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Post Info TOPIC: Medium D plans progress


Legend

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RE: Medium D plans progress
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Not sure if it will make a noticeable difference, but after checking measurements (again!) for turret alterations, I have started a third set of plans - but with a slightly different method.

The first two plans were started by drawing a bounding box 254mm long and 78mm high, then marking the idler and sprocket axle positions, adding the idler and sprocket, then drawing the track return to act as a reference for vertical measurements when marking out the profile of the side armour. This method produced a good result, as shown by the paper model - but with a bit of guesswork over the axle heights, I actually ended up with the proportions of the mud-chutes and the side plating slightly out of kilter. It was also difficult to try to match up the vertical panel lines without needing to make the tank longer...

So this time I have changed the reference point, choosing instead the bottom edge of the mud chute,where it meets the side plating. I drew this before adding the axle positions, ignoring the upper and lower height limits. The measurement problems I had on the last plan have been sorted, and the height that the axle lines end up at I have yet to discover; once the lower edge of the side plating is marked out, the ground line can be determined - which will allow the height of the tank to be marked in.

Hopefully this will produce the best accuracy I can manage; the second plans, from which the paper test model was made, were not far off - they just need some tweaks here and there to improve them. After that, the question which remains is whether or not the front/rear rake looks right, or whether it needs to be changed. The first plans had the same rake as the Medium D mockup - about 4.6 degrees; the second plans, thus the paper model, had a lower rake of about 3.8 degrees because I had wondered if the mockup was slightly steeper than the prototypes, and 3.8 degrees is roughly the right angle for a 30ft-long vehicle designed to cross a 4-ft high obstacle at one end and 6-ft at the other (which is what the design requirements called for, apparently).

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Legend

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I think I am now happy with the track profile: I have used lines of perspective to try to work out the height of the rear of the tracks and conclude that the front-rear rake is about the same as on the mockup - as long as the suspension isn't sagging at the back! This means a rake for the mud-chutes and track return of about 4.6 degrees (or a little more).

The turret is now starting to make some sense - and there might even be adequate space for the cupola! So I have started to add some details to the side view, and will need to check the last plan view to see if it can be amended easily; likewise the front view.

Once those three views are sufficiently far advanced, I intend to make another small test model - preferably in card, as paper is flimsy for a long, narrow vehicle.

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Legend

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Thanks Wayne! The plans won't be ready for CAD/3D modelling for a while - the three views i have been working on need to be finished, and there is still the back view to do, plus the inside of the track frame.

There are two or three more pics I want to add, but I shall try adding these later.



-- Edited by TinCanTadpole on Thursday 21st of February 2013 06:25:19 PM

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Legend

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Okay, I have made another basic test model - in card this time, as paper is flimsy for such long and narrow track frames. Not sure it has been entirely successful, as somewhere the model has turned out slightly taller than it supposed to be (easy at 1:72). You may not see much difference in proportions, but here are some photos anyway.



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Legend

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Front quarter view.



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Legend

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Slightly lower angle - hopefully similar to the well-known pic of a 'D' driving along a town street.



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Legend

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Rear quarter.



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Legend

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Another quarter view, showing increased turret size.



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Colonel

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It looks really good to me. Let me know when you need help with CAD/3D modeling.



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Legend

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Back view of first and second test models: increased height of second model can be seen - it should be lower than this.

Tracks are also taller at the back - this time intentional.



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Hero

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I keep meaning to post these photos from 'An illustrated record of the development of the British armoured fighting vehicle tanks 1915-18'.

Blimey, that title's so long I forgot what I was doing. :)

Oh yeah, photos of Medium D's... wasn't sure if you had seen them... and it makes a good excuse to see how you're doing with your plans.

Helen x

Oh poop! photos didn't attach..... grrrrr....... try again!



-- Edited by MK1 Nut on Thursday 11th of April 2013 11:17:14 PM

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Legend

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Helen: the top photo in your post is the Medium D* - an amphibious variant widened about a foot to 8' 5", with the turret moved forwards closer to the idlers. This photo is one of the few commonly found online, but the other one, which I understand is the even wider (9 feet), plus longer (31' 10" rather than 30ft) Medium D** (two star), is one I only saw recently for the first time. I've only seen it as a small pic in an image search, so thanks for posting a larger version! Both of these tanks were built by the Wolseley division of Vickers; the D* was considered lacking in stability, but the D** was okay and seems to be the basis for the final D Modified, which had a new turret with an additional cupola. Wolseley-built tanks can be identified (regardless of whether D, D*, or D**) by the pent-roof top of the turret; the Fowler-built Mk Ds had a curved roof, the D Modified (built by ROF Woolwich) had a flat roof, whilst the mockup appears to have had a three-section roof like that used later on the Light Infantry Tank - a flat centre panel with sloped ones to either side.

Charlie: that's interesting about the notes the Americans made. I am aware of the M1921 and M1922 from the Tanks website, also that an M1922 survives - although somewhat shabby from years outside at APG. Not