¼ scale Hurricane tailwheel

[kenking-King Design]

I've been asked to make a sprung tailwheel unit for a ¼ scale Hurricane and this thread will show the various stages in manufacture. I sketched out some ideas and studied and measured photographs to come up with a reasonably accurate full-size drawing, which I then scaled down. My client provided a standard 3" wheel, (correct scale size) which came with glossy sidewalls and moulded tread, so that need changing for a start. Pleasingly, a few minutes experimenting in the lathe with various abrasives soon produced a typical well-worn, smooth tyre.

Most encouraging. Discs will be added later to hide the machined alloy hub.

The wheel linkage must sit correctly compressed on the ground , and as the model is not yet built some approximate calculations (guesses) have to be made with regard to spring rate, preload etc. More of that later.

I decided to use thin wall steel tube for the outer, fixed tube which resides almost completely within the fuselage.

[kenking-King Design]

Continued

Pivoting within that will be an inner alloy tube, and within THAT, a plunger and spring arrangement, hooked up to the swinging arm in as realistic looking way as possible. A real Hurricane has a short clevis link between the swing arm and the piston rod of the hydraulic damper,with pivots at each end of course. I decided it wasn't practical nor strictly necessary to reproduce the link action exactly, so I'm making the link to pivot only at the swing arm end whilst the other end will be solidly fixed to the spring plunger shaft. The slight arc that will be followed will be accommodated by making the spring-carrying piston (which is a slack fit anyway), with a part-spherical rim to allow such motion without jamming.

The outer tube has been turned to length and a a piece of phosphor bronze bonded into each end as a bearing bush, using Loctite496. I bored the bushes in situ, the adhesive holding them with complete security.

I may mill away some of the tube wall to reduce weight later.

[kenking-King Design]

Next came the castor arm (my title) milled from a block of light alloy. The first step was to drill a hole to take pivot bushes, then it went into the 4-jaw chuck on the lathe to have a top collar turned and to be bored for the inner tube.

After that I marked out and milled a small step, and the two sloping faces, as well as forming the radiused lower end.

The castor arm and inner tube were then bonded together. The tube had previously been skimmed and lightly relieved over the middle part. you can't see it here, but the arm has been significantly relieved on the underside. I also turned two PB bushes and the pivot spindle, which is titanium (all exotic materials here, you'll note), from a shaft out of a chemical process machine. the spindle is threaded internally M4, the idea being that it will be gripped solidly between the forward arms of the wheel fork by end screws, and so be unable to turn. At the end of the milling exercises all the arms and the wheel fork (as I now call it) will be hand carved to give the nice curves of the real thing.

[kenking-King Design]

Now to the wheel fork, again this is milled from a block of light alloy. Firstly, I drilled the principal pivot holes, then milled the top half of the side profile, followed by boring for the wheel aperture

After much rotating this way and that in the mill vise, and producing copious amounts of silvery swarf, I had a rather cubist looking fork.

[kenking-King Design]

continued

Here's the fork in its present state.

Ive made the wheel spindle in a similar way to the pivot spindle, i.e. titanium, internally threaded each end for clamping screws. The wheel will run on its own hub, with short spacer tubes fitted to centralise it in the fork. Further detailing will be added later, such as the rearward projecting lugs for a manoeuvring handle.

At this point I just couldn't resist putting bits together to get an idea of how the finished job will look.

and finally ...

That's as far as I've got at present. Next I'll be dealing with the internal spring arrangement, and a lower travel stop to ensure the correct angle of dangle in the air.

[Depron Daz]

I do like a well engineered piece of metal. It always fascinates me just what can be made.

Who was it that said that inside every block of stone there is a statue waiting to be found? Or words to that effect.... Same here, inside every block of metal........ Well done!

[Plummet]

Thanks for this thread. I have watched metalworking from afar. I have seen things going on, but so often I have only seen one stage of a complicated procedure. It is great to see the whole lot stage by stage.

