Fig. 51. and Fig. 52.
ELEVATING PLANES.—We must now have the general side elevation of the frame, the planes, their angles, the tail and the rudder support, and the frame for the forward elevator.
To this may be added the forward elevating plane L, the rear elevator, or tail M, and the vertical steering rudder N.
The frame which supports the structure thus described, may be made in a variety of ways, the object being to provide a resilient connection for the rear wheel O.
Fig. 52 shows a frame which is simple in construction and easily attached. The lower fore and aft side bars P have the single front wheel axle at the forward end, and the aft double wheels at the rear end, a flexible bar Q, running from the rear wheel axle to the forward end of the lower plane.
A compression spring R is also mounted between the bar and rear end of the lower plane to take the shock of landing. The forward end of the bar P has a brace S extending up to the front edge of the lower plane, and another brace T connects the bars P, S, with the end of the forwardly- projecting frame.
Fig. 53. Plan view.
The full page view, Fig. 53, represents a plan view, with one of the wings cut away, showing the general arrangement of the frame, and the three wheels required for support, together with the brace bars referred to.
The necessity of the rear end elevation will now be referred to. The tail need not, necessarily, be located at a point on a horizontal line between the planes. It may be higher, or lower than the planes, but it should not be in a position to touch the ground when the machine is about to ascend.
Fig. 54. Alighting.
The angle of ascension in the planes need not exceed 25 degrees so the frame does not require an angle of more than 17 degrees. This is shown in Fig. 54, where the machine is in a position ready to take the air at that angle, leaving ample room for the steering rudder.
ACTION IN ALIGHTING.—Also, in alighting, the machine is banked, practically in the same position thus shown, so that it alights on the rear wheels O.
The motor U is usually mounted so its shaft is midway between the planes, the propeller V being connected directly with the shaft, and being behind the planes, is on a medial line with the machine.
The control planes L, M, N, are all connected up by means of flexible wires with the aviator at the set W, the attachments being of such a character that their arrangement will readily suggest themselves to the novice.
THE MONOPLANE.—From a spectacular standpoint a monoplane is the ideal flying machine. It is graceful in outline, and from the fact that it closely approaches the form of the natural flyer, seems to be best adapted as a type, compared with the biplane.
THE COMMON FLY.—So many birds have been cited in support of the various flying theories that the house fly, as an example has been disregarded. We are prone to overlook the small insect, but it is, nevertheless, a sample which is just as potent to show the efficiency of wing surface as the condor or the vulture.
The fly has greater mobility than any other flying creature. By the combined action of its legs and wings it can spring eighteen inches in the tenth of a second; and when in flight can change its course instantaneously.
If a sparrow had the same dexterity, proportionally, it could make a flight of 800 feet in the same time. The posterior legs of the fly are the same length as its body, which enable it to spring from its perch with amazing facility.
Fig. 55. Common Fly. Outstretched Wings.
The wing surface, proportioned to its body and weight, is no less a matter for wonder and consideration.
In Fig. 55 is shown the outlines of the fly with outstretched wings. Fig. 56 represents it with the wing folded, and Fig. 57 is a view of a wing with the relative size of the top of the body shown in dotted lines.
Fig. 56. Common Fly. Folded Wings.
The first thing that must attract attention, after a careful study is the relative size of the body and wing surface. Each wing is slightly smaller than the upper surface of the body, and the thickness of the body is equal to each wing spread.
Fig. 57. Relative size of wing and body.
The weight, compared with sustaining surface, if expressed in understandable terms, would be equal to sixty pounds for every square foot of surface.
STREAM LINES.—The next observation is, that what are called stream lines do not exist in the fly. Its head is as large in cross section as its body, with the slightest suggestion only, of a pointed end. Its wings are perfectly flat, forming a true plane, not dished, or provided with a cambre, even, that upward curve, or bulge on the top of the aeroplane surface, which seems to possess such a fascination for many bird flight advocates.
It will also be observed that the wing connection with the body is forward of the line A, which represents the point at which the body will balance itself, and this line passes through the wings so that there is an equal amount of supporting surface fore and aft of the line.
Again, the wing attachment is at the upper side of the body, and the vertical dimension of the body, or its thickness, is equal to four-fifths of the length of he wing.
The wing socket permits a motion similar to a universal joint, Fig. 55 showing how the inner end of the wing has a downward bend where it joins the back, as at B.
THE MONOPLANE FORM.—For the purpose of making comparisons the illustrations of the monoplane show a machine of 300 square feet of surface, which necessitates a wing spread of forty feet from tip to tip, so that the general dimensions of each should be 18 1/2 feet by 8 1/2 feet at its widest point.
First draw a square forty feet each way, as in Fig. 58, and through this make a horizontal line 1, and four intermediate vertical lines are then drawn, as 2, 3, 4, 5, thus providing five divisions, each eight feet wide. In the first division the planes A, B, are placed, and the tail, or elevator C, is one-half the width of the last division.
Fig. 58. Plan of Monoplane.
The frame is 3 1/2 feet wide at its forward end, and tapers down to a point at its rear end, where the vertical control plane D is hinged, and the cross struts E, E, are placed at the division lines 3, 4, 5.
The angles of the planes, with relation to the frame, are usually greater than in the biplane, for the reason that the long tail plane requires a greater angle to be given to the planes when arising; or, instead of this, the planes A, B, are mounted high enough to permit of sufficient angle for initiating flight without injuring the tail D.
Some monoplanes are built so they have a support on wheels placed fore and aft. In others the tail is supported by curved skids, as shown at A, Fig. 59, in which case the forward supporting wheels are located directly beneath the planes. As the planes are at about eighteen degrees angle, relative to the frame, and the tail plane B is at a slight negative angle of incidence, as shown at the time when the engine is started, the air rushing back from the propeller, elevates the tail, and as the machine moves forwardly over the ground, the tail raises still higher, so as to give a less angle of incidence to the planes while skimming along the surface of the ground.
Fig. 59. Side Elevation, Monoplane.
In order to mount, the tail is suddenly turned to assume a sharp negative angle, thus swinging the tail downwardly, and this increases the angle of planes to such an extent that the machine leaves the ground, after which the tail is brought to the proper angle to assure horizontal flight.
The drawing shows a skid at the forward end, attached to the frame which carries the wheels. The wheels are mounted beneath springs so that when the machine alights the springs yield sufficiently to permit the skids to strike the ground, and they, therefore, act as brakes, to prevent the machine from traveling too far.
THIS is a phase of the flying machine which has the greatest interest to the boy. He instinctively sees the direction in which the machine has its life,—its moving principle. Planes have their fascination, and propellers their mysterious elements, but power is the great and absorbing question with him.
