Optimising Glider Thermal Centring
I have been around gliding for a number of years now and am starting to compete. I spent this off season thinking about how best to centre a thermal and how to verify and communicate this.
Speaking to others on this point, I have heard variations of "turn when you feel the kick", "turn tighter in the good air" or the BGA's "Less lift: reduce the bank and increase the bank as the lift improves".
The issue I have with all these is: Assuming you are not already centered, the "kick", "good air" or "lift" only comes once per circle. This means your feedback loop takes a circle to resolve (about 25s according the the BGA Instructor Manual). The main variant of this technique involves opening the turn in the bad air, which makes this problem worse.
Before I give you the results of my musing, I should preface this with: Centring a thermal is a complex problem that has many more facets than can be dealt with by a simply defined technique. This is either a place to start, or a backup that can live in your subconcious for when you are eating a sandwich, having a pee, making strategic decisions, or all three at once. If you have the ability and capacity to model the thermal and plot an intercept to the optimal path round the centre, do that. If not...
This is what I came up with:
Open the turn while the needle rises, close while it falls.
More formally:
Your turn rate should be your desired turn rate with an additive proportional to your downward acceleration.
This allows you to receive and incorporate immediate feedback, it also has only two parameters (proportional weight and desired turning rate), making it fast to optimise for your aircraft/conditions.
Examples
Heading diagonally towards the core, the vario value increases -> open the turn.
Heading diagonally away from the core, the vario value decreases -> close the turn.
Heading at a tangent to a circle concentric with the thermal, the vario is constant (irrespective of value) -> fly your desired turn rate.
In all situations, the prescribed technique offers an immediate bettering of the glider's position in the thermal.
Turn Rate Vs Control Input
The ideal proportionality is between the rate of change in vertical speed and the turn rate, not the control input. Therefore you would ideally preempt the vario reading to accommodate for the time it takes to change the turn rate. This may be why the technique of increasing the turn rate in the lift works well for many people. The control input may be proportional to vertical jerk, but I found this too complex to use in flight.
Simulations
I built a simulator to test this out. If you think you have a better technique, please contact me and I might be able to test it!
Vario Lag
Varios lag, but only a tiny bit - the time it takes for the pressure to get through the plumbing and to accelerate the instrument's mechanism. This is, as far as I can tell, a negligible amount of time.
If you have modern instruments with IMU integration (HAWK etc.), your instruments probably already account for the below.
As you fly into rising (or sinking) air, the difference in vertical velocity between the air and your glider exerts a force, accelerating you, but not instantly changing your velocity to match the airmass. That force is proportional to the difference between the airmass velocity and the glider's velocity. This is the largest element of what is commonly referred to as "vario lag".
Lets look at a graph of airmass vs glider vertical velocity for a simulated glider flying straight through a thermal (don't pay much mind to the values here, they are fabricated). We are interested in the airmass velocity, but our vario is telling us the glider's velocity.
The graph shows the "lag" between the peek of the airmass velocity and the glider's. It also shows that the acceleration you feel is proportional to the difference between your vario reading and the airmass movement. This means you can use your senses to compensate for "vario lag" if you're good enough!
If you're feeling a "kick" up, your vario is reading low. If you feel a "drop" down, your vario is reading high.
06/06/2026