We’re moving into the time of year when weather doesn’t have to be ideal to provide excellent soaring at Crystal — especially if you like your thermals blue. Though ten degrees cooler than these last few days, the next spell will bring more uninterrupted sun and the westerly wind will gradually moderate. Sometime very soon, we’ll be due for a rash of those teensy little cumulus that form a full mile above the highest mountaintops… Maybe next week.
If this doesn’t start an argument no one’s listening. It might be the least discussed phenomenon in soaring, so ubiquitous, yet also relatively minor, that it has no name, and apparently most pilots never notice it. The concept is simple: where lift is evenly distributed, gliding into the wind will keep you higher than going downwind through the very same air. Said differently, zero sink to windward can yield a loss going downwind, where zero on a downwind pass becomes a gain of height to windward – all at the same airspeeds!
I didn’t believe it either at first, but repeated, admittedly non-scientific test runs in scores of various situations leave no doubt. It’s usually subtle, but can be observed in all kinds of conditions that offer consistent – not to say constant – lift in level flight. Examples include slope lift angled with a quartering headwind one way and tailwind the other, and shearlines of several kinds. The effect becomes more obvious when dolphining in dynamic stuff, such as straight through closely linked thermals, or in luckily timed rotor, where it’s sometimes possible to climb on a roaring headwind by aggressively pounding into it. Wave too? Presumably, but that’s difficult to monitor.
This is no brainy hypothesis, mind you, only empirical observation. I have queried a few aerodynamic luminaries on occasion, and some agreed that it’s a thing, but all blithely let the topic drop with no attempt at analysis… which leaves an open field for the rest of us, right? Hopefully you’ll gather enough data to hatch a proper theory of your own, but because I can never resist noodling, here’s a little thought grenade for openers. (Aerodynamic fundamentalists might wanna cover your eyes, I’m about to pull the pin.)
First, let’s stipulate that air molecules rising in a steady wind do so diagonally — and each one has its own inertia. They approach the point of impact from angles that are vertically oblique, acute while tacking one direction and obtuse the other.
What if this distinction somehow affects a subtle relativity between pitch angle and angle of attack? What if airfoils respond differently from other parts of the aircraft, such as the belly for example? And what role might CG play? Or differential friction? Don’t ask me.
Yeah yeah, Einstein on a train, the infamous downwind turn and all those other tasty conundra. But they’ve all been dissected and explained, and so far as I know, this one hasn’t. Folks can shake their heads and never think of it again, and maybe no one will ever explain it, but it’s still REALITY whether we comprehend it or not, and we should know more.
So do us a favor. When finally you do learn the secret, please don’t keep it, okay?
This week, Friday will be the warmest day of the year so far, with light wind and likely some fine blue thermals. Saturday is scheduled for mostly cloudy, so look for the best lift at the downwind edge of sunny areas. And on Sunday expect clear again but five degrees cooler, and windier due to the lowest pressure of the cycle. A wild card, you could say…
SEE YOU SOON!
A good tow pilot will bring you right to best lift, and the very good can even center thermals on tow. However, if one or more gliders are already working a thermal, the tow should avoid their immediate airspace, not only for safety reasons, but also as a courtesy. Those pilots need room to maneuver in unpredictable directions. Also, some believe the violent action of a propeller can disrupt or even destroy thermals, somewhat like popping a bubble. Whether or not this is scientifically true, towing through (or under) other people’s thermals is generally frowned upon.
Towing into traffic on a ridge requires similar planning and tact to accommodate ships already soaring there. They do have the right-of-way! If ever you see that you’re being towed too close to other gliders – or traffic of any kind – sooner is better than later for steering your tow pilot out of harm’s way.