Thanks to birdwatching, I’ve often watched turkey vultures, gulls, and hawks quietly circling higher and higher within the warm updraft of a thermal. While I was down in Southern California, an avid soarer took me up in a sailplane. A towplane pulled us up into the sky. Time aloft is dependent on finding and using thermals to prolong one’s glide. Tom dropped the tow early when he felt a strong thermal and began circling.
The sailplane has three main instruments. One tells the airspeed (usually around 60-70 mph), one tells the altitude, and the third (the variometer) tells how quickly the plane is rising or sinking in feet per minute (fpm).
A sixty-foot wingspan sailplane cannot circle as tightly as a six-foot wingspan turkey vulture. So the 65 mph sailplane tends to pass through the thermal repeatedly. Because the invisible thermal often drifts, the sailplane sometimes passes through the strong center of it, sometimes nearer the weaker edge. The pilot has both the instruments, the kinesthetic experience of which wing is experiencing the strongest updraft, and sometimes the visual aids of circling birds or a cumulus cloud growing at the top of the thermal to help keep the plane within the thermal as much as possible.
The strongest updraft is near the center of the thermal. Outside of the thermal, there tends to be a downdraft of the air that is flowing out from the top of the thermal. So a sailplane passing through a thermal will first experience a downdraft, then an updraft, then an increasingly strong updraft which then drops off as the plane passes through and a downdraft on the other side. To give it some plausible numbers, imagine a downdraft of 100 fpm turning into a 100 fpm updraft as we enter the thermal. The updraft around us grows to 200 fpm and then crescendoes to a rushing roar of 700 fpm for several seconds and then diminishes down to 200, then 100 fpm until we pass out into the downdraft beyond the thermal.
The point of this article is to focus on my experience of moving from the 700 fpm rising air at the core of the thermal to the 200 fpm rising air beyond it. The plane was experiencing continual uplift throughout this transition. But I felt a strong downward jerk even though I was going up. Similarly, when we passed from a 400 fpm downdraft into a milder 100 fpm downdraft, I felt an upward jerk even though the plane was still sinking. My kinesthetic sense did not match what the instruments were saying about our flight. This created a slight kinesthetic queasiness that gradually grew over time.
I didn’t learn about jerk in my college Physics 101-102 class. I did learn how velocity was “change in position divided by the amount of time required for that change” (v=∆ position / ∆ time) and how acceleration was “change in velocity divided by the amount of time required for that change” (a = ∆ v / ∆ time). But I did not learn about jerk which is the “change in acceleration divided by the amount of time required for that change”5yr (j = ∆ a / ∆ time). Jerk is the right name. Steady, unchanging acceleration – such as in a car – can feel very smooth. But a change in acceleration, such as the shifting of gears, the turning of the wheel, or hitting the brakes, creates jerks – as does passing through a thermal.
So we passed through air rising at various rates. The up and down jerks that my body experienced were not related to whether we are going up or down but to whether we were going up or down as fast as we were before. Only the variometer (and the altimeter) let me know whether I was actually rising or sinking. Now I know why pilots learn to fly by their instruments.
This kinesthetic experience fascinated me because I’ve been thinking about jerk theoretically in terms of how we shift our current downward spiral into an upward spiral. Jerk is prominent in the last chapters of Roaming Upwards in terms of navigating what I call the Fifth Dimension. Soaring gave a new perspective on it.
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