A Few Hours of Daylight

for the flow of air over the sur face of its wing. If the flow is fast enough, it will fly, if not, it will stall and descend vertically. What is happening in the air only A F®w IHI@lillIr~ a few feet away is of no concern, far less what is happening at the earth's surface below. Insofar as ©~ [))©lyllii!lffitt the glider is concerned, it is al ways descending at a constant rate J. S. Brennan for any given airspeed, and that airspeed is measured only by the rate of flow of the air over its wing. But what if a wind is blowing? Wind is assume<;l here to be a Articles on the sport of soar "gliding" bears a connotation of horizontal flow of air with respect ing, or motorless flight, are of descending, and "soaring" of as to the fixed earth. The glider, iso many kinds, ranging from the cending. Common usage also dis lated in its immediately surround highly technical to the highly tinguishes between a "glider" as ing air, is not concerned with the aesthetic, from low-speed aero being a motorless aircraft of low wind. The fact that the mass of dynamics to Jonathan Living performance, and a "sailplane" as air, through which the glider is stone Seagull. This article ad being one of high performance. flying at its constant airspeed and dresses the basic mechanics of We speak more of "performance" rate of descent, is moving with soaring, the weather conditions later. respect to the ground has abso and phenomena which permit Influence of Wind lutely no effect on the flight con soaring flight, but not the tech ditions of the glider. The pilot nique of flying a sailplane. It It is not difficult to accept the may be concerned that his path attempts to supply sufficient in fact that a motorless aircraft, be over the ground is modified by formation for the reader to gain it glider or sailplane, in stilI air wind-induced drift, or that his a reasonable comprehension not will be constantly descending. So speed over the ground is increased only of the sources of energy long as it is unaffected by out or decreased by wind components, which permit soaring, but, by way side influences, the glider will be but this is a matter of navigation, of fallout, an appreciation of converting the potential energy and has nothing to do with flight what the sport is all about. Hope of altitude into the kinetic energy performance. fully it will convey to the reader of forward motion, through which awareness of the fallacy con aerodynamic lift is generated to Vertical Flow of Air tained in the often-heard ques sustain the glider in flight, while tion, "Why did you land, did the some portion of the energy is Instead of the horizontal mo wind quit?" dissipated to the surrounding air tion of the atmosphere which we Throughout this article the through aerodynamic drag. Un call wind, let us stipulate the ex terms "gliding" and "soaring" will less energy is added to the sys istence of vertical motion. Vertical be used without precise distinc tem, the glider will eventually motion necessarily entails both up tion, although in common usage exp~nd all of its altitude and must ward and downward flow, so that land. In the case of a powered the general mass distribution of the J . S. Brennan, author of "A Few Hours aircraft, energy is continually atmosphere is maintained. Ignor of Daylight," was born in Utica, N.Y. He being fed into the system by the ing for the moment what might be received a B.S. degree in Optics from the the source of such vertical cur University of Rochester in 1943, and a engine. The glider must extract Juris Doctor degree from Georgetown Uni its energy from its natural sur rents of air, we can accept the versity in 1951. Since joining APL in 1965, roundings. The rate of this energy fact that a glider, flying in a mass Mr. Brennan served for several years as Acting Head of the Budget Office and is extraction is a function of the of air which is rising at a speed now the Administrator for Grants and Con existing meteorological conditions greater than the natural descend tracts. His interest in aviation was evident and of the skill of the pilot. ing speed of the glider, will gain at an early age, and he learn"ed to fly powered .aircraft in 1944. He received his The glider is extremely self altitude. These rising currents of private license in gliders in 1960, and has centered. It will fly straight or air are generically called "lift," been an active member of the Mid-Atlantic Soaring Association based in Frederick, turn, fly fast or slowly at the be while the accompanying down Maryland, since 1962. hest of the pilot, but it cares only ward currents are called "sink." Volume 12, Number 3 11 Depending on the nature of the knots or more at the crest of the meteorological phenomenon which ~::=~===- ridge, and increasing with alti gives rise to the currents, the tech tude; and (c) the existence of a nique of extracting its energy, that stable layer (e.g. a temperature is, the pattern of flight, will differ. inversion) in the atmosphere sep The pilot's task is to remain in arating layers of low stability the lift until he has reached some above and below. desired altitude, and then to ex Schematically the flow pattern pend the altitude in straight flight which accompanies the wave is along his intended path. shown in Fig. 2. While the wave The vertical currents can result pattern is stationary with respect from a number of naturally oc to the ridge, the air is flowing curring conditions. The principal Fig. I-Air flow against an obstruction through it. Typically the horizon sources of such lift are known as resulting in the generation of ridge lift. tal component of the flow at "ridge lift," "mountain wave," some altitude above the crest of and "thermals." In addition, there Altitude is gained, or main the ridge will be near or above are coldfronts, shear lines, oro tained, in ridge lift by flying back the stalling speed of the glider. graphic thermals, anabatic winds, and forth across the face of the By flying into the wind, the pilot thermal wave, and all varieties terrain feature which causes it. can control his position with re of combinations of some of these Generally it is necessary to fly spect to the wave by changing his effects. Almost all soaring in this quite close to the face of the airspeed, or if the wind velocity country uses one or more of the slope to find the strongest lift, so is too low, he can tack back and first three named effects, and an that as a safety precaution, all forth at a small angle to the wind understanding of them will suffice turns are made away from the direction. for all practical purposes. slope. Rules of the road have been devised to minimize interference when more than one glider is ~15r------------------' Ridge Lift Ql involved. ~ Ridge lift is the simplest of the '0 three to understand. It is the re Lee Wave sult of a wind of some velocity, perhaps ten knots or more, blow Mountain wave, or lee wave as ing against the face of a sharp it is frequently called, provides an w o opportunity of flying to relatively ::J rise in the terrain, such as a cliff I- or a mountain ridge. Since the high altitudes. Like ridge lift, it is i= ....J air must rise to get over the ob generated by a wind blowing « struction, there is a vertical com nearly perpendicularly to a ridge, ponent to the wind flow, and it but it is found on the lee side of is this vertical component that the ridge in the form of a stand Fig. 2-Air flow over a mountain or ridge resulting in lee wave. provides the lift. The flow is ing wave. It can be visualized by shown diagrammatically in Fig. analogy to a log lying submerged 1. The location of the maximum across a stream. The water rises While it is possible under most lift will vary somewhat with the to cross the obstruction, falls on conditions to climb in any of the shape of the rise in the ground, the downstream side, and then upward flowing parts of the wave, with the velocity of the wind, and may rise and fall several more the greatest climb rate and peak with altitude. Characteristically it times before quiescent flow is altitude is usually found in the will be located along some line, again established. In a similar primary wave, i.e., the wave such as the line x-x' in the figure, fashion, under the appropriate closest to the ridge. If a glider and always on the upwind side conditions, a wave is formed in is introduced into the wave sys of the slope. Corresponding to the the air downwind from a ridge. tem at some point A in the figure, lift on the upwind side of a ridge, While the appropriate conditions the vertical component of the air there is nearly always sink on the are not precisely defined, they flow will permit the glider to downwind side at low altitudes. consist essentially of the follow ascend not only along the stream Ridge lift is usually associated ing: (a) wind direction nearly line, but also from one streamline with a terrain feature of some perpendicular to the length of the to another until some point B is length, although it might be found ridge, and nearly constant with reached, at which point the lift near an isolated hill.

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