DH Comet model

[Footless Halls]

Thought people might be interested in this little Comet 1 (or 2?) model I picked up in a junk shop on the King's Road. It was presumably a promotional ashtray dating from before disaster overtook the Comet. Its made of aluminium with a granite base. Ironically the mounting, which is also aluminium, has broken and it looks like fatigue....

http://i1119.photobucket.com/albums/k638/Ctesibius/WP_20140906_005.jpg

[Cubs2jets]

Comet down due to metal fatigue? Surley you jest !?!

C2j

[Proplinerman]

I've got a Dinky Toys Comet 1 in BOAC colours, G-ALYX, rectangular windows, but, strange to say, blue tail-ie not 100% accurate. I wasn't actually born when the Comet 1 disasters happened, so presumably someone gave me this model (diecast metal) when I was a child in the 1960s.


Must have been something of a rarity even then-and today! If anyone wants to see a photo of this, let me know here, but I won't be able to put it up on my Flickr photostream, with link to it here, till two weeks hence, as I'm going on holiday in a couple of hours.

[Footless Halls]

Hi C2j,

I've just spent quite a lot of time polishing it up and I'm going to work out how to reconnect it to the mount. The thing is, it does seem to be hand made - to a very high standard but still hand-made. And it is SO light. I would like to think it must have been made by someone at de Havillands and presumably of aluminium from the works. And yes, the mounting arm failed at its most stressed point and does appear (to my untutored eye at least) a little crystallised. So maybe it was fatigue!

[joy ride]

Nice model. What you saw sounds like fatigue and or porous casting. I am not an expert on Ali but that could be a tricky as whatever you do might have consequences. Get expert advice (unless you are an expert!), my guess is that it <<might>> be better to drill both ends and put in a pin with epoxy or Loctite 603 Retainer. Heat on that metal might just add to the problem.

[tdracer]

I second using a pin and epoxy - drill the holes for the pin a bit oversize (unless you have access to some pretty fancy equipment you'll never get the holes perfectly aligned). Dry fit with the pin - if necessary open up the hole enough that you can get perfect alignment with the pin in place. I'd recommend "JB Weld" epoxy - after mixing, fill the pin holes before installing the pin then assemble and support so it'll stay aligned until cured (JB Weld is super strong but it's slow cure - I'd give it overnight).
You probably want to figure out how you'll hold it in alignment before you add the epoxy :)

[Noyade]

Hello FH.

Does the Comet have a chunky arse where it broke from the mounting? Is it fat enough to drill and tap a thread? Is the mounting easily removed so a thread can be cut? It looks fairly cylindrical at the tip?

If it's possible, that would keep it in the air forever! :)

http://i59.tinypic.com/35nc6ec.jpg

[PPRuNe Pop]

Footless Halls, please adjust your pic size to no more than 850x850. This is PPRuNe's required size for posting photo's.


Thanks.

[joy ride]

JB Weld is not widely available over here, but Chemical Metal is the same. Also Araldite would be fine.

Drilling and tapping is possibly a good idea but cutting threads in poor metal might weaken it further, but if this route IS recommended by an expert it is probably best to use Ferret Sh*t cutting compound like Trefolex or Temaxol to ease the cutting, and take it easy. Happy to send you a spoonful if wanted.

[semmern]

Pray tell, if the tap and thread method is used, how does one screw the model back on, with the wings and all?

[joy ride]

Well spotted, you would have to cut the wings off or cut the stem off the base!

[BOAC73]

Noyade's sketch assumes the mount is bolted to the base. That makes it work.

[Cubs2jets]

"Pray tell, if the tap and thread method is used, how does one screw the model back on, with the wings and all?"

Aileron rolls...:eek:

C2j

[Footless Halls]

Ha, you are a clever lot.

We've already tried epoxy with a 'wrap' of thin aluminium sheet. Lasted about 30 seconds.

