kenking-King Design

cont'd. Three weeks have passed since the last entry, so you may have been wondering what was or was not happening. Firstly, both 'castings' have been brought up to similar condition i.e. linered, and painted silver. Then, following Ben's revision of weight range, I revisited my spring tables and eventually came up with a series of combinations, each of which gave the required 'at rest' compression at an increment within that range. You'd think, wouldn't you, that a spring is about the simplest mechanism to deal with, but sums can get suprisingly complicated and time consuming ....... or maybe it's just me. After that I turned my attention to the retraction linkage with a view to finalising lengths and establishing how all this gubbins was going to be fixed into the nacelles in suitably robust manner. Obviously that was going to require close collaboration between Ben and I, so I fired off a requests for detailed nacelle views and dimensions, which he promised to send in a day or two. Days passed and I realised Ben had gone awfully quiet, in fact he'd dropped off the radar completely and remained out of touch for quite a while. I feared the worst, but thankfully he eventually 'returned to the land of the living' as he put it. Quite how far he'd travelled in the opposite direction I don't know, and he isn't saying. At any rate, a drawing did then arrive, also showing the original sprung wire U/C arrangement. I copied the side view and I plotted my best estimate of the scale casting pivot point (quite different to the wire one) and the rear pivot of the main retraction link. A few twirls of my compasses later I realised there was a major problem. The slightly shorter scale legs, coupled with repositioned pivot, meant that the retracted wheel sat further forward in the nacelle and conflicted heavily with Ben's planned position for the 2" diameter wing joining tube (seen below, faintly coloured green, as I then was). The paper cutout over Ben's plan drives the point home. Knowing Ben already had the laser-cut parts I gloomily feared I might have to remake and lengthen the steel tube legs, thereby also sacrificing the scale veracity we'd sought. Ben, however, was much more relaxed about the news than I expected, and has pretty much undertaken to reposition the tube, lifting my black cloud. As a result I'm now back planning the linkage parts and mounts, based on my estimated scale dimensions, so watch this space. Ken.

No typos Ian, we've been talking kgs throughout, but you're right about the likely model weight of course. Ben was testing me I think, perhaps ensuring a good built-in safety factor. He has since revised his estimate to between 20 and 30 kgs, so you are both in the same ballpark. Maybe he is planning a full bomb load. Ken

I know what you mean alan p, Let's hope we don't also see even the high estimate being grossly exceeded, in the usual manner of such things. In fairness to Ben, he deliberately gave a very wide range, within which the weight was pretty certain to be contained, in order to ensure the design of the U/C would not be a restriction, and I'm pleased to say it's not. BTW, if anyone disagrees with my analysis and predictions about the way the damper will work, do please speak up. I'm very aware that I'm shooting from the hip here, and a different viewpoint will be interesting. For instance, I haven't really taken into account the effect of displaced oil above the piston being in a confined space. The internal cylinder volume, and initial fill, must leave sufficient top space to keep ALL oil contained within the cylinder despite the intrusion of the extra volume of the piston rod during compression. Will air be expelled or inhaled through the 'O'ring under such circumstances ? What do you think ? Ken

