Last month we discussed how lightplane pilots can take advantage of rising air to improve climb and cruise performance. These techniques include operating on the windward sides of hills, slowing in updrafts, and capitalizing on the benefits offered by thermals. We also discussed how to minimize the deleterious effects of sinking air by speeding through downdrafts and avoiding leeward slopes.
Sailplane pilots, of course, are intimately familiar with techniques that consider vertical currents of air; otherwise, their flights would be frustratingly short. With practice, they learn to take maximum advantage of rising air and avoid sinking air.
But a sailplane is not needed to gain familiarity with the vertical movements of the atmosphere. There are two fascinating and challenging procedures that can be practiced in an airplane that come close to simulating the flight of a glider. The idea is to configure an airplane so that it flies like a glider. Thankfully, this does not require shutting down the engine. An airplane — even with power off — cannot begin to approach the glide performance of a sailplane. The typical lightplane, for example, has a glide ratio of 8:1, which means that it glides 8 feet forward for each foot lost during descent. On the other hand, a typical sailplane has a glide ratio of 30:1, which means that it can glide 30 nm in any direction from an altitude of only 1 nm (6,280 feet) in still air.
On a smooth, stable day, reduce the speed of the airplane to its best glide speed and then use whatever power is needed to maintain altitude. This places the airplane in a zero-sink configuration.
Flying an airplane in this manner, however, might result in an uncomfortably nose-high attitude. In such a case, extend the flaps to the first notch or detent (10 to 20 degrees). The result is a much flatter deck angle that improves over-the-nose visibility, which is why partial flaps should be used when maintaining altitude in a traffic pattern. Finally, readjust and note the power required to maintain the best glide speed at a constant altitude.
All that is required now is to proceed at cruise speed toward an area where you suspect even the mildest vertical currents of air might be found. Once there, establish the aircraft in its zero-sink configuration and attempt to locate the updrafts, the presence of which is confirmed by a climb, even a shallow one. Conversely, attempt to avoid losing altitude by remaining clear of downdrafts, no matter how mild or subtle they might be. Doing this under a variety of conditions will begin to teach you where the rising and sinking air can be found.
If you really want to accept a challenge and have some fun at the same time, consider converting your airplane into a sailplane. I don't mean this literally, of course. I am suggesting only that you configure the airplane to simulate the gliding performance of a sailplane.
Assume, for example, that we want to use a 1982Cessna 172P to simulate a sailplane with a glide ratio of 30:1. The first step requires expressing the normal glide speed of a Skyhawk (65 KIAS) in terms of feet per minute. To do this, multiply 65 (nm per hour) by 6,280 feet (per nm), which results in an airspeed of 408,200 feet per hour, or 6,800 feet per minute.
If a Skyhawk is to have a 30:1 glide ratio (like that of a typical glider), it must be capable of gliding 30 times as far forward per minute as it descends per minute. Therefore, divide the forward speed of 6,800 fpm by 30 to obtain the desired sink rate of 227 fpm. In other words, a glider or an airplane with an indicated glide speed of 65 knots and a glide ratio of 30:1 descends at only 227 fpm. To simplify matters, let's use a sink rate of 200 fpm.
The next step requires extending the flaps to the first notch or detent and then applying and noting the power required to establish and stabilize the Skyhawk in a 200-fpm descent while maintaining exactly 65 knots. Voilà! You have just converted your Cessna into a flying machine that has "glide" characteristics similar to those of a sailplane with a 30:1 glide ratio. In other words, as long as you do not allow airspeed or power to vary, you will be flying a simulated glider and are ready for some serious fun. (Soaring purists might turn up their noses at this imitation of their sport, but at least airplane drivers are not faced with the possibility of an off-airport landing when rising currents of air cannot be found.)
The challenge now is to proceed at normal cruise speed toward an area where it is suspected that rising air might be found. Once there, place the airplane in its simulated-soaring configuration and attempt to maintain altitude by seeking out rising air and avoidingsinking air. With luck, you'll either maintain or gain altitude for significant periods of time.
Most important is that you will gain an appreciation for the amount of vertical movement in the atmosphere and discover effects that often are camouflaged during normal flight. Learning to find and use them to advantage can provide quite a boost (from below) at a time when it might be needed.
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