Ace Posted May 16, 2020 Share Posted May 16, 2020 Very creative solution Quote Link to comment Share on other sites More sharing options...
kenking-King Design Posted May 19, 2020 Author Share Posted May 19, 2020 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 Quote Link to comment Share on other sites More sharing options...
alan p Posted May 19, 2020 Share Posted May 19, 2020 Finished model weight looks like a government civil service estimate. Quote Link to comment Share on other sites More sharing options...
kenking-King Design Posted May 19, 2020 Author Share Posted May 19, 2020 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 Quote Link to comment Share on other sites More sharing options...
reg shaw Posted May 19, 2020 Share Posted May 19, 2020 Loving this topic, but how heavy is the model going to be, up to 45kg!!!!??? A Halifax is similar in size to a Lancaster (I obviously could google!!) so 1/9th scale is about 11.5 feet or so isn't it? This model will be nothing like that weight unless something has gone badly wrong. Is there a thread on the build of the model anywhere. It shouldn't need to come out a right lot over 20kg at worst. Here's hoping there's some typo's here, kg instead of lb for example? Lovely to see and keep up the splendid work! Ian. Quote Link to comment Share on other sites More sharing options...
kenking-King Design Posted May 20, 2020 Author Share Posted May 20, 2020 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 Quote Link to comment Share on other sites More sharing options...
kenking-King Design Posted June 13, 2020 Author Share Posted June 13, 2020 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. Quote Link to comment Share on other sites More sharing options...
alan p Posted June 13, 2020 Share Posted June 13, 2020 Must admit I was wondering about the hiatus,(being the times we exist in at present). Must have been a ooh bother moment or similar quote!!! Glad to see it reconciled. One of the problems with distanced design/production. Keep safe Alan p Quote Link to comment Share on other sites More sharing options...
kenking-King Design Posted June 19, 2020 Author Share Posted June 19, 2020 cont'd. A change of plan, quite literally, and for the better. Following the previous exchange with Ben I re-examined the photo evidence on which my efforts to date were based, because I couldn't understand why things were not a better fit within the nacelle. I'll cut a VERY long story short by saying that the plan depiction of the U/C door was a little inaccurate and hence adversely affecting positioning of the casting pivot. A lot more eyeballing, measuring and scaling led to significant revisions and a clearerer understanding on my part of how things should be arranged. Suffice it to say that a revised scheme now has the existing casting and leg sitting where it should, and retraction linkage is free to operate without conflict anywhere - no changes to U/C parts, wing tube position - nothing ! Relief doesn't begin to describe my feeling, nor that of Ben I suspect, although he maintained his cool throughout. Best of all, there is now remarkably good agreement with scaled photo evidence. You may just be able to pickout the labelled points where the casting emerges from the lower edge of the door, and the extent of the exposed tyre when retracted. On that happy note, making the linkage can now proceed with confidence, and I'll simply HAVE to make a dummy nacelle well so that I can play uppy-doony ( as well as experiment with actuators). Ken Quote Link to comment Share on other sites More sharing options...
alan p Posted June 19, 2020 Share Posted June 19, 2020 Makes my suck it and see/ seat of the pants approach, using bits of card and wire look very naïve More strength to your arm Alan p Quote Link to comment Share on other sites More sharing options...
