David P Williams Posted June 9, 2014 Share Posted June 9, 2014 As I've bored you with in the past, I'm building a SLEC/Apache RAF Chipmunk. It is one of the first kits SLEC produced after taking over the Apache Aviation range last year, so we've had a few headscratching moments along the way but Ian and Chris at SLEC have been great at resolving problems. I really should have done a build log, but this is my first build in a while and I was shy about revealing the likely cockups learning opportunities along the way. Anyway, encouraged by Danny, and not wanting to clutter his Chip Shop thread any further, I thought I would give a bit of retrospective info about the oleos I've just made for this model. The kit includes standard 4swg wire undercarriage legs, and a suggestion to use Robart strut covers to replicate the Chipmunk's prominent scissor links, but I treated myself to a new Chinese lathe last year and was itching to make something on it, so I desided to try to make some working oleo legs. The model is also roughly 1/5 scale, so the Robart covers are too small. These pics of the full size show what I was trying to replicate:- Many of the ex-Portuguese Airforce Chipmunks that I was involved in restoring had these gaiters, but most were perished and removed. I didn't really know how to rate or source suitable springs, so I purchased a pair of 60-120 size oleos from HobbyKing to copy and cannibalise them. Pressing these down on my scales gave about 7lb to compress halfway, so I though the springs might be OK for a 13-14lb model - we'll see. So, having measured the HK oleo internals, and scaled the externals from photos, I fired up my CAD program and drew some plans:- The main bodies and 'plunger' were pretty straightforward turning jobs, using sections of aluminium bar purchased in a 'variety pack' from Noggin End Metals, who had a stand at the Nats last year (and I got some more from them at the Long Marston show a couple of weeks ago). The original HK units are on the left, and my new larger ones on the right. The scissor links had me headscratching for a while and were eventually made from 12mm square aluminium bar from B&Q. I borrowed a vertical slide from a friend to bolt to the lathe cross slide, and a milling cutter or two, and set about learning to mill. The results were OK, if a bit crude round the edges where I had to file the rounded bosses to shape, but it all gets painted grey in the end. 2mm screws and nylocs from good old Modelfixings completed them. The difficult bit was how to mount them to the legs. In the end I milled 4mm slots across the legs and epoxied mounting bushes into them. The pin in the vertical slot takes the twisting and end-stop loads, so there should be little load on the links. The 4swg legs from the kit were cut down so that these oleos will mount onto them, and the cut off portions cut down to make axles. Spacers were turned up to give clearance for the wheels and hopefully it will all work OK. Just need to trim the screws to length, make the fairings and brake flexi pipes, and they should be good to go. Time will tell how they stand up to our patch! Quote Link to comment Share on other sites More sharing options...
Danny Fenton Posted June 9, 2014 Share Posted June 9, 2014 Excellent well done that man, and thank you for showing how you made them. Can I ask how you made the slotted hole in the upper leg section, was that also milled on the lathe? Cheers Danny Quote Link to comment Share on other sites More sharing options...
David P Williams Posted June 9, 2014 Author Share Posted June 9, 2014 Yes it was, with a 3mm slot drill - a slot drill allows you to plunge into the material as it has one or more cutting edges that spans the centre, whereas a standard end mill has a gap in the centre. It still cuts along the side as well. Quote Link to comment Share on other sites More sharing options...
Rich too Posted June 10, 2014 Share Posted June 10, 2014 Excellent David. I just ordered some replacements for my H9 P47, you could have made them for me!! Rich Quote Link to comment Share on other sites More sharing options...
David P Williams Posted June 10, 2014 Author Share Posted June 10, 2014 Took me several days, several scrap pieces and lots of language to make these Rich, so I don't think I'll be offering engineering services in the near future ! Quote Link to comment Share on other sites More sharing options...
Rich too Posted June 10, 2014 Share Posted June 10, 2014 Haha! No worries - but I was surprised at how difficult it is to locate the correct length (160mm), in the end I got them from Giantshark - and they happened to be by far the cheapest.. Rich Quote Link to comment Share on other sites More sharing options...
Edward Jennings Posted June 10, 2014 Share Posted June 10, 2014 Good job man! I like every details of it. Quote Link to comment Share on other sites More sharing options...
