Comet metal fatigue

At the risk of bringing the thread back to the original subject, fatigue cracks are a simple fact of life in metal aircraft, now as then, and much was already known about the subject at that time.

What the Comet design was missing was a knowledge of the propagation of these cracks and in particular the need to ensure any crack is limited in its ultimate length by some design feature: panel joint, panel thickening etc.

I wasn’t aware of tyhe pressurised Mossies – but did know of the pressurised Wellingtons, Waricks, Spitfires and, of course, the Viscount. Boeing had built the pressurised 307 Stratoliner before the war so were probably the company with the longest history of pressurising large fuselages by the late 1940s.

I still think that De Havilland, by their nature, were a more prone to “risk taking” with their designs than some of the other British aircraft manufacturers.

I think that was my point Bruce.

I think its fair to say that pressurised craft, and particularly passenger airliners were still in their infancy. de Havilland had no more or less experience than anyone else!

Bruce

As James has already mentioned, some marks of the Mossie were pressurised too. With the PR.32 and 34 having service ceilings of over 40,000 feet I imagine they must have been reasonably experienced with valves and control runs.

Westland did a lot of Pressurisation Development on the Welkin, things like the effects of very low temperature on control valves and control runs from inside to outside of the cockpit.
Were early Meteors and Vampires Pressurised?

No – early Meteors and Vampires were not pressurised, as far as I recall. Edit! – later ones were though.

The Spitfire VI, VII, PRXI and PRXIX were other early pressurised types, of course.

Westland did a lot of Pressurisation Development on the Welkin, things like the effects of very low temperature on control valves and control runs from inside to outside of the cockpit. And at the same time the Vickers 432 was flying using a similar Pressure cabin to the Wellington V and VI.
Were early Meteors and Vampires Pressurised?

I mentioned the high altitude Wellington at the end of my post. I think the Mk number was higher than V – VII perhaps.

I’m surprised the capsule experience helped with future projects. I’ve no idea how big it was but compared to a complete fuselage it was obviously much smaller. IIRC, the Wellington capsule had, for want of a better description, a sort of astrodome for the pilot, rather than conventional windows. The Windsor always looked odd too, I wonder if it had the same arrangement.

I would imagine it gave some insight into the problems although little was a direct technology transfer from Wellington to Viscount.

The (Mk V and VI) Wellington pressure cabin itself was a self contained unit that was grafted onto the foreshortened nose of the geodesic structure in a way that allowed the unit to contract and expand. There are shots of the machines in construction in the Ian Allen publication “Wellington special”.

The photo in this link – http://www.fortunecity.com/tattooine/farmer/120/welly.html – shows the rather ungainly result and you can easily visualise how the cabin sat on the end of the original structure.

I mentioned the high altitude Wellington at the end of my post. I think the Mk number was higher than V – VII perhaps.

I’m surprised the capsule experience helped with future projects. I’ve no idea how big it was but compared to a complete fuselage it was obviously much smaller. IIRC, the Wellington capsule had, for want of a better description, a sort of astrodome for the pilot, rather than conventional windows. The Windsor always looked odd too, I wonder if it had the same arrangement.

Dhfan, there were some experimental versions of the Wellington (Mk V?), which had a hight altitude preasure cabin.

Steve

I can’t see the capsule in the Windsor or high-altitude Wellington having much relevance to a fully pressurised fuselage.

….although George Edwards did say to his biographer that the experience with the high altitude Wellingtons was of value to the later development of fully pressurised aircraft….

What had pressurised aircraft had Vickers built? I can only think of the Viscount, which flew almost exactly a year before the Comet.

The Viscount’s cruising ceiling was only roughly half that of the Comet, presumably giving a much lower pressure differential.

I can’t see the capsule in the Windsor or high-altitude Wellington having much relevance to a fully pressurised fuselage.

Yes, some variants, particularly the PR versions of the Mosquito were pressurised.

I am trying to think what else was being built at the time of the Comet, or just prior, that was pressurised in the same manner. Vickers did build that huge four engined thing, with a pressure cockpit in, which was essentially just a capsule. I cant immediately think of many others.

Its easy to be wise after the event. de Havilland were always at the forefront of design – which could be one reason why they ended up with the odd problem.

Bruce

I do recognise that De Havilland were entering new territory with the Comet – especially for them as I am pretty sure that they had never built a pressurised aircraft up to that time.

Couple of odd footnotes to the discussion:

I was looking at the RAAF Museum’s Mosquito PR.XVI restoration the other day, and noted the canopy structure, and was told that it was pressurised… It wasn’t the only pressurised Mozzie either.

The Canadians had built a perfectly sound (but less innovative or advanced) jet airliner at the time of the Comet’s early days. It didn’t go anywhere, despite interest from the mercurial Howard Hughes among others. Nothing wrong with it, but…

http://en.wikipedia.org/wiki/Avro_Jetliner

http://www.avroarrow.org/Jetliner/JetlinerIntro.html

From:
http://www.jetliner.purpleglen.com/

I doubt Boeing (or anybody) contributed towards RAE’s tank cost: in 1954 there were no procedures for MoS to work under contract for aliens, or to put £-cost on in-house effort. There were (are) procedures for Treasury to receive cash cross-referenced to an expense ledger. Thus US’ say in disposal of Hunters &tc. part-funded under MDAP/MSP. What we did do was sell Cohen’s Report for £0.125. US did the same with Turk/DC-10 pylon fitting, duly acted on for A300B pod. Aero, generally, disseminates its lessons, to the benefit of us all.

Being an ex QANTAS sheety I can agree it all depends where the crack is.There are repair schedules in place to tell you where and where not to stop drill cracks.As long as the hole is deburred and not in a structural member things should be Ok.

I do recognise that De Havilland were entering new territory with the Comet – especially for them as I am pretty sure that they had never built a pressurised aircraft up to that time. However, other UK companies had already made pressurised aircraft with Vickers probably being the most experienced in Britain. I can think of NO accident to any other pressurised aircraft caused by structural failure due to inadequate consideration of fatigue stress factors – even in earlier designs than the Comet.

When you look at DH’s history regarding structuralk failures, you will see that it was a weak point in many of their designs. In my opinion they were the “Lotus” of the aircraft world, lightness for performance at the expense of robustness for longevity.

What prompted de Havilland to use buried engines in the Comet 1-4 and what specifications did the proposed Comet 5 (with podded underwing engines) have (size, performance etc).

Yes, reading through the report suggests that this is not necessarily a bad technique – depends on where it is…

Bruce

Bruce
A certain operator of BAC 1-11s had a crack in a fuselage skin. Requested a repair scheme from Bae. The answer – stop drill the crack with one-eighth drill and fill the hole with a solid rivet. This was in the pressurised section.
This aircraft is still flying regularly and the rivet can still be seen!

CS