Designing & making main U/C with springs, dampers and brakes
|kenking-King Design||21/03/2020 16:25:37|
285 forum posts
A client is planning the build of a 1/9th scale Handley-Page Halifax with a span of around nine feet, so acres of space for detailing. As the main U/C units were such a distinctive feature he would like them to be realistically represented, and has asked me to look at making them. Oh, and he'd like them to be fully sprung (of course) and if possible to incorporate effective damping (hmm, bit more involved then) and if left/right braking were included that would be nice (now this is getting really serious !).
Ever a sucker for a challenge, I started looking into what it would require. Firstly a look at the aircraft itself, and I could immediately see what he meant about the mains. They were housed in a whopping great magnesium casting by the manufacturers, Messier, and it is certainly eye-catching.
As you can see, I've been scribbling over this view to glean some basic dimensions, knowing the tyre diameter was 63". I purchased the Haynes Manual for the Halifax (yes, really) and therein found all sorts of useful photos and dimensional data. There was a lot more scribbling to come before any metal was cut, but more later .....
|kenking-King Design||21/03/2020 19:25:34|
285 forum posts
That view is of a Merlin powered aircraft, though Bristol engines gave more power, and gave a top speed a little better than the Lancaster. Here's an artist's impression of the Gear 'Down' and a ghost image of it 'Up' showing how two rearward rams did the moving. I have to say they look quite puny compared to the bulk of that casting and its wheel.
After hours and hours, spread over several weeks, spent trawling the web for photgraphs from which reasonable scaling could be carried out, a composite sketch of the casting was prepared as the basis for manufacture. Unfortunately we didn't find any original HP drawings, so this is the best approximation I could come up with ...
Another very helpful view is this part-sectioned illustration from Messier documentation; lots of detail, and hints at construction and assembly methods which would prove useful .....
Another vital measure needed was an estimate of the weight to be carried on each wheel, and for that I needed two things - a guesstimate of the finished model weight, and a best guess as to the weight distribution. For the first, my client offered a very wide range of 25 to 45 kgs, displaying a degree of uncertainty but at least giving me some limits within which to work. For the second I found a good 3-view drawing on which I superimposed a CoG point. Then, by taking moments about mains and tailwheel I arrived at a figure of 44% weight on each mainwheel, meaning each will carry 5.5 - 9.9 kgs, or for old Imperialists like me, 12.1 - 21.8 lbs.
Sorry it's sideways.
Using these numbers, spring catalogues were consulted, and min/max spring diameters determined. Then I could give serious thought as to hown things might be tied together. That's when even more serious scribbling began ....
|Don Fry||21/03/2020 19:48:11|
4557 forum posts
|Timo Starkloff||21/03/2020 22:03:53|
393 forum posts
I’ve seen the Halifax at the Yorkshire Air Museum in Elvington, very impressive plane!
Good luck for building this plane
|kenking-King Design||21/03/2020 23:20:53|
285 forum posts
Thanks Don, and Timo - I'm only trying to build the U/C, it's my client who will be heaving baulks of timber around. Whilst I started covering page after page with drafts of potential ways to make these units, he was busy looking for 7" wheels, (scale size) and correct proportions. After much searching the best fit he found were Airtops from the USA, which he ordered. They eventually cleared customs, and I received one of them. Diameter is fine but the tyres are several millimetres too wide, so I sketched a revised hub profile, hoping to squeeze them in a bit, and also allowing space for a brake.
Wooden forms, turned to the profile and assembled on studding with the tyre between them, were pushed together, but the shape didn't perform as hoped so it was revised again, and this time worked better, though I'd lost some brake space.
At about the same time we both discovered that our respective tyres were leaking air at the inflation point, so that is being investigated by the purchaser. We may yet have to find an alternative. Coincidentally I recently stumbled upon some foam samples acquired a couple of years ago with home-made tyres in mind, a project which may be resurrected if all else fails.
In the meantime a method of construction was settled upon. That took very little time to write, but there were around six other schemes dreamed up in hope and rejected in despondency, before that point was reached. To design the unit meant not only gathering dimensional information, assessing likely spring sizes and so on, it also meant materials searches and internet trawls for suppliers to find out what was available and if it was possible to translate my ideas into physical objects. I actually thoroughly enjoy those aspects, frustrating and time consuming though they may be.
