Much of the efficiency of the motor is due to the form and gearing of the propeller. Here again, as in other vital parts of flying-machine mechanism, we have a wide divergence of opinion as to the best form. A fish makes progress through the water by using its fins and tail;a bird makes its way through the air in a similar manner by the use of its wings and tail. In both instances the motive power comes from the body of the fish or bird.
In place of fins or wings the flying machine is equipped with a propeller, the action of which is furnished by the engine. Fins and wings have been tried, but they don't work.
While operating on the same general principle, aerial propellers are much larger than those used on boats.
This is because the boat propeller has a denser, more substantial medium to work in (water), and consequently can get a better "hold," and produce more propulsive force than one of the same size revolving in the air.
This necessitates the aerial propellers being much larger than those employed for marine purposes. Up to this point all aviators agree, but as to the best form most of them differ.
Kinds of Propellers Used.
One of the most simple is that used by Curtiss. It consists of two pear-shaped blades of laminated wood, each blade being 5 inches wide at its extreme point, tapering slightly to the shaft connection. These blades are joined at the engine shaft, in a direct line. The propeller has a pitch of 5 feet, and weighs, complete, less than 10 pounds. The length from end to end of the two blades is 6 1/2 feet.
Wright uses two wooden propellers, in the rear of his biplane, revolving in opposite directions. Each propeller is two-bladed.
Bleriot also uses a two-blade wooden propeller, but it is placed in front of his machine. The blades are each about 3 1/2 feet long and have an acute "twist."Santos-Dumont uses a two-blade wooden propeller, strikingly similar to the Bleriot.
On the Antoinette monoplane, with which good records have been made, the propeller consists of two spoon-shaped pieces of metal, joined at the engine shaft in front, and with the concave surfaces facing the machine.
The propeller on the Voisin biplane is also of metal, consisting of two aluminum blades connected by a forged steel arm.
Maximum thrust, or stress--exercise of the greatest air-displacing force--is the object sought. This, according to experts, is best obtained with a large propeller diameter and reasonably low speed. The diameter is the distance from end to end of the blades, which on the largest propellers ranges from 6 to 8 feet. The larger the blade surface the greater will be the volume of air displaced, and, following this, the greater will be the impulse which forces the aeroplane ahead. In all centrifugal motion there is more or less tendency to disintegration in the form of "flying off" from the center, and the larger the revolving object is the stronger is this tendency. This is illustrated in the many instances in which big grindstones and fly-wheels have burst from being revolved too fast. To have a propeller break apart in the air would jeopardize the life of the aviator, and to guard against this it has been found best to make its revolving action comparatively slow. Besides this the slow motion (it is only comparatively slow) gives the atmosphere a chance to refill the area disturbed by one propeller blade, and thus have a new surface for the next blade to act upon.
Placing of the Motor.
As on other points, aviators differ widely in their ideas as to the proper position for the motor. Wright locates his on the lower plane, midway between the front and rear edges, but considerably to one side of the exact center. He then counter-balances the engine weight by placing his seat far enough away in the opposite direction to preserve the center of gravity. This leaves a space in the center between the motor and the operator in which a passenger may be carried without disturbing the equilibrium.
Bleriot, on the contrary, has his motor directly in front and preserves the center of gravity by taking his seat well back, this, with the weight of the aeroplane, acting as a counter-balance.
On the Curtiss machine the motor is in the rear, the forward seat of the operator, and weight of the horizontal rudder and damping plane in front equalizing the engine weight.
No Perfect Motor as Yet.
Engine makers in the United States, England, France and Germany are all seeking to produce an ideal motor for aviation purposes. Many of the productions are highly creditable, but it may be truthfully said that none of them quite fill the bill as regards a combination of the minimum of weight with the maximum of reliable maintained power. They are all, in some respects, improvements upon those previously in use, but the great end sought for has not been fully attained.
One of the motors thus produced was made by the French firm of Darracq at the suggestion of Santos Dumont, and on lines laid down by him. Santos Dumont wanted a 2-cylinder horizontal motor capable of developing 30 horsepower, and not exceeding 4 1/2 pounds per horsepower in weight.
There can be no question as to the ability and skill of the Darracq people, or of their desire to produce a motor that would bring new credit and prominence to the firm. Neither could anything radically wrong be detected in the plans. But the motor, in at least one important requirement, fell short of expectations.
It could not be depended upon to deliver an energy of 30 horsepower continuously for any length of time.
Its maximum power could be secured only in "spurts."This tends to show how hard it is to produce an ideal motor for aviation purposes. Santos Dumont, of undoubted skill and experience as an aviator, outlined definitely what he wanted; one of the greatest designers in the business drew the plans, and the famous house of Darracq bent its best energies to the production. But the desired end was not fully attained.
Features of Darracq Motor.
