Popular Science Monthly/Volume 81/July 1912/Holes in the Air
| HOLES IN THE AIR |
By W. J. HUMPHREYS, Ph.D.
PROFESSOR OF METEOROLOGICAL PHYSICS, U. S. WEATHER BUREAU, WASHINGTON, D. C.
THE bucking and balking, the rearing, plunging and other evidences of the mulish nature of the modern Pegasus soon inspired aerial jockeys to invent picturesque terms descriptive of their steeds and of the conditions under which their laurels were won or lost. One of the best of these expressions, one that is very generally used and seems to be a permanent acquisition, is "holes in the air." There are, of course, no holes in the ordinary sense of the term in the atmosphere—no vacuous regions—but the phrase "holes in the air" is brief and elegantly expressive of the fact that occasionally at various places in the atmosphere there are conditions which, so far as flying is concerned, are mighty like unto holes. Such conditions are indeed real, and it is the purpose of this paper to point out what some of them are, when and where they are most likely to occur and how best to avoid them.
Suppose for a moment that there was a big hole in the atmosphere, a place devoid of air and of all pressure. The surrounding air would rush in to fill this space with the velocity pertaining to free particles of the atmosphere at the prevailing temperature; that is to say, at the
velocity of sound in air at the same temperature, and therefore, at ordinary temperature, of about 1,100 feet per second, or 750 miles per hour. Even then, if such a hole existed, it would be impossible for an aeronaut to get into it—he couldn't catch up with it.
But, according to the claims of some, if there are no complete holes in the atmosphere there are, at any rate, places where the density is much less than that of the surrounding air; so much less indeed that when an aeroplane runs into one of them it drops quite as though it was in a place devoid of all air and without support of any kind.
This too, like the actual hole, is a pure fiction that has no support in barometrical records. Indeed, such a condition, as every scientific man knows, could be established and maintained only by a gyration or whirl of the atmosphere, such that the "centrifugal force" would be sufficient to equal the difference in pressure, at the same level, between the regions of high and low density.
Appropriate equations can be written to express the balance between pressure gradient and centrifugal force in any sort of winds, and at any part of the world (it depends slightly upon latitude). Therefore it is possible with certain conditions given to compute the wind velocity, or with other conditions given to compute the pressure gradient. But in the present case numerical calculations are not necessary. We know that an elevation of half a mile, easily reached by any aeroplane, produces roughly a 1 per cent, decrease in pressure; and we know too that a greater pressure difference than this seldom exists even between center and circumference of violent tornadoes. Hence a drop in density, or pressure, to which the density is directly proportional, sufficient to cause an aeroplane to fall, would require a tornadic whirl of the most destructive violence. Xow there were no whirlwinds of importance in the air, certainly none that could be called tornadoes, at the times and places where aeronauts have reported holes, and therefore even half holes, in the sense of places sufficiently vacuous to cause a fall, must also be discarded as unreal, if not impossible.
Along with these two impossibles, the hole and the half hole, the vacuum and the half vacuum, should be consigned to oblivion that other picturesque fiction,—the "pocket of noxious gas." Probably no other gases, certainly very few, have, at ordinary temperatures and pressures, the same density as atmospheric air. Therefore a pocket of foreign gas in the atmosphere would almost certainly either bob up like a balloon, or sink like a stone in water: it could not float in mid air. It is possible, of course, as will be discussed a little later, to run into columns of rising air that may contain objectionable gases and odors, but these columns are quite different from anything likely to be suggested by the expression, "pockets of gas."
The above are some of the things that fortunately, alike for those who walk the earth and those who fly the air, do not exist. We will now consider some of the things that do exist and produce effects such as actual holes and half holes would produce—sudden drops, and occasional disastrous falls.
Aerial Fountains
A mass of air rises or falls according as its density is less or greater respectively than that of the surrounding atmosphere, just as, and for the same reason that a cork bobs up in water and a stone goes down. Hence warm and therefore expanded and light air is buoyed up whenever the surrounding air at the same level is colder; and as the atmosphere is heated mainly through contact with the surface of the earth, which in turn has been heated by sunshine, it follows that these convection currents, or vertical uprushes of the atmosphere, are most numerous during warm clear weather.
The turbulence of some of these rising columns is evident from the numerous rolls and billows of the large cumulus clouds they produce, and it is obvious that the same sort of turbulence, probably on a smaller scale, occurs near the tops of those columns that do not rise to the cloud level. Further, it is quite possible, when the air is exceptionally quiet, for a rising column to be rather sharply separated from the surrounding quiescent atmosphere, as is evident from the closely adhering long columns of smoke occasionally seen to rise from chimneys.
