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Comment by WalterBright | Hacker News Reader

The corner of a square hole in a flat plate has 4x the stress of the surrounding material. It's called a "stress riser". If you look at a concrete slab with such an inside corner, there's usually a crack there.

Boeing solved the problem by using rounded windows, and riveting on a forged doubler around the window frame. The skin was also organized so a crack will not propagate far. The success of the latter has shown itself in various incidents where an airliner has lost a panel in flight, but went on to land safely.

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> Boeing solved the problem by using rounded windows, and riveting on a forged doubler around the window frame.

Boeing was by no means the first to use radiused windows in pressurised airliners; the Avro Tudor was doing so in 1946 and there may well have been earlier examples.

Conversely the Boeing Stratoliner was flying with square windows in the early 1940s and didn't suffer fatigue failures, nor did the later DC-6. Corner stresses were understood at the time.

What really undid the Comet was that the fuselage skin gauge was insufficient to accommodate the increased rate of change of pressure differential that was introduced with jet flight. Whereas the preceding piston-engined airliners were lucky to hit 1000 feet per minute climbing to 20,000 ft, the Comet climbed at three times that rate to 35,000 ft +.

The Comet had skin reinforced to 20 swg along the window belt, whereas the succeeding Comet IV had 18 swg with 16 swg framing.

> Avro Tudor

Curiously, this did not extend to the cockpit windows:

http://smg.photobucket.com/user/Coridano/media/TB01.jpg.html

suggesting that stress cracking was not the reason they did it. (There are other reasons for round windows - such as portholes on ships which are easier to make a seal.)

The 707 shows a more radiused corner for the cockpit windows.

Fatigue failure was poorly understood at the time. It still is incompletely understood, but much better models have been constructed to predict it. Fatigue damage goes up as the cube of the load, and the Comet was pressurized at 35,000 feet rather than 20,000 for the Stratoliner, meaning much more load.

I assume I am wrong on this, but surely if it pressurized to 35000 feet, then that would be less load? As they pressurized it to that it would be closer to the air at its altitude?

The cabin pressure inside is 10,000 feet, the air pressure outside is 35,000 feet, making for more of a pressure differential than at 20,000 feet altitude.

In the past couple of years I've been updating my CAD skills, one of the most fascinating discoveries for me was how fillets and rounds distribute the stresses that affect sharp corners. First-year engineering stuff, sure, but pretty amazing to learn what a difference even the smallest details can make.

Real Engineering has a nice explainer on this topic:

https://youtu.be/7rXGRPMD-GQ

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