Part 17 (1/2)
_Fig. 81. Marking the Side._
This line is on the rear side of the propeller, and is perfectly straight. Along the front of this line is a bowline E, which indicates the front surface of the propeller blade.
PROPELLER OUTLINE.--While the marks thus given show the angles, and are designed to indicate the two faces of the blades, there is still another important element to be considered, and that is the final outline of the blades.
_Fig. 82. Outlining._
It is obvious that the outline may be varied so that the entire width at 1, Fig. 82, may be used, or it may have an outline, as represented by the line 2, in this figure, so that the widest part will be at or near the dotted line 3, say two-thirds of the distance from the center of the blade.
This is the practice with most of the manufacturers at the present time, and some of them claim that this form produces the best results.
FOR HIGHER SPEEDS.--Fig. 83 shows a propeller cut from a blank, 4” x 6” in cross section, not laminated.
_Fig. 83. Cut from a 4” x 6” Single Blank._
It should be borne in mind that for high speeds the blades must be narrow. A propeller seven feet in diameter with a six foot pitch, turning 950 revolutions per minute, will produce a pull of 350 pounds, if properly made.
Such a propeller can be readily handled by a forty horse power motor, such as are specially constructed for flying machine purposes.
INCREASING PROPELLER EFFICIENCY.--Some experiments have been made lately, which, it is claimed, largely increase the efficiency of propellers.
The improvement is directed to the outline shape of the blade.
The typical propeller, such as we have ill.u.s.trated, is one with the wide part of the blade at the extremity. The new type, as suggested, reverses this, and makes the wide part of the blade near the hub, so that it gradually tapers down to a narrow tip.
Such a form of construction is shown in Fig.
84. This outline has some advantages from one standpoint, namely, that it utilizes that part of the blade near the hub, to produce a pull, and does not relegate all the duty to the extreme ends or tips.
_Fig. 84. A Suggested Form._
To understand this more fully, let us take a propeller six feet in diameter, and measure the pull or thrust at the tips, and also at a point half way between the tip and the hub.
In such a propeller, if the blade is the same width and pitch at the two points named, the pull at the tips will be four times greater than at the intermediate point.
CHAPTER XIV
EXPERIMENTAL GLIDERS AND MODEL AEROPLANES
AN amusing and very instructive pastime is afforded by constructing and flying gliding machines, and operating model aeroplanes, the latter being equipped with their own power.
Abroad this work has been very successful as a means of interesting boys, and, indeed, men who have taken up the science of aviation are giving this sport serious thought and study.
When a machine of small dimensions is made the boy wonders why a large machine does not bear the same relation in weight as a small machine.
This is one of the first lessons to learn.
THE RELATION OF MODELS TO FLYING MACHINES.
--A model aeroplane, say two feet in length, which has, we will a.s.sume, 50 square inches of supporting surface, seems to be a very rigid structure, in proportion to its weight. It may be dropped from a considerable height without injuring it, since the weight is only between two and three ounces.