Three threads of change
An aircraft from 1910 and one from 2010 share the same physics, but almost nothing else looks alike. The changes can be grouped into three threads that advanced together: the structure that carries the loads, the engine that supplies power, and the controls that let a pilot keep the aircraft stable and steer it. A gain in one often forced improvements in the others. A stronger airframe was worth little without a lighter engine, and a faster airplane needed more reliable controls.
Structure: from sticks and fabric to stressed skin
The machines that flew in the first decade after 1903 were built mostly of wood, wire and cloth. Biplanes were the norm because two wings braced with struts and wires gave strength at low weight, even though the drag from all that bracing limited speed. During the First World War, the demand for faster and sturdier fighters pushed designers to refine this form, and by the 1920s some aircraft used welded steel tube frames covered with fabric.
A major turning point was the move to metal. Hugo Junkers built an all-metal monoplane in 1915, and aluminum alloys, which became practical to use partly because of industrial aluminum smelting, gradually replaced wood. The more important idea was the stressed skin, or monocoque: instead of a frame with a covering, the outer shell itself carries much of the load. This allowed cantilever wings with no external bracing, cleaner shapes and higher speeds. By the 1930s all-metal airliners such as the Douglas DC-3, which first flew in 1935, were common.
In recent decades, builders have used more composite materials, such as carbon fiber mixed with resin, which can be strong and light. Some modern airliners use composites for a large share of their structure, and designers also continue to refine wing design with computer modeling. Each of these changes came with trade-offs in cost, repair methods and inspection.
Engines: more power for less weight
The Wright brothers had to build their own engine in 1903, a four-cylinder design producing on the order of a dozen horsepower. Over the next few decades, aircraft engines improved sharply. Rotary engines, in which the cylinders spun around a fixed crankshaft, were popular in the First World War. Radial engines, with cylinders arranged in a circle, became widespread in the 1920s and 1930s for their reliability and cooling, while liquid-cooled inline engines suited streamlined fighters.
Propellers improved as well. Fixed-pitch wooden blades gave way to metal blades and then to variable-pitch and constant-speed propellers, which let the engine and propeller work efficiently across takeoff, climb and cruise. These details of airplane propellers shaped what piston-powered aircraft could do.
The next leap came with the gas turbine. Frank Whittle in Britain and Hans von Ohain in Germany developed jet engines in the 1930s, and the first jet-powered flight took place in Germany in 1939. Jets allowed speeds and altitudes that piston engines could not match.
Controls: keeping the aircraft stable
The Wrights’ great insight was that a pilot had to control the airplane in three axes: pitch, roll and yaw. Their solution used wing twisting for roll, a movable rudder and a forward elevator. Within a few years, hinged ailerons on the trailing edges of the wings became the standard way to control roll, and the tail-mounted elevator and rudder replaced the front elevator on most designs.
As airplanes grew faster, the forces on control surfaces became too large for pilots to handle by muscle alone. Designers added trim tabs and then hydraulic systems to assist movement. Autopilots, which use gyroscopes to hold a heading or altitude, appeared in the 1910s and became standard on larger aircraft. In the 1970s and 1980s, electronic fly-by-wire systems began replacing mechanical linkages in some aircraft, with computers interpreting the pilot’s inputs and limiting maneuvers that could overstress the airframe. Airliners such as the Airbus A320, introduced in the late 1980s, used this approach.
Shapes that followed speed
Speed changed the look of aircraft. As designs approached the speed of sound, engineers found that thin, swept-back wings reduced the sharp rise in drag. Research on swept wings in Germany before and during the Second World War was studied and built upon after 1945, and swept wings became standard on jets. Airplane cockpits changed alongside, moving from open seats with a few instruments to enclosed, pressurized cabins and eventually screen-based displays built around integrated circuits.
Navigation also changed. Pilots once flew by landmarks and compass, then relied on radio beacons and aircraft navigation aids, and later on satellite signals, so that flight crews could concentrate on managing the flight rather than finding the way.
What the pattern shows
Looking back, no single feature defines a modern airplane. It is the accumulation of better materials, lighter and more powerful engines, and more capable control systems, each building on the last. Many of these steps came out of wartime urgency, others out of the economics of airline travel, and some, such as the push for lower fuel use, from simple cost pressure. The century since 1903 is best seen as a series of overlapping improvements rather than a straight line from one machine to the next.