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July 1, 2012
By Barry Schiff
While attending (and speaking at) this year’s convention of the Soaring Society of America in Reno, Nevada, a frequent topic of conversation related to the state of soaring. These days every aspect of general aviation seems concerned about its future. A serious problem that soaring has always had is that glider schools and soaring sites are small in number and relatively far from major cities, which discourages many from becoming involved in this educational and challenging sport.
This led me many years ago to develop a fun procedure that enables power pilots to sample soaring in their own airplanes (without having to shut down their engines). The fundamentals of soaring involve flying gliders—enthusiasts refer to them as sailplanes—at their best glide speeds in rising air, such that altitude can be gained or maintained in the process. In other words, the sailplane needs to be flown in air that rises more rapidly than the glider would otherwise descend.
Although a high-performance sailplane can have a glide ratio well in excess of 50 to 1, a training glider has a much smaller glide ratio, about 30 to 1. This means that while being flown at its best glide speed of 60 knots, for example, it would descend in still air at two knots, or 200 fpm. A typical light airplane, however, has a glide ratio of only 8 or 9 to 1. It is not a particularly efficient glider. How, then, can an airplane be used in place of a glider to practice soaring? It is really quite easy.
Assume that you want to practice soaring in a Cessna 172P, for example. This airplane has a glide speed of 65 knots. For it to have a glide ratio of 30 to 1, it must have a sink rate of 2.2 knots (65 divided by 30), which is equal to 13,376 feet per hour or about 225 fpm. To simulate a training glider in a Cessna 172P, therefore, it is necessary only to glide at 65 knots while descending at only 225 fpm. This is going to require using a bit of power.
To determine the power required, establish a normal, power-off glide at 65 knots. Then slowly, very slowly, add power until the airplane is descending at only 225 fpm while maintaining 65 knots. That is all there is to it. Then note the required power setting for future use, adjust the friction lock to maintain that power setting, and, voilà! you’re flying a glider with a glide ratio of 30 to 1.
If this airspeed/power combination results in an uncomfortably nose-high attitude, extend the flaps to the first notch, setting, or position. Additional power will be required to maintain the same airspeed/sink-rate combination, but body angle likely will be reduced, and this improves over-the-nose visibility.
Once the required power setting has been determined (with the flaps up or extended partially, your choice), wait for a day of good thermal activity (convective lift) or proceed to an area where local winds are forced to rise because of a mountain slope (ridge lift).
The idea is to fly into an area of suspected lift (rising air) while strictly maintaining best glide speed and the power setting previously determined to simulate a glider with a 30:1 glide ratio. If you can then maintain altitude or climb, you’re soaring. If the airplane descends at more than 225 fpm (using the Cessna 172 example), you are in sinking air. With practice and the proper conditions, you will find it possible—and a great deal of fun—to gain altitude without varying airspeed or power. The goal, though, is not simply to sample the exhilaration of soaring, but to learn where and under what circumstances lift can be used to advantage, and to confirm in a very realistic manner the workings of the atmosphere and its effect on flight.
Learning how and where to find lift is not only invaluable to sailplane pilots but also to power pilots operating at heavy weights or high density altitudes. Knowing where to find rising air and how to avoid sinking air can go a long way toward maximizing the performance of any light airplane.
A discussion of up- and downdrafts leads to the question, “Can a strong downdraft force an airplane to the ground?”
Many pilots—including some professionals—do not believe that it can. Their reasoning is that a descending column of air approaching the ground must spread laterally. In other words, the vertical component of a downdraft weakens rapidly near the ground and affords the airplane an opportunity to escape the grip of involuntary descent. This might sound logical but is often incorrect.
Assume, for example, that an automobile is cruising along the highway. The relative wind it creates is horizontally analogous to a strong downdraft. Enter a large bug. Does the unsuspecting insect follow the deflected airstream around the windshield to safety? Probably not. The bug and the airplane have inertia (momentum), and this can make each go splat.
Visit the author’s website.
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