We shall try to make its application plain in the following pages. We have nothing to do here with the construction and operation of the motor itself, as, to do that justice, would require pages.
FEATURES IN POWER APPLICATION.—It will be more directly to the point to consider the following features of the power and its application:
1. The amount of power necessary.
2. How to calculate the power applied.
3. Its mounting.
WHAT AMOUNT OF POWER IS NECESSARY.—In the consideration of any power plant certain calculations must be made to determine what is required. A horse power means the lifting of a certain weight, a definite distance, within a specified time.
If the weight of the vehicle, with its load, are known, and its resistance, or the character of the roadway is understood, it is a comparatively easy matter to calculate just how much power must be exerted to overcome that resistance, and move the vehicle a certain speed.
In a flying machine the same thing is true, but while these problems may be known in a general way, the aviator has several unknown elements ever present, which make estimates difficult to solve.
THE PULL OF THE PROPELLER.—Two such factors are ever present. The first is the propeller pull. The energy of a motor, when put into a propeller, gives a pull of less than eight pounds for every horse power exerted.
FOOT POUNDS.—The work produced by a motor is calculated in Foot Pounds. If 550 pounds should be lifted, or pulled, one foot in one second of time, it would be equal to one horse power.
But here we have a case where one horse power pulls only eight pounds, a distance of one foot within one second of time, and we have utilized less than one sixty-fifth of the actual energy produced.
SMALL AMOUNT OF POWER AVAILABLE.—This is due to two things: First, the exceeding lightness of the air, and its great elasticity; and, second, the difficulty of making a surface which, when it strikes the air, will get a sufficient grip to effect a proper pull.
Now it must be obvious, that where only such a small amount of energy can be made available, in a medium as elusive as air, the least change, or form, of the propeller, must have an important bearing in the general results.
HIGH PROPELLER SPEED IMPORTANT.—Furthermore, all things considered, high speed is important in the rotation of the propeller, up to a certain point, beyond which the pull decreases in proportion to the speed. High speed makes a vacuum behind the blade and thus decreases the effective pull of the succeeding blade.
WIDTH AND PITCH OF BLADES.—If the blade is too wide the speed of the engine is cut down to a point where it cannot exert the proper energy; if the pitch is very small then it must turn further to get the same thrust, so that the relation of diameter, pitch and speed, are three problems far from being solved.
It may be a question whether the propeller form, as we now know it, is anything like the true or ultimate shape, which will some day be discovered.
EFFECT OF INCREASING PROPELLER PULL.—If the present pull could be doubled what a wonderful revolution would take place in aerial navigation, and if it were possible to get only a quarter of the effective pull of an engine, the results would be so stupendous that the present method of flying would seem like child's play in comparison.
It is in this very matter,—the application of the power, that the bird, and other flying creatures so far excel what man has done. Calculations made with birds as samples, show that many of them are able to fly with such a small amount of power that, if the same energy should be applied to a flying machine, it would scarcely drive it along the ground.
DISPOSITION OF THE PLANES.—The second factor is the disposition or arrangement of the planes with relation to the weight. Let us illustrate this with a concrete example:
We have an aeroplane with a sustaining surface of 300 square feet which weighs 900 pounds, or 30 pounds per square foot of surface.
DIFFERENT SPEEDS WITH SAME POWER.—Now, we may be able to do two things with an airship under those conditions. It may be propelled through the air thirty miles an hour, or sixty miles, with the expenditure of the same power.
An automobile, if propelled at sixty, instead of thirty miles an hour, would require an additional power in doing so, but an airship acts differently, within certain limitations.
When it is first set in motion its effective pull may not be equal to four pounds for each horse power, due to the slow speed of the propeller, and also owing to the great angle of incidence which resists the forward movement of the ship.
INCREASE OF SPEED ADDS TO RESISTANCE.—Finally, as speed increases, the angle of the planes decrease, resistance is less, and up to a certain point the pull of the propeller increases; but beyond that the vacuum behind the blades becomes so great as to bring down the pull, and there is thus a balance,—a sort of mutual governing motion which, together, determine the ultimate speed of the aeroplane.
HOW POWER DECREASES WITH SPEED.—If now, with the same propeller, the speed should be doubled, the ship would go no faster, because the bite of the propeller on the air would be ineffective, hence it will be seen that it is not the amount of power in itself, that determines the speed, but the shape of the propeller, which must be so made that it will be most effective at the speed required for the ship.
While that is true when speed is the matter of greatest importance, it is not the case where it is desired to effect a launching. In that case the propeller must be made so that its greatest pull will be at a slow speed. This means a wider blade, and a greater pitch, and a comparatively greater pull at a slow speed.
No such consideration need be given to an automobile. The constant accretion of power adds to its speed. In flying machines the aviator must always consider some companion factor which must be consulted.
HOW TO CALCULATE THE POWER APPLIED.—In a previous chapter reference was made to a plane at an angle of forty-five degrees, to which two scales were attached, one to get its horizontal pull, or drift, and the other its vertical pull, or lift.
PULLING AGAINST AN ANGLE.—Let us take the same example in our aeroplane. Assuming that it weighs 900 pounds, and that the angle of the planes is forty-five degrees. If we suppose that the air beneath the plane is a solid, and frictionless, and a pair of scales should draw it up the incline, the pull in doing so would be one-half of its weight, or 450 pounds.
It must be obvious, therefore, that its force, in moving downwardly, along the surface A, Fig. 60, would be 450 pounds.
The incline thus shown has thereon a weight B, mounted on wheels a, and the forwardly-projecting cord represents the power, or propeller pull, which must, therefore, exert a force of 450 pounds to keep it in a stationary position against the surface A.
In such a case the thrust along the diagonal line E would be 900 pounds, being the composition of the two forces pulling along the lines D, F.
THE HORIZONTAL AND VERTICAL PULL.—Now it must be obvious, that if the incline takes half of the weight while it is being drawn forwardly, in the line of D, if we had a propeller drawing along that line, which has a pull of 450 pounds, it would maintain the plane in flight, or, at any rate hold it in space, assuming that the air should be moving past the plane.
Fig. 60. Horizontal and Vertical pull.
The table of lift and drift gives a fairly accurate method of determining this factor, and we refer to the chapter on that subject which will show the manner of making the calculations.
THE POWER MOUNTING.—More time and labor has been wasted, in airship experiments, in poor motor mounting, than in any other direction. This is especially true where two propellers are used, or where the construction is such that the propeller is mounted some distance from the motor.