I finally settled on fabricating the casting shape from three constituents. The first, the top part which I call the head, and which provides the leg pivot bearing and transmits most loads into the airframe, is to be machined from solid aluminium alloy. The oleos, incorporating springs and oil dampers are to be housed in 3/4"square aluminium tube lined with thin-walled brass tubing to provide a bearing surface, and the rest of the casting wil be assembled from formed aluminium sheet parts. All three will be soldered together, a process I've used once before with very pleasing results. Some soldering tests were carried out to remind myself of how not to do it, but also to check compatibility of sheet and tube for soldering ........
Pleasingly all seems well, and the joints are really strong, particularly when internal corners give a nice fillet, and provided one allows sufficient flow time for the alloy mixing to penetrate to a good depth. The wall and sheet thicknesses are shown, and are not dissimilar. I have yet to test solder the solid bar element in conjunction with the other parts, but it better be all right as I've already started making the heads.
2" x 3/4" bar has been cut to length .....
and has been drilled for the leg pivot shaft (note wood chips from turning trial hubs) ....
then bored to form part of the top valley ......
after which the blanks were marked out and bandsawed toroughly the right shape ....
Perhaps I'd better go and do that last soldering test before I go much further. It may already be too late .....
|kenking-King Design||22/03/2020 00:46:51|
285 forum posts
Still not tried soldering the block material, but full of optimism I've continued with filing the flanks and rounding over the tops. This one's finished externally, apart from removing more metal to show off the small ribs which curve over the top surfaces.
At this stage the blank weighed 146 grams. I wanted to reduce the weight further, and also the mass of metal to be soldered to sheet components, so the process of hollowing out began on the mill .......
That hollowing follows the external shape but the depth doesn't show up very well in this shot. It took away another 50 gms or so, but I'll reduce the wall thickness a little more, to around 1.8 mms, which will still leave a pretty robust form. The leg pivot holes will eventually be fitted with brass sleeves for wear resitance, once the hot processing is over.
The next stage will involve sheet components and soldering elements together. Some careful jigging will be required if I'm to produce flush joints and avoid much sanding. It's also pretty important that things are kept square and parallel, particularly the square tubes as the two sliding oleos housed within them, and moving in tandem, must not bind.
You may have gathered that the events described so far have occurred over a period of several months, and this evening I've been writing it all up in very condensed form. Now that the account is up to date the true snail-like rate of progress will be revealed as the venture lurches from one crisis to another, or not, if I'm lucky.
More later, and thanks for reading thus far,
|kenking-King Design||22/03/2020 00:57:20|
285 forum posts
Just spotted a Whoops in earlier text describing weight distribution calcs. I worked out the weight on each mainwheel as 44% of aircraft weight, then quoted figures which referred to half that, being the weight carried by EACH LEG and therefore each leg spring, which was the purpose of the exercise. Thanks for not pointing out my error, but please, less diplomacy and more insistence on accuracy of reporting.
|Piers Acland||22/03/2020 08:31:41|
18 forum posts
I will be following this thread with real interest. Ken helped me out with some retracts for a Mossie. He's a genius!
|alan p||22/03/2020 09:33:48|
|257 forum posts|
Watching with interest. I soldered a home made exhaust recently and the biggest problem was to maintain the heat within the structure without damaging it, to enable the solder to flow. Used Durafix
Good luck Ken
|kenking-King Design||25/03/2020 00:31:09|
285 forum posts
Piers, I can't possibly live up to that ! but thanks anyway.
I milled some more metal out of the interiors which brought the weight of each head down to 86gms. The original cut-to-length blank weighed 300, so that's a good reduction. Forgive the unsavoury thumbnail, if you will.
The deck plates are 2mm sheet, cut to size, with two apertures milled out to give access for interior as well as exterior soldering of the heads ...
Pairs of components were clamped centrally whilst some soldering at each end fixed them in register, then soldering continued along each join line, inside and out. This is not something I want coming apart in a heavy landing.
Here are both assemblies, awaiting cleanup ...
I realised too late that I could have achieved a neater job in less time if I had left the deck plates a little oversize so that the external side and front joints were made in an internal angle between the two parts (something I'd earlier learned gave very neat beads) and could then have trimmed the plate. Unfortunately I'd promptly forgotten the lesson (what does that tell you ?) and so made life a little harder for myself. Something for you to remember though, if you make anything similar.
The next soldering stage will be attachment of the square tube oleo housings to the underside of the decks, but to do that I must first make a jig to hold each pair parallel. Maybe tomorrow.
