Review: XC in Welsh wave
Back | HomeWell, it would seem I am not a plonker 
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There was good wave over Wales and downstream and a number of flights have been uploaded to the ladder. Several are short XCs, and it’s especially good to see flights from Talgarth, the Mynd, and Denbigh (though the Denbigh flights could have been uploaded as XCs, making it more obvious to any passing CAA types where they originated from…). Stand-out flights have to be Diana King’s 139 km on Saturday and Phil King’s 238 km on Sunday.
Although the wave had been well-forecast all week, the main problem was the strong wind on Saturday which I totally failed to mention. To figure out just how long XCs in wave can be done, I decided to look at profiles for John William’s three longest flights last year. (It took me a while to find a data archive!) The profiles have been drawn using the IDV.
The first is from 8 April 2007 when John flew 1,000 km on a north-south track across Scotland:
The classic bulge of a stable layer is there, and the wind gradient increases from 15 knots at the surface to about 40 knots at flying height. Above that, it slooowly increases to 70 knots at the tropopause. John averaged 130 kph running up and down the bars in his Antares (handicap 113, which helps!).
The next is from 30 August when John flew 770 km—-twice. At nearly 150 kph. A Libelle pilot did 500 km the same day too!
This shows a fairly small stable layer which is also quite high up. The wind gradient is slightly less than before, from around 10 knots at the surface to only 30 knots at height, with higher speeds of 60 knots or so not coming until near the tropopause. Speculating, I expect it was the lower wind speed that allowed gliders like a Libelle to cover such long distances.
Finally here’s the sounding from 10 September when John flew another 1,000 km, this time at near 140 kph:
Pretty damn big isothermal layer on that one! John flew most of the flight around the 700 mb level, which was at 40 knots again. This time the 50 knot winds came much lower, as far down as about 12,000 ft. I wonder if the stronger stability increased the strength of the wave to offset the extra wind?
Judging from those three examples it seems that the best wave in Scotland at least often sees around 30 knots at flying height. So if we look at Saturday’s profile for Wales:
You can see where things went a bit wrong, with winds of 50 knots at 6,000 ft! The stable layer isn’t all that strong either, so the wave wasn’t the strongest it could be. Here’s a sat pic from Saturday (around 1pm) showing the results of all that:
Looks nice and defined on the satellite (some high-level cirrus, as forecast, obscuring things a little), but of course you can never tell the windspeed or wave strength on a satpic, so it can be deceptive. You can see that Nymsfield also benefitted from the wave, with a couple of folk soaring for over 5 hours there.
Sunday didn’t look, or at least I thought it didn’t look, so promising for wave. Here’s the sounding for Sunday:
What worried me was that although the wind increased with height until around 850 mb, it then began decreasing. My (unspoken) suspicion was that it would result in breaking waves rather than trapped standing waves, and indeed the overnight F215 warned of severe turbulence in the area, while Sunday’s made no mention of neither wave nor turbulence! However, look at that stable layer! Much bigger than on Saturday. And more significantly, those winds are much lower—-max 30 knots on that sounding. The result was wave which looked like this:
Quite smaller bars (and also a lump of annoying cloud to the north). (Also note the with-wind roll clouds in west Wales.) The lower speed allowed Phil King to do his 238 km flight. (I did want to try over-laying Phil’s flight on a 250 m/pixel MODIS image, which has worked very well before, but hardware failures at NASA at the weekend mean that there’s no data available at the moment 
).
So in future I’ll be paying more to what the wind speeds actually are, rather than just what the gradient is doing!
One other boo-boo I made last week was not pointing out that Thursday’s windspeed and direction was just right for Nympsfield’s ridges. Indeed two 177 km flights were flown that day! One of the pilots also took a thermal to 4,000 ft —- not long now to the thermal season…
Thanks Dan for this very interesting and useful analysis. It is great to have all this information brought together in one place. I would have responded sooner only I wanted to make time to have a good look first.
I would like to add a few comments. You say: “Sunday didn’t look, or at least I thought it didn’t look, so promising for wave.” My thoughts on Saturday evening were exactly the same and for roughly the same reasons. If I had known how good it was going to be I might have made more effort to take an early launch (the tuggy Sean tells me he was at Shobdon by 8:30 and could have come earlier if I wanted). Also I would have benefited from water ballast, and what’s more there wouldn’t have been any danger of freezing because the lowest outside air temperature I saw was +6C.
So a few learning points for me there! I shall no longer dismiss a day simply because the upper winds are very light. I should have known that anyway from an experience many years ago of climbing over the Black Mountains in a strong easterly and finding that above FL200 I could continue climbing slowly to FL230 while circling in very light winds. A met man with access to a tephigram for that day told me later that above FL230 the winds were westerly!
It’s interesting than weather Jack’s 5th rule of thumb for wave that the temperature at 500mb should be no lower than -24C applies to all these days.
Weather Jack’s wave tutorial is at:
http://s214580749.websitehome.co.uk/tutorials/tut-various/wave/wave.html
“Also I would have benefited from water ballast, and what’s more there wouldn’t have been any danger of freezing because the lowest outside air temperature I saw was +6C.”
I’ve been thinking about how fast the Scottish pilots manage to go in wave and whether they used ballast. I know nowt about actually flying in wave so e-mailed John Williams (who then also put me in touch with Santiago Cervantes). I expect you/others may already know this but I found it very interesting.
John explained that when he used to fly a LS8-18 he did use water in it. He put about 30% antifreeze into the tail tank, but left the wing tanks plain water. I imagine the thermal mass was large enough that not much froze (or there’s room for expansion). John achieved 150.1 kph around a 500 with the LS8 on 15m tips using 100 kg of water. He did mention that the dump system often froze, so typically landed with the ballast on board, but this was “not normally a problem with the wind strength needed for wave”. Kevin Hook flies a DG400 so is always carrying the fairly heavy engine, but mainly flies in 15m mode to increase the wing loading further.
John and Sant (who flies a dry Libelle) also had the same tips for maximum speed: don’t stop to climb, and fly in lift as much as possible, not least as the wind speed is usually lower there. Sant says his Libelle is happy in up to 50 knots! Towards 70 knots things get tricky, especially crossing bars.
Sant mentions that the hardest part of wave flying is simply getting into it, and this is where turbo and self-launch gliders have the advantage. Once in though so much is possible: a 500 has been done in an Astir too, and one pilot has even done a 300 km OR in a K6e from Aboyne!
The comments by John and Sant are right on the ball as you can see from the flights they do. However I think we may need to modify their techniques in Wales. I have been flying the Welsh wave for 38 years, and the Scottish wave for 7 years, so I feel qualified to say this. The main difference is one of scale — the mountains are lower in Wales, so the strength and height of the lift tends to be less. This probably favours gliders with a low sink rate and good gliding angle, or to use Platypus’es phrase “there’s no substitute for span”. On the other hand it is well worth using water ballast because this has a very small effect on the min sink. The increase in stalling speed and circling radius is of little importance for wave flying.