| Comet 4 |
|---|
Enormous effort had been made to restore the Comet’s reputation. A new tank was built for the Comet 4, and the structure was tested rigourously: an automated rig simulated loads by pressurisation, and additional stresses were simulated using hydraulic rams acting on the wings. The whole process was accelerated so one “flight” occurred every 4 minutes.
Meanwhile, Boeing were selling an airliner concept based on the KC-135 Stratotanker that they were developing for the military. This would become the 707, with 70% more power than the Comet, 179 passengers and twice as much fuel. A 35º swept wing increased speed by 100 mph, and the fuel capacity gave it 30% longer range.
Even so, the Comet had been designed for existing runways on Empire routes, and the Series 4 maintained the ability to fly to smaller airports. For BOAC, this was a considerable advantage; they had trained crews, and had equipped airports
Capitalising on its strengths, a special version was planned for internal American routes, called the 4A. It sacrificed fuel capacity for more seats, and had shorter wings to reduce drag at lower altitudes. The 4A didn’t materialise, but a European equivalent, the 4B, was developed specifically for BEA. It had no pinion tanks, up to 100 seats, and an economical cruising speed of 530 mph at 23,500 ft; ideal for short city-hops. 18 Comet 4Bs were built. Their success lead to the specification of a purpose-built aircraft for BEA, the very high-speed, low-drag DH-121 Trident.
The Comet 4C combined the long fuselage of the 4B with the long-range wings from the 4. It was produced in both civil and military (RAF) versions. Although it had neither the full range of the 4, nor the speed of the 4B, it wasn't far short. Many operators found it the most useful of all, especially Dan Air. A single aircraft had the flexibility to be brimmed with fuel for long range, or filled with tourist-class seats on shorter journeys, without much difference in efficiency.