SECURING THE PROPELLER TO THE SHAFT.—But even where the propeller is mounted on the engine shaft, too little care is exercised to fix it securely. The vibratory character of the mounting makes this a matter of first importance. If there is a solid base a poorly fixed propeller will hold much longer, but it is the extreme vibration that causes the propeller fastening to give way.
VIBRATIONS.—If experimenters realized that an insecure, shaking, or weaving bed would cause a loss of from ten to fifteen per cent. in the pull of the propeller, more care and attention would be given to this part of the structure.
WEAKNESSES IN MOUNTING.—The general weaknesses to which attention should be directed are, first, the insecure attachment of the propeller to the shaft; second, the liability of the base to weave; or permit of a torsional movement; third, improper bracing of the base to the main body of the aeroplane.
If the power is transferred from the cylinder to the engine shaft where it could deliver its output without the use of a propeller, it would not be so important to consider the matter of vibration; but the propeller, if permitted to vibrate, or dance about, absorbs a vast amount of energy, while at the same time cutting down its effective pull.
Aside from this it is dangerous to permit the slightest displacement while the engine is running. Any looseness is sure to grow worse, instead of better, and many accidents have been registered by bolts which have come loose from excessive vibration. It is well, therefore, to have each individual nut secured, or properly locked, which is a matter easily done, and when so secured there is but little trouble in going over the machine to notice just how much more the nut must be taken up to again make it secure.
THE GASOLINE TANK.—What horrid details have been told of the pilots who have been burned to death with the escaping gasoline after an accident, before help arrived. There is no excuse for such dangers. Most of such accidents were due to the old practice of making the tanks of exceedingly light or thin material, so that the least undue jar would tear a hole at the fastening points, and thus permit the gasoline to escape.
A thick copper tank is by far the safest, as this metal will not readily rupture by the wrench which is likely in landing.
WHERE TO LOCATE THE TANK.—There has been considerable discussion as to the proper place to locate the tank. Those who advocate its placement overhead argue that in case of an accident the aeroplane is likely to overturn, and the tank will, therefore, be below the pilot. Those who believe it should be placed below, claim that in case of overturning it is safer to have the tank afire above than below.
DANGER TO THE PILOT.—The great danger to the pilot, in all cases of accidents, lies in the overturning of the machine. Many have had accidents where the machine landed right side up, even where the fall was from a great height, and the only damage to the aviator was bruises. Few, if any, pilots have escaped where the machine has overturned.
It is far better, in case the tank is light, to have it detached from its position, when the ship strikes the earth, because in doing so, it will not be so likely to burn the imprisoned aviator.
In all cases the tank should be kept as far away from the engine as possible. There is no reason why it cannot be placed toward the tail end of the machine, a place of safety for two reasons: First, it is out of the reach of any possible danger from fire; and, second, the accidents in the past show that the tail frame is the least likely to be injured.
In looking over the illustrations taken from the accidents, notice how few of the tails are even disarranged, and in many of them, while the entire fore body and planes were crushed to atoms, the tail still remained as a relic, to show its comparative freedom from the accident.
In all monoplanes the tail really forms part of the supporting surface of the machine, and the adding of the weight of the gasoline would be placing but little additional duty on the tail, and it could be readily provided for by a larger tail surface, if required.
THE CLOSED-IN BODY.—The closed-in body is a vast improvement, which has had the effect of giving greater security to the pilot, but even this is useless in case of overturning.
STARTING THE MACHINE.—The direction in which improvements have been slow is in the starting of the machine. The power is usually so mounted that the pilot has no control over the starting, as he is not in a position to crank it.
The propeller being mounted directly on the shaft, without the intervention of a clutch, makes it necessary, while on the ground, for the propeller to be started by some one outside, while others hold the machine until it attains the proper speed.
This could be readily remedied by using a clutch, but in the past this has been regarded as one of the weight luxuries that all have been trying to avoid. Self starters are readily provided, and this with the provision that the propeller can be thrown in or out at will, would be a vast improvement in all machines.
PROPELLERS WITH VARYING PITCH.—It is growing more apparent each day, that a new type of propeller must be devised which will enable the pilot to change the pitch, as the speed increases, and to give a greater pitch, when alighting, so as to make the power output conform to the conditions.
Such propellers, while they may be dangerous, and much heavier than the rigid type, will, no doubt, appear in time, and the real improvement would be in the direction of having the blades capable of automatic adjustment, dependent on the wind pressure, or the turning speed, and thus not impose this additional duty on the pilot.
THE ANEMOMETER.—It requires an expert to judge the force or the speed of a wind, and even they will go astray in their calculations. It is an easy matter to make a little apparatus which will accurately indicate the speed. A device of this kind is called an Anemometer.
Two other instruments have grown out of this, one to indicate the pressure, and the other the direction of the moving air current.
THE ANEMOGRAPH.—While these instruments indicate, they are also made so they will record the speed, the pressure and the direction, and the device for recording the speed and pressure is called a Anemograph.
All these instruments may be attached to the same case, and thus make a handy little device, which will give all the information at a glance.
THE ANEMOMETROGRAPH.—This device for recording, as well as indicating the speed, pressure and direction, is called an Anemometrograph, The two important parts of the combined apparatus, for the speed and pressure, are illustrated, to show the principle involved. While the speed will give the pressure, it is necessary to make a calculation to get the result while the machine does this for you.
Fig. 61. Speed Indicator.
THE SPEED INDICATOR.—Four hemispherical cups A are mounted on four radiating arms B, which are secured to a vertical stem C, and adapted to rotate in suitable bearings in a case, which, for convenience in explaining, is not shown.
On the lower end of the stem C, is a small bevel pinion, which meshes with a smaller bevel pinion within the base. This latter is on a shaft which carries a small gear on its other end, to mesh with a larger gear on a shaft which carries a pointer D that thus turns at a greatly reduced speed, so that it can be easily timed.
Fig. 62. Air Pressure Indicator.
AIR PRESSURE INDICATOR.—This little apparatus is readily made of a base A which is provided with two uprights B, C, through the upper ends of which are holes to receive a horizontally-disposed bar D. One end of the bar is a flat plane surface E, which is disposed at right angles to the bar, and firmly fixed thereto.
The other end of the bar has a lateral pin to serve as a pivot for the end of a link F, its other end being hinged to the upper end of a lever G, which is pivoted to the post C, a short distance below the hinged attachment of the link F, so that the long end of the pointer which is constituted by the lever G is below its pivot, and has, therefore, a long range of movement.
A spring I between the upper end of the pointer G and the other post B, serves to hold the pointer at a zero position. A graduated scale plate J, within range of the pointer will show at a glance the pressure in pounds of the moving wind, and for this purpose it would be convenient to make the plane E exactly one foot square.
