In the early twenties the design of the airplane underwent very little change. The biplane with an enclosed fuselage remained standard in both military and civil aircraft. With the exception of a few Navy flying boats, the biplane was a two-place plane capable of carrying the pilot and one passenger, or 300 pounds of cargo or mail. There were some attempts at streamlining to eliminate drag, but they consisted mainly of using fewer wing struts and wire bracings.

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.

During this time the Curtiss Company continued to build successful airplanes for both the Army and the Navy, including the first of the famous Hawk fighters, completed in 1923. Martin worked on improved types of Army bombers and Douglas built planes for both branches of the service. In Seattle, Washington, the Boeing Company had started its first aircraft for the Army. New names such as Beech, Cessna, Sikorsky, Vought, Fairchild, Northrop, and others began to appear on the nameplates of new planes.

In the early twenties, with transcontinental mail service well under way, there were many attempts made to establish air transport and cargo services. Most of these ventures were undertaken by former military aviators, using cast-off Army airplanes. Their airports usually were cow pastures. They planned their own air routes and got their weather reports from the newspapers. Bad weather would often ground a flight and passengers were almost as uncertain as the weather. Many of those pioneer operators had to depend on the dollar-a-ride hops of Sunday sightseers to “keep the wolf from the door.” One service operated 14-passenger converted Navy seaplanes on a route between New York and Havana, and another route between Cleveland and Detroit. Most of these pioneer air transport Operations lasted for only a short time, due to the heavy cost of maintaining the planes and the lack of properly marked air routes.

Difficulties had arisen in the air mail service by 1921. It had become apparent that air mail would not be valuable to the Government unless it could be flown by night as well as by day. It had been standard practice for the mail to be flown only during daylight hours and to be carried by train at night. The Government was about to abandon the air mail service when the pilots pointed out that all that was needed was a chain of airway beacons and lights for the landing fields and planes.

To prove their point a group of pilots volunteered to make a continuous night-and-day flight from San Francisco to New York. Flying in relays and guided at night by bonfires tended by friendly farmers along the route, the pilots flew the mail across the country in 33 hours and 21 minutes. The Post Office Department immediately arranged for the installation of lighted airways and the planes were equipped with navigation and landing lights.

By July, 1924, a continuous chain of lighted airway beacons marked the air mail route from coast to coast. Lighted landing fields were established at 250-mile intervals and through transcontinental air mail service, with night-and-day flying, was an accomplished fact.