HP42 Posted January 10, 2015 Share Posted January 10, 2015 Hi all, a few very quick questions about colours on the Short Stirling. 1) What colour should the inside of the engine cowlings be? 2) What's the best way of reproducing the collector ring at the front of the cowling? I think the instructions have you use a fairly garish bronze colour, but this always looks too red for me. I made a bit of progress on the Stirling over Christmas and would really like to get it finished. Link to comment Share on other sites More sharing options...
Aeronut Posted January 10, 2015 Share Posted January 10, 2015 No idea about the cowlings but I've taken to painting the collector ring in Humbrols metalcote Steel and following a buffing airbrushing a brown wash over them to the finish I'm after. All I can say is check your references, as you can get all shades from bright steel to very burnt and some even had a heat resistant paint coating and double skinning which didn't burn. The one colour they aren't is Bronze. Link to comment Share on other sites More sharing options...
old thumper Posted January 10, 2015 Share Posted January 10, 2015 Collector rings were painted black later in the war but would become rusty as the paint wore off. Earlier in the war collector rings were left unpainted and gradually became rusty. There was also a circular plate added to the front of the collector ring that was made of a different metal, if you look at old pictures of the Sterling where the collector ring has not been painted you will see that this ring is more of a silver colour. Most painting instructions tell us to paint collector rings copper or some similar colour, collector rings were steel and never copper. This misunderstanding comes from old colour pictures where rust looks like copper. I'm not 100% sure what colour the inside of the cowlings were but I would paint them black. At some point in the war reflective surfaces were ordered to be painted black as were the cockpits of night bombers, plus the engines coughed up a lot of black stuff anyway. You can use humbrol antique bronze with a black wash to create a rust effect though. Link to comment Share on other sites More sharing options...
woody37 Posted January 10, 2015 Share Posted January 10, 2015 As far as I'm aware, the interior of the cowlings is interior green in colour. Link to comment Share on other sites More sharing options...
Cees Broere Posted January 10, 2015 Share Posted January 10, 2015 i don't think exhaust collector rings got a chance to rust. The colour was due to heat staining. Cheers Cees Link to comment Share on other sites More sharing options...
Work In Progress Posted January 10, 2015 Share Posted January 10, 2015 (edited) Correct: they discolour all sorts of pretty shades, but don't rust. as such. Edited January 10, 2015 by Work In Progress Link to comment Share on other sites More sharing options...
Troy Smith Posted January 10, 2015 Share Posted January 10, 2015 usual caveats regarding WW2 colour film, but here's the Stirling pics on Etienne's Flickr, some great shots here, inclding post war in transport camo, naturalmetal and a one with D-Day stripes - https://www.flickr.com/search/?w=8270787@N07&q=stirling this one appears to have some kind of paint on the collector rings, anyone know more? HTH Link to comment Share on other sites More sharing options...
HP42 Posted January 10, 2015 Author Share Posted January 10, 2015 Many thanks for all the tips and advice, much appreciated! I'm sat here blending a variety of shades to do the job now. This aircraft may well escape the shelf of doom in the coming weeks! Link to comment Share on other sites More sharing options...
dogsbody Posted January 10, 2015 Share Posted January 10, 2015 The exhaust collector was painted over to reduce the glow on night missions over Occupied Europe, in an attempt to reduce detection by enemy nightfighters. On Bristol engine powered Coastal Command aircraft with white under-surfaces, the collector was sometimes painted in a white stove paint, to add the camouflage effect. Chris Link to comment Share on other sites More sharing options...
old thumper Posted January 11, 2015 Share Posted January 11, 2015 i don't think exhaust collector rings got a chance to rust. The colour was due to heat staining. Cheers Cees The collector rings were made of pretty much the same kind of material as modern stainless steel car exhausts, I think they call it nickel chrome steel or something similar. Anyway depending on the quality of the material used in manufacture and other factors stainless steel exhausts can rust to varying degrees. Often you get a sort of fine poo coloured dust that will stick to your fingers when you rub along the surface. I would google picture search rusty stainless steel car exhausts or used stainless steel car exhausts, that should give a pretty good likeness. Link to comment Share on other sites More sharing options...
dogsbody Posted January 11, 2015 Share Posted January 11, 2015 A few images from the Imperial War Museum collection, showing various Bristol exhaust collectors. http://zoom.iwm.org.uk/view/55283&cat=photographs&oid=object-205212315 http://zoom.iwm.org.uk/view/31616&cat=photographs&oid=object-205188201 http://www.iwm.org.uk/collections/item/object/205123807 http://www.iwm.org.uk/collections/item/object/205123801 http://www.iwm.org.uk/collections/item/object/205126543 http://www.iwm.org.uk/collections/item/object/205188209 http://www.iwm.org.uk/collections/item/object/205212817 Link to comment Share on other sites More sharing options...
old thumper Posted January 11, 2015 Share Posted January 11, 2015 Very interesting to see those photos, I wonder what colours the Matador refueler is painted in? it looks like a green pattern on a grey or blue background? I thought the background or base colour was an army brown similar to dark earth at that time? The pictures show collector rings with two types of silver inner ring. On early Hercules and other Bristol engines the inner part of the collector ring remained silver because at that point the steel was folded over and joined. Being thicker at that point it didn't get so hot and so didn't tarnish as quickly as the outer part of the ring, if you look at close up pictures of these collector rings you can see a line of what appears to be rivets where this join is. The second type of inner silver ring can be seen on the later picture where the Stirling has hedgehog exhausts. In this picture the inner ring is a separate piece of metal that was spaced out from the collector ring. I have never been able to find out why these separate pieces of metal were added or what they were for. I think these assemblies were added at some point in the war to four engine heavies, but not Beaufighters or other aircraft, again later they do not seem to be present on post-war aircraft such as the Hastings. I have seen pictures of wrecked engines that have been recovered where this assembly has come away from the collector ring, even after many years of being exposed to the weather on a mountain or moor they have only turned a white colour. Link to comment Share on other sites More sharing options...