DETERMINING THE PRESSURE FROM THE SPEED.— These two instruments can be made to check each other and thus pretty accurately enable you to determine the proper places to mark the pressure indicator, as well as to make the wheels in the anemometer the proper size to turn the pointer in seconds when the wind is blowing at a certain speed, say ten miles per hour.
Suppose the air pressure indicator has the scale divided into quarter pound marks. This will make it accurate enough for all purposes.
CALCULATING PRESSURES FROM SPEED.—The following table will give the pressures from 5 to 100 miles per hour:
Velocity of wind in Pressure Velocity of wind in Pressuremiles per hour per sq. ft. miles per hour per sq ft5 .112 55 15.12510 .500 60 18.00015 1.125 65 21.12520 2.000 70 22.50025 3.125 75 28.12530 4.600 80 32.00035 6.126 86 36.12640 8.000 90 40.50045 10.125 95 45.12550 12.5 100 50.000
HOW THE FIGURES ARE DETERMINED.—The foregoing figures are determined in the following manner: As an example let us assume that the velocity of the wind is forty-five miles per hour. If this is squared, or 45 multiplied by 45, the product is 2025. In many calculations the mathematician employs what is called a constant, a figure that never varies, and which is used to multiply or divide certain factors.
In this case the constant is 5/1000, or, as usually written, .005. This is the same as one two hundredths of the squared figure. That would make the problem as follows:
45 X 45 = 2025 / 200 = 10.125; or, 45 X 45 - 2025 X .005 = 10.125.
Again, twenty-five miles per hour would be 25 X 25 = 625; and this multiplied by .005 equals 2 pounds pressure.
CONVERTING HOURS INTO MINUTES.—It is sometimes confusing to think of miles per hour, when you wish to express it in minutes or seconds. A simple rule, which is not absolutely accurate, but is correct within a few feet, in order to express the speed in feet per minute, is to multiply the figure indicating the miles per hour, by 8 3/4.
To illustrate: If the wind is moving at the rate of twenty miles an hour, it will travel in that time 105,600 feet (5280 X 20). As there are sixty minutes in an hour, 105,600 divided by 60, equals 1760 feet per minute. Instead of going through all this process of calculating the speed per minute, remember to multiply the speed in miles per hour by 90, which will give 1800 feet.
This is a little more then two per cent. above the correct figure. Again; 40 X 90 equals 3600. As the correct figure is 3520, a little mental calculation will enable you to correct the figures so as to get it within a few feet.
CHANGING SPEED HOURS TO SECONDS.—As one- sixtieth of the speed per minute will represent the rate of movement per second, it is a comparatively easy matter to convert the time from speed in miles per hour to fraction of a mile traveled in a second, by merely taking one-half of the speed in miles, and adding it, which will very nearly express the true number of feet.
As examples, take the following: If the wind is traveling 20 miles an hour, it is easy to take one-half of 20, which is 10, and add it to 20, making 30, as the number of feet per second. If the wind travels 50 miles per hour, add 25, making 75, as the speed per second.
The correct speed per second of a wind traveling 20 miles an hour is a little over 29 feet. At 50 miles per hour, the correct figure is 73 1/3 feet, which show that the figures under this rule are within about one per cent. of being correct.
With the table before you it will be an easy matter, by observing the air pressure indicator, to determine the proper speed for the anemometer. Suppose it shows a pressure of two pounds, which will indicate a speed of twenty miles an hour. You have thus a fixed point to start from.
PRESSURE AS THE SQUARE OF THE SPEED.—Now it must not be assumed that if the pressure at twenty miles an hour is two pounds, that forty miles an hour it is four pounds. The pressure is as the square of the speed. This may be explained as follows: As the speed of the wind increases, it has a more effective push against an object than its rate of speed indicates, and this is most simply expressed by saying that each time the speed is doubled the pressure is four times greater.
As an example of this, let us take a speed of ten miles an hour, which means a pressure of one- half pound. Double this speed, and we have 20 miles. Multiplying one-half pound by 4, the result is 2 pounds. Again, double 20, which means 40 miles, and multiplying 2 by 4, the result is 8. Doubling forty is eighty miles an hour, and again multiplying 8 by 4, we have 32 as the pounds pressure at a speed of 80 miles an hour.
The anemometer, however, is constant in its speed. If the pointer should turn once a second at 10 miles an hour, it would turn twice at 20 miles an hour, and four times a second at 40 miles an hour.
GYROSCOPIC BALANCE.—Some advance has been made in the use of the gyroscope for the purpose of giving lateral stability to an aeroplane. While the best of such devices is at best a makeshift, it is well to understand the principle on which they operate, and to get an understanding how they are applied.
THE PRINCIPLE INVOLVED.—The only thing known about the gyroscope is, that it objects to changing the plane of its rotation. This statement must be taken with some allowance, however, as, when left free to move, it will change in one direction.
To explain this without being too technical, examine Fig. 63, which shows a gyroscopic top, one end of the rim A, which supports the rotating wheel B, having a projecting finger C, that is mounted on a pin-point on the upper end of the pedestal D.
Fig. 63. The Gyroscope.
When the wheel B is set in rotation it will maintain itself so that its axis E is horizontal, or at any other angle that the top is placed in when the wheel is spun. If it is set so the axis is horizontal the wheel B will rotate on a vertical plane, and it forcibly objects to any attempt to make it turn except in the direction indicated by the curved arrows F.
The wheel B will cause the axis E to swing around on a horizontal plane, and this turning movement is always in a certain direction in relation to the turn of the wheel B, and it is obvious, therefore, that to make a gyroscope that will not move, or swing around an axis, the placing of two such wheels side by side, and rotated in opposite directions, will maintain them in a fixed position; this can also be accomplished by so mounting the two that one rotates on a plane at right angles to the other.
Fig. 64. Application of the Gyroscope.
THE APPLICATION OF THE GYROSCOPE.—Without in any manner showing the structural details of the device, in its application to a flying machine, except in so far as it may be necessary to explain its operation, we refer to Fig. 64, which assumes that A represents the frame of the aeroplane, and B a frame for holding the gyroscopic wheel C, the latter being mounted so it rotates on a horizontal plane, and the frame B being hinged fore and aft, so that it is free to swing to the right or to the left.
For convenience in explaining the action, the planes E are placed at right angles to their regular positions, F being the forward margin of the plane, and G the rear edge. Wires H connect the ends of the frame B with the respective planes, or ailerons, E, and another wire I joins the downwardly-projecting arms of the two ailerons, so that motion is transmitted to both at the same time, and by a positive motion in either direction.
Fig. 65. Action of the Gyroscope.
