What happens when the motor stops? The velocity of the machine gradually decreases: the resistance to forward movement stops its forward movement and the excess of weight over upward pressure due to velocity causes it to descend. It behaves like a projectile, but the details of behavior are seriously complicated by the variation in head resistance and sustaining force due to changes in the angle of the planes. The “angle of inclination” is now not the angle made by the planes with the horizontal, but the angle which they make with the path of flight. Theory indicates that this should be about two-thirds the angle which the path itself makes with the horizontal: that is, the planes themselves are inclined downward toward the front. The forces which determine the descent are fixed by the velocity and the angle between the planes and the path of flight. Manipulation of the rudders and main planes or even the motor may be practised to ensure lancing to best advantage; but in spite of these (or perhaps on account of these) scarcely any part of aviation offers more dangers, demands more genius on the part of the operator, and has been less satisfactorily analyzed than the question of “getting down.” It is easy to stay up and not very hard to “get up,” weather conditions being favorable; but it is an “all-sufficient job” to come down. Under the new rules of the International Aeronautic Federation, a test flight for a pilot’s license must terminate with a descent (motor stopped) in which the aviator is to land within fifty yards of the observers and come to a full stop inside of fifty yards therefrom. The elevation at the beginning of descent must be at least 150 feet.

Gliders

If the motor and its appurtenances, and some of the purely auxiliary planes, be omitted, we have a glider. The glider is not a toy; some of the most important problems of balancing may perhaps be some day solved by its aid. Any boy may build one and fly therewith, although a large kite promises greater interest. The cost is trifling, if the framework is of bamboo and the surfaces are cotton. Areas of glider surfaces frequently exceed 100 square feet. This amount of surface is about right for a person of moderate weight if the machine itself does not weigh over fifty pounds. By running down a slope, sufficient velocity may be attained to cause ascent; or in a favorable wind (up the slope) a considerable backward flight may be experienced. Excessive heights have led to fatal accidents in gliding experiments.

Models

The building of flying models has become of commercial importance. It is not difficult to attain a high ratio of surface to weight, but it is almost impossible to get motor power in the small units necessary without exceeding the permissible limit of motor weight. No gasoline engine or electric motor can be made sufficiently light for a toy model. Clockwork springs, if especially designed, may give the necessary power for short flights, but no better form of power is known just now than the twisted rubber band. For the small boy, a biplane with sails about eighteen inches by four feet, eighteen inches apart, anchored under his shoulders by six-foot cords while he rides his bicycle, will give no small amount of experience in balancing and will support enough of a load to make the experiment interesting.

Some Details: Balancing

It is easily possible to compute the areas, angles, and positions of auxiliary planes to give desired controlling or stabilizing effects; but the computation involves the use of accurate data as to positions of the various weights, and on the whole it is simpler to correct preliminary calculations by actually supporting the machine at suitable points and observing its balance. Stability is especially uncertain at very small angles of inclination, and such angles are to be avoided whether in ordinary operation or in descent. The necessity for rotating main planes in order to produce ascent is disadvantageous on this ground; but the proposed use of sliding or jockey weights for supplementary balancing appears to be open to objections no less serious. Steering may be perceptibly assisted, in as delicately a balanced device as the aeroplane, by the inclination of the body of the operator, just as in a bicycle. The direction of the wind in relation to the required course may seriously influence the steering power. Suppose the course to be northeast, the wind east, the independent speed of the machine and that of the wind being the same. The car will head due north. By bringing the rudder in position (a), the course may be changed to north, or nearly so, the wind exerting a powerful pressure on the rudder; but if a more easterly or east-northeast course be desired, and the rudder be thrown into the usual position therefor (b), it will exert no influence whatever, because it is moving before the wind and precisely at the speed of the wind.

It might be thought that, following analogies of marine engineering, the center of gravity of an aeroplane should be kept low. The effect of any unbalanced pressure or force against the widely extended sails of the machine is to rotate the whole apparatus about its center of gravity. The further the force from the center of gravity, the more powerful is the force in producing rotation. The defect in most aeroplanes (especially biplanes) is that the center of gravity is too low. If it could be made to coincide with the center of disturbing pressure, there would be no unbalancing effect from the latter. It is claimed that the steadiest machines are those having a high center of gravity; and the claim, from these considerations, appears reasonable.

