We all hear lots of talk about wave lift in praise of climb rates, preposterous smoothness and all the rest, yet seldom much mention of wave sink. That’s understandable, but sink is always half of any big picture. Open and endless as wave lift may feel, it’s normally sandwiched between sink of similar volume and strength, upwind beyond the trough and probably downwind behind the crest. And once you’re in it, wave sink feels plenty open and endless too.
In wave sink, whatever your achieved climb rate has been, the corresponding descent will feature an equivalent vertical speed downward plus the 100+ fpm aerodynamic sink rate you had while climbing — think carefully about this, or you won’t believe it. That’s if you remain at the low-end speed you maintained while climbing, and almost anything you might want to do in sink necessitates higher airspeeds.
Theoretically, wave requires at least twenty knots of wind across the hilltop that generates it (and on wave days, 60-knot winds aloft are not rare). For this discussion, let’s stipulate an achieved climb of 5 knots in ambient wind of 30 knots. Response to the equivalent sink, by itself, will roughly double speed-to-fly, and penetrating to the next lift upwind will demand also adding half the headwind. This puts you up around maneuvering speed in any glider, and quickly compounds to 15 knots DOWN, maybe more…
To estimate achieved glide during a wave penetration, subtract the approximate headwind from your airspeed and divide that by the total rate of descent. Example: 75 kts airspeed minus 30 kts headwind equals 45 kts ground speed — and if you’re coming down at 15 knots, you get an achieved glide of 3/1. This is not an exceptional situation; it’s ordinary in wave soaring. For stronger conditions or low performance gliders, the numbers will be MUCH WORSE!
Having fun yet?
Gliding downwind in wave sink is a very different story. Moderate sink in a tailwind of thirty or less requires only moderate increase in speed, which an old-fashioned MacCready ring will give you. For stronger tailwinds and the heaviest sink it gets numerically complicated, but going downwind in wave usually doesn’t take long, whatever the situation, and by the time you calculate exact speed-to-fly, that question may be moot.
Gliding crosswind in wave sink is just a bad idea for almost any purpose, unless you’re intentionally letting it cast carry you down to wherever you wish to go. But this brings us to using wave sink for descent, which can be an efficient, entertaining, and educational way to finish a flight. There are many ways to do this, some not at all recommended, while some are lots of fun. Here’s just one.
Imagine a free standing lenticular, say four thousand feet deep. Climb by its windward face to just a few hundred feet higher than the top, then turn and glide downwind toward it, still slowly rising as you approach the crest. From horizontal flow there, you drift into the reciprocal of all that lift, so softly you may never feel the vertical transition from huge up to huge down. Proceed a few moments further as the cloud below drops away, and once your vario’s pegged on down, turn back into the wind again to face the falling cloud like a helicopter autorotating before a waterfall! Cool stuff, so long as there’s some clear route to safety — or a convenient harmonic waiting a few seconds further downwind…
Now you want a real challenge? Rassle rotor beneath that cloud (bladder permitting), all the way upstream to the windward side again, and climb from there back up into the wave you started from. For that achievement I always award myself an imaginary lennie pin. An inverted one of course, pewter, not brass!