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. a.s.suming 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 s.p.a.ce, a.s.suming 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 ill.u.s.trations 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.

CHAPTER XI

FLYING MACHINE ACCESSORIES

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 ill.u.s.trated, 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.