World War II provided the impetus to develop the modern ejection seat, which uses rocket power and a seat on rails to propel a pilot away from the aircraft before deploying a parachute. The U.S. Air Force tested what would have been a major enhancement to the basic ejection seat in the late 1960s—a turbojet-powered, steerable recovery system that would carry the pilot up to 50 nm to avoid landing in enemy territory and save the military from conducting dangerous search-and-rescue operations.
Researchers toward the end of World War I discovered that the power of a piston engine at sea level could be significantly boosted by precompressing air before it was ingested into the engine, usually by a centrifugal compressor. The system also compensated for the effect of decreasing air density with altitude, preventing power levels from declining as the aircraft climbed. The true benefits of turbo-superchargers for improved high-altitude performance became apparent in the 1930s with the development of the Boeing B-17 strategic bomber and the first airliners with pressurized cabins.
German aerodynamicist Adolf Busemann first proposed using swept wings to reduce high-speed drag in 1935. Germany began a secret research program with high-speed wind tunnel testing, but by the end of World War II a prototype swept-wing fighter, the Messerschmitt P.1101, was only 80% complete. The U.S. seized the aircraft and data. Meanwhile, U.S. National Advisory Committee for Aeronautics (NACA) aerodynamicist Robert T. Jones had independently discovered the high-speed advantage of swept wings. The results were the North American F-86 fighter and Boeing B-47 bomber (pictured), flown within months of each other in 1947. The Soviet Union also flew the MiG-15 that year. Britain was slower to follow suit, but in 1949 the de Havilland Comet became the first swept-wing, jet-powered airliner to fly.
The concept of an aircraft powered by a gas turbine engine with an inlet, compressor, combustor, turbine and nozzle was conceived in 1929 by Frank Whittle, a Royal Air Force pilot and engineer who filed a patent in January 1930. Almost simultaneously, while Whittle struggled to gain serious support for his idea in Britain, a completely independent turbojet design was developed by Hans von Ohain, a researcher at Gottingen, Germany.
Backed by aircraft developer Ernst Heinkel, the first HeS 1 engine (fueled by hydrogen) ran within weeks of the first Whittle engine in early 1937. An improved version of Ohain’s engine, the diesel-fueled HeS 3, powered the Heinkel He 178 experimental aircraft on its first flight on Aug. 27, 1939. The Whittle W.1-powered Gloster E.28/39 experimental prototype flew May 15, 1941, from RAF Cranwell, England.
Both Ohain and Whittle used centrifugal compressors, but BMW and Junkers used axial compressors, which proved to have greater growth potential.
In the mid-1930s, U.S. airlines grew increasingly interested in the potential of flying at higher altitudes. Trans World Airlines (TWA) launched its own “‘over-the-weather”’ research program with a modified DC-1 and later a Northrop “Gamma” testbed fitted with oxygen for the crew and supercharged engines. In 1937, Lockheed also conducted high-altitude tests of a specially modified Electra with an all-new pressurized fuselage. In addition, Boeing began development of the Model 307 Stratoliner, the first airliner with a pressurized passenger cabin using air from General Electric Type B superchargers fitted to the aircraft’s Wright Cyclone engines.
Opening of the large Propeller Research Tunnel at NACA’s Langley site in 1927 allowed testing of full-scale fuselages and confirmed that fixed landing gear was a major source of drag. Retractable landing gear appeared on the Boeing Monomail in 1930 and Lockheed Orion in 1931. At first hand-cranked, then electrically driven, landing gear become hydraulically actuated after the O-ring was patented in the U.S. in 1937.
Mention airships, and inevitably the Hindenburg disaster of 1937 comes to mind. The fiery destruction of the Zeppelin rigid airship while attempting to moor at NAS Lakehurst, New Jersey, after a transatlantic flight killed 36 people and ended the era of passenger airships.
U.S. airships of the time used helium rather than hydrogen for buoyancy because of the discovery by the University of Kansas in 1905 that helium could be extracted from natural gas. The U.S. Navy used airships for anti-submarine warfare and airborne early warning into the 1960s.
Small airships continue to be used for aerial advertising, sightseeing flights, surveillance and research. Three 12-passenger Zeppelin NTs will become the latest in the iconic line of Goodyear Blimps.
The U.K.’s Hybrid Air Vehicles bought the canceled LEMV hybrid airship prototype back from the U.S. Army and will return the vehicle to flight this year as the Airlander 10. Lockheed Martin has secured a launch customer for its LMH-1 hybrid cargo airship.
Development of numerical control (NC) machining enabled complex structures such as bulkheads and integrally stiffened wing skins to be cut from solid blocks of alloy, rather than assembled from sheet metal—improving quality, reducing weight, and saving time and cost. NC machining was conceived in 1942 by machinist John Parson. Today five-axis high-speed precision machining is standard for metal structures.
