THE HELICOPTER—THE ORNITHOPTER—WING SURFACE—FLYING SPEED—LANDING SPEED—EFFECT OF MOTORS—THE SEAPLANE
The heavier-than-air machines are divided into three classes. The helicopter is a machine which theorists of that school believe can fly straight up into the sky because its air screw propeller works on a vertical axis. This type of aircraft has never been successful, for the reason that the propeller does not lift. It simply pulls a stream-lined surface through the air. The lifting must be done by planes.
The ornithopter is another heavier-than-air craft which seeks to fly by flapping wings like a bird. The effort to build this type of machine is as old as human desire to imitate the fowls of the air and it has been as unsuccessful as the helicopter.
Before we begin to discuss the aeroplane we must remember that before a modern machine leaves the ground it must be moving at least thirty-five miles an hour with respect to the air. This forcing of the edges of these broad-pitching, curved surfaces through the air at such a velocity naturally drives the air downward and these particles of atmosphere react in exactly the same degree upward, thus forcing the planes and the attached apparatus upward. Therefore, as long as the aeroplane rushes through the air at that or greater speed the thousands of cubic feet of air forced down beneath the wings deliver up a reaction that results in complete support. When an aircraft fails to move at that velocity it loses “flying speed” and falls to the earth. The net result of this reaction is called “lift,” and as long as the machine sweeps forward at that momentum it has lift. The engine, of course, must supply this forward movement, and when it stalls, the heavier-than-air machine must glide to a landing-place or fall perpendicular to the ground.
To understand why a heavier-than-air machine flies it is necessary to remember that air or atmosphere has many of the characteristics of water. Indeed, like the ocean, its pressure varies at different altitudes. At sea-level a cubic foot in dry weather weighs 0.0807 pounds, but at a mile above sea-level it weighs only 0.0619 pounds, and at five miles 0.0309 pounds per cubic foot and so on up. Therefore machines designed to fly at sea-level often fail to get off the ground at 12,000 feet above the sea in such countries as Mexico.
Air also has motion. Its tendency to remain motionless is called inertia, and its characteristic desire to reoccupy its normal amount of space is known as its elasticity, and the tendency of the particles of air to resist separation is described as its viscosity. Thus we see that air has practically the same characteristics as water, only it is much lighter.
Without going into a technical discussion of all the forces that enter into the flight of an aeroplane we must, however, realize that if the pressure of the atmosphere is uniform in all directions, in order to make the air forced under a wing or plane lift more than the air above forces down, the wing of the plane must be curved in such a way that the forward motion of the edge of the wing causes the air underneath to force any particle of the surface upward, while the upper surface is relieved of the pressure. This is done by curving the surface of the planes so that the under surface is concave while the upper part is almost convex, like the outspread wing of a bird. When this wing is forced horizontally through the air it creates a vacuum immediately behind the upper or convex part, the under pressure is still constant and the surface is lifted upward. That is why a plane covered with a curved surface will fly and a plane with a flat surface will not. In short, a curved surface when moving through atmosphere causes eddies in the air, and if the curvature of the wings is properly calculated, it leaves a vacuum near the rear edge of the surface of the plane and it climbs upward. The smaller the angle the smaller the lift or climbing power of the plane. Thus a 15-degree angle will lift one pound; if reduced to 10 degrees it will only lift two-thirds of a pound, but because a wing is curved a plane could fly at several degrees less than 0 degree, but its “stalling” or critical angle beyond which it is not safe to go is 15 degrees.
It must be borne in mind that the larger the wing surface the larger load the aeroplane can carry, for the lift of a heavier-than-air machine depends entirely on the number of square feet of surface in the plane or wings. The larger the planes the more power is required to force them through the air and the less easy they are to manœuvre and land. The Nieuports, Spads, Sopwiths, and Fokkers, with their small wing spread of less than 30 feet, made them much easier to fly, even though they land faster than the “big busses.” Therefore every pound of weight added to an aeroplane decreases its speed proportionately and requires an equivalent increase in horse-power to force it through the air. Of course, an increase of speed gives an increase in lift, so by doubling the speed of a plane you increase the lift just four times.
There are, however, a number of factors which tend to decrease the progress of a machine through the air: the head resistance of the fuselage, the motor, the struts, the wires, the landing-gear, etc. These things do not add to the lift and are described as “dead-head” resistance. Stream-line, or the tapering of all surfaces which resist the air, helps reduce this resistance, so that the design of the plane has much to do with its speed, also as to whether the plane can climb faster than fly straight ahead. Naturally the horse-power of the motor determines the flying speed of the aeroplane as much as any other factor.
To lift a plane off the ground it must be travelling at least 35 miles an hour with respect to the air, as we have pointed out before. So if a gale is blowing 20 miles an hour the aeroplane may be lifted off the ground when moving no faster than 15 miles an hour with respect to the earth. Likewise unless a machine is moving 35 miles an hour it will lose flying speed and fall to the ground.