Landing gears were made stronger and the oleo landing strut was introduced. The oleo landing strut was made by two sleevelike cylinders which operated as does a piston. The upper cylinder was filled with heavy oil. The landing wheels were attached to the lower cylinder. On landing, the weight of the airplane caused the cylinder to push up, as a piston, into the oil-filled upper cylinder. This produced a pressure on the oil. A small opening in the cylinder allowed the oil slowly to slip out of the cylinder. This reduced the pressure gradually as the gear absorbed the landing shock. If you take a bicycle pump and hold your finger over the valve, then build up pressure in the pump and at the same time allow just a little air to escape from under your finger, you will readily see how the oleo landing works. The oleo shock-absorbing type of landing gear is standard with all modern planes.
Fuselage construction of wooden stringers and posts, with the wire bracing so familiar in all early airplanes, gave way to the use of veneered wood covering. The first Douglas planes, the DH-4’s, the Curtiss Orioles, and the L. W. F. of the early twenties used veneer covering instead of fabric for their fuselages. This was followed by the introduction of welded steel tubing for fuselage framework. Several attempts were made to develop a monoplane in those days but none was very successful. In Germany, in 1922, the Junkers JL6 was the first plane successfully to use an internally braced monoplane wing. In this country it was several years before an aircraft designer dared to attempt to overcome the prejudiced aviators against the monoplane design.
During the middle twenties the names of Wright, Curtiss, and Martin were still to the fore. The Wright Aëronautical Corporation was the leader in its field. Its liquid-cooled engines had grown from 120-horsepower to 300-, 400-, 675-horsepower. It also had begun to experiment with and develop an air-cooled radial airplane engine. This engine, invented by Charles L. Lawrance, was a result of his study of the Manley radial engine built for Professor Langley’s _Aerodrome_. The Manley engine was far ahead of its time. What might have happened had the first Wright plane and the Manley engine come together in the early days is pure guesswork. The original Manley radial engine weighed only 3.6 pounds per horsepower. In the early twenties, when Lawrance started to work with the Manley engine as a guide, airplane engines weighed about 10 pounds per horsepower. The Manley engine used in the _Aerodrome_ was water-cooled and Lawrance went to work to eliminate the extra weight caused by radiator and water-cooling equipment. So successful were his first experiments that he joined the Wright Aëronautical Corporation to collaborate in developing an aircraft engine that was to have a profound influence on world aviation.