Ta.

Plummet

[kenking-King Design]

Thank you Depron Daz and Plummet, I'll post more as it's done. With reference to what's inside a block, I loved the explanation given by a hugely talented old carver of exquisitely finished decoy waterfowl, pieces of art in their own right. When asked how he carved one he said " I take a block of wood and cut off anything that doesn't look like a duck".

Cheers,

Ken.

[Wiltshire Flyer]

Amazing engineering! I am a tad jealous if your skills and tools!

[kenking-King Design]

A bit more done this morning. Out came the Dremel and a cutter and next minute it was snowing flakes of alloy. After a while, and with some judicious hand filing in odd corners the cubist fork was transformed into representational art, and much more like the real thing.

Then I turned up a couple more small PB bushes, this time for the pivot at the lower end of the dummy clevis link to the spring pushrod. Trial fitting them here, they will be bonded in place later with a drop of Loctite. The spindle here will be ⅛" silver steel, and I intend to fix it into the clevis by having a small drilled hole into which a drop of Loctite will be introduced in the hope that it locks the spindle and nothing else. I may grease the bush faces before this final assembly to prevent stray Loctite doing the unthinkable.

Now I'm waiting for information on the CG position so I can calculate the likely parked load on the tailwheel and from that calculate the spring force required at the desired state of compression. Then I can source a spring of the desired characteristics. The spring housing will incorporate a means of adjustment through the use of shim washers so that variation from projected model weight can be catered for.

[Tony Bennett]

wow

a work of art sir.

i love watching skilled workmen in action.

[Bob Cotsford]

wot Tony said, real craftsmanship

[kenking-King Design]

Ohh, you guys, just stop it will you ? But thanks, a bit of encouragement is always welcome, especially from talented people such as yourselves. But now I'm asking myself why I elected to use silver steel for that last mentioned spindle, it doesn't make sense does it ? I'll use titanium again, it just means I have to turn it rather than cutting a piece off a stock rod, lazy beggar.

[Phil Cooke]

That really is an impressive piece of work kenking, fantastic.

How did you hold the piece whilst you were Dremeling and hand filing - was it handheld?? True craftsmanship to take it from the form in your previous picture to how it looks today.

[Martian]

Excellent craftsmanship pleasure to read and view

[kenking-King Design]

Well Phil, before I sculpted the wheel recess I was able to insert my spacer block and then clamp the fork in a small vice to work other areas, but after that, and for much of the final touches it was hand held. I expect to do a little more rubbing down here and there. The eagle-eyed among you will have spotted that two triangular fillets have been added to the lugs carrying the pushrod pivot. I made an oops on the mill and picked a wrong dimension off my sketch, taking away ⅛" I shouldn't. Feet of clay after all, you see.

Next came the link to the pushrod, which is only pivoted at the bottom end, on said lugs, and yes, I have made the spindle from titanium. The alloy tube pushrod will be bonded into an axial hole at one end. Here's the oversize block, having been drilled and reamed for pushrod and pivot hole. A piece of brass tube and fine grinding paste are creating the impression of an upper pivot pin. Here's the result, awaiting final sizing.Here we are after cutting off the bits that didn't look like a link, and adding the pushrod which has been turned down and polished for a short length to suggest a hydraulic cylinder rod. I've roughly outlined a rod-end, and if I can think of a way to dig out material I'll try to improve this feature.

[Martian]

excellent stuff you seem to be just over 7 years in the past

[Josip Vrandecic -Mes]

Sir .....all my respect...

Joe

[kenking-King Design]

To Martian, well spotted but the camera date is correct. You see, I have so much to do each day it won't fit into 24 hours, so I'm running two parallel existences and popping back and forth in my time machine (plans to be made available, at some time in the future, naturally), however, I'll reset the camera to my present day existence, as it bothers you.