In the second figure, 65, the frame of the aeroplane is shown tilted at an angle, so that its right side is elevated. As the gyroscopic wheel remains level it causes the aileron on the right side to change to a negative angle, while at the same time giving a positive angle to the aileron on the left side, which would, as a result, depress the right side, and bring the frame of the machine back to a horizontal position.
FORE AND AFT GYROSCOPIC CONTROL.—It is obvious that the same application of this force may be applied to control the ship fore and aft, although it is doubtful whether such a plan would have any advantages, since this should be wholly within the control of the pilot.
Laterally the ship should not be out of balance; fore and aft this is a necessity, and as the great trouble with all aeroplanes is to control them laterally, it may well be doubted whether it would add anything of value to the machine by having an automatic fore and aft control, which might, in emergencies, counteract the personal control of the operator.
ANGLE INDICATOR.—In flight it is an exceedingly difficult matter for the pilot to give an accurate idea of the angle of the planes. If the air is calm and he is moving over a certain course, and knows, from experience, what his speed is, he may be able to judge of this factor, but he cannot tell what changes take place under certain conditions during the flight.
For this purpose a simple little indicator may be provided, shown in Fig. 66, which is merely a vertical board A, with a pendulum B, swinging fore and aft from a pin a which projects out from the board a short distance above its center.
The upper end of the pendulum has a heart- shaped wire structure D, that carries a sliding weight E. Normally, when the aeroplane is on an even keel, or is even at an angle, the weight E rests within the bottom of the loop D, but should there be a sudden downward lurch or a quick upward inclination, which would cause the pendulum below to rapidly swing in either direction, the sliding weight E would at once move forward in the same direction that the pendulum had moved, and thus counteract, for the instant only, the swing, when it would again drop back into its central position.
Fig. 66. Angle Indicator.
With such an arrangement, the pendulum would hang vertically at all times, and the pointer below, being in range of a circle with degrees indicated thereon, and the base attached to the frame of the machine, can always be observed, and the conditions noted at the time the changes take place.
PENDULUM STABILIZER.—In many respects the use of a pendulum has advantages over the gyroscope. The latter requires power to keep it in motion. The pendulum is always in condition for service. While it may be more difficult to adjust the pendulum, so that it does not affect the planes by too rapid a swing, or an oscillation which is beyond the true angle desired, still, these are matters which, in time, will make the pendulum a strong factor in lateral stability.
Fig. 67. Simple Pendulum Stabilizer.
It is an exceedingly simple matter to attach the lead wires from an aileron to the pendulum. In Fig. 67 one plan is illustrated. The pendulum A swings from the frame B of the machine, the ailerons a being in this case also shown at right angles to their true positions.
The other, Fig. 68, assumes that the machine is exactly horizontal, and as the pendulum is in a vertical position, the forward edges of both ailerons are elevated, but when the pendulum swings both ailerons will be swung with their forward margins up or down in unison, and thus the proper angles are made to right the machine.
STEERING AND CONTROLLING WHEEL.—For the purpose of concentrating the control in a single wheel, which has not alone a turning motion, but is also mounted in such a manner that it will oscillate to and fro, is very desirable, and is adapted for any kind of machine.
Fig. 68. Pendulum Stabilizers.
Fig. 69 shows such a structure, in which A represents the frame of the machine, and B a segment for the stem of the wheel, the segment being made of two parts, so as to form a guideway for the stem a to travel between, and the segment is placed so that the stem will travel in a fore and aft direction.
The lower end of the stem is mounted in a socket, at D, so that while it may be turned, it will also permit this oscillating motion. Near its lower end is a cross bar E from which the wires run to the vertical control plane, and also to the ailerons, if the machine is equipped with them, or to the warping ends of the planes.
Fig. 69. Steering and Control Wheel.
Above the cross arms is a loose collar F to which the fore and aft cords are attached that go to the elevators, or horizontal planes. The upper end of the stem has a wheel G, which may also be equipped with the throttle and spark levers.
AUTOMATIC STABILIZING WINGS.—Unquestionably, the best stabilizer is one which will act on its own initiative. The difficulty with automatic devices is, that they act too late, as a general thing, to be effective. The device represented in Fig. 70 is very simple, and in practice is found to be most efficient.
In this Fig. 70 A and B represent the upper and the lower planes, respectively. Near the end vertical standards a, D, are narrow wings E E, F F, hinged on a fore and aft line close below each of the planes, the wings being at such distances from the standards C D that when they swing outwardly they will touch the standards, and when in that position will be at an angle of about 35 degrees from the planes A B.
Fig. 70. Automatic Stabilizing Wings.
Fig. 71. Action of Stabilizing Wings.
Inwardly they are permitted to swing up and lie parallel with the planes, as shown in Fig. 71 where the planes are at an angle. In turning, all machines skid,—that is they travel obliquely across the field, and this is also true when the ship is sailing at right angles to the course of the wind.
This will be made clear by reference to Fig. 72, in which the dart A represents the direction of the movement of the aeroplane, and B the direction of the wind, the vertical rudder a being almost at right angles to the course of the wind.
Fig. 72. Into the Wind at an Angle.
In turning a circle the same thing takes place as shown in Fig. 73, with the tail at a different angle, so as to give a turning movement to the plane. It will be seen that in the circling movement the tendency of the aeroplane is to fly out at a tangent, shown by the line D, so that the planes of the machine are not radially-disposed with reference to the center of the circle, the line E showing the true radial line.
Referring now to Fig. 71, it will be seen that this skidding motion of the machine swings the wings E F inwardly, so that they offer no resistance to the oblique movement, but the wings E E, at the other end of the planes are swung outwardly, to provide an angle, which tends to raise up the inner end of the planes, and thereby seek to keep the planes horizontal.
Fig. 73. Turning a Circle.
BAROMETERS.—These instruments are used for registering heights. A barometer is a device for measuring the weight or pressure of the air. The air is supposed to extend to a height of 40 miles from the surface of the sea. A column of air one inch square, and forty miles high, weighs the same as a column of mercury one inch square and 30 inches high.
Such a column of air, or of mercury, weighs 14 3/4 pounds. If the air column should be weighed at the top of the mountain, that part above would weigh less than if measured at the sea level, hence, as we ascend or descend the pressure becomes less or more, dependent on the altitude.
Mercury is also used to indicate temperature, but this is brought about by the expansive quality of the mercury, and not by its weight.
Fig. 74. Aneroid Barometer.
ANEROID BAROMETER.—The term Aneroid barometer is frequently used in connection with air- ship experiments. The word aneroid means not wet, or not a fluid, like mercury, so that, while aneroid barometers are being made which do use mercury, they are generally made without.
