National Geographic : 1967 Jan
the glider. Twenty-four years before, I had taken three short instruction flights in a power plane, so the stick and rudder pedals weren't total strangers to me. Now he let me try my hand at flying the ship, and we wobbled unsteadily along. Above us, patches of mist were developing into cumulus clouds that resembled giant cauliflowers. Sudden turbulence jostled the controls, and I felt Bernie's firm hand take over. "Watch the variometer," he said as he put the ship into a circle. The green pellet rose in its tube and hovered halfway to the top. "We're circling in a thermal now," Bernie com mented. "As long as the green pellet stays up in the tube, we're climbing. If it sinks and the red one pops up, we'll be descending." Thermals come in all sizes. Judging from our wide circle, this one must have been more than half a mile across. Instrument Reveals Invisible Air Currents As we spiraled up, Bernie stressed the importance of these two glass tubes to a soaring pilot. Until the late 1920's, sailplane pilots were con fined to riding the rising winds that swept up slopes. They could not locate thermals, because neither the altimeter nor the pilot's sense of vertical movement was precise enough to register thermal lift, second by second. Then the ingeniously simple variometer came along (page 57). It freed soaring pilots from ridge flying, for it could tell them when they had flown into an invisible updraft. "There's an air flask behind the instrument panel, connected to the variometer," Bernie explained. "As we climb, the outside air pressure drops and air leaks out of the flask, blowing the feather-light green pellet up in its tube. Going down, air leaks in through another tube and lifts a red pellet." Bernie pointed out that high-performance sail planes use more sophisticated variometers, includ ing electric versions. "But they're all based on the same simple principle-measuring air that leaks into or out of a flask. "Competitive soaring pilots add an audio unit, so they can hear their variometer without having to look at it. The unit sends out an audible tone that varies in pitch as the lift changes. With one less instrument to look at, the pilot can concentrate on precision flying in the thermal." Early gliders were designed on the natural as sumption that the slower a glider descended, the longer it would stay aloft. Minimum weight and large wingspan-those were the goals of early sail plane builders. But thermal soaring brought a whole new concept. Sailplanes must be able to circle slowly in a thermal -and then speed miles across stable air to the next BY HOWLAHVSUHLHMN; KVVACHKUMkbBY LMORYKK15TOFU N.( .5 .