[Martian]

ah the old parallel existence eh some use it to run 2 wives others to work and holiday at same time some to watch football and do household chores but your use is much more fruitful but yes I would appreciate the plans my time machine is constructed form some old H G Wells plans and we know what happened there

[kenking-King Design]

cont'd.

At this point I had to sit down and do some SUMS, real, hard arithmetic, concerning the choice of spring, before I could do more engineering. As the finished model weight is unknown I had to build-in some means of allowance for variation from the target of 25Kgs. so that we could still achieve the correct sit-down position of the linkage when the aircraft is parked. Briefly, a parked Hurricane side-view had the expected C.G. added, them measurements were taken along the ground to give moments of C.G. and tailwheel about the main-wheel centre. For the target 25 Kgs this gave a tailwheel load of a little under 2Kgs, exactly 4 lbs. (that's better, good old Imperial units), then the castoring linkage had to be taken into account to give the force required at the main shaft, which is where the spring will be. This more than doubled the force to about 8 ½ lbs. I also had to consider the full stroke required, as it would be nice to have a little spring load remaining at full extension. I was surprised by just how low a Hurricane tailwheel dangles in flight, something I'd never noticed before. It will take about 1.3g to achieve full travel and compression. With spring O.D. limited to about 7/16" I found Morris Springs had just the job. Their catalogue lists all the essential data for each spring, very handy. I've designed in some packing washers which can be removed if the model is lighter, or more added if heavier, and so can cope with a range from 20 - 30 Kgs., each washer of .014" catering for 1Kg.

Now I had dimensions for the spring so I turned a dummy one from L.A. , and used it to help fix the length of the pushrod, which in turn fixed the position of down-stops, needed to hold the correct angle of dangle. These plates intrude into the bore, each covering a quarter of the circumference. Once bonded in they were cleaned up and internal profiles adjusted to allow passage of the spring during assembly. The dummy spring can be seen alongside the pushrod in the background of the next photo.Hard to photograph, but you can just make out the intrusions.To allow the piston to also pass during assembly it was modified by removal of corresponding portions. Once in, a 90 degree turn prevents escape. The conical nose automatically centres the spring, a feature repeated on the upper spring cap.Here is the dummy spring located between the two.The spring-cap which is threaded into the inner tube, acts as the retainer at the top bearing. Its central threaded hole is for steering arm retention (see later pic.). Alongside is the pushrod with dummy link, the cross-pin now retained by a grubscrew which doesn't bear directly on the pin, as that would chew the surface and score a bush upon withdrawal. Instead the pin now has a slight waist (unlike me), through which the screw passes, in a blind threaded hole, so it can be locked firmly home. The waist is visible below. The steering arm will be retained by one central bolt. To positively transmit torque it will be housed in a shallow recess, yet to be milled across the spring cap flange (that's why it is so thick). Oh, and please note, from hereon the dateline has been correctly set. You know who I'm talking to.

Next are some photos of the bits put together. Linkage is in the dangle position, and you may note that some further sculpting has been done, this time on the castor arm. Finally, I revisited the wheel with a view to making centralising spacers. I measured the hub width, subtracted from the gap, divided by two and made spacers accordingly. It was then I discovered the wheel was 1/32" off centre, making 1/16" difference between sides in the fork., and the tyre was wobbling sufficiently to rub. Back to the lathe to true the wheel, improve the profile, and make new the spacers as required. I fitted the now centralised wheel, and look forward to adding discs to mask the modern hub.

[kenking-King Design]

This photo should have been attached at the end of my previous text but I must not have pressed the right button.

[kenking-King Design]

O.K. nitpicker, before you point it out, I know two more past-dated photos slipped through the net.

[Phil Cooke]

That KK, is a quality piece of engineering - fantastic work!!

If the rest of the airframe is built to anything approaching this standard its going to be a superb model for sure!!!

[Martian]

Who me never..fantastic work to witness Ken