One such form is illustrated in Fig. 74, which represents a cylindrical shell A, which has at each end a head of concentrically formed corrugations. These heads are securely fixed to the ends of the shell A. Within, one of the disk heads has a short stem C, which is attached to the short end of a lever D, this lever being pivoted at E. The outer end of this lever is hinged to the short end of another lever F, and so by compounding the levers, it will be seen that a very slight movement of the head B will cause a considerable movement in the long end of the lever F.
This end of the lever F connects with one limb of a bell-crank lever G, and its other limb has a toothed rack connection with a gear H, which turns the shaft to which the pointer I is attached.
Air is withdrawn from the interior of the shell, so that any change in the pressure, or weight of the atmosphere, is at once felt by the disk heads, and the finger turns to indicate the amount of pressure.
HYDROPLANES.—Hydro means water, hence the term hydroplane has been given to machines which have suitable pontoons or boats, so they may alight or initiate flight from water.
There is no particular form which has been adopted to attach to aeroplanes, the object generally being to so make them that they will sustain the greatest amount of weight with the least submergence, and also offer the least resistance while the motor is drawing the machine along the surface of the water, preparatory to launching it.
SUSTAINING WEIGHT OF PONTOONS.—A pontoon having within nothing but air, is merely a measuring device which determines the difference between the weight of water and the amount placed on the pontoon. Water weighs 62 1/2 pounds per cubic foot. Ordinary wood, an average of 32 pounds, and steel 500 pounds.
It is, therefore, an easy matter to determine how much of solid matter will be sustained by a pontoon of a given size, or what the dimensions of a pontoon should be to hold up an aeroplane which weighs, with the pilot, say, 1100 pounds.
As we must calculate for a sufficient excess to prevent the pontoons from being too much immersed, and also allow a sufficient difference in weight so that they will keep on the surface when the aeroplane strikes the surface in alighting, we will take the figure of 1500 pounds to make the calculations from.
If this figure is divided by 62 1/2 we shall find the cubical contents of the pontoons, not considering, of course, the weight of the material of which they are composed. This calculation shows that we must have 24 cubic feet in the pontoons.
As there should be two main pontoons, and a smaller one for the rear, each of the main ones might have ten cubic feet, and the smaller one four cubic feet.
SHAPES OF THE PONTOONS.—We are now ready to design the shapes. Fig. 75 shows three general types, A being made rectangular in form, with a tapering forward end, so constructed as to ride up on the water.
The type B has a rounded under body, the forward end being also skiff-shaped to decrease as much as possible the resistance of the water impact.
Fig. 75. Hydroplane Floats.
The third type C is made in the form of a closed boat, with both ends pointed, and the bottom rounded, or provided with a keel. Or, as in some cases the body may be made triangular in cross section so that as it is submerged its sustaining weight will increase at a greater degree as it is pressed down than its vertical measurement indicates.
All this, however, is a matter left to the judgment of the designer, and is, in a great degree, dependent on the character of the craft to which it is to be applied.
THE novice about to take his first trial trip in an automobile will soon learn that the great task in his mind is to properly start the machine. He is conscious of one thing, that it will be an easy matter to stop it by cutting off the fuel supply and applying the brakes.
CERTAIN CONDITIONS IN FLYING.—In an aeroplane conditions are reversed. Shutting off the fuel supply and applying the brakes only bring on the main difficulty. He must learn to stop the machine after all this is done, and this is the great test of flying. It is not the launching,— the ability to get into the air, but the landing, that gives the pupil his first shock.
Man is so accustomed to the little swirls of air all about him, that he does not appreciate what they mean to a machine which is once free to glide along in the little currents which are so unnoticeable to him as a pedestrian.
The contour of the earth, the fences, trees, little elevations and other natural surroundings, all have their effect on a slight moving air current, and these inequalities affect the air and disturb it to a still greater extent as the wind increases. Even in a still air, with the sun shining, there are air eddies, caused by the uneven heating of the air in space.
HEAT IN AIR.—Heat is transmitted through the air by what is called convection, that is, the particles of the air transmit it from one point to the next. If a room is closed up tight, and a little aperture provided so as to let in a streak of sunlight, it will give some idea of the unrest of the atmosphere. This may be exhibited by smoke along the line of the sun's rays, which indicates that the particles of air are constantly in motion, although there may be absolutely nothing in the room to disturb it.
MOTION WHEN IN FLIGHT.—If you can imagine a small airship floating in that space, you can readily conceive that it will be hurled hither and thither by the motion which is thus apparent to the eye.
This motion is greatly accentuated by the surface of the earth, independently of its uneven contour. If a ball is thrown through the air, its dynamic force is measured by its impact. So with light, and heat. In the space between the planets it is very cold. The sunlight, or the rays from the sun are there, just the same as on the earth.
Unless the rays come into contact with something, they produce no effect. When the beams from the sun come into contact with the atmosphere a dynamic force is exerted, just the same as when the ball struck an object. When the rays reach the earth, reflection takes place, and these reflected beams act on the air under different conditions.
CHANGING ATMOSPHERE.—If the air is full of moisture, as it may be at some places, while comparatively dry at other points, the reflection throughout the moist area is much greater than in the dry places, hence evaporation will take place and whenever a liquid vaporizes it means heat.
On the other hand, when the vapor is turning to a liquid, condensation takes place, and that means cooling. If the air should be of the same degree of saturation throughout,—that is, have the same amount of moisture everywhere, there would be few winds. These remarks apply to conditions which exist over low altitudes all over the earth.
But at high altitudes the conditions are entirely different. As we ascend the air becomes rarer. It has less moisture, because a wet atmosphere, being heavier, lies nearer the surface of the earth. Being rarer the action of sunlight on the particles is less intense. Reflection and refraction of the rays acting on the light atmosphere do not produce such a powerful effect as on the air near the ground.
All these conditions—the contour of the earth; the uneven character of the moisture in the air; the inequalities of the convection currents; and the unstable, tenuous, elastic nature of the atmosphere, make the trials of the aviator a hazardous one, and it has brought out numerous theories connected with bird flight. One of these assumes that the bird, by means of its finely organized sense, is able to detect rising air currents, and it selects them in its flight, and by that means is enabled to continue in flight indefinitely, by soaring, or by flapping its wings.
ASCENDING CURRENTS.—It has not been explained how it happens that these particular "ascending currents" always appear directly in the line of the bird flight; or why it is that when, for instance, a flock of wild geese which always fly through space in an A-shaped formation, are able to get ascending air currents over the wide scope of space they cover.
