The air at the top and bottom of these whirls is moving in diametrically opposite directions, at the top with the wind, at the bottom against it, and since they are close to the earth they may therefore, as explained under "wind layers," be the source of decided danger to aeronauts. There may be danger also at the forward side of the eddy where the downward motion is greatest. When the wind is blowing strongly landings should not be made, if at all avoidable, on the lee sides of and close to steep mountains, hills, bluffs, or even large buildings; for these are the favorite haunts, as just explained, of treacherous "holes in the air." The whirl is best avoided by landing in an open place some distance from bluffs and large obstructions, or, if the obstruction is a hill, on the top of the hill itself. If, however, a landing to one side is necessary and the aeronaut has choice of sides, he should, other things being equal, take the windward and not the lee side. Finally, if a landing close to the lee side is compulsory he should, if possible, head along the hill, and not toward or from it; along the axis of the eddy and not across it. Such a landing would be safe, unless made in the down draft, since it would keep the machine in winds of nearly constant (zero) velocity with reference to its direction, whatever the side drift, provided the hill was of uniform height and slope and free from irregularities. But as hills seldom fulfill these conditions lee side landings of all kinds should be avoided. AERIAL TORRENTS. Just as water torrents are due to drainage down steep slopes, so, too, aerial torrents owe their origin to drainage down steep narrow valleys. Whenever the surface of the earth begins to cool through radiation or otherwise the air in contact with it becomes correspondingly chilled and, because of its increased density, flows away to the lowest level. Hence of clear still nights there is certain to be air drainage down almost any steep valley. When several such valleys run into a common one, like so many tributaries to a river, and especially when the upper reaches contain snow and the whole section is devoid of forest, the aerial river is likely to become torrential in nature along the lower reaches of the drainage channel. A flying machine attempting to land in the mouth of such a valley after the air drainage is well begun is in danger of going from relatively quiet air into an atmosphere that is moving with considerable velocity at times amounting almost to a gale. If one must land at such a place he should head up the valley so as to face the wind. If he heads down the valley and therefore runs with the wind he will, on passing into the swift air, lose his support, or much of it, for reasons already explained, and fall as though he had suddenly gotten into an actual "hole in the air." AERIAL BREAKERS. The term "aerial breakers" is used here in analogy with water breakers as a general name for the rolling, dashing and choppy winds that accompany thunderstorm conditions. They often are of such violence, up, down, and sideways in any and every direction that an aeroplane in their grasp is likely to have as uncontrolled and disastrous a landing as would be the case in an actual hole of the worst kind. Fortunately aerial breakers usually give abundant and noisy warnings, and hence the cautious aeronaut need seldom be, and, as a matter of fact seldom is caught in so dangerous a situation. However, more than one disaster is attributable to just such winds as these to aerial breakers. CLASSIFICATION. The above nine types of atmospheric conditions may conveniently be divided into two groups with respect to the method by which they force an aeroplane to drop. 1. The vertical group.-All those conditions of the atmosphere, such as aerial fountains, cataracts, cascades, breakers, and eddies (forward side), that, in spite of full speed ahead with reference to the air, make it difficult or impossible for an aeronaut to maintain his level, belong to a common class and depend for their effect upon a vertical component, up or down, in the motion of the atmosphere itself. Whenever the aeronaut, without change of the angle of attack and with a full wind in his face, finds his machine rapidly sinking, he may be sure that he has run into some sort of a down current. Ordinarily, however, assuming that he is not in the grasp of storm breakers, this condition, bad as it may seem, is of but little danger. The wind can not blow into the ground and therefore any down current, however vigorous, must somewhere become a horizontal current, in which the aeronaut may sail away or land as he chooses. 2. The horizontal group.-This group includes all those atmospheric conditions-wind layers, billows, eddies (central portion), torrents and the like-that, in spite of full speed ahead with reference to the ground, abruptly deprive an aeroplane of a portion at least of its dynamical support. When this loss of support, due to a running of the wind more or less with the machine, is small and the elevation sufficient there is but little danger, but, on the other hand, when the loss is relatively large, especially if near the ground, the chance of a fall is correspondingly great. CONCLUSIONS. 1. Holes in the air, in the sense of vacuous regions, do not exist. 2. Conditions in the atmosphere favorable to precipitous falls, such as would happen in holes, do exist, as follows: (a) VERTICAL group. 1. Aerial fountains.-Uprushes of air, most numerous during warm clear weather and over barren soil, especially above conical hills, are disconcerting and dangerous to the novice, but do not greatly disturb an experienced aviator. 2. Aerial cataracts.-Downrushes of the free air, like the uprushes with which they are associated in a vertical circulation, though less violent, must also be most frequent during warm weather when the ground is strongly heated. They, too, however annoying to the beginner, should not be dangerous to the experienced man, because even when strong enough to carry the machine down for a distance. their descent necessarily becomes slower and their chief velocity horizontal before the surface is reached. Downrushes of weighted air over precipices, analogous to waterfalls, must be strictly avoided. 