Stepping up to a higher performance glider or switching to a race harness could earn you a better glide ratio by 0.5 point. But what does this actually mean in terms of flying cross-country in the UK? I decided to investigate the theoretical answer with a few “back of the envelope” calculations. And I discovered that—on the average glide in the UK—a 0.5 pt inferior glide ratio will cost you only one circle in the next thermal.
Here’s why. Imagine I set off on from cloudbase at 1500m on my LTF 2 glider, with my friend on his LTF 2-3 glider. We glide at the same speed (say, trim), but my glide ratio is 9:1, whereas his is 9.5:1. We glide a distance of 3750m—2.5 times the height of cloudbase1—through the airmass to the next thermal. We arrive at the same time, but he is 22m higher. With a 3 m/s average climb rate in the next thermal, it takes me only 7 seconds—about the time taken for one 360—to regain the lost height. It’s such a small difference that if I managed to average a climb rate only 0.1 m/s better in the next thermal, I would gain a greater advantage in the climb than I had lost on the glide.
The catch? Say, at the top of the next thermal, there’s a sure-fire cloud forming 3.75 Km downwind. My competitive friend with the tricked-out kit decides to show me who’s the boss and he steps on the gas. This time he glides at 49 Km/h but still matches my 9:1 glide. So he arrives at the next thermal 81 seconds faster than I do. By the time I arrive, he has already climbed 240m higher. By the next thermal, he’s left me for dead. If he maintains this sterling performance over a 4 hour flight, he will fly 120 Km to my 106 Km. It’s a hands-down win for higher performance gliders at high gliding speeds.
But what is the actual optimum gliding speed between thermals on a typical UK XC flight?
The answer depends on your confidence in finding the next thermal. If you’re heading for a nearby working cloud, you’ll fly a bit above trim. But if there are no obvious indicators of lift, you’ll fly a bit below trim to maximize your time in the air and coverage of the ground downwind.
As current World Champion Bruce Goldsmith says in Cross Country magazine No.118:
…speed to fly theory gives speeds that are surprisingly fast..STF assumes that you have the performance to be able to find the next piece of lift. On a paraglider this is often not the case. The next thermal may be simply out of glide’s reach or the time between thermals may be so long that you are on the ground before the next thermal is triggered…If I am flying XC in the UK and cloudbase is low, I fly slow. As soon as I see a climb I speed up. It is all about assessing the risk of going down.
In the UK, you are likely to feel that there is a high risk of going down. Often, conditions are weak and patchy. On your downwind course, there are few perfectly forming fluffy clouds, few ridges with house thermals, few gaggles of gliders and few soaring birds.
Only once this year have I flown in conditions that you could confidently stay up in, and even that was only for the middle third of the flight.
That’s why—if you don’t race around short circular courses in strong Alpine conditions—0.5 point of glide rarely matters.
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From Reichmann, Cross Country Soaring 7th Ed., SSA ↩