ASPIRATE CURRENTS.—Some years ago, in making experiments with the outstretched wings of one of the large soaring birds, a French sailor was surprised to experience a peculiar pulling motion, when the bird's wings were held at a certain angle, so that the air actually seemed to draw it into the teeth of the current.
It is known that if a ball is suspended by a string, and a jet of air is directed against it, in a particular way, the ball will move toward the jet, instead of being driven away from it. A well known spraying device, called the "ball nozzle," is simply a ball on the end of a nozzle, and the stream of water issuing is not effectual to drive the ball away.
From the bird incident alluded to, a new theory was propounded, namely, that birds flew because of the aspirated action of the air, and the wings and body were so made as to cause the moving air current to act on it, and draw it forwardly.
OUTSTRETCHED WINGS.—This only added to the "bird wing" theory a new argument that all flying things must have outstretched wings, in order to fly, forgetting that the ball, which has no outstretched wings, has also the same "aspirate" movement attributed to the wings of the bird.
The foregoing remarks are made in order to impress on the novice that theories do not make flying machines, and that speculations, or analogies of what we see all about us, will not make an aviator. A flying machine is a question of dynamics, just as surely as the action of the sun on the air, and the movements of the currents, and the knowledge of applying those forces in the flying machine makes the aviator.
THE STARTING POINT.—Before the uninitiated should attempt to even mount a machine he should know what it is composed of, and how it is made. His investigation should take in every part of the mechanism; he should understand about the plane surface, what the stresses are upon its surface, what is the duty of each strut, or brace or wire and be able to make the proper repairs.
THE VITAL PART OF THE MACHINE.—The motor, the life of the machine itself, should be like a book to him. It is not required that he should know all the theories which is necessary in the building, as to the many features which go to make up a scientifically-designed motor; but he must know how and why it works. He should understand the cam action, whereby the valves are lifted at the proper time; what the effect of the spark advance means; the throttling of the engine; air admission and supply; the regulation of the carbureter; its mechanism and construction; the propeller should be studied, and its action at various speeds.
STUDYING THE ACTION OF THE MACHINE.—Then comes the study on the seat of the machine itself. It will be a novel sensation. Before him is the steering wheel, if it should be so equipped. Turning it to the right, swings the vertical tail plane so the machine will turn to the right. Certainly, he knows that; but how far must he turn the wheel to give it a certain angle.
It is not enough to know that a lever or a wheel when moved a certain way will move a plane a definite direction. He should learn to know instinctively, how FAR a movement to make to get a certain result in the plane itself, and under running conditions, as well.
Suppose we have an automobile, running at the rate of ten miles an hour, and the chauffeur turns the steering wheel ten degrees. He can do so with perfect safety; but let the machine be going forty miles an hour, and turn the wheel ten degrees, and it may mean an accident. In one case the machine is moving 14 1/2 feet a second, and in the other instance 58 feet.
If the airship has a lever for controlling the angle of flight, he must study its arrangement, and note how far it must be moved to assume the proper elevating angle. Then come the means for controlling the lateral stability of the machine. All these features should be considered and studied over and over, until you have made them your friends.
While thus engaged, you are perfectly sure that you can remember and act on a set of complicated movements. You imagine that you are skimming over the ground, and your sense tells you that you have sufficient speed to effect a launching. In your mind the critical time has come.
ELEVATING THE MACHINE.—Simply give the elevator lever the proper angle, sharp and quick and up you go. As the machine responds, and you can feel the cushioning motion, which follows, as it begins to ride the air, you are aware of a sensation as though the machine were about to turn over to one side; you think of the lateral control at once, but in doing so forget that the elevator must be changed, or you will go too high.
You forget about the earth; you are too busy thinking about several things which seem to need your attention. Yes, there are a variety of matters which will crowd upon you, each of which require two things; the first being to get the proper lever, and the second, to move it just so far.
In the early days of aeroplaning, when accidents came thick and fast, the most usual explanation which came from the pilot, when he recovered, was: "I pushed the lever too far."
Hundreds of trial machines were built, when man learned that he could fly, and in every instance, it is safe to say, the experimenter made the most strenuous exertion to get up in the air the first time the machine was put on the trial ground.
It is a wonder that accidents were not recorded by the hundreds, instead of by the comparatively few that were heard from. It was very discouraging, no doubt, that the machines would not fly, but that all of them, if they had sufficient power, would fly, there can be no doubt.
HOW TO PRACTICE.—Absolute familiarity with every part of the machine and conditions is the first thing. The machine is brought out, and the engine tested, the machine being held in leash while this is done. It is then throttled down so that the power of the engine will be less than is necessary to raise the machine from the ground.
THE FIRST STAGE.—Usually it will require over 25 miles an hour to raise the machine. The engine is set in motion, and now, for the first time a new sensation takes possession of you, for the reason that you are cut off from communication with those around you as absolutely as though they were a hundred miles away.
This new dependence on yourself is, in itself, one of the best teachers you could have, because it begins to instill confidence and control. As the machine darts forward, going ten or fifteen miles an hour, with the din of the engine behind you, and feeling the rumbling motion of the wheels over the uneven surface of the earth, you have the sensation of going forty miles an hour.
The newness of the first sensation, which is always under those conditions very much augmented in the mind, wears away as the machine goes back and forth. There is only one control that requires your care, namely, to keep it on a straight course. This is easy work, but you are learning to make your control a reflex action,—to do it without exercising a distinct will power.
PATIENCE THE MOST DIFFICULT THING.—If you have the patience, as you should, to continue this running practice, until you absolutely eliminate the right and left control, as a matter of thought, occasionally, if the air is still turning the machine, and eventually, bringing it back, by turning it completely around, while skimming the ground, you will be ready for the second stage in the trials.
THE SECOND STAGE.—The engine is now arranged so that it will barely lift, when running at its best. After the engine is at full speed, and you are sure the machine is going fast enough, the elevator control is turned to point the machine in the air. It is a tense moment. You are on the alert.
The elevator is turned, and the forward end changes its relation with the ground before you. There was a slight lift, but your caution induces you to return the planes to their normal running angle. You try it again. You are now certain that the machine made a leap and left the ground. This is the exhilarating moment.
With a calm air the machine is turned while running, by means of the vertical rudders. This is an easy matter, because while going at twenty miles an hour, the weight of the machine on the surface of the ground is less than one-tenth of its weight when at rest.
Thus the trial spins, half the time in the air, in little glides of fifty to a hundred feet, increasing in length, give practice, practice, PRACTICE, each turn of the field making the sport less exciting and fixing the controls more perfectly in the mind.