3. Aerial cascades.-The lower wind, in following as it must surface contours, sweeps down to the leeward of hills and mountains in cascade-like falls, and the stronger the wind the more rapid the cascades. But they are of no danger to the aeronaut so long as he takes the precaution to keep above the eddies and other surface disturbances. 4. Aerial breakers.-The choppy, breaker-like winds of thunderstorms that surge up and down and in all sorts of directions are as much to be avoided by aerial craft as are ocean breakers by water craft. Hence a flight should positively not be attempted under any such circumstances. 5. Wind eddies (forward side).—The air on the forward side of a strong eddy has a rapid downward motion and therefore should be avoided. If caught in the down current of an eddy the aeronaut should head lengthwise of the hill or mountain to which the eddy is due. By heading away from the mountain he might, to be sure, get entirely out of the whirl, but the chances are just as great that instead of getting out he would only get the deeper in and thus encounter downward currents swifter and still more dangerous than those he had sought to shun. (b) HORIZONTAL GROUPS. 1. Wind layers.-The atmosphere is often made up of two or more superimposed layers moving each with its own velocity and direction. Such a condition is a source of danger to the aeronaut because transition from one of these layers to another more nearly coincident in direction and velocity with his aeroplane is certain to result in at least a sudden decrease in the magnitude of its supporting pressure and in the effectiveness of the balancing devices. Under certain extreme conditions this transition, even when the winds are parallel, is well nigh inevitably disastrous. When the layers move more or less across each other, as they usually do, the turmoil of the resulting short and choppy billows, by rendering equilibrium difficult, add an additional danger all their own. Dangerous wind layers are most frequent at flying levels during the transition of fair to foul weather. 2. Wind billows.-Wind waves analogous to water waves are set up at the interface between two layers that are moving with different velocities. If both layers are moving in the same direction, the resulting waves are long and regular; if in different directions, they are short and choppy. Therefore, other things being equal, it obviously is advisable to keep within the lower layer, or at least to get away from the billowy interface, either above or below, and to avoid crossing it oftener than is absolutely necessary. 3. Wind gusts.-The horizontal velocity of the wind near the surface of the earth is exceedingly irregular and fluctuates from second to second at times by as much as the whole of the average velocity. In such a wind, if at all swift, the support to an aeroplane is exceedingly erratic and both its launching and its landing full of danger. Obviously, too, the stronger the wind the higher, because of these surface disturbances, one should fly, if at all. 4. Wind eddies (central portion).-Eddies, or horizontal rolls in the atmosphere, are found on both the windward and lee sides, especially the latter, of cliffs and steep hills and mountains. When the wind is strong a landing should not be attempted in any such place. If forced to land in a place of this kind, the machine should be headed along and not at right angles to the direction of the hill. 5. Aerial torrents.-Steep barren valleys, especially of clear still nights and when the upper reaches are snow covered, are the beds of aerial drainage rivers that at times amount to veritable torrents. Therefore, however quiet the upper atmosphere and however smooth its sailing, it would be extremely dangerous to attempt to land an aeroplane at such a place and such a time. NOTE. All the above sources of danger, whether near the surface like the breakers, the torrents, and the eddies, or well up like the billows and the wind sheets, are less and less effective as the speed of the aeroplane is increased. But this does not mean that the swiftest machine necessarily is the safest; there are numerous other factors to be considered, and the problem of minimum danger or maximum safety, if the aeronaut insists, can only be solved by a proper combination of theory and practice, of sound reasoning and intelligent experimentation. REVIEW OF APPLIED MECHANICS.1 By L. LECORNU, Member of the Institute, Inspector General of Mines, Professor at l'École Polytechnique. The domain of applied mechanics is great. The number of publications devoted to it, in Europe as well as in America, forms an immense bibliography. I can not analyze here even briefly this mass of material. I shall limit myself, as in former years (1903, 1905, and 1909), to describing a few of the most interesting results. I. STEAM APPARATUS. Leprince-Ringuet published in the Revue de Mécanique (1911) a research upon the transmission of heat between a fluid in movement and a metallic surface. After examining various theories and experiments upon the subject he found that when a gas flows through a tube the quantity of heat transmitted per hour per square meter can be given by the expression B(CVp)", where B is the coefficient of transmission, varying inversely as the 0.13 power of the length of the tube; C is a coefficient increasing proportionately with the temperature; V the velocity of the gas in meters; p the weight of a cubic meter of the gas in kilograms; and n is an exponent which depends upon the relation of the cross section to the perimeter. For a circular tube of the diameter d it is practically The coefficients B and C vary only slightly from one liquid to another. The author concludes that the exchange of heat can be considerably increased by increasing the flow, the eddies, and the ratio of the perimeter to the cross section. It is also advantageous, whenever possible, to promote the condensation upon the metallic surfaces. Bone developed the idea of applying to the heating of steam boilers a process suggested by an old invention of Lude, which consisted in injecting through a contrivance formed of porous surfaces 1 Translated by permission from Revue générale des Sciences pures et appliqués, Paris, July 30, 1912, pp. 548-557. |