Weights

It has been found not difficult to keep down the weight of framework and supporting surfaces to about a pound per square foot. The most common ratio of surface to total weight is about one to two: so that the machinery and operator will require one square foot of surface for each pound of their weight. On this basis, the smallest possible man-carrying aeroplane would have a surface scarcely below 250 square feet. Most biplanes have twice this surface: a thousand square feet seems to be the limit without structural weakness. Some recent French machines, designed for high speeds, show a greatly increased ratio of weight to surface. The Hanriot, a monoplane with wings upwardly inclined toward the outer edge, carries over 800 pounds on less than 300 square feet. The Farman monoplane of only 180 square feet sustains over 600 pounds. The same aviator’s racing biplane is stated to support nearly 900 pounds on less than 400 square feet.

Motor weights can be brought down to about two pounds per horse-power, but such extreme lightness is not always needed and may lead to unreliability of operation. The effect of an accumulation of ice, sleet, snow, rain, or dew might be serious in connection with flights in high altitudes or during bad weather. After one of his last year’s flights at Étampes Mr. Farman is said to have descended with an extra load of nearly 200 pounds on this account. With ample motor power, great flexibility in weight sustention is made possible by varying the inclination of the planes. In January of this year, Sommer at Douzy carried six passengers in a large biplane on a cross-country flight: and within the week afterward a monoplane operated by Le Martin flew for five minutes with the aeronaut and seven passengers, at Pau. The total weight lifted was about half a ton, and some of the passengers must have been rather light. The two-passenger Fort Myer biplane of the Wright brothers is understood to have carried about this total weight. These records have, however, been surpassed since they were noted. BrÉguet, at Douai, in a deeply-arched biplane of new design, carried eleven passengers, the total load being 2602 pounds, and that of aeronaut and passengers alone 1390 pounds. The flight was a short one, at low altitude; but the same aviator last year made a long flight with five passengers, and carried a load of 1262 pounds at 62 miles per hour. And as if in reply to this feat, Sommer carried a live load of 1436 pounds (13 passengers) for nearly a mile, a day or two later, at Mouzon. One feels less certain than formerly, now, in the snap judgment that the heavier-than-air machine will never develop the capacity for heavy loads.

Miscellaneous

French aviators are fond of employing a carefully designed car for the operator and control mechanism. The Wright designs practically ignore the car: the aviator sits on the forward edge of the lower plane with his legs hanging over.

It has been found that auxiliary planes must not be too close to the main wings: a gap of a distance about 50% greater than the width of the widest adjacent plane must be maintained if interference with the supporting air currents is to be avoided. Main planes are now always arched; auxiliary planes, not as universally. The concave under surface of supporting wings has its analogy in the wing of the bird and had long years since been applied in the parachute.

The car (if used) and all parts of the framework should be of “wind splitter” construction, if useless resistance is to be avoided. The ribs and braces of the frame are of course stronger, weight for weight, in this shape, since a narrow deep beam is always relatively stronger than one of square or round section. Excessive frictional resistance is to be avoided by using a smoothly finished fabric for the wings, and the method of attaching this fabric to the frame should be one that keeps it as flat as possible at all joints.

The sketches give the novel details of some machines recently exhibited at the Grand Central Palace in New York. The stabilizing planes were invariably found in the rear, in all machines exhibited.

The Things to Look After

The operator of an aeroplane has to do the work of at least two men. No vessel in water would be allowed to attain such speeds as are common with air craft, unless provided with both pilot and engineer. The aviator is his own pilot and his own engineer. He must both manage his propelling machinery and steer. Separate control for vertical rudders, elevating rudders and ailerons, for starting the engine; the adjustment of the carbureter, the spark, and the throttle to get the best results from the motor; attention to lubrication and constant watchfulness of the water-circulating system: these are a few of the things for him to consider; to say nothing of the laying of his course and the necessary anticipation of wind and altitude conditions.

These things demand great resourcefulness, but—for their best control—involve also no small amount of scientific knowledge. For example, certain adjustments at the motor may considerably increase its power, a possibly necessary increase under critical conditions: but if such adjustments also decrease the motor efficiency there must be a nice analysis of the two effects so that extra power may not be gained at too great a cost in radius of action.

The whole matter of flight involves both sportsman’s and engineers problems. Wind gusts produce the same effects as “turning corners”; or worse—rapidly changing the whole balance of the machines and requiring immediate action at two or three points of control. Both ascent and descent are influenced by complicated laws and are scarcely rendered safe—under present conditions—by the most ample experience. A lateral air current bewilders the steering and also demands special promptness and skill. To avoid disturbing surface winds, even over open country, a minimum flying height of 300 feet is considered necessary. This height, furthermore, gives more choice in the matter of landing ground than a lower elevation.

When complete and automatic balance shall have been attained—as it must be attained—we may expect to see small amateur aeroplanes flying along country roads at low elevations—perhaps with a guiding wheel actually in contact with the ground. They will cost far less than even a small automobile, and the expense for upkeep will be infinitely less. The grasshopper will have become a water-spider.