DH Comet model
 

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/...140906_005.jpg

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

C2j

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.

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!

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.

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 :)

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

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


Thanks.

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.

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

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

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

"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

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?

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.

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 . . .

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.

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

Yes mate. Thanks for appreciating that assumption. :)

Well, if the mount doesn't come off, improvise! :)

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

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.