By the end of 1926, there were sixteen air transport operators holding air mail contracts. Most of the flying was still done in single-engined planes. Up to that time the weight of the big water-cooled engines in multi-engined transports left little to spare for pay loads. It was not until the development of the radial engine that commercial aviation really started. [Illustration: AIR-COOLED RADIAL ENGINE] The in-line engine required a long, heavy crankshaft with sections for each cylinder. This required that separate crankshaft bearings be used for each cylinder. The whole crankshaft assembly was heavy and cumbersome. When extra cylinders were added, the engine’s weight increased and it became longer. In the radial engine a single crankshaft hearing was used. The radial air-cooled engine immediately showed many advantages over the in-line, water-cooled engines of that time. The use of aluminum in its construction made it lighter. It was cooled by allowing air to rush through finely spaced fins on cylinder heads and barrels. The weight of the cooling liquid (water) and the pump and mechanism to circulate it was avoided. [Illustration: THE ACTION OF THE CRANKSHAFT IS THE SAME AS WINDING UP THE WELL ROPE THAT HOISTED THE OLD OAKEN BUCKET. THE CRANKSHAFT’S JOB IS TO TRANSLATE STRAIGHT-LINE MOTION INTO ROTARY MOTION. IN THE RADIAL ENGINE, ALL HANDS TRANSMIT THEIR POWER TO ONE MASTER ROD IN THE IN-LINE ENGINE THE POWER OF EACH CYLINDER IS TRANSMITTED BY A SEPARATE CRANK. THIS MAKES THE CRANKSHAFT GROW LONGER WITH EACH ADDITIONAL CYLINDER. SHORT, LIGHT, 9-CYLINDER, RADIAL CRANKSHAFT LONG, HEAVY CRANKSHAFT OF A FOUR-CYLINDER IN-LINE ENGINE EACH ADDITIONAL CYLINDER NOT ONLY INCREASES THE LENGTH OF THE CRANKSHAFT, BUT IT ALSO MAKES IT NECESSARY TO CONSTRUCT A HEAVIER BASE TO SUPPORT THE CYLINDERS, CRANKSHAFT, AND CRANKSHAFT BEARINGS. THE RADIAL ENGINE, THE POWER OF ALL CYLINDERS IS TRANSLATED INTO ROTARY MOTION BY RODS CONNECTED TO THE MASTER ROD, WHICH TURNS THE SHORT CRANKSHAFT.] [Illustration: LINDBERGH’S RYAN MONOPLANE _SPIRIT OF ST. LOUIS_] BETTER POWER FOR AMERICA’S AIRPLANES The Wright Brothers’ first airplane engine had weighed 170 pounds and had produced 12 horsepower. It had used twenty-five per cent of its energy propelling itself. With the introduction of the air-cooled, radial engine twenty years later, a pound and a half of engine had been made to produce one horsepower. Thus the new 350-pound radial engine of 200 horsepower put all but a fraction of weight into load-carrying power.