faster and more banked up in the same radius? It probably all depends on what type of thermal it is.
According to cross country, you want to fly wider and flatter to maximize lift and find a better core. This might mean also speeding up somewhat as you will cover more ground hunting for the thermal, so fly closer to trim speed with minimal brake for sensitivity.
On the other hand, in a really light thermal, it probably pays to fly as slow and sensitively as possible, making slow flat turns that are not too big to minimize the chance of losing lift. This is the low save thermal, where you bring every ounce of concentration to the table to feed your addiction to lift.
In a strong, small and punchy thermal like we fly here often in the heat of Southern California or in somewhere with strong conditions like Valle de Bravo, you probably want to get banked up in the core and turn as tightly as possible. This also implies slowing down, using quite a lot of brake on both sides of the wing and probably controlling the bank angle using outside brake only. Best not to overdo inside brake in a tight and slow thermal to avoid a spin.
If the thermal is strong and large like say in Piedrahita, or Valadares, you might fly faster and wide to increase thermal efficiency as much as possible by reducing excessive bank.
According to cross country, you want to fly wider and flatter to maximize lift and find a better core. This might mean also speeding up somewhat as you will cover more ground hunting for the thermal, so fly closer to trim speed with minimal brake for sensitivity.
On the other hand, in a really light thermal, it probably pays to fly as slow and sensitively as possible, making slow flat turns that are not too big to minimize the chance of losing lift. This is the low save thermal, where you bring every ounce of concentration to the table to feed your addiction to lift.
In a strong, small and punchy thermal like we fly here often in the heat of Southern California or in somewhere with strong conditions like Valle de Bravo, you probably want to get banked up in the core and turn as tightly as possible. This also implies slowing down, using quite a lot of brake on both sides of the wing and probably controlling the bank angle using outside brake only. Best not to overdo inside brake in a tight and slow thermal to avoid a spin.
If the thermal is strong and large like say in Piedrahita, or Valadares, you might fly faster and wide to increase thermal efficiency as much as possible by reducing excessive bank.
This post wouldn't be complete without asking...what was your most epic thermal?
Mine was at the Rat Race Sprint in 2012...from less than 100m above the 'Rabies Ridge' where I was about to head for the landing field, I was blasted up 1500m in less than 5 minutes in a convergence line thermal, in just thirteen 360s. Each 360 was an vertical gain of around 120 meters....
Mine was at the Rat Race Sprint in 2012...from less than 100m above the 'Rabies Ridge' where I was about to head for the landing field, I was blasted up 1500m in less than 5 minutes in a convergence line thermal, in just thirteen 360s. Each 360 was an vertical gain of around 120 meters....