cont'd. Several hours yesterday were spent poring over my spring tables and doing sums to determine which springs could be used to cover the range of finished model weights given by Ben, the builder, anything from 25 to 45 kgs. I'd earlier calculated wheel loadings and hence individual leg loads, so knew each leg needs springs to give from 5.5 to 9.5 kgs in the 'at rest' position. Selections worked out alright and I was pleased to find no dimensional changes were necessary within the assembly. That allowed me to start on the hydraulic dampers, which are to be located within the lower leg sections. Each damper comprises a thin-walled brass cylinder, incorporating a piston on a rod which passes through the cylinder cap, with 'O' ring seal, and extends through the springs to the top of the leg housing and a disc cap. Springs are trapped between the two caps, so we end up with almost a 'coil-over-shock' arrangement. A very simple test cylinder was made by soldering a length of tube to a brass block, robust enough to be clamped in my vise ..... An equally simple piston was turned on the end of some LA rod, initially to just comfortably slide in the tube, a bore of .375" with maybe a thou or so clearance ....... The silicone oil purchased some time ago for this very purpose was eventually tracked down, and a good fill-up added. Just to get a feel for things I inserted the piston and pressed down against fairly solid resistance, but the piston did very slowly sink . The BIG surprise came when I went to pull it back up, expecting similar resistance, but NO ! Up it came quite readily, though it felt as if I was pulling against a spring. - most intriguing. I eventually worked out what was happening, confirmed by calculation. The piston area is so small that a pull of only .74 kgs is sufficient to overcome atmospheric pressure and create a vacuum zone below it, which acts like a constant-force tension spring. I admit that this is an aspect which had not occurred to me, at all. The effect lasts until the oil above the piston has seeped back through the clearance gap to fill the void, something it does even more slowly because a pressure differential of only 1 bar is now motivating it. To get some measure of performance I used a fixed weight of 2kgs acting on the piston rod, steadied by hand ... and timed the piston fall over a distance of 25mm, taking 10 seconds. Next, having reduced to piston diameter to give .005" clearance, I repeated the test, this time logging only 2 seconds. Pulling the piston up was also quite different because the oil return flow was a lot quicker and the vacuum effect short-lived. What a difference those few thou made ! Thinking about the impact all this has upon the great scheme of things I came to the conclusion that my idea of having a clever piston valve is now quite redundant, and so my secret design will remain just that - a secret. I reckon in practice this performance may prove to be about right. In a hard landing the tyre takes the initial blow and transmits force to the legs. They compress as fast as the damper will allow, further softening shock to the airframe. If the aircraft bounces upward the legs will begin to quickly expand again, creating the vacuum space as described. If they have not reached full extension before the next impact they will immediately shorten, the vacuum space will disappear, and at that point normal damping will resume. In the VERY unlikely event of a series of reducing, damped bounces the legs will go through repeats of the above cycle, to return in stages to their original length, ergo, no valve needed ! Making up the dampers just became much simpler, and with them, the spring enclosures. I have some springs to hand for trying things out, but the REAL ones needn't be ordered until we have the model weight firmed up. Sighs of relief all round, Ken

cont'd. Repair of the 'casting' took priority today, after having thought hard for a while, and a method being settled upon. The leg tube needs a really stout lid as it takes all the spring thrust, that's why 2mm plate spans the area, and so the repair needs to be equally solid. First step was to mill a flanged plug for the hole at the leg top ...... A little doctoring of the flange ensured it would sit flush, then it had to be bored out to accept the replacement liner tube. As the bore equalled the square size, a cunning plan was needed. A short section of leg tube material was fitted around it whilst most of the interior was taken away, then a hacksaw cut allowed removal of the tube support, leaving a four-legged insert .... The insert is now bonded in place with THAT Loctite, and I reckon there's more than enough contact area for the strength required. I'll also try to have the liner tube bonded into it so it will be held from both ends, so to speak. There followed more surface restoration work and epoxy filling, and the unit is supended fro the shed roof to cure nicely overnight, to be followed by sanding and primer painting. I can't yet make the replacement liner or sliding tube as I haven't any spare material to hand; it's on order though, so with luck a few more days will see this unit back where it was a week ago. That's progress ?? There was enough time to make eight small steel bushes for the retract linkage brackets on the back of the castings ....... These will be bonded in place with the flanges to the inside, giving wear resistant facings for the link within. I ordered twenty dowels, 3mm x 10mm long, but instead received 10 dowels, 3mm x 20 long, a very minor irritation at this stage. Ken .