The 'drill and pin' idea is a good one, but I think the arm is too thin for it to work. Either the pin would be too small to hold, or the hole would be so large that the arm would break (fatigue?) again. So my solution has been to ask a friend's husband (he's a professional metal worker) to make up an iron sleeve to fit over the mounting arm. This will support the thinnest part of the mounting arm and I can then pass a bolt up vertically through the sleeve into the existing threaded hole underneath the centre section of the model. I think that should do it. It ought to be strong enough to reinforce the arm, which we know is susceptible to fatigue, while not being too unsightly from above and holding the aircraft in the right place relative to the ashtray. I suppose we should call this mounting the Mk IV...

The most interesting thing about this model, though, and I wondered whether anyone can tell me anything about it, is that it does look rather 'handmade' and I wonder how many were made originally? The wings have no aerodynamic section - they are more or less flat - yet the area around the engines looks definitely hand-tooled.

I always assumed it was a promotional gift from de Havilland's very successful sales campaign for the Comet 1 and 2, but perhaps it was a one-off made by someone who worked at de Havillands and mounted on an off-the-shelf ashtray?

[joy ride]

A ferrous sleeve might over time cause catalytic oxidation, so perhaps hold the sleeve in place with an all over layer of Epoxy?

I guess it is sand cast, hence granular/"crystalised" look of interior metal. Sand casting is not too expensive for an individual to commission as a one-off, but more likely to be for producing a batch.

[Allan Lupton]

We had a foundary at DHs where they cast press tools. Whilst the metal used was not aluminium alloy, it wouldn't have been too difficult to melt a bit of scrap ally and pour it into a mold.
Sand molds are easy enough if your pattern-making is up to it, so the aeroplane could be an amateur job but I'm sure the stone part isn't. There's a lot of polishing required to get that finish on a sand casting of course.

I know a man who might know and have asked him . . .

[A30yoyo]

Footless Halls...Have you looked at the smaller diameter non-ferrous pipes available from garages and central heating stores?...brake pipes, Citroen suspension pipes, small diameter copper tubing.

[joy ride]

^ That's what I would look for, Model shops often have small brass tubing.

[Noyade]

Noyade's sketch assumes the mount is bolted to the base.
Yes mate. Thanks for appreciating that assumption. :)

Well spotted, you would have to cut the wings off or cut the stem off the base! Well, if the mount doesn't come off, improvise! :)

http://i62.tinypic.com/eb5x05.jpg

[joy ride]

Chortle!

Very slight thread drift: reference has been made to epoxy adhesives and to Loctite Retainer.

Retainer/Retaining Solution is for holding pins into holes, gear wheels onto shafts etc.. It works in close fitting cylindrical pieces by crystallizing, effectively forming thousands of mechanical pins which grow into either face and make an incredibly strong union. Not well known, but amazing stuff. I use 603 for general purpose, but other viscosities are available.

Araldite, Aerolite, Redux and various others were developed at Norman de Bruyn's labs at Duxford, and many of the glues, paints and modeling putty we use nowadays are thanks to this unsung hero, who should be MUCH better known for his contribution to the modern world.

It is my understanding that R E Bishop' design team intended for the Comet's windows to be Reduxed (an epoxy + high temperature and pressure process) but de H management could not bring themselves to trust this new-fangled process fully, and insisted on adding rivet plates and rivets....belt and braces.

Sadly it was the riveted reinforcements that led to pressure concentrations and the eventual fatigue and failures.

The beautiful Sud Aviation Caravelle had rounded triangular windows and the L1011 had rounded rectangular ones very much like the Mk 1 Comet, but purely bonded into place.

[A30yoyo]

Why is it that 60+ years after the Comet I disasters there is still confusion over what happened? I was under the impression that the failures initiated from an ADF window (NOT initially the passenger windows) on the front of the aircraft roof and that rivetting had been used in production without design study and authority. The switch to round passenger windows was just a symbol and not copied by Boeing or Douglas. Shoot me down if I'm wrong

[DaveReidUK]

The switch to round passenger windows was just a symbol and not copied by Boeing or Douglas.As far as I'm aware, neither Boeing nor Douglas ever designed/built a jet airliner that had passenger windows with corners, so the question of switching to round ones didn't arise.

[joy ride]

I too understand that the problems had initiated in the roof ADF window, but assumed they had gone through the same design and revision process as the cabin windows, ie originally spec'd for Redux then revised to redux + rivets, is that correct, or were were they just riveted without Redux?