kenking-King Design Posted July 9, 2020 Author Share Posted July 9, 2020 cont'd. Hello again Dear Readers. Did you think I'd deserted you? Never, though I admit to having been absent for a while, due mainly to this and that, but nothing earth-shaking, and now here we are, off adventuring once again. Martyn K asked if I'd seen any pneumatic damping, and I have now. The leg/wheel assembly moves under gravity like one of those slow drawer closers, which is gratifying as long as I can preserve that freedom and fit. And alan p, note that a few pragraphs ago I was using a paper cutout and pin, the more elegant drawing followed on from that vital first exercise, so we share a common approach. So, what's been happening ? Well, for a start, I've ordered the springs for a target weight of 25 kg. Each leg will contain two springs mounted concentrically. The inner is 8mm OD, 51mm long (the full housing length) whilst the outer is 11mm OD but only 38mm long. Both have about the same solid length i.e. when fully compressed, at which point the inner exerts a force of 3.2 kg, and the outer a heavier 9kg. In practice, from full extension only the inner is acting upon the leg for the first 13mm of travel, with force climbing from 0 to 1.2 kg, so very gentle. At that point the outer spring also comes into play, and over the next 7 mm travel to the 'Aircraft at Rest' position the combined spring force climbs to 5.5 kg, which nicely supports a 25 kg aircraft. (Remember there are four legs, each of which supports 22% of the weight. The remaining 12% rests on the tailwheel). The aircraft would have to experience a 2.2 'g' vertical deceleration in order to exercise the further 11 mm compression available, when the leg force will have climbed steeply to 12 kg. One fervently hopes this situation will not arise, and indeed it should not, because interposed twixt U/C and terra firma will be a 7" diameter pneumatic tyre and its shock absorbency, plus the gentle deceleration occasioned by the early stage leg compression, should have slowed things down a bit. I've some aluminium in the lathe as I write, being turned into the first of four upper spring caps, and to retain them on the central spindle I've received some dinky little 'C' clips to click into a groove. The'll carry almost no load so can afford to be dinky. The other thing I've done is cut the first 3mm thick actuating arm, measuring 237mm centre to centre, from some large extruded aluminium angle which has lingered in my metal rack for yonks. I like extruded aluminium sections because the material seems to be inherently stiffer than cut sheet, and I'm sure there is a good reason for that, buried within the extrusion process conditions. Anyway, these arms are solidly linked in pairs, hingeing at the rear of the nacelles, and at the fore ends connecting via 60mm links to the lugs on the rear of the castings. As the arms swing upward the links pull the castings back and up into the nacelle, as per the drawing of June 19th. Ben and I share a desire to have the motion imparted via worm and wheel from electric motors. Still quite a lot of work required in that department, and a reproduction of Ben's nacelle construction will be needed to house a sample U/C assembly and allow measurements of torque required, followed by games with gears, motors and microswitches. I look forward to that, but it's some way off. That has brought you up to date; not very exciting perhaps, and no pictures to look at, but there will be more soon. Thanks for revisiting, Regards, Ken Quote Link to comment Share on other sites More sharing options...
alan p Posted July 10, 2020 Share Posted July 10, 2020 Hi Ken Having read the above the calculator may be needing some new batteries, as the difference in OD is only 3mm assuming the wire dia is about 1mm to allow clearance to prevent binding, if the same expertise that has gone into the under carriage goes into the airframe it will be an impressive replica. Any info/pictures of the airframe. Regards Alanp Quote Link to comment Share on other sites More sharing options...
kenking-King Design Posted July 10, 2020 Author Share Posted July 10, 2020 Alan, you are right, there is little clearance between the two springs, 0.2mm in fact, but I'm hoping that is sufficient to ensure snag-free operatiion and I'll be looking at that very carefully as soon as the springs arrive. I'll let you know ! Regards, Ken Quote Link to comment Share on other sites More sharing options...
kenking-King Design Posted July 11, 2020 Author Share Posted July 11, 2020 cont'd. Here are four LA spring caps plus a spare, lightly counterbored to locate the top end of the 8mm OD inner spring, and drilled for a 2.3mm stainless spindle. In the foreground is one of the 'E' clip retainers. I've yet to try turning a .028" wide groove in the spindle material to take a clip. The plan is to also counterbore the other side, to the OD of the clip, so that once engaged and bedded in there it can't get off. Also today experimented with making the short links from LA tube, as simply as possible. The end was flattened n two stages, the first to expand the bore width sufficiently to insert a brass blade, then the second to flatten onto the blade, thus achieving the desired end thickness to fit within the casting lugs. After removing the blade,the end was rounded and drilled, and a 3mm hardened steel dowel fitted through. The open slot accepted a drop of Loctite, and after a decent interval the grip was tested - it was very strong - perfect. In practice the link will be fitted in the lugs, then the dowel will be fed through and bonded. All being well, dowel will rotate freely in lug bushes, and link will swing happily. A similar arrangemnet at the other end will connect link to operating arm. It was nice to be back in the shed after quite a break, more soon, Ken Quote Link to comment Share on other sites More sharing options...