Morning All. I've just surfaced after a VERY late blog session last night, and I see there are a couple of questions to answer. Firstly, Martyn K; There are split keeper plates at the leg ends, retained on the four studs. You'll find details earlier in the blog on 2/4 and 9/4. I've not noticed any pneumatic damping during dry operation, but I guess a thin lubricant film might change that. Secondly, Alan P., I was beginning to have serious qualms on this issue too, but since revising the foot/clamp arrangement there are no problems whatsoever. (Why do I feel vaguely uncomfortable as I write that ? Tempting Providence, perhaps ?) In truth there shouldn't be problems because the feet/clamps are pretty accurately squared to the sliding diameter of their legs, and the housings in the 'castings' came out parallel to within a couple of thou, measured across legs at full extension, thanks to the solidly made build jig mkII. Axle units now slide in and out under the action of gravity alone, (even without the wheel weight), which is as it should be, and is why I spent so much frustrating time chasing down the friction problems I was seeing. Thanks to all for your continued interest, Ken

cont'd. I had a Loctite product to bond them in, and at the same time I would apply a little bead of epoxy to the top of each foot to fill the slight irregular gap where it abutted the endface of the 'casting'. I mixed some epoxy and applied it, ran some Loctite the full length of the first liner surface, and proceeded to insert it, taking care to align it rotationally. Unfortunately I hadn''t appreciated just how quicky the bonding would take effect, and before fully home it was stuck - really stuck. In trying to turn the liner with grips the stud flange bent and finally broke off, and there I was, up to my neck in poo. Talk about snatching defeat from the jaws of victory ! Of course, I'd inadvertently left the sliding tube inside and had started bending stuff before I realised it, by which time it was impossible to simply slide it out. I now had a double thickness tube to deal with. I shall draw a veil over the activities of the following hours, the overiding concern being the potential destruction of the 'casting', but eventually, after much hammering of shaped probes and ordeal by fire, surgery was resorted to, and the sorry wreckage removed. THE COMPONENTS BEFORE THE COMPONENTS AFTER I now have no reservations about the strength of my built 'castings', and the greatest possible respect for the strength of the Loctite bond - phenomenal. No irreparable damage was done to the 'casting', thank goodness, though restoration will take some thought and time, and parts have to be remade. They say if you fall off a horse you should immediately remount, and so, applying the same principle plus the hard-earned lessons of the above debacle, I took the second 'casting' and, by using a much shorter bond length and not hanging about, successfully incorporated both liners in short order. Rejoicing was tempered by thoughts of behind-the-scenes restoration work to come, but nevertheles I celebrated by adding the wheel on its axle, which really puts the whole U/C assembly into pleasing perspective. FULLY EXTENDED 'AT REST' COMPRESSION Scale hubs will be a major improvement and will appear in due course. There you have it for the time being - I did promise this would be 'warts and all' didn't I ? Thanks for laughing in such a sympathetic manner. More later, much later, Ken

cont'd. Thanks for the kind comments chaps, but you flatter me undeservedly, as you will see later on. My reamer did in fact turn up the next day, so I was able to forge ahead, getting square tubes ready to accept liners freely, and proceeding to dry run assembly of the parts, at which point some problems arose. Leg/axle assemblies would not slide consistently, and parallelism was an issue. It turned out that the 'collar in a groove' idea gave more trouble than benefit, although it took me ages to finally work out what was going on. A tiny bit of excess solder, collar positioning and fit in the groove, as well as some clamp issues, were all combining to induce very slight cant in the legs as the clamps were tightened, so I took the decision to scrap the idea. To restore accuracy I put each leg into the lathe in turn and faced the brass foot, making it absolutely square to the sliding axis. I then put two feet together face to face, and bored right through, giving me a true semicircular trough in each. The aluminium clamps were modified too, the 6mm diameter groove being opened up to an 8mm curve but retaining the same depth. This new arrangement gave true axle bedding in the feet, with no possible interference to alignment from the clamps. Sorry, that's perhaps too blurred to see the effect. At any rate, it solved the problems and I was able to put things together and repeatedly achieve very free leg movement. Great ! A lot of time and head scratching but I was finally at the point where I could permanently affix the liners, ... and THAT, dear readers, is where it all went wrong, horribly, HORRIBLY wrong.