Anti-G wear—pressurized suits worn by aviators and astronauts to prevent blood from pooling in the lower part of the body during high-acceleration conditions to prevent G-induced loss of consciousness—was first used by British Hurricane and Spitfire pilots in the early 1940s. Those devices—water-filled bladders that encapsulated a pilot’s legs—evolved into air-filled bladders around the calves, thighs and abdomen in the gradient pressure suits developed by researchers at the Mayo Clinic. The David Clark Co. fabricated the early suits, which the U.S. military began using in 1943.
The company continues to build the suits 70 years later and is evolving the design. In 2012, it built a free-fall suit (pictured) for Felix Baumgartner, the adventurer who jumped from a balloon at 128,100 ft. and traveled at supersonic speeds in free fall until deploying a parachute at a lower altitude.
Radar’s history reaches back to the late 19th century, but Britain was first to demonstrate radio detection and ranging of aircraft in 1935. The Royal Air Force (RAF) fielded the first VHF-frequency airborne radars in 1940 on Bristol Blenheims. Development of the cavity magnetron in the U.K. enabled smaller, higher-resolution microwave-frequency radars, which became operational in 1941 on RAF Bristol Beaufighters.
After World War II, Hughes Aircraft Co. developed the E-1 fire control system for the U.S. Air Force’s Northrop F-89 and Lockheed F-94 interceptors. This led to the MA-1 weapon control system, including the first U.S. guided missile, the semiactive radar-homing Hughes AIM-4 Falcon, which became operational on the Convair F-106 in 1959.
Perhaps the first aircraft to have a high-lift system was a de Havilland DH4 testbed (pictured) modified by Handley Page to a monoplane with full-span leading-edge slats and trailing-edge flaps to improve low-speed performance. Fowler flaps that extend backward and downward to increase area and camber were invented in the U.S. in 1924 and used on the Lockheed Electra. Forward-hinged Kruger leading-edge flaps were invented in Germany in 1943 and used on the Boeing 707.
Acrylic, a transparent plastic, was introduced in 1933 in Germany as Plexiglas and first used for aircraft windshields shortly before World War II. Plastic was lighter than glass, and the acrylic bubble canopy dramatically improved visibility for fighter pilots. Today’s F-35 has a thermoformed acrylic canopy.
Widely believed to be the inspiration for the term “drone,” de Havilland’s DH82B Queen Bee was a radio-controlled target aircraft for anti-aircraft gunnery training, derived from the Tiger Moth and first flown in 1935. But the most influential designs have been the Ryan Firebee, Israel’s IAI Scout and Tadiran Mastiff battlefield UAVs and General Atomics’ MQ-1/9 Predator family.
The BQM-34 Firebee was a successful jet-powered target drone produced by Ryan Aeronautical Co. and developed into the Model 147A Fire Fly and 147B Lightning Bug long-range reconnaissance UAVs. The Scout and Mastiff launched a line of Israel Aerospace Industries tactical UAVs, including the widely used Searcher. The RQ-1 Predator was developed from the Gnat long-endurance UAV produced for the CIA and led to the armed MQ-9 Reaper now flown by the U.S., U.K. and others.
The Aug. 21, 1951, Aviation Week described a new ice protection system. The aircraft was the Canadian-built Avro Jetliner, a four-engine airliner that first flew 11 days before the magazine came out but was canceled two years later. The Goodyear ice protection system, called Iceguard, is no longer in use, but the technique—electro-thermal heat via electrical resistance in wires running over the wing, horizontal stabilizer and vertical stabilizer leading edges—remains a viable option today.
A more popular technique for jet aircraft is to route hot bleed air from engine compressors through ducts in the leading edge. For lighter aircraft, pneumatic deicing boots on propellers, wing and tail leading edges is generally the most effective, although some aircraft use anti-icing fluids pumped up through tiny holes in the wings, tail and propeller to prevent ice buildup.
The first transistorized airborne digital computer, called Tradic (pictured), was developed in 1954 by Bell Labs for the U.S. and used in the Boeing B-52 bomber. Another early milestone was the MIT-developed, Raytheon-built Apollo Guidance Computer of the 1960s. But aircraft really entered the digital age when the Mil-Std-1553 avionics databus was defined in 1973. This was first used in the General Dynamics (later Lockheed Martin) F-16. The civil-avionics equivalent, Arinc 429, was first used in the early 1980s in the Boeing 757/767 and Airbus A310.
Real-time inertial navigation moved rocketry from impressive fireworks to guided missiles and Moon landings. The basic navigational strategy is that of dead reckoning—keeping track of one’s position, direction and velocity without reference to landmarks or guide stars.
Germany’s V-2 system in World War II combined gyroscopes, an accelerometer and a primitive computer to determine and adjust the rocket’s flight path. Advances led to packaged inertial measurement units (IMU) for spacecraft and aircraft. IMU work continues today. The Defense Advanced Projects Agency is working toward an IMU on a chip to allow precise navigation without the aid of GPS.
