360 FORMS OF ENERGY Much more of the power of an engine is used in overcoming the force of gravity when climbing than is used to maintain a given altitude during flight, just as is true of climbing mountains with an automobile. Higher speeds are possible at high altitudes partly because the air has less resistance due to its lower density at higher altitudes. New advances in petroleum technology have not only made available large quantities of gasoline of 100 octane number or higher, thus considerably increasing the potential power of airplane engines designed to use such fuels, but they have also made possible a high-octanenumber fuel no more volatile than kerosene with a flash point (temperature at which the vapors will ignite) above 100° F. in contrast with flash points below room temperature for gasolines. Such fuels would Fig. 153. Starting an airplane engine by means of a cartridge. (Photograph by Rudy Arnold.) be much safer to use, but their use would require that the fuel be injected into the engine as in the Diesel engine rather than be mixed with air in a carburetor. Diesel engines have already competed successfully w'ith air-cooled gasoline engines for use in airplanes. Perhaps the airplane engine of the future will represent a compromise between the gasoline engine and the Diesel engine. The maintenance of Diesel engines is more costly than that of gasoline engines, and for that reason Diesel airplane engines have not been developed in the United States, where gasoline is plentiful. Aeronautical engineers predict that engines of 5000 horsepower, liquid-cooled, of 24 or more cylinders, may be developed within the decade 1940-1950. Many planes are now equipped with self-starters consisting of explosive cartridges that fire a charge of gas into the motor. Another starter used for light planes is the hydraulic type of enginestarter, which uses oil compressed by a hand pump in the plane cabin. Plastics Have Made Many Contributions to Modem Airplanes. There has been a gradual change from the wire sticks and fabrics used to construct the first airplanes through the use of stainless steel and aluminum to the use of plastics and plywood.
AIRPLANE A TRIUMPH OF MODERN SCIENCE 361 Experimental airplanes have demonstrated that the wings and fuselage may be molded in large quantities at low cost from plastics laminated with plywood. The strength for a given weight of such plastic materials exceeds by ten times that of stainless steel in local buckling, but not in tensile strength. Fig. 154. Plastic-plywood plane. (Courtes> of the Langley Aviation Corporation.) The surface of such planes may be made very smooth, thus reducing skin friction. Plastic surfaces are resistant to weather, impervious to water, reasonably fireproof, and corrosion-proof. Plastics enter modern aircraft in many minor ways, such as in their use for instrument cases. Windows are sometimes made of acrylic resin sheets. Neoprene Gasoline Tanks Have Many Advantages. Neoprene-rubber gasoline tanks, called Mareng cells, have the following advantages: 1. They eliminate corrosion. 2. They are not affected by vibration or the surging of the gasoline in them.