Work In Progress Posted January 11, 2015 Share Posted January 11, 2015 (edited) On early Hercules and other Bristol engines the inner part of the collector ring remained silver because at that point the steel was folded over and joined. Being thicker at that point it didn't get so hot and so didn't tarnish as quickly as the outer part of the ring, if you look at close up pictures of these collector rings you can see a line of what appears to be rivets where this join is. It's not the thickness of the metal: it's because that thin inner lip is just a thin lip rather than hollow, and does not carry the exhaust gases like the main section does. Edited January 11, 2015 by Work In Progress 1 Link to comment Share on other sites More sharing options...
Graham Boak Posted January 11, 2015 Share Posted January 11, 2015 Both black and white paints were developed to carry the camouflage over the exhaust collector rings, which also had the effect of reducing the visibility in the infra-red. The AEC refueller is possibly in the earlier Army colours of G3 and G5 greens. There are a number of photos taken in this session, and It does seem to vary from view to view. Link to comment Share on other sites More sharing options...
dogsbody Posted January 11, 2015 Share Posted January 11, 2015 A small diagram to help explain the exhaust collector. Chris 3 Link to comment Share on other sites More sharing options...
old thumper Posted January 12, 2015 Share Posted January 12, 2015 The diagram of the later collector ring shows the baffle. Any idea what the baffle was for? Link to comment Share on other sites More sharing options...
Work In Progress Posted January 12, 2015 Share Posted January 12, 2015 (edited) It takes cool(ish) and fairly 'dead' air from inside the cowling and directs it around both surfaces of the stinking hot exhaust. The exiting air from the baffle is very hot so goes out with a lot more energy and speed than it entered at the top. By the time it exits it is blowing with vigour in a direction that makes aerodynamic sense. It blows over the hottest part of the outer exhaust surface, and the energy of the airflow has a rearward vector which will both help energy recovery of cooling drag to a small degree, and help to tidy up the boundary layer over the rest of the cowlings. It's a pretty clever bit of design, tbh. Edited January 12, 2015 by Work In Progress 1 Link to comment Share on other sites More sharing options...
Graham Boak Posted January 12, 2015 Share Posted January 12, 2015 The flow of the hot air will create an effect rather like a syphon to suck the cooler air out. Link to comment Share on other sites More sharing options...
Work In Progress Posted January 12, 2015 Share Posted January 12, 2015 Yes, it's really rather good. Even with the aeroplane stationary on the ground it will have quite a decent cooling effect on the exhaust. Link to comment Share on other sites More sharing options...
HP42 Posted January 12, 2015 Author Share Posted January 12, 2015 This is quite fascinating! There's so much design thinking in this. It started off as a colour question then grew into a science lesson, thanks chaps! Link to comment Share on other sites More sharing options...
Navy Bird Posted January 13, 2015 Share Posted January 13, 2015 While the professors are present, what was the reason for the collector ring in the first place, and why was it not used, or used differently, in other engine designs? For example, the P&W Wasp collector ring, if that's the proper term in this case, is behind the engine and inside the cowling. Cheers, Bill, your aero-engine technology deficient servant Link to comment Share on other sites More sharing options...
old thumper Posted January 13, 2015 Share Posted January 13, 2015 While I feel guilty for departing from the original subject even further can anyone tell me how the introduction of spinners on later Stirling Hercules engines helped stop the engine from over heating? I have read that spinners were added for this purpose as the Hercules engine was prone to over heat whilst towing gliders. I believe the Halifax glider tug also had spinners added, at least post war while in the middle east. Link to comment Share on other sites More sharing options...
dogsbody Posted January 13, 2015 Share Posted January 13, 2015 While the professors are present, what was the reason for the collector ring in the first place, and why was it not used, or used differently, in other engine designs? For example, the P&W Wasp collector ring, if that's the proper term in this case, is behind the engine and inside the cowling. Cheers, Bill, your aero-engine technology deficient servant This idea goes back to the introduction of the Townend Ring ( http://en.wikipedia.org/wiki/Townend_ring) to reduce the drag of radial engine cylinders sticking out in the slipstream. Someone at Bristols had the idea to incorporate an exhaust collector into the ring and as they say, Bob's yer uncle! When NACA devised their cowling design, Bristol just kept the collector in front. It also uncluttered the rear face of the engine. Chris Link to comment Share on other sites More sharing options...
woody37 Posted January 13, 2015 Share Posted January 13, 2015 While I feel guilty for departing from the original subject even further can anyone tell me how the introduction of spinners on later Stirling Hercules engines helped stop the engine from over heating? I have read that spinners were added for this purpose as the Hercules engine was prone to over heat whilst towing gliders. I believe the Halifax glider tug also had spinners added, at least post war while in the middle east. I presume the spinner created a venture effect increasing the speed of the air over the cylinders? Link to comment Share on other sites More sharing options...
Work In Progress Posted January 13, 2015 Share Posted January 13, 2015 Yes, it smooths the available airflow and directs it where you want it, and makes it go faster past the cylinder finning. The crankcase and/or gearbox are cool anyway. The single biggest factor constraining the power you can get sustainably from a radial engine of a given size is the cooling of the cylinders, hence the remarkable 'finning war' that you can see as finning density went through the roof during WW2. And for a given finning density, the most cooling effect comes from the greatest possible transfer of air through that finning. 1 Link to comment Share on other sites More sharing options...
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