The velocity of ascent of such fountains of air is, at times, surprisingly great. Measurements on pilot balloons, and measurements taken in manned balloons, have shown vertical velocities, both up and down, of as much as 10 feet per second. The soaring of large birds is a further proof of an upward velocity of the same order of magnitude, while the fact that in cumulus clouds water drops and hailstones often are not only temporarily supported, but even carried to higher levels, shows that uprushes of 25 to 30 feet per second not merely may but actually do occur.
There are, then, aerial fountains of considerable vertical velocity whose sides at times and places may be almost as sharply separated from the surrounding air as are the sides of a fountain of water, and it is altogether possible for the swiftest of these to produce effects more or less disconcerting to the aeronaut. The trouble may occur:
1. On grazing the column, with one wing in the rising and the other in the stationary air; a condition that interferes with lateral stability, and produces a sudden shock both on entering the column and on leaving it.
2. On plunging squarely into the column; thus suddenly increasing the angle of attack, the pressure on the wings, and the angle of ascent.
3. On abruptly emerging from the column; thereby causing a sudden decrease in the angle of attack and also abruptly losing the supporting force of the rising mass of air.
That flying with one wing in the column and the other out must interfere with lateral stability and possibly cause a fall as though a hole had been encountered, is obvious, but the effects of plunging squarely into or out of the column require a little further consideration.
Let an aeroplane that is flying horizontally pass from quiescent air squarely into a rising column. The front of the machine will be lifted, as it enters the column, a little faster than the rear, and the angle of attack, that is to say, the angle at which the wing is inclined to the horizon, will be slightly increased. This, together with the rising air, will rapidly carry the machine to higher levels, which, of itself, is not important. If, however, the angle of attack is so changed by the pilot as to keep the machine, while in the rising column, at a constant level, and if, with this new adjustment, the rising column is abruptly left, a rapid descent must begin—the half hole is met. But even this is not necessarily harmful. Probably the real danger under such circumstances arises from over adjustments by the aeronaut in his hasty attempt to correct for the abrupt changes. Such an adjustment might well cause a fall so sudden as strongly to suggest an actual hole in the air.
Rising columns, of the nature just described occur most frequently during clear summer days and over barren ground. Isolated hills, especially short or conical ones, should be avoided during warm still days, for on such occasions their sides are certain to be warmer than the adjacent atmosphere at the same level, and hence to act like so many chimneys in producing updrafts. Eising air columns occur less frequently and are less vigorous over water, and over level green vegetation, than elsewhere. They are also less frequent during the early forenoon than in the hotter portion of the day, and practically absent before sun rise, and at such times as the sky is wholly covered with clouds.
Aerial Cataracts
The aerial cataract is the counterpart of the aerial fountain, and js most likely to occur at the same time. It is seldom rapid, save in connection with thunder storms, and such effect as it may have is exactly similar to, but in the opposite direction from, that of the rising column.
Aerial Cascades
The term "aerial cascade" may, with some propriety, be applied to the wind as it sweeps down the lee side of a hill or mountain. It is most pronounced when the wind is at right angles to the direction of the ridge, and when the mountain is rather high and steep. The swift downward sweep of the air when the wind is strong may carry the aeroplane with it and lead observers, if not the pilot, to fancy that another hole has been encountered, when of course, there is nothing of the kind. Indeed such cascades should be entirely harmless so long as the aeronaut keeps his machine well above the surface and therefore out of the treacherous eddies, presently to be discussed.
Wind Layers
It is a common thing to see two or more layers of clouds moving in different directions and at different velocities. Judgment of both the actual and the relative velocities of the cloud layers may be badly in error—the lower seems to be moving faster, and the higher slower, than is actually the case. Accurate measurements, however, are possible and have often been made.
These differences in direction and velocity of the winds are not confined to cloud layers, nor even to cloudy weather, as both pilot and manned balloons have often shown-Occasionally balloons float for long intervals with a wind in the basket, showing that the top and the bottom of the balloon are in currents of different velocities. Another evidence of wind layers moving with different velocities is the waves or billows so often seen in a cloud layer. A beautiful example of the?e cloud waves, both regular, when the directions are the same but the velocities different, and irregular, when the winds are more or less crossed, is shown in the accompanying picture, taken by Professor A. J. Henry, of the U. S. Weather Bureau, and kindly lent for this illustration.
It was explained by Helmholtz as far back as 1889 that layers of air differing in density are of frequent occurrence, and that they glide, sharply divided and with but little intermingling, the one over another in much the same manner that air flows over water, and with the same general wave-producing effect. These air waves are "seen" only when the humidity at the interface is such that the slight difference in temperature between the crests and the troughs is sufficient to keep the one cloud-capped and the other free from condensation. In short, the humidity condition must be just right, and therefore, though such clouds are often seen, air billows must be of far more frequent occurrence.
