Thermals close to the ground in wind
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Some beginners acquire a belief that they gain height by turning into wind and lose height when turning down wind. I and some others explained to one such pilot that an aircraft in flight, having no connection with the ground, has no ‘concept’ of ground speed and therefore no concept of up-wind or down-wind. I find a useful way of imagining wind is to view a stationary cube of air with the ground sliding by under it; its hills, valleys, and other obstructions disturbing that air in exactly the same way as when you view it in a more conventional way (that is, viewing it as air flowing over stationary ground).
However, this guy, perhaps because he was older and more certain of his ground (and air) would not give in: “But if I closed my eyes, I’d be able to tell when I turn into wind because my climb rate definitely increases — according to my vario.” (A variometer emits an audible succession of beeps when you are going up. See Variometer in Hang gliding 1976 part 1.)
Sometimes it takes a persistent counter revolutionary to persuade knowledgeable pilots to question their hard-won beliefs. In the end I had to admit that, even according to my own experiences that day — thermaling from a 2000 foot high ridge in semi desert conditions — he was right!
‘Take home’ message: When thermaling (circling in patchy lift) close to the ground, to stay in the strongest lift and, moreover, to prevent falling out the back of the thermal, you must extend the upwind portion of your circling.
Adopting Einstein’s approach to life, I then sought to reconcile these observations with known physical laws and I came up with the following theory, which was published in the September 1992 edition of the UK hang gliding magazine.
According to one acknowledged expert(*) the smallest thermal usable by a hang glider has the mass (weight) of a large freight train or a small ship. When such a mass of air rises from the ground, its inertia resists it being accelerated down-wind. Instead, it blocks the wind, much like a hill does. The incipient thermal constitutes a temporary (and invisible) hill.
|Incidentally, while ridge soaring, you can often discern when a thermal is rising ahead of you by an increase in ‘penetration’; you suddenly start moving out away from the hill, as if the wind speed has decreased. It has. The thermal is blocking the wind. Moreover, when the thermal lifts off, it draws air in from around it, adding to the effect. This phenomenon is particularly noticeable in a paraglider, perhaps because of its slower speed compared to a hang glider.|
Rising air is rising air, almost regardless of the cause of its rising. In this case, the rising air ‘inside’ the thermal and the rising air ‘outside’ the thermal at its up-wind edge become one. In other words, the thermal grows up-wind.
|The reference to Drawing 3 in the diagram refers to the preceding diagram. This diagram would be better if the leading edge (at right) was less inclined and the X correspondingly shifted right, that is, up-wind.|
Even when the thermal breaks free of the ground and starts to accelerate down-wind, its center of gravity moves up-wind relative to the air mass, and possibly even up-wind over the ground. (‘Breaks free’ is too simple a model, I know, but the core that rises strongly enough to carry us upward does not extend to the ground for long.)
Therefore, when you turn into wind, you move into increasing lift; you travel towards the core of the thermal, which has moved up-wind since you were last flying into wind. You need to level out for a few seconds to move into the core. (How many seconds? Listen to your variometer.) Conversely, turning down wind, you recede from the ever-moving core. The result is that, when thermaling close to the ground, you need to extend the up-wind portion of your 360s and tighten up the down-wind arcs so that you keep moving forward to the the shifting core.
At altitude, after the thermal has had time to accelerate to something approaching the general wind speed, such effects lessen. Your 360s become more circular to center on the strongest lift.
Feeling that I could not be the first to come up with this theory, after a long search, I discovered an article in I think the 1960s written by a sailplane pilot in (I think) New Zealand putting forward the same argument. (I will add details if I come across them in my files.)
Thermaling a hang glider: Big Blue Sky 2008 documentary by Bill Liscomb on YouTube starting at 40 minutes 12 seconds. The sequence is brief, but illustrative. The glider is a Seagull 10 Meter or 11 Meter of the late 1970s. Narration is by Pete Brock of Ultralight Products.
Dennis Pagen, The Heavy Air, in Hang Gliding, August 1989