9196 forum posts
I can forgive the dirty thumbnail Ken, it’s the mark of a craftsman
|kenking-King Design||27/03/2020 22:19:32|
285 forum posts
I knocked up the jig from oddments kicking around in my shed. At the heart of it is a block of laminate, an offcut from a laboratory workbench manufacturer. It's easy to machine, drill and tap, so made the job straightforward. The block was cut to a width of 76mm, being the desired leg spacing, and to it I screwed pieces of angle fitted with grubscrews for clamping. Here are a couple of views of it, loaded with a machined head, and two leg tubes ...
So far, so good, and full of self-congraulation I fired up the torch and proceeded to heat up the head/leg joint lines, and in fact made the first couple of soldered joints before the smoke became noticeable. In the view above you will note the rectangular void, one face of which is the end surface of my laminate. Due to lack of protection it caught fire. Why I was surprised I cannot now imagine nor, I suspect, can you. Anyway, it was extinguished and the show went on, as it always must. One clamping piece was removed to allow soldering all around each leg top, and the assembly and jig were seperated. The first 3-part assembly is shown below, before any joints were cleaned up.
Sadly, I found the laminate was not only scorched, it had changed size and bulged in places, requiring dressing on the sander, retruing, and the addition of thin packing strips. Before soldering the second assembly I filled that guilty void with an offcut of the square tube - what a pity I didn't think of doing it at the start - numpty !
The next instalment will cover some sheet metalwork, as I make the profiled side and front panels which will provide the necessary bracing between head plate and legs, and some experimental applications of glued paper !
|Barnstormer 52||28/03/2020 16:03:02|
67 forum posts
A bit off topic, but this might be of interest to your client.
In WW II my father was a flight engineer on a Halifax mk III.
Stationed out in Heliopolis Egypt.
Attached is a picture of him underneath the nose of of a Mk III a veteran of 2 Squadron.
I'm not sure if it was named
'Digger & The Free French'
or if he was just referring to the crew.
You can clearly see a Kangaroo and the insignia of the Free French Air force painted on the nose.
The back of the photograph also says it had completed 13 night missions and 7 day time - and is obviously still in one piece
There is also an interesting video on Youtube which I imagine your client may have found, all taken inside a Halifax.
Edited By Barnstormer 52 on 28/03/2020 16:03:43
Edited By Barnstormer 52 on 28/03/2020 16:10:41
|kenking-King Design||31/03/2020 01:01:21|
285 forum posts
Geoff, just repeating what I said in my email to you, we all owe a huge debt of gratitude to fellows like your father whose courage and bravery under great duress helped win the war. Thanks for the insight, and the link, which I had already seen.
Meanwhile, back in my shed, I came to my senses, junked the scorched laminate, and remade the soldering jig around a core of aluminium tooling plate. Then a critical look at the soldered U/C unit revealed that the second leg had moved due to the effects of the small fire and was nicely soldered but wrongly aligned. Time to take a second bull by the horns, cut off the leg, clean up and fix a new leg.
Feeling pleased with the new jig I loaded another set of parts and soldered legs to head. Great stuff, we're on a roll here. Next came the arch braces, two per unit, cut and filed from 1.5mm sheet. Repositioning the unit partly out of the jig, and having made the brace a lovely push fit, I soldered it to one leg and the underside of the deck, at which point enough stray heat had travelled ahead of me to cause the first leg to drop off.
Well, you can imagine my disappointment. Ooooh, I was cross. Of course, it wasn't my fault, it was that feeble, underpowered gas torch that took so long to heat the scene of activity that all points in a large radius were also approaching soldering temperature.
Cutting a depressing story short, I switched to oxy-acetylene. Bold or foolhardy? I just knew I had to heat things up more quickly, and keep it local, so a spray bottle of water was also at hand to cool nearby susceptible joints. So far it's going better than expected .....
and the purpose of the rear arch, seen loosely fitted here, is not only to hide the less than pretty resoldering of the front one, it also further braces the legs. I'm sure it will all look a lot better after an overall sand.
At this point I'd had enough aluminium soldering for a while, so for a change turned my attention to the internal leg arrangements. Briefly, a 5/8" diameter thin-walled brass tube will be bonded into the square leg to form a cylindrical housing. At the bottom it will carry a rectangular brass foot with a steel stud protruding downward, from each corner. In the photo you can see how neatly the two tubes fit. Below is a stack of four foot blanks, temporarily soft soldered together for milling and drilling ......
