In addition to bombing operations, these large machines are also used for the protection of "spotting" aeroplanes, or for the direct protection of the lines against land attacks. These heavily armed bombers are very difficult to attack, even for the smaller and more agile "Chasers," as they can fire from below as well as from the front, top, or sides. In the bombers which have only a single gun in the rear, the gunner is working at a disadvantage if his adversary forces him to continually raise and lower his gun from the top of the body to the lower trap door. This is very tiring to the rear gunner, and if the chaser's tactics are carried out for a sufficient length of time, it can wear out the gunner by continually rising and dropping at the tail of the bombing plane. In regard to the front gun, the twin motor type offers many of the advantages of the pusher, and as a whole, the twin arrangement will nearly double the field of fire of either the tractor or pusher.
The bombing planes must have a very large radius of action, particularly those that are used in night bombing operations. The Gothas in bombing London fly several hundred miles from their base, and recently a Handley-Page bombing plane flew from London to Constantinople, Turkey, making only a few stops on the way. Starting out from Hendon, England, the Handley-Page machine flew to Paris, down the Rhone valley to Lyons and Marseilles, and then to Pisa, and Rome (Italy), where they landed. From Rome the machine passed over Naples, over Oranto and then over the Albanian Alps to the base at Salonica. Making preparations at this base they flew the final stage of the trip to Constantinople, a distance of 250 miles over hostile country. The bombing of the Turkish capital was done at night after a flight of 2 1/2 hours from Salonica. When over the sea of Marmora, the ship "Goeben" was bombed, and in addition a hit was scored on the two submarines lying at her side. Four bombs struck the "Goeben" directly, from an altitude of 800 feet. Two more bombs were dropped on the German ship, "General," which was the headquarters of the German staff. Finally, after 30 minutes over the city of Constantinople, the Turkish War Office was the recipient of two more bombs. In the words of the Turkish communiqué this "Was not entirely destroyed." On its return to Salonica it was found that the machine had been struck by 26 shrapnel bullets. This disabled one of the power plants so that the greater part of the return journey was made on a single motor.
From London to Salonica five men were carried. In addition was their luggage, bedding, two tool boxes, spare parts equivalent in weight to one engine, and two 11'-6" spare propellers. Complete, the machine weighed over 6 tons, with a useful load of about 6,000 pounds. In crossing the Albanian Alps the machine frequently was at an altitude of 10,000 feet. The power plant consisted of two 275 horsepower Rolls-Royce motors, and even at this high altitude, and with the heavy loading, no trouble was experienced. During the bombing, only three men were carried, the remainder of the useful weight being made up of bombs and other ammunition. While this record will probably be beaten before this book goes to press, it will at least give an idea as to the requirements and capabilities of the bombing type aeroplane.
Military Training Machines. The military training machines used in the United States are generally of the two seater tractor type, similar in external appearance to the reconnaissance type machines already described. They are low powered, 90 to 125 horsepower, and will have an average span of 40'-0". The controls are in duplicate so that the student's controls move in unison with the instructor's.
CHAPTER III. ELEMENTARY AERODYNAMICS
Definition. Aerodynamics treats of the forces produced by air in motion, and is the basic subject in the study of the aeroplane. It is the purpose of this chapter to describe in detail the action of the wing in flight, and the aerodynamic behavior of the other bodies that enter into the construction of the aeroplane. At present, aerodynamic data is almost entirely based on experimental investigations. The motions and reactions produced by disturbed air are so complex and involved that no complete mathematical theory has yet been advanced that permits of direct calculation.
Properties of Air. Air being a material substance, possesses the properties of volume, weight, viscosity and compressibility. It is a mechanical mixture of the two elementary gases, oxygen and nitrogen, in the proportion of 23 per cent of oxygen to 77 per cent of nitrogen. It is the oxygen element that produces combustion, while the nitrogen is inert and does not readily enter into combination with other elements, its evident function being to act as a dilutant for the energetic oxygen. In combustion, the oxygen enters into a chemical combination with the fuel while the nitrogen passes off with the products of combustion unchanged.