kenking-King Design Posted July 13, 2020 Author Share Posted July 13, 2020 cont'd. Only got a little work donr today, so just a short update. Found and reground a tiny lathe tool to suit the .025" thick 'E' clips, then established the required depth of cut to get a good snapon, and proceeded to groove four spindles. When I have to lick my finger in order to pick up a component it signals that I'm operating at the lower limit of feasiblity ........ .... and quite often outside the accepted bounds of good hygiene, but eventually managed to get all four on. I then turned recesses in the caps to contain each clip and its short spindle protrusion, giving a flush fit .... ...... and was very pleased with how solidly they retain the caps, despite their diminutive size. The overlong spindles now await me figuring out what is needed at the other ends for damper pistons etc. Regards, Ken Quote Link to comment Share on other sites More sharing options...
alan p Posted July 13, 2020 Share Posted July 13, 2020 Unless you have an unusually large digit we can now see the scale you are working at, definitely not for the visually impared.!! Quote Link to comment Share on other sites More sharing options...
kenking-King Design Posted July 13, 2020 Author Share Posted July 13, 2020 Hi Alan, digits normal but I had to go down one size of clip in order to get strong retention on the 3/32" spindle, hence groove is .025" rather than .028" quoted previously, and yes, a bench magnifier played an important role. Ken Quote Link to comment Share on other sites More sharing options...
kenking-King Design Posted July 18, 2020 Author Share Posted July 18, 2020 A celebratory lunch today (wedding anniversary) required an extended siesta afterwards, restricting shed time. There was sufficient, however, to conclude manufacture of the four retraction links using the method established a few days ago i.e. aluminium tube flattened and drilled at both ends. Four pieces were cut and faced to 68mm long and were marked up. The first ends, in turn, were entered into the machine vice on the mill, as far as the scribed line. Processed as described earlier, but with a second stage squeeze using a second, thinner blade formed a nice flat end, avoiding the usual dumbell section of a squashed tube ...... As it was important for the second end to occupy the same plane (no pun intended) a small toolmakers vise was propped on parallels to hold the first end vertical whilst the othe end was similarly dealt with ...... The two brass blades can be seen here, as can a rusty block of steel used to give a horizontal sighting line - simple things work best, don't they ? The pressed blanks were drilled in another simple setup, unfortunately appearing on its side, sorry, forgot that would result from rotating the camera ........ To get the flat truly horizontal I balanced that brass tube on it and visually aligned the tube with the machine column - simplicity again you see. Both ends were drilled and reamed, pitched at 60mm centres via machine leadscrew, then all that remained was to radius them. For this a filing guide was used, comprising two short pieces of rod, 8mm diameter, centrally drilled, and bolted either side of each flat. Filing down to the rod was done in the hand, and finished off with a rub of wet and dry paper ..... I think they'll look alright between the arms and castings, and should be stiff enought to withstand compression loads from the bumpiest of landings. More later, Ken Quote Link to comment Share on other sites More sharing options...
alan p Posted July 18, 2020 Share Posted July 18, 2020 Hi Ken Congratulations on anniversary, celebrated our 50th in April with a raucous party of 2!!. You servived the siesta then Can see the alignment rod in the background to ensure both holes are in the same plane. Edited By alan p on 18/07/2020 09:16:53 Quote Link to comment Share on other sites More sharing options...
kenking-King Design Posted July 19, 2020 Author Share Posted July 19, 2020 Hello Alan, thanks for congrats. In the background is actually the 3mm reamer in the drill chuck. Brass tube is in the foreground, laid on the vise jaw, Ken Quote Link to comment Share on other sites More sharing options...