Into this fixed tube will go another brass tube retained by the foot plate, acting as a bearing for the sliding leg, which, would you believe it, is formed from two tubes. This time they are steel. The upper, larger one houses the springs and lives inside the leg, whilst the lower protrudes to represent the ram of the oleo. A polished finish may help the illusion. If I can get the hydraulic damper sorted out, it will sit within the lower part. Removing the foot plate will allow all the internals to be removed if, for example, the spring needs adjusting, or changing, to get the correct sitting height.
I'm getting well ahead of myself here - there's more aluminium soldering tomorrow, including making the front shrouds to more or less complete the shape of the original Messier casting.
This is a warts and all sort of blog, as you have seen. I make mistakes and poor decisions sometimes, and hope I learn from them. If laughing at them helps you avoid something similar, sometime, then it will have served its purpose.
575 forum posts
It makes it so much more interesting too. My modelling efforts are nowhere near this skilful but I still go through the same process. The idea, the realisation that the first plan won’t or cannot work, finishing with a successful result. But surely this whole process is what we all do, if you had just presented the fully finished legs and left it at that, we may have imagined that it was the first effort.
|kenking-King Design||02/04/2020 11:24:54|
285 forum posts
In the previous post I referred to removing the foot plates to release the internals - this was not strictly accurate as the footplate will be permanently fixed to the large brass tube which is also permanently fixed, within the square leg. what I should have said, and illustrated below, is that there will be a keeper plate to do the retaining.
It was such a relief to work with metal of a different colour that I carried on with them. You've seen the four foot plates in a soldered block, and the keeper plates were made in similar fashion, the difference being that the keepers need to be split, as the central hole won't be big enough to clear the axle mounting block on the lower leg.
Two strips were soldered together edge to edge .....
then marked out and a small central hole drilled in each before cutting the assembly into four pieces. These blanks were bolted to the foot blanks, four corner holes were drilled through 6BA tapping size, and the external profiles milled to match. Retainers had the corner holes opened up and were bolted together as a block for insertion in the lathe, where the central hole was bored to size ....
after which the foot block was bored to its slightly larger size, to accept the largest brass tube. Although yet to be seperated into four seperate feet, the arrangement with keepers can be seen. Keepers were unsoldered and cleaned up, taking just a few minutes.
The four-corner-bolt arrangement echoes that of the full-size U/C, but true scale fasteners, even if available, would be too tiny to be of use, so I'm using 6BA bolts, but they're purchased with one-size-smaller hexes, 7BA in other words, which makes them a little less conspicuous.
Now yellow metal must be put aside, and it's back to the aluminiun shrouds. More later,
|kenking-King Design||02/04/2020 23:47:35|
285 forum posts
Managed to grab a bit more shed time today, and finished soldering in the last arch piece, still using oxy-acetylene. At this point I should make it clear that, despite my earlier outburst, there is absolutely nothing wrong with my small gas torches; in fact they are very convenient to use. The trouble comes from the way I've made up these U/C assemblies with so many conjoined seams that any stray heat spells trouble. For a simpler assembly such a gas torch is fine, so don't be put off the process.
Moving on to the front shrouds; these really define the unique shape of the Messier casting, so I'm keen to see them attached. They also form the crucial brace between the head/deck and the legs. I spent some time marking out the developed shape on 1.5mm sheet, and after the usual short period of confusion between what was waste and what needed to be retained, I transferred to the bandsaw, complete with new fine-tooth blade.
Here's one of the cut blanks ....
I plan to saw out the arch shape later after folding, as the sheet is more stable that way and the bends will benefit. The sides were folded up, leaving one more fold to give the extra angle on the front lower portion, with two short front seams to complete the shaping .......
At this point I couldn't resist sitting a part-finished shroud into position, as one does with such things. Of course, one then has to half close eyes, defocus, and imagine the rest, but we all do it, don't we ?
Once soldered up there will be some fettling and filing to ensure a good fit to the main assembly, then some heart-in-the-mouth moments with the torch, around the head/deck shroud junction. Joining to the legs doesn't worry me too much, but if anything should happen to the machined head pieces !!! It doesn't bear thinking about.
9196 forum posts
Great write up and very detailed, thanks for that.
|306 forum posts|
I'm no metal engineer but absolutely fascinated -
|kenking-King Design||03/04/2020 12:16:50|
285 forum posts
Thanks Cymaz and Ace, I was beginning to worry that It was becoming quite boring - well, perhaps it really is for some folk, but not the more discerning ones eh ? I'll just carry on then.
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