_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.
An aeroplane twenty times the length of this model, however strongly it may be made, if dropped the same distance, would be crushed, and probably broken into fragments.
If the large machine is twenty times the dimensions of the small one, it would be forty feet in length, and, proportionally, would have only seven square feet of sustaining surface. But an operative machine of that size, to be at all rigid, would require more than twenty times the material in weight to be equal in strength.
It would weigh about 800 pounds, that is, 4800 times the weight of the model, and instead of having twenty times the plane surface would require one thousand times the spread.
It is this peculiarity between models and the actual flyers that for years made the question of flying a problem which, on the basis of pure calculation alone, seemed to offer a negative; and many scientific men declared that practical flying was an impossibility.
LESSONS FROM MODELS.--Men, and boys, too, can learn a useful lesson from the model aeroplanes in other directions, however, and the princ.i.p.al thing is the one of stability.
When everything is considered the form or shape of a flying model will serve to make a large flyer. The manner of balancing one will be a good criterion for the other in practice, and experimenting with these small devices is, therefore, most instructive.
The difference between gliders and model aeroplanes is, that gliders must be made much lighter because they are designed to be projected through the air by a kick of some kind.
FLYING MODEL AEROPLANES.--Model aeroplanes contain their own power and propellers which, while they may run for a few seconds only, serve the purpose of indicating how the propeller will act, and in what respect the sustaining surfaces are efficient and properly arranged.
It is not our purpose to give a treatise on this subject but to confine this chapter to an exposition of a few of the gliders and model forms which are found to be most efficient for experimental work.
AN EFFICIENT GLIDER.--Probably the simplest and most efficient glider, and one which can be made in a few moments, is to make a copy of the deltoid kite, previously referred to.
This is merely a triangularly-shaped piece of paper, or stiff cardboard A, Fig. 84, creased in the middle, along the dotted line B, the side wings C, C, being bent up so as to form, what are called diedral angles. This may be shot through the air by a flick of the finger, with the pointed end foremost, when used as a glider.
_Fig. 85. Deltoid Glider._
THE DELTOID FORMATION.--This same form may be advantageously used as a model aeroplane, but in that case the broad end should be foremost.
_Fig. 86. The Deltoid Racer._
Fig. 86 shows the deltoid glider, or aeroplane, with three cross braces, A, B, C, in the two forward braces of which are journaled the propeller shaft D, so that the propeller E is at the broad end of the glider.
A short stem F through the rear brace C, provided with a crank, has its inner end connected with the rear end of the shaft D by a rubber band G, by which the propeller is driven.
A tail may be attached to the rear end, or at the apex of the planes, so it can be set for the purpose of directing the angle of flight, but it will be found that this form has remarkable stability in flight, and will move forwardly in a straight line, always making a graceful downward movement when the power is exhausted.
It seems to be a form which has equal stabilizing powers whether at slow or at high speeds, thus differing essentially from many forms which require a certain speed in order to get the best results.
RACING MODELS.--Here and in England many racing models have been made, generally of the A-shaped type, which will be explained hereinafter.
Such models are also strong, and able to withstand the torsional strain required by the rubber which is used for exerting the power.
It is unfortunate that there is not some type of cheap motor which is light, and adapted to run for several minutes, which would be of great value in work of this kind, but in the absence of such mechanism rubber bands are found to be most serviceable, giving better results than springs or bows, since the latter are both too heavy to be available, in proportion to the amount of power developed.
Unlike the large aeroplanes, the supporting surfaces, in the models, are at the rear end of the frames, the pointed ends being in front.
_Fig. 87. A-Shaped Racing Glider._
Fig. 87 shows the general design of the A- shaped gliding plane or aeroplane. This is composed of main frame pieces A, A, running fore and aft, joined at their rear ends by a cross bar B, the ends of which project out slightly beyond their juncture with the side bars A, A. These projecting ends have holes drilled therein to receive the shafts a, a, of the propeller D, D.
A main plane E is mounted transversely across this frame at its rear end, while at its forward end is a small plane, called the elevator. The pointed end of the frame has on each side a turnbuckle G, for the purpose of winding up the shaft, and thus twisting the propeller, although this is usually dispensed with, and the propeller itself is turned to give sufficient twist to the rubber for this purpose.