Seeing as Caravelle and L1011 managed well enough with "cornered" windows suggests to me that de H wanted to put visible distance between early and later models so that everyone could see that "the problem had been fixed" even if many then and now do not fully understand the exact cause.

Whenever I hear people blaming the window shape alone as the cause I refer to Caravelle and L1011 windows to show that things are not always as simple as they seem!

[WHBM]

The irony in referring to the Caravelle is that its whole nose section, including the ADF aperture, was designed for Sud by De Havilland, based on the Comet itself (look and compare), and the first couple of Caravelle noses were actually manufactured for them at Hatfield.

Among the more extreme designs that followed was the Handley Page Herald, which had circular/oval apertures throughout, including where the flight deck is fixed into the front upper fuselage.

I never understood why De Havilland came back with the substantially redesigned Comet 4 but with the same name, instead of choosing a different model name for it.

[joy ride]

I have always felt that keeping the Comet name was a big mistake. The later ones were virtually different machines, but never shook off that name's reputation. Even now when I tell people that one of my earliest memories is flying in a Comet the vast majority show an instant expression of concern and alarm and say things like "You're lucky to be alive".

Perhaps when they were operational more airlines might have bought them but thought they would loose passengers with a plane whose name STILL seems almost as cursed as Titanic. Shame.

[Allan Lupton]

I think it a pity that threads like this are always hijacked by people who feel the need to express views on matters that they had no part in and which took place when ideas, methods, knowledge and people were very different from anything they can have experienced.

I have no need to join in so will just say that, had DH not called the Comet 4 a Comet, we would have been accused of trying to hide the fact that it was one.

[Planemike]

I think it a pity that threads like this are always hijacked by people who feel the need to express views on matters that they had no part in and which took place when ideas, methods, knowledge and people were very different from anything they can have experienced.

AL......

On that basis we would have a very limited view of history. As an example, no one alive fought at the battle of Waterloo but it is possible to research it and then have an opinion about it.

In the case of more modern history we do still have some of the participants with us so they can give their perspective on the event. Others who were not closely involved can still have very valid opinions and it is often interesting to hear them, in this case: the development of the Comet.

[A30yoyo]

Yes, sorry for the hi-jacking....I've opened a new thread on the topic which I hope will be factual

[joy ride]

I like to discuss things with an open mind, stating what I believe to be true but happy to be corrected by those more knowledgeable. Interesting to hear Alan Lupton's view about changing the name also being taken as an attempt to hide the fact. Good and valid point! Sometimes whatever decision you take might lead to repercussions and/or debate.

Nothing will ever persuade me that the Comet was anything less than one of the most beautiful planes ever made, a marvel of its age, a glimpse of the future and a stunning trail-blazer which left the rest of the world a decade behind, and all that from a shattered and bankrupt country, I LOVE it!

[Wodrick]

Never worked on it then did you :)

[Windy Militant]

Back to the OP It's hard to tell from just a photo but the colour and the mention of how light it is makes me suspect it's MAZAK which is great for casting but an absolute nightmare to do any thing else with. It was much beloved by Uncle Joe Lucas Prince of Darkness. As mentioned it may be a promotional item but it has the look of a work shop retirement piece. Helped to make a few myself as an apprentice, slate mounts and machined etched brass faces for clocks, that kind of thing. Not really done nowadays but used to be common when we still had workshops where people served for many years.

[A30yoyo]

MAZAK mostly Zinc is denser than Aluminium roughly 7 vs.2.7...I'd forgotten about 'Joseph Lucas, Prince of Darkness' :-)

[tonytales]

DeHavilland built beautiful airliners pre-war. To my knowledge, only the Flamingo was all-metal. They certainly had no experience in pressurized metal structures. Boeing had its Stratoliner, B-29 and Stratocruisers, Lockheed itsL949 Aand Later Connirs and Douglas the DC-6.
De Havilland had to cope with metal, pressurization and very high altitudes (compared with pistons) so the learning curve for them was much steeper. Perhaps it is not so surprising they stumbled in the structural area.
They were gutsy to leap ahead of everyone..A beautiful machine with deep flaws.