John Howitt 1 Posted August 5, 2014 Share Posted August 5, 2014 David I have looked long and hard at your article with admiration and have purchased some of the HobbyKing oleos which you used in producing your scaled up versions. I intend trying something similar and would love to have a copy of your CAD plan which would help to make my life a lot easier. I wonder if the oleo shaft needs to be increased in diameter or whether you could merely lengthen the lower piston element. If the lower shaft was noticeably non scale it might be possible to disguise this fact with a suitable gaiter. Maybe I am just being idle but the time I will have to allocate to rivets and rib tapes etc means that I can't spend too much time enjoying myself with the metalwork. I find the oleos that the boys are rushing out to buy look a bit clumpy around the scissors and feel that I could design and make a rather more delicate version. Perhaps a bit of sheet metalwork and some silver soldering would be suitable. Jings I have just re-read this and hope I don't come across as full of conceit. I took up model engineering on retirement and feel comfortable in that environment - on the other hand I have returned to aeromodelling only recently and am still finding my feet in the scale arena. I would love to hear your comments on my suggestions and if possible a larger copy of your CAD plan. Yours John Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted August 5, 2014 Share Posted August 5, 2014 David P A very neat piece of work but I would have to say that whilst a 'pin and slot' is simple to do a scissor link is much better able to resist rotation (and wear) particularly at the very short radius available to the pin in the oleo leg and especially as you went to the trouble to make the links as well! Quote Link to comment Share on other sites More sharing options...
David P Williams Posted August 5, 2014 Author Share Posted August 5, 2014 John and Simon - I'm just off out for an 8 mile walk to the pub (!) but I'll sort out a better reply tomorrow. Quote Link to comment Share on other sites More sharing options...
John Howitt 1 Posted August 5, 2014 Share Posted August 5, 2014 Wow David, 16miles there and back. I enjoy a drink very much but preferably within zimmer distance. Perhaps I saw you performing in the Commonwealth Games arena?? Cheers John Quote Link to comment Share on other sites More sharing options...
David P Williams Posted August 6, 2014 Author Share Posted August 6, 2014 John - it was only 8 miles, we got a lift back! Our village pub has closed so 5 of us retired chaps from the village go out on Tuesdays for beer and banter taking it in turns to drive and sometimes 2 or 3 of us walk there to meet the others. Usually hard work - we're at 600ft AMSL right by the halfway point on the Offa's Dyke path, so going north or south involves a series of steep climbs and descents up to 1300ft or so. Makes the pint or three taste even better. We were in the pub attached to the oldest continually licensed brewery in the UK. Anyway - the CAD drawing is in AutoCad2007 .dwg format - can you handle that? It's actually created in IntelliCad, a (very) much cheaper AutoCad clone, which gives me various options for exporting the file, including earlier versions of AutoCad format, or ASCII or Binary .dxf, or Drawing Template .dwt. Have to confess I don't know what these are, but are any of them aby use to you? (I was too stingy to buy the add-on to export as a .pdf). If you want the file PM me with your email address and I'll send it over. I should add that the drawing was to get the main dimensions and proportions right for 1/5 scale, and I can't guarantee the accuracy of any of it as I probably made bits up as I worked. Simon - you're probably right, but most of the commercial units at this size use the pin in slot method. My scissor links were meant to be scale size and mostly decorative and I didn't think they would be robust enough on their own, so I did the belt and braces thing as the slot will be covered by a fairing anyway. The scissor links on the full size Chipmunk wear a LOT and most checks and inspections require shimming of the links to take out play. I used to spend a lot of time grinding washers down to suit. Cheers, David Edited By David P Williams on 06/08/2014 10:59:18 Quote Link to comment Share on other sites More sharing options...
David P Williams Posted October 12, 2014 Author Share Posted October 12, 2014 Well I deleted the photos that went with this thread from my album, to leave it containing only Chipmunk 'walkaround pics, so of course the links from this thread are broken. I've put the oleo pics in a new album here for thos of you who have asked for them. Please don't forget that these were to a larger scale (1/5) than the current mass Chipmunk build requires. Quote Link to comment Share on other sites More sharing options...
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