ALLAN WINN / LONDON
Flight International looks at 10 technologies that changed the face of powered flight, 10 that failed to have an impact, and another 10 that may yet
In the first 100 years of heavier-than-air aviation, the principles have stayed the same. Aircraft may now use ailerons instead of wing-warping, but the basic aerodynamic principles still apply. The materials, instrumentation, control laws, and enabling technologies such as hydraulics and electronics are the things that have changed.
These are 10 ideas that changed the face of powered flight:
1 The gas turbine
Before the Second World War, there were few piston-powered twins, for example, that could maintain height (far less climb as is now the certification requirement) on one engine. The most powerful piston engines weighed well over 2,000kg (4,400lb), yet struggled to deliver 1,000lb (4.45kN) of thrust through their propellers at altitude - even the meanest and crudest of gas turbines matched that. The most reliable big piston engines ever - the sleeve valve Bristol Hercules and Centaurus - managed a maximum of 3,000h between overhauls. Today, engines like the Rolls-Royce RB211-535 and Pratt & Whitney PW2000 are achieving 10 times that. The GE90 has a diameter of 3m (10ft) - as wide as the fuselage of the Boeing single-aisle family - but at 30,000ft it delivers 50 times the thrust of a P&W Wasp Major that weighed almost as much and was nearly half the diameter.
2 Pressurisation
Most weather is at under 20,000ft altitude, and the air is generally smoother above that. In addition, a cabin altitude of 8,000ft is so much more comfortable than one of 18,000ft, which is where the last non-pressurised airliners were operating. Before the Boeing 307 Stratoliner heralded the start of the pressurisation era, most airliners owed more to bridge construction techniques. There was no great drive to build round, tapered fuselages before pressurisation - a square section was easier to build, and gave better space at the expense of drag. But pressurisation came at a price, because even a minor failure of a stressed, pressurised skin could be catastrophic - the story of de Havilland's DH106 Comet is all too familiar.
3 Global positioning system
No dead reckoning, no searching for a clear patch of sky to shoot the stars - just plug in the co-ordinates of your waypoints and destination, and Uncle Sam's satellite will take you there. The sight of a Tomahawk cruise missile proceeding down Baghdad High St and turning left at Al Rashid's supermarket is a compelling reminder of the power of this technology. The real advantage, however, comes with technologies such as the enhanced ground proximity warning system, where navigational and situational awareness are combined through the power of GPS - there is now no excuse for controlled flight into terrain (CFIT). But the GPS satellite constellation is still funded by, and under the control of, the US military. And what happens when the GPS-reliant pilot spills his coffee down his strapdown and is left with no idea where he is, or how to work it out by other means?
4 Stressed skin construction
The Douglas DC-3 was not the first aircraft to employ stressed-skin construction, but it was the most significant early application of the technique. Its adoption meant that strong structures were now light enough for engine power to be devoted to hauling fuel and payload - rather than unproductive bulk - around. Loads were spread throughout the structure, not pinpointed on specific nodes as they had been with earlier girder-like structures. Before this, redundant load paths came at the expense of complexity like Barnes Wallis's geodetic structure. The downside was, of course, that these new structures were much more difficult to analyse and stress. But stressed skins also gave the torsional strength required for successful swept wings.
5 Powered flying controls
Hydraulic powered controls really started to surface during the Second World War, as bombers and transport aircraft doubled in size over their pre-war predecessors. But the real advances came with post-war aircraft designed with them from scratch. The shining example was de Havilland's DH106 Comet, which had one of the most beautifully harmonised, balanced and precise sets of flying controls ever. The contrast with the last of the big non-powered-control piston aircraft was immense. The Avro Shackleton of 1949 was a lumbering, heavy, unresponsive beast to fly: the Comet (the basis of the Shackleton's Nimrod successor and which also first flew in 1949) eventually had the same span but 50% greater wing area and almost double the all-up weight of the Shackleton, but could be flown with the fingertips. Powered controls were also the essential enabler of the relaxed-stability and inherently unstable agile aircraft of today. That said, powered controls brought weight and complexity, and still needed mechanical back-up, at least until the Airbus fly-by-wire generation came along.
6 The supercharger
Early experiments with superchargers for aviation date back to the First World War, notably at General Electric with turbo-superchargers. The attraction was simple - a supercharger boosts absolute power, and helps maintain that power at altitude. Without the supercharger, we would never have had the 350kt (650km/h) piston-engined fighter, nor the transatlantic piston airliner like the Douglas DC-7 and Lockheed Constellation. And, of course, the technology carried over into the turbine era, as anybody who compares a supercharger impeller from the Rolls-Royce Griffon to the centrifugal compressor of the Dart turboprop can clearly see. Superchargers did not give their advantages for free, however - 2bar (30lb/in2) boost pressures were too much for the metallurgy of the 1940s and 1950s, leading to very low mean times between failure on the big piston engines. Also, the 130-octane fuels they demanded in prodigious quantities were lethal cocktails, and at full power they had to run heavily over-fuelled to keep combustion temperatures down.
7 Radio navigation
In the 1920s, people were still talking about aerial lighthouses to act as visual beacons for aircraft flying between London and Paris. Over water, at night or in cloud, the stars were not much good for navigation, and dead-reckoning is also chancy if you cannot see what your drift indicator is trying to tell you. Radio direction-finding and navigation changed all that, using principles identical to those that still hold with the global positioning system. But setting up the infrastructure across the globe was expensive, so that private ventures - such as Decca with its Navigator - had to take the risk, and pre-VHF and UHF, radios did not have the range to make it easy.
8 The disk brake
Tricycle undercarriages revolutionised take-offs for large aircraft, because of low drag, but imposed huge loads on landing because of the same low drag, especially before the arrival of the reverse-thrusting turbofan. Disk brakes gave the sustained stopping power required, and they also enabled anti-skid braking through their fast response times which allowed rapid cycling. Disk brakes for aircraft go back at least as far as the Polikarpov I-15 of 1933. The Lockheed aircraft disks of the early 1950s fitted to Brooklands Museum's Napier Railton racing car still do sterling service, stopping over 2,000kg of motor car from over 160km/h (100mph).
9 Automatic landing
Getting rid of the uncertainty of operation into fog-bound airports was a major priority in Western Europe - and Europe is still the home of Category IIIB landings. Automatic landing has been given much less importance in the Americas and Asia - although judging by the continuing number of incidents due to repeated missed approaches in poor visibility, especially in eastern Europe and Asia, a lot more use of Cat III equipment could be justified. The first commercial automatic landing was performed with a de Havilland Trident 1C of British European Airways in 1963, and it is now routine.
10 Composite helicopter rotor blade
With many fewer moving parts in the notoriously maintenance-heavy rotor system, and with a much greater degree of control and better aerodynamic design enabled by composites, the modern rigid rotor is much quieter as well as having superior performance to the early wood and metal ones. Although Lockheed demonstrated the promise of the bearingless rotor as early as 1959 with its wooden-rotor CL475 (and later, in 1966, with its much-more sophisticated Model 286), bearingless rotors only really took off with the development of the composite blade. In the early days, they were difficult to build - getting consistency of lay-up, and making them without voids were real problems - but those problems were overcome at least in part by the development of superior non-destructive testing techniques. n
Allan Winn is a former editor and publisher of Flight International. He is now the director of Brooklands Museum Trust.
Source: Flight International
