Why were square windows a problem for the Comet, but not for the Super Constellation?

Question

Famously, one of the problems dooming the Comet was the square windows. But the contemporary Super Constellation also had square windows but I can't find any articles detailing similar problems to the Comet. What was the difference?

Answer 1

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The difference was in the cruise altitude of the two aeroplanes. The Super Connie was powered by piston engines and had a service ceiling of 25,700 ft, usually cruising at about 20,000ft as mentioned in this article about the Connie. The Comet cruised at an altitude of 42,000 - 45,000 ft, with its fuselage pressurised at equivalent altitude of 8,000 ft.

The much larger pressure differential at cruise altitude added up to fatigue cracks in the fuselage. Not necessarily at the window corners, as per the wiki:

…. the fuselage had ripped open at a bolt hole, forward of the forward left escape hatch cutout. The failure then occurred longitudinally along a fuselage stringer at the widest point of the fuselage (accident report Fig 7).

Reconstruction of one of the wrecks revealed a fatigue crack starting at a rivet hole, leading to a change in riveting practice on the Comets - no more punch riveting, the holes were drilled.

The Comet was the first airliner with jet engines, enabling it to cruise above the weather. But unfortunately it was also the first airliner to experience metal fatigue from repeated cycles of pressurisation.

Answer 2

There is a huge misconception here. The problem was NOT window shape in itself. It was errors in the calculations that determined the material and fastener requirements for the window frames and local structure as used, combined with the lack of a long term pressurization fatiugue test before entry into service that would have uncovered the error.

They were simply under-built for their particular frame profile and the pressure differential being used. If you made the corners beefy enough, with sufficient material thickness and enough rivets, you can meet fatigue strength requirements with any shape.

At the extremes, you could make window frames from heavy, thick high strength steel, and use a window with razor sharp corners like your house, if you could live with the weight. At the other extreme, if you want the lightest possible structure, you make the windows perfect circles so that almost all the stress around frame is pure tension.

So the bottom line is the Connie's windows had adequate skin thickness, frame cross-section, and fastener type and count for the stress levels imposed by its pressurization requirements, and the Comet didn't, due to undetected calculation mistakes. Had the proper calculations been done, DeHavilland would have either increased the corner radius, or added more rivets and material if it could live with the weight increase. In the end it chose to go with oval windows to avoid the weight increase (and probably an abundance of caution along with avoiding the whole "square window" meme that had taken hold by the media coverage), but in theory it could have simply reinforced the corners by whatever it took.

Modern airliner windows are also mostly square, but with a larger radius at the corners and a larger vertical dimension than horizontal dimension, which minimizes the "missing meat" in critical areas of the fuselage between the windows, and this allows the best compromise between pleasing window shape and adequate structural margin while keeping the structure reasonably light. Make the frame too thin or undercount the rivets, and you'll have 737s or A320s blowing up from window corner fatique as well.

Answer 3

The hull openings on the Comet were cut with hydraulic shears, which introduced stress raisers all around the periphery with each jaw closure. Square windows then focused the overall stress distribution at the window corners and where the corners coincided with a "sknitch" produced by the shears, you'd get fatigue cracking.

After the Comet crashes, Boeing and Douglas switched to chem milling of hull panels (in which a hull panel was painted with resist except for the hole areas and then dipped in a chemical bath until all the unprotected areas had been etched through and eaten away) This produced atomistically smooth edges without shear damage. I'm willing to bet that Lockheed chem milled the Constellation's hull panels too.