kenking-King Design

cont'd. Another week of lockdown, and some more done, though not as much as I'd hoped. Still waiting for the reamer which I need before I can assemble the liner tubes into the 'castings'; ten days in transit, 2nd class post from down South, and still no sign, most frustrating, In the meantime the castings just hang there in their grey primer, waiting, waiting .. Transferring my attentions to the legs, I bonded upper and lower segments using a lathe setup to ensure alignment, then gave thought to axle location. You may recall the planned arrangement of a split foot for axle clamping between a brass upper plate and LA lower, with half diameter troughs - actually .005" less in each, giving .010" clamp gap. I want the axles to play an active part in maintaining rigid leg parallelism and uniform movement, in other words to hold the leg spacing at the design centre distance at all times in a foolproof manner. A groove machined into 6mm (simulated axle) rod was supposed to locate on a ridge left in the half-bore of the brass part, but achieving a satisfactory ridge proved too difficult, so the half-bores went straight through, using a slot drill. Ridges was added later by parting off rings from a brass tube, and soldering them into the bores at the outer end. The rings are .070" wide for no reason other than to match the groove width from my parting tool. Holding them in location whilst soldering required lateral thinking in the jig department, but a solution was found - two down, two to go ..... Two rings are visible on the left, adjacent to an axle groove. The first one done was fiddly and did not behave itself at all well, the reworking reintroduced heat stain to the lower leg, so that's got to be cleaned up, again, and water spray urgently needed further along to prevent the bonded joint from degrading. Steep learning curve, and the second was much simpler. It was pleasing to assemble the first clamp and feel the rigidity achieved. That little brass collar will carry no real load, it just positively locates each axle end in legs at the correct centres, until the four clamp bolts are tightened ... Perhaps my reamer will arrive tomorrow?? If not, its hydraulic damper time, and I've had an idea for a very simple valve to change damping rate according to direction, much better than my original vague notion of spring loaded ball valves from soap bottles. The aim is to have but slight hydraulic resistance as the U/C compresses on a hard landing, but significantly more resistance to the U/C expanding again, making it slower to recover full length and hopefully limiting or even eliminating bouncing. Of course, I have no control over how the tyre reacts, but springs I hope to control with a firm hand. Anyone care to comment on this approach ? I'd love to hear of your experiences with damping mechanisms, Ken

cont'd. Thanks Ben. A little more done over the past couple of days with the legs themselves. Each leg comprises two steel tubes, a larger one internal to the casting, and a smaller external one ....... By reaming the large bore and slightly reducing the smaller O.D. they can be persuaded to nest together, and they will be bonded with a loctite product .... . You may remember that earlier in the blog I made up a short section to see how a polished tube would look emerging from the leg, and those pieces were bonded. I've tried knocking them apart and can't, so I'm confident in the bonding. At the end of the leg is a two-piece axle clamp, a brass upper plate, and a L.A. lower, with a 6mm transverse hole on the joint line to take the axle.The upper is brass for reasons of joining, firstly to the steel leg, and secondly to accept a torque arm from the brake drum. Here you can see the uppers, with shallow sockets for the legs, and in the background the L.A. lower plates, yet to be seperated. Finally remembered the philately term for this (but stamps, obviously) - se tenant, holding hands I guess. Gravity provides a most reliable clamping method. In the lathe the plates were turned, forming a collar whilst shedding some weight .... Those black tubes are the bits I said I would polish ! Me and my big mouth. On another front, the grey primer arrived. It was all of five minutes before the lid came off the can, and now spraying and rubbing down is in progress. Still waiting for the reamer though, so can't fit the lining tubes yet. More soon, Ken

Hello SR71 (fantastic looking model btw), I understand what you are saying, and frankly I share some of your concern, but I'm also mindful of the forces involved when a model weighing as much as 45 kgs smacks down at speed onto hard, bumpy terrain, and I'm intent on the undercarriage not being a weak link in the chain of force transmission into the aircraft structure. It may turn out that I've overemphasised the strength required and that too severe a weight penalty has been incurred, we shall see. Redesign is not a dirty word, and I'm always happy to receive informed opinion, and I respect the valued experience of others, so don't hold back. I agree that the last photo looks like something from a boilermakers' convention, but in reality those brass tubes are only .015" wall thickness, the plate is about 1mm and there's not a lot of it. When you see the next photos, of the steel tube legs, then you may justifiably have conniptions. When working out how to build something, such as these U/C units, one faces a series of compromises between material desired and what is actually available, further hampered by lack of time, facilities and funds to carry out destructive testing of alternatives, so conservative gut-feeling holds sway. Certainly, by the time the hollow legs have been filled with springs, and damping cylinders of oil, and the hubs carry braking equipment, then these WILL be heavy units, but on the other hand fragility will not be a concern, and I hope even severe descent rates into terrain might be survived by the model as a result. Thanks for your interest, and how about emailing me some info on that model (rather than in this blog)?

cont'd. Having finished the clevis brackets, thanking my lucky stars I'd remembered to make L.H and R.H. versions .... I soldered them on in what I hope is the scale position. I'm going to fit a tiny brass bush in each eye, flange facing inward, to prevent undue wear, and house a 3mm stainless dowel pivot pin for the retraction links. That was the end of hot processing, meaning I could give my attention to block balsa infill of the arches, and a 1.5mm sheet on the rear faces. Balsa was shaped to fit the spaces, but left oversize, and was epoxied in place, a task which took a lot longer than I had anticipated. When all was hardened, the bandsaw was used to cut and nibble off most of the excess, but wanting the arches to be smooth and true prompted me to make a sanding drum of sorts, actually a bit of old fencepost ....... but it did the job, whilst adding more layers of wood debris to the lathe .... Please overlook the yawning gaps between blocks if you will.You should have seen the amount of rubbish epoxied to my fingers after the exercise of trying to slide each into place without dislodging the previous ones. After sanding, the arches were lined with - well - lining paper, what else ? and would have looked quite tidy but for this new adhesive also getting all over my fingers, then transferring the grime to the paper with faultless adhesion. I really CAN'T wait for the spray primer to arrive. A new day dawned, and a start was made on the spring/damper cartridges, the first elements of which are the soldered assemblies of 5/8" brass tubes, bottom plates and studs. I used lead-free silver bearing solder for this as it's stronger and harder than tin/lead. For neatness I wanted the studs to protrude only enough to carry a nut, and show just the tip of the stud, and of course the bottom plates needed to sit square to the tubes, so a soldering jig was made from scrap aluminium bar and rod. It held the tube and plate square, whilst a turned recess allowed bolts to be threaded through the correct amount ... After soldering the bolts were cut off behind the plate. They are steel 6BA, and the only load they carry is that of the remaining spring force when legs reach full extension i.e. aircraft aloft, or on a supporting stand. Nuts are 7BA hexagon size. The four finished tube assemblies will be permanently bonded into the 'castings' once I've received a 5/8" reamer to dress inside the square legs. These brass tubes will push in at the moment, but are quite tight, so I reason there must be slight deformation of their thin walls and that could affect the fit of the next tubes in the sequence, removable elements which should be an easy in/out when required. The reamer will just scrape off a thou or two in strategic places. It's on its way, same as the spray primer ..... Ken