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 cha.s.sis is liable to be crushed by the impact.

FLYING ALt.i.tUDE.--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 alt.i.tudes 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 alt.i.tude 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.

But explanations and statements, and the experiences of pilots might be detailed in pages, and still it would be ineffectual to teach the art of flying.

The only sure course is to do the work on an actual machine.

Many of the experiences are valuable to the learner, some are merely in the nature of cautions, and it is advisable for the beginner to learn what the experiences of others have been, although they may never be called upon to duplicate them.

All agree that at great elevations the flying conditions are entirely different from those met with near the surface of the ground, and the history of accidents show that in every case where a mishap was had at high alt.i.tude it came about through defect in the machine, and not from gusts or bad air condition.

On the other hand, the uptilting of machines, the accidents due to the so-called "Holes in the air," which have dotted the historic pages with accidents, were brought about at low alt.i.tudes.

At from two to five thousand feet the air may be moving at speeds of from twenty to forty miles an hour,--great ma.s.ses of winds, like the trade stream, which are uniform over vast areas. To the aviator flying in such a field, with the earth hidden from him, there would be no wind to indicate that he was moving in any particular direction.

He would fly in that medium, in any direction, without the slightest sense that he was in a gale.

It would not affect the control of the machine, because the air, though moving as a ma.s.s, would be the same as flying in still air. It is only when he sees fixed objects that he is conscious of the movement of the wind.

CHAPTER XIII

THE PROPELLER

BY far the most difficult problem connected with aviation is the propeller. It is the one great vital element in the science and art pertaining to this subject which has not advanced in the slightest degree since the first machine was launched.

The engine has come in for a far greater share of expert experimental work, and has advanced most rapidly during the past ten years. But, strange to say, the propeller is, essentially, the same with the exception of a few small changes.

PROPELLER CHANGES.--The changes which have been made pertaining to the form of structure, princ.i.p.ally, and in the use of new materials. The kind of wood most suitable has been discovered, but the lines are the same, and nothing has been done to fill the requirement which grows out of the difference in speed when a machine is in the act of launching and when it is in full flight.

PROPELLER SHAPE.--It cannot be possible that the present shape of the propeller will be its ultimate form. It is inconceivable that the propeller is so inefficient that only one sixty-fifth of the power of the engine is available. The improvement in propeller efficiency is a direction which calls for experimental work on the part of inventors everywhere.

The making of a propeller, although it appears a difficult task, is not as complicated as would appear, and with the object in view of making the subject readily understood, an explanation will be given of the terms "Diameter," and "Pitch," as used in the art.

The Diameter has reference to the length of the propeller, from end to end. In calculating propeller pull, the diameter is that which indicates the speed of travel, and for this reason is a necessary element.

Thus, for instance, a propeller three feet in diameter, rotating 500 times a minute, has a tip speed of 1500 feet, whereas a six foot propeller, rotating at the same speed, moves 3000 feet at the tips.

PITCH.--This is the term which is most confusing, and is that which causes the most frequent trouble in the mind of the novice. The term will be made clear by carefully examining the accompanying ill.u.s.tration and the following description:

In Fig. 76 is shown a side view of a propeller A, mounted on a shaft B, which is free to move longitudinally. Suppose we turn the shaft so the tip will move along on the line indicated by the arrow C.

Now the pitch of the blade at D is such that it will be exactly in line with the spirally-formed course E, for one complete turn. As the propeller shaft has now advanced, along the line E, and stopped after one turn, at F, the measure between the points F and G represents the pitch of the propeller.

Another way to express it would be to call the angle of the blade a five, or six, or a seven foot pitch, as the pitches are measured in feet.

_Fig. 76. Describing the Pitch Line._

In the ill.u.s.tration thus given the propeller shaft, having advanced six feet, we have what is called a six foot pitch.

Now, to lay out such a pitch is an easy matter.

a.s.sume, as in Fig. 77, that A represents the end of the blank from which the propeller is to be cut, and that the diameter of this blank, or its length from end to end is seven feet. The problem now is to cut the blades at such an angle that we shall have a six foot pitch.

_Fig. 77. Laying out the Pitch._

LAYING OUT THE PITCH.--First, we must get the circ.u.mference of the propeller, that is, the distance the tip of the propeller will travel in making one complete turn. This is done by multiplying 7 by 3.1416. This equals 21.99, or, practically, 22 feet.

A line B is drawn, extending out horizontally along one side of the blank A, this line being made on a scale, to represent 22 feet. Secondly, at the end of this line drawn a perpendicular line C, 6 feet long. A perpendicular line is always one which is at right angles to a base line. In this case B is the base line.

Line C is made 6 feet long, because we are trying to find the angle of a 6 foot pitch. If, now, a line D is drawn from the ends of the two lines B, C, it will represent the pitch which, marked across the end of the blank A, will indicate the line to cut the blade.

PITCH RULE.--The rule may, therefore, be stated as follows: Multiply the diameter (in feet) of the propeller by 3.1416, and draw a line the length indicated by the product. At one end of this line draw a perpendicular line the length of the pitch requirement (in feet), and join the ends of the two lines by a diagonal line, and this line will represent the pitch angle.

Propellers may be made of wood or metal, the former being preferred for the reason that this material makes a lighter article, and is stronger, in some respects, than any metal yet suggested.

LAMINATED CONSTRUCTION.--All propellers should be laminated,--that is, built up of layers of wood, glued together and thoroughly dried, from which the propeller is cut.

A product thus made is much more serviceable than if made of one piece, even though the laminated parts are of the same wood, because the different strips used will have their fibers overlapping each other, and thus greatly augment the strength of the whole.

Generally the alternate strips are of different materials, black walnut, mahogany, birch, spruce, and maple being the most largely used, but mahogany and birch seem to be mostly favored.

LAYING UP A PROPELLER FORM.--The first step necessary is to prepare thin strips, each, say, seven feet long, and five inches wide, and three- eighths of an inch thick. If seven such pieces are put together, as in Fig. 78, it will make an a.s.semblage of two and five-eighth inches high.

_Fig. 78. A Laminated Blank._

Bore a hole centrally through the a.s.semblage, and place therein a pin B. The contact faces of these strips should be previously well painted over with hot glue liberally applied. When they are then placed in position and the pin is in place, the ends of the separate pieces are offset, one beyond the other, a half inch, as shown, for instance, in Fig. 79.

This will provide ends which are eight and a half inches broad, and thus furnish sufficient material for the blades. The ma.s.s is then subjected to heavy pressure, and allowed to dry before the blades are pared down.

_Fig. 79. Arranging the Strips._

MAKING WIDE BLADES.--If a wider blade is desired, a greater number of steps may be made by adding the requisite number of strips; or, the strips may be made thicker. In many propellers, not to exceed four different strips are thus glued together. The number is optional with the maker.

An end view of such an a.s.semblage of strips is ill.u.s.trated in Fig. 80. The next step is to lay off the pitch, the method of obtaining which has been explained.

_Fig. 80. End view of Blank._

Before starting work the sides, as well as the ends, should be marked, and care observed to place a distinctive mark on the front side of the propeller.

Around the pin B, Fig. 81, make S-shaped marks C, to indicate where the cuts on the faces of the blades are to begin. Then on the ends of the block; scribe the pitch angle, which is indicated by the diagonal line D, Fig. 80.