Consider now the effect on an aeroplane as it passes from one such layer into another. For the sake of illustration let the case be an extreme one. Let the propeller be at rest and the machine be making a straight away glide to earth, and let it suddenly pass into a lower layer of air moving in the same horizontal direction as the machine and with the same velocity. This of course is an extreme case, but it is by no means an impossible one. Instantly on entering the lower layer, under the conditions just described, all dynamical support must cease and with it all power of guidance. A fall, for at least a considerable distance, is absolutely inevitable, and a disastrous one highly probable. To all intents and purposes a hole, a perfect vacuum, has been run into.
The reason for the fall will be understood when it is recalled that, for all ordinary velocities, wind pressure is very nearly proportional to the square of the velocity of the wind with respect to the tiling against which it is pressing. Hence, for a given inclination of the wings, the lift on an aeroplane is approximately proportional to the square of the velocity of the machine with reference, not to the ground, but to the air in which it happens to be at the instant under consideration. If then it glides, with propellers at rest, into air that is moving in the same horizontal direction and with the same velocity it is in exactly the condition it would be if dropped from the top of a monument in still air. It must inevitably fall to ruin, unless indeed rare skill in balancing or, possibly, mere chance should bring about a new glide after additional velocity had been acquired as the result of a considerable fall. Warping of wings, turning of ailerons, dipping and twisting of rudders, and all the other devices of this nature would be utterly useless at first, totally without effect so long as wind and machine have the same velocity, for, as already explained, there would be no pressure on them in any position and consequently nothing that could be done with them would at first have any effect on the behavior of the machine. However, as stated above, a skillful pilot may secure a new glide with a properly constructed machine, and finally, if high enough, make a safe landing.
Of course, such an extreme case must be of rare occurrence, but cases less extreme are met with frequently. On passing into a current where the velocity of the wind is more nearly that of the aeroplane, and in the same direction, more or less of the supporting force is instantly lost, and a corresponding drop or dive inevitable. Ordinarily, however, this is a matter of small consequence, for the new speed necessary to support the machine is soon acquired, especially if the engine is in full operation. Occasionally though the loss in support may be large and occur but a short distance above the ground, and therefore be distinctly dangerous.
If the new wind layer is against and not with the machine an increase instead of a decrease in the sustaining force is the result, and but little occurs beyond a mere change in the horizontal speed of the machine with reference to the ground, and a slowing up of its rate of descent.
"Wind sheets, within ordinary flying levels, are most frequent during weather changes, especially as fine weather is giving way to stormy. This then is a time to be on one's guard against the most dangerous of-all "holes in the air." It is also well to avoid making great changes in altitude since wind sheets, of whatever intensity, remain roughly parallel to the surface of the earth, and the greater the change in altitude the greater the risk of running into a treacherous "hole." Also, lest there might be a wind sheet near the surface, and for other good reasons, landings should be made, if possible, squarely in the face of the surface wind.
Wind Billows
It was stated above that when one layer of air runs over another of different density billows are set up between them, as illustrated by the cloud picture. Of course, as above explained, the warning clouds are comparatively seldom present, and therefore even the cautious aeronaut may, with no evidence of danger before him, take the very level of the billows themselves, and before getting safely above or below them encounter one or more sudden changes in wind velocity and direction due, in part, to the eddy-like or rolling motion within the billows, with chances in each case of being suddenly deprived of a large part of the requisite sustaining force—of encountering a "hole in the air." There may be perfect safety in either layer, but, unless headed just right, there necessarily is some risk in going from the one to the other, and therefore, since flying at the billow level would necessitate frequent transitions of this dangerous nature, it should be strictly avoided.
Wind Eddies
Eddies and whirls exist in every stream of water, from tiny rills to the great rivers and even the ocean currents, wherever the banks are such as greatly to change the direction of flow and wherever there is a pocket of considerable depth and extent on either side. Similar eddies, but with horizontal instead of vertical axes, occur at the bottoms of streams where they flow over ledges that produce abrupt changes in the levels of the beds.
The inertia of the stream of water, its tendency to keep on in the direction it is actually moving and with unchanged velocity, together with its viscosity, necessitate these whirls with which nearly all are familiar. Similarly, and for the same general reasons, horizontal eddies occur in the atmosphere, and the stronger the wind the more rapid the rotation of the eddy. They are most pronounced on the lee side of cuts, cliffs and steep mountains, but occur also, to a less extent, on the windward side of such places.
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 side 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 thunder-storm 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 eight 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.—Down rushes of air, like the up rushes 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 slow and their chief velocity horizontal before the surface is reached.
3. Aerial Cascades.—Rapid falls of air are found to the lee sides of hills and mountains, and the stronger the wind the more rapid the cascade. 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 thunder storms 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 encounter downward currents of higher speed.
b. Horizontal Group
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 a sudden decrease in the magnitude of its supporting pressure and in the effectiveness of the balancing devices. Under certain extreme conditions this transition is well nigh inevitably disastrous.
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 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.
4. 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 practise, of sound reasoning and intelligent experimentation.