At this point let us become familiar with the idea of power in connection with flying. When the wind blows it often possesses enor mous power and when it blows suitably against a windmill or the sail of a yacht or the wing of a bird, it may transfer some of its energy into mechanical movement as grinding, sail ing, and flying, respectively, in the three in stances cited.
When the air itself does not move, or does not move in a suitable way to enable the transformation of its energy into soaring flight, the power necessary to the continuance of flying must be supplied by the object which flies. The bird flaps its wings, the aeroplane starts its engine. There may be little resem blance between the action of a bird's wing and that of the propeller which is driven by the engine of an aeroplane. Scientifically, there is a close analogy between them. One of na ture's masterpieces is the perfect articulation of the joints of an anatomy that permits such smooth-acting to-and-fro movement as is manifested by the legs of an animal when walking and of a bird's wing when in flapping flight. Reciprocating motion is anathema to engineering, but the engineer finds a great compensation in the principle of rotation, and wherever it is possible to do so the mechanism of mechanical power is confined to the con tinuously revolving shaft. Thus, on an aero plane, you find an engine which generates the power, a revolving shaft, which transmits the power, and a propeller on the shaft, which transforms the power into thrust or traction, and so pushes or pulls the engine as a whole through the air.
Power is essential to flight, but when the engine stops in mid-air the aeroplane does not fall to the ground, nor is the pilot in any need of a balloon. The aeroplane itself, when properly designed and flown, possesses an in herent quality that is better than any artifice, and without which flying by aeroplane would be quite out of the question.
The principles of aeroplane propulsion and sustentauon are shown graphically in Fig. 1. The case considered is that of a biplane. A single large propeller serves to pull the machine in the direction of flight and the air immediately ahead of the machine, being displaced by the action of the propeller, flows backward hori zontally at high velocity and presses against the under surface of the aerofoil.
The following tables, giving the air resist ance and corresponding horse-power and the air pressure at different velocities, will be found of value in calculations pertaining to the de signing of any class of aircraft: The effect of varying areas and forms of planes is marked, and with planes of different form but of the same area results are obtained which are so different as to be the cause of comment. There is no question that the form
of the wing of the bird gives proportions which can be followed in the aeroplane with good results. This means that the best results will be obtained with a plane which is wide when viewed from the front and short when seen from the side. The exact proportions have never been definitely determined, and vary in all of the successful creations, but it was de termined by Professor Langley that a plane with a wide advancing edge was the most efficient, not only by calculation, but by actual experiments. His tests were made with various forms and areas of planes, and for illustration it will be said that a plane with an advancing edge or width of six inches and a length of 18 inches, when moving at a horizontal velocity of 65.61 feet per second, or at the rate of 45 miles per hour, as shown in Fig. 2, A fell vertically four feet in seven-tenths of a second. On the other hand a plane with an advancing edge of 18 inches and a length of six inches, obviously having the same supporting area as the other, and moving at the same velocity, fell four feet vertically in two seconds, demon strating that the sustaining power of the form shown in B was about three times as great as that of the form with the narrow advancing edge.
edge. Two of the most important accessory features of the supporting planes are the ele vator and the rudder. Since the aeroplane sails in, instead of on, the aerial sea, it must be equipped with two sorts of rudder, one for steering to the right and left in the ordinary way, and another for steering up and down. The latter we now call an elevator in order to Constructional Features.— Like birds, aeroplanes possess bodies, wings and tails; they also have undercarriages that serve the purpose of legs when alighting. The class of machine that most closely approximates to the bird type is the typical monoplane; biplanes often have no resemblance whatever, in ap pearance, to nature's best flyers. The technical difference between a monoplane and a biplane, however, is merely that a monoplane, in com mon with the bird, possesses only one pair of wings, while the biplane is provided with two main supporting surfaces, one situated above the other.