First patented by Short Brothers, the concept of folding wing mechanisms was developed to reduce the amount of deck space taken up by shipborne aircraft. Various systems have been developed, including simple folds along a chordwise hinge line, more complex aft folding wings, and double folds including wingtips and rotating systems, such as used by the V-22, that rotate the entire wing around a fuselage-mounted axis point. Boeing’s 777X will have a fold that will allow the outer 12 ft. of each wing to hinge up for ground operations at space-limited airports.
Satellite designers quickly realized that deriving power directly from sunlight offered huge potential for weight savings and spacecraft life extension. The driving force for the concept of photovoltaic power in space was Hans K. Ziegler, one of the associates of Wernher von Braun brought to the U.S. after World War II under Operation Paperclip.
In the early applications, solar cells were mounted on the skin of spacecraft. Vanguard 1, launched in 1958, featured them first.
But when engineers mastered the origami of space-deployable structures, “wings” appeared on spacecraft, and they began to realize the full potential of solar power. The solar arrays on the International Space Station (lower photo) cover one-third of a hectare, nearly an acre, and produce enough electricity to power 40 typical houses.
Like that other canal we all must pass through, the jet bridge now seems a universal passageway. This innovation took away the thrill passengers experienced when they left the airport lounge to go outside amid the movement, noise and fumes to climb the stairs to their aircraft. But these moveable hallways made boarding airliners more accessible, comfortable and efficient.
An afterburner augments the thrust of a turbojet by burning additional fuel in the jetpipe. This technique increases the rate of change of momentum of the exhaust by combusting unburned air, but uses large volumes of fuel. Canceled in 1946, the U.K.’s Miles M.52 supersonic aircraft was to have one of the first afterburners.
The evident need for improved fuel efficiency and higher thrust in the 1950s drove manufacturers to study a new form of engine that divided the airflow into a bypass stream and a core stream. In early low- and medium-bypass engines, a small percentage of the air from the low-pressure compressor is directed around the core to provide a propulsive force. The portion of air directed into the high-pressure compressor supercharges the core, increasing pressure ratio and thermal efficiency. Because the bypass air has not been heated and accelerated to higher jet speeds in the core, its lower overall jet velocity more closely matches the relative velocity of the aircraft and therefore has better propulsive efficiency.
The Rolls-Royce Conway was the first bypass engine to enter service on the Boeing 707 in 1960, followed by the Pratt & Whitney JT3D. The Conway had a small bypass ratio (BPR) of only 0.3, whereas the more successful JT3D included two extended fan stages that produced a BPR of 1.5. General Electric produced the CJ805-23B, the first U.S. turbofan, by adding a fan stage to the aft section of a derivative of the military J79 engine. Although the venture had limited success, powering only the Convair 990, the resulting 28% reduction in cruise fuel burn proved higher BPR was the right direction.
Increasingly efficient high-pressure ratio cores and the advent of Pratt & Whitney’s gear-driven turbofan family have led to the development of a new generation of engines with higher relative bypass than was previously possible. These include the 9.3 bypass ratio Rolls-Royce Trent XWB and the 10:1 bypass ratio GE9X which, with a 134-in.-dia. fan, is the largest turbofan ever made.
Aircraft rivets initially had dome-shaped heads, but as the importance of reducing drag by streamlining became evident in the 1930s, the flush rivet increasingly came into use. Innovative pneumatically driven riveting guns were developed to insert rivets and work in combination with a bucking bar, a heavy block of steel that provides reaction to the riveting gun. Various specialist rivets have been developed with improved fatigue life that, in most cases, did not depend on plastic deformation to fill the hole. These include various shear fasteners, such as the Hi-Lok, which combined a threaded pin and collar. Combining the rivet and bolt-and-nut (collar) system, the Hi-Lok is tightened until the collar breaks off when the required torque is applied.
World War II and the years immediately before and after were ones of soaring sophistication in aviation. Aircraft gained retractable gear, pressurized cabins, high-lift systems, ice protection, and eventually airborne radar, inertial navigation and digital computers. Pilots gained ejection seats and G suits. Propulsion technology advanced from turbocharged pistons to afterburning turbojets and bypass turbofans. They were decades of transition, the airship fading away and swept wing becoming dominant. They also heralded the future, from unmanned aircraft to solar-powered spacecraft.
Here are some of the most important technologies, innovations and novel ideas that have made aviation and space what they are today.
This gallery was originally published on May 6, 2016 as part of Aviation Week & Space Technology's centennial issue. It is part of a series of top 100 technologies in the last century.
Guy is a Senior Editor for Aviation Week, covering technology and propulsion. He is based in Colorado Springs.
Graham leads Aviation Week's coverage of technology, focusing on engineering and technology across the aerospace industry, with a special focus on identifying technologies of strategic importance to aviation, aerospace and defense.