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1/9th Halifax U/C

Designing & making main U/C with springs, dampers and brakes

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kenking-King Design09/07/2020 23:48:34
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294 forum posts
843 photos

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

alan p10/07/2020 09:26:48
272 forum posts
7 photos

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

kenking-King Design10/07/2020 12:25:21
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294 forum posts
843 photos

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

kenking-King Design11/07/2020 02:08:10
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294 forum posts
843 photos

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.

spring capsdsc04256.jpg

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.

Link end test piecedsc04252.jpg

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

kenking-King Design13/07/2020 01:32:20
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294 forum posts
843 photos

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 ........

dsc04257.jpg

.... 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 ....

dsc04258.jpg

...... 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

alan p13/07/2020 09:10:21
272 forum posts
7 photos

Unless you have an unusually large digit we can now see the scale you are working at, definitely not for the visually impared.!!

kenking-King Design13/07/2020 13:31:59
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294 forum posts
843 photos

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

kenking-King Design18/07/2020 01:29:03
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294 forum posts
843 photos

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 ......

link sectiondsc04260.jpg

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 ......

links second enddsc04259.jpg

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 ........

link drillingdsc04262.jpg

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 .....

finished linksdsc04263.jpg

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

alan p18/07/2020 09:16:01
272 forum posts
7 photos

Hi Ken

Congratulations on anniversary, celebrated our 50th in April with a raucous party of 2!!. You servived the siesta thenwink

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

kenking-King Design19/07/2020 01:30:36
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294 forum posts
843 photos

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

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