THE THIRD STAGE.—Thus far you have been turning on the ground. You want to turn in the air. Only the tail control was required while on the ground. Now two things are required after you leave the ground in trying to make a turn: namely, putting the tail at the proper angle, and taking charge of the stabilizers, because in making the turn in the air, the first thing which will arrest the attention will be the tendency of the machine to turn over in the direction that you are turning.
After going back and forth in straight-away glides, until you have perfect confidence and full control, comes the period when the turns should be practiced on. These should be long, and tried only on that portion of the field where you have plenty of room.
OBSERVATIONS WHILE IN FLIGHT.—If there are any bad spots, or trees, or dangerous places, they should be spotted out, and mentally noted before attempting to make any flight. When in the air during these trials you will have enough to occupy your mind without looking out for the hazardous regions at the same time.
Make the first turns in a still air. If you should attempt to make the first attempts with a wind blowing you will find a compound motion that will very likely give you a surprise. In making the first turn you will get the sensation of trying to fly against a wind. Assuming that you are turning to the left, it will have the sensation of a wind coming to you from the right.
FLYING IN A WIND.—Suppose you are flying directly in the face of a wind, the moment you begin to turn the action, or bite of the wind, will cause the ends of the planes to the right to be unduly elevated, much more so than if the air should be calm. This raising action will be liable to startle you, because up to this time you have been accustomed to flying along in a straight line.
While flying around at the part of the circle where the wind strikes you directly on the right side the machine has a tendency to climb, and you try to depress the forward end, but as soon as you reach that part of the circle where the winds begin to strike on your back, an entirely new thing occurs.
As the machine is now traveling with the wind, its grip on the air is less, and since the planes were set to lower the machine, at the first part of the turn, the descent will be pretty rapid unless the angle is corrected.
FIRST TRIALS IN QUIET ATMOSPHERE.—All this would be avoided if the first trials were made in a quiet atmosphere. Furthermore, you will be told that in making a turn the machine should be pointed downwardly, as though about to make a glide. This can be done with safety, in a still air, although you may be flying low, but it would be exceedingly dangerous with a wind blowing.
MAKING TURNS.—When making a turn, under no circumstances try to make a landing. This should never be done except when flying straight, and then safety demands that the landing should be made against the wind and not with it. There are two reasons for this: First, when flying with the wind the speed must be greater than when flying against it.
By greater speed is meant relative to the earth. If the machine has a speed of thirty miles an hour, in still air, the speed would be forty miles an hour going with the wind, but only twenty miles against the wind. Second, the banking of the planes against the air is more effective when going into the wind than when traveling with it, and, therefore, the speed at which you contact with the earth is lessened to such an extent that a comparatively easy landing is effected.
THE FOURTH STAGE.—After sufficient time has been devoted to the long turns shorter turns may be made, and these also require the same care, and will give an opportunity to use the lateral controls to a greater extent. Begin the turns, not by an abrupt throw of the turning rudder, but bring it around gently, correcting the turning movement to a straight course, if you find the machine inclined to tilt too much, until you get used to the sensation of keeling over. Constant practice at this will soon give confidence, and assure you that you have full control of the machine.
THE FIGURE 8.—You are now to increase the height of flying, and this involves also the ability to turn in the opposite direction, so that you may be able to experience the sensation of using the stabilizers in the opposite direction. You will find in this practice that the senses must take in the course of the wind from two quarters now, as you attempt to describe the figure 8.
This is a test which is required in order to obtain a pilot's license. It means that you shall be able to show the ability to turn in either direction with equal facility. To keep an even flying altitude while describing this figure in a wind, is the severest test that can be exacted.
THE VOLPLANE.—This is the technical term for a glide. Many accidents have been recorded owing to the stopping of the motor, which in the past might have been avoided if the character of the glide had been understood. The only thing that now troubles the pilot when the engine "goes dead," is to select a landing place.
The proper course in such a case is to urge the machine to descend as rapidly as possible, in order to get a headway, for the time being. As there is now no propelling force the glide is depended upon to act as a substitute. The experienced pilot will not make a straight-away glide, but like the vulture, or the condor, and birds of that class, soar in a circle, and thus, by passing over and over the same surfaces of the earth, enable him to select a proper landing place.
THE LANDING.—The pilot who can make a good landing is generally a good flyer. It requires nicety of judgment to come down properly. One thing which will appear novel after the first altitude flights are attempted is the peculiar sensation of the apparently increased speed as the earth comes close up to the machine.
At a height of one hundred feet, flying thirty miles an hour, does not seem fast, because the surface of the earth is such a distance away that particular objects remain in view for some moments; but when within ten feet of the surface the same object is in the eye for an instant only.
This lends a sort of terror to the novice. He imagines a great many things, but forgets some things which are very important to do at this time. One is, that the front of the machine must be thrown up so as to bank the planes against the wind. The next is to shut off the power, which is to be done the moment the wheels strike the ground, or a little before.
Upon his judgment of the time of first touching the earth depends the success of safely alighting. He may bank too high, and come down on the tail with disastrous results. If there is plenty of field room it is better to come down at a less angle, or even keep the machine at an even keel, and the elevator can then depress the tail while running over the ground, and thus bring the machine to rest.
Frequently, when about to land the machine will rock from side to side. In such a case it is far safer to go up into the air than to make the land, because, unless the utmost care is exercised, one of the wing tips will strike the earth and wreck the machine.
Another danger point is losing headway, as the earth is neared, due to flying at too flat an angle, or against a wind that happens to be blowing particularly hard at the landing place. If the motor is still going this does not make so much difference, but in a volplane it means that the descent must be so steep, at the last moment of flight, that the chassis is liable to be crushed by the impact.
FLYING ALTITUDE.—It is doubtful whether the disturbed condition of the atmosphere, due to the contour of the earth's surface, reaches higher than 500 feet. Over a level area it is certain that it is much less, but in some sections of the country, where the hill ranges extend for many miles, at altitudes of three and four hundred feet, the upper atmosphere may be affected for a thousand feet above.
Prof. Lowe, in making a flight with a balloon, from Cincinnati to North Carolina, which lasted a day and all of one night, found that during the early morning the balloon, for some reason, began to ascend, and climbed nearly five thousand feet in a few hours, and as unaccountably began to descend several hours before he landed.
Before it began to ascend, he was on the western side of the great mountain range which extends south from Pennsylvania and terminates in Georgia. He was actually climbing the mountain in a drift of air which was moving eastwardly, and at no time was he within four thousand feet of the earth during that period, which shows that air movements are of such a character as to exert their influence vertically to great heights.
For cross country flying the safest altitude is 1000 feet, a distance which gives ample opportunity to volplane, if necessary, and it is a height which enables the pilot to make observations of the surface so as to be able to judge of its character.