CHAPTER III
WHY AN AEROPLANE FLIES
THE HELICOPTER—THE ORNITHOPTER—WING SURFACE—FLYING SPEED—LANDING SPEED—EFFECT OF MOTORS—THE SEAPLANE
THE HELICOPTER—THE ORNITHOPTER—WING SURFACE—FLYING SPEED—LANDING SPEED—EFFECT OF MOTORS—THE SEAPLANE
Theheavier-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 inexactly 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, aswe 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.
Machines do not all land at the same speed. The famous Morane monoplane skimmed along the ground at anywhere from 45 to 90 miles an hour. It is manifestly impossible to do more than suggest the fundamental principles of aeroplane flight here. To be sure, the type of aircraft has, as we have indicated, much to do with why and how it flies. Because of its similarity to the bird and owing to the lack of struts, etc., to increase the head resistance the monoplane or single-wing plane is the fastest machine. The absence of struts and the few bracing wires brings a greater strain on the wings and increases its chances of breaking. The biplane, with its two parallel wings separated by struts, is more easily braced and proportionately stronger. The lift is also greater, due to the additional wing surface. The vacuum made over the lower wing is interfered with by the upper plane, and thus neutralizes somewhat the lifting and flying efficiency of the upper wing. Since a plane must reverse all its stresses when looping, the double supports of the biplane make it less susceptible to doubling up and falling. These are some of the reasons for the popularity of the biplane.
The triplane is so called because it has three tiers ofwing surfaces set one above the other. This allows for even greater strength in construction, and despite the resistance several very fast-climbing triplanes have been built. The famous Caproni triplanes with three motors have a wing spread of 127 feet. Many biplanes and flying-boats also have approximately 126-foot wing spread. The well-known Handley Page bomber and the NC-1, NC-2, NC-3, NC-4 Naval Flying Boats, which tried the Atlantic flight, had a similar wing spread.
In the war the small aeroplane of the monoplane or biplane type with a small wing spread and equipped with a rotary motor, whose nine or more cylinders revolved with the propeller, or a small V-type motor, was called a scout. These biplanes seldom had a wing spread of over 28 feet and the horse-power of the rotary motors seldom developed more than 150 horse-power, whereas the stationary motors for these same machines generated as much as 300 horse-power, as in the case of the Hispano-Suiza. These machines were used for fighting because they made as high as 150 miles an hour and responded so easily to the slightest movement of the “joy stick” and, consequently, manœuvred so readily. Since trick flying was absolutely essential to air duels these machines were best for this purpose and for quickly getting information of troop movements.
The next larger size, seating two men and driven by the same types of motors or even larger twelve-cylinder Rolls-Royce or Liberty motors, but with a wing spreadof from 34 to 48 feet, was used for taking photographs, directing artillery-fire, and general reconnaissance in war. The multimotored machines, with a wing spread of anywhere from 48 to 150 feet, were used for bombing at night or during the day. Owing to the size of these machines and because of their slow-flying speed they were easy to land. Some of the scouts weighed, with petrol and two hours’ fuel, less than 1,000 pounds, whereas the four-motored bombers, with 127-foot wing spread, weighed over six tons and could carry a useful load of three tons.
The hydroaeroplane does not differ fundamentally from the aeroplane as regards flying principles. In structure it may be a biplane or triplane, but owing to the supports necessary to carry the pontoons it cannot be easily attached to a monoplane. Structurally, it differs from the aeroplane only in having pontoons or a boat substituted for wheels and landing chassis. Owing to the surfaces presented by the pontoons or the hull of the boat, looping is practically eliminated and the spread of these flying craft is much slower than land machines.
Although M. Fabre conducted experiments with aeroplanes carrying floats instead of wheels, Mr. Glenn H. Curtiss was the first to successfully construct and fly a hydroplane. At the time of his flight down the Hudson River from Albany to New York he equipped his plane with a light boat to protect himself in case of a forced landing on the water. Encouraged by this experiment under the Alexander Graham BellAerial Experiment Association, and by later attaching a canoe, he succeeded in landing and getting off the water. Later he built a hydroaeroplane and flew successfully at San Diego, Cal., thus establishing America as the land which invented and developed the seaplane and flying-boat.
Structurally, the modern seaplane has two small pontoons on the end of each wing and a small boat in the centre, or sometimes only two pontoons in all which are side by side near the fuselage. The flying-boat has one large boat instead of a fuselage, with a small pontoon on the end of each wing. The former is used for fast flying, but owing to the air resistance to the pontoons, and especially to the boats, the speed cannot be compared to that of the scout aeroplanes. Moreover, they are much harder to do stunts with and few are known to have looped the loop. Like the big land bombers the flying-boats may be equipped with as many as three motors. One of these has carried as many as fifty passengers at one time.
Contrary to the accepted notion, these flying-boats are very hard to land on the sea because it is so difficult to calculate the position of the wave when you strike—both are moving so rapidly.
As we have already seen that due to the fact that a heavier-than-air machine must be moving at least 35 miles an hour to get off the ground or water, a strong and powerful motor is absolutely essential to make aeroplane flying possible. We have already discovered that the Wrights had to construct their own motor because none was light enough for an aeroplane. Their 16 horse-power single-cylinder engine weighed over 200 pounds. To-day the Liberty is rated at from 400 to 450 horse-power, and it weighs less than two pounds per horse-power. An Italian aeronautical engine develops 700 horse-power, and one sixteen-cylinder American motor generates 900 horse-power. This shows the tremendous development of the motor for modern flying.