kenking-King Design Posted August 13, 2020 Author Share Posted August 13, 2020 cont'd. An even longer absence, for which I apologise. Let me start by tying up a couple of loose ends, or rather, by restating the situation. Firstly springs. I had selected two concentric springs, and Alanp quite reasonably worried about the mere 0.2mm clearance between them. In the event there was no problem - with the longer inner spring compressed the outer was still free. We have moved on, however, and I have since found I can satisfy all requirements with a single 64mm long spring, saving some weight and better still, gaining the ability to adjust the setup to match final aircraft weight. This is achieved by adding to the precompression in 1mm stages via washers at the end where it presses against the damper cylinder, 1mm equating to around 1/2 kg in aircraft weight. Speaking of dampers- therein lies the cause of my prolonged absence, or some of it. I had previously decided that the unintended generation of vacuum voids in a partly filled cylinder rendered my use of a piston valve pointless - the partial filling being due to the need to cater for introduced piston rod volume. After much thought mostly at subconscious level, the conclusion was thrust upon me that a through-rod cylinder was the way to go i.e. a cylinder in which the rod emerges at both ends. One advantage is that the internal volume remains constant, and so the cylinder can be completely filled with oil, thus avoiding any vacuum void generation, and giving fully controlled movement at all times. A second is that this then allows piston valves to act as desired, and so I have reinstated the idea to give easier compression but slower extension of the U/C legs. I hope thereby to reduce the chances of a bouncy landing whilst still damping and softening blows from terra firma. That's my idea, but if you think otherwise please say so - I'm flying by the seat of my pants on this, and mey be barking up the wrong tree. Discuss. Having wasted a lot of time and effort in trying to make dismountable Oring housings, for easy seal changes, I realised the issue was becoming far too complicated, so opted for a simple soldered housing with the hope that leakage will be negligible. Cylinder endcaps are shown in early stage of manufacture. Two different O.D.'s serve to centralise the cylinder as it spans the change from upper leg to lower. One cap has its Oring fitted whilst another is pressed into the end of the 3/8" tube for cylinder bodies ...... The Oring seen more clearly here, is 2.29mm ID and 1.1mm section, Viton rubber. There's a lot of spare brass still to be removed btw, so caps will not be so chunky when finished ..... Two-part piston valves are also being made, comprising aluminium piston and valve ring. Pistons have just a couple of thous. clearance in the bore, whilst the valve ring has a few more. Oil must flow either around the piston for slow extension, or via a ring of feed holes (not yet drilled) to flow around the ring for easier compression. In the photo below one pair is in the closed position whilst the pair nearest the camera is spaced apart. The difference in diameters of piston and ring are barely detectable. but have a marked influence on flowrate (see earlier trials on 19/05) Hopefully manufacture and assembly of all the above will be completed in a few days for your perusal, and attention will return to retraction linkages. Until then, Ken . Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted August 13, 2020 Share Posted August 13, 2020 You invite comment Ken, where we look on and daydream, you have to resolve the problems within this complex project by pouring hours of work into it. Do we 're-invent the wheel, or do we look to sealed dampers on door exits, cars etc etc. The usual damper solution is serviceable by dismantling of sealed unit, like the one on your car tailgate. If the tailgate damper leaks, we throw it away and replace it. McPherson struts have a replaceable inner, so the strut and spring carrier do their work and at end of life, the centre piston and oil contraption is removed and replaced I believe that to construct a damper, then one end of the tube is sealed, oil is added, then a piston regulator, valve? Is added, and a serviceable end screwed in. We then toil with the idea of speed of flow and damping resistance and we just don't know, but an experienced modeller once told me to make it all just as stiff as you can get it to operate, because the forces change with ever single landing. Edited By Denis Watkins on 13/08/2020 07:22:43 Quote Link to comment Share on other sites More sharing options...
alan p Posted August 13, 2020 Share Posted August 13, 2020 Hi Ken We tend to look for solutions that at the time of conception are logical, when we eventually settle on the KISS solution . You are unfortunately working on a abstract target(unknown weight of the finished model)Nice to see you are still surviving. Keep Safe Alanp Quote Link to comment Share on other sites More sharing options...
kenking-King Design Posted August 14, 2020 Author Share Posted August 14, 2020 cont'd. The endcaps have lost some weight as promised, and the components required to complete the Oring housings therein have also been produced - just simple washers .030" thick. They are soldered into the larger recess, creating pockets behind them for the Orings to live in. The photo shows on the left an endcap for processing, and below it a washer and solder ring, the latter formed by winding turns of fine solder around a mandrel and slicing through. On the right is a soldered item. A weighted probe held the washer firmly down in its recess during heating and subsequent cooling to maintain spacing, as otherwise such small items tend to bounce around and settle in arbitrary positions of their own choosing ...... Here are the eight finished endcaps, although the ugly one (that's why it is at the back of the group) may need to be reworked .... My next trick will hopefully be the insertion of one of those Orings into each housing, with the aid of some lubrication and dexterous manipulation of cocktail sticks. Holes in the washers are larger than spindle diameter, covering only the outer 75% of the Oring, thereby giving me a little more room for manoeuvre. In the next posting I'll let you know how I got on, Ken Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted August 15, 2020 Share Posted August 15, 2020 That looks like " fire stain " Ken, which you may already know. If the surface needs to be seen Then just polish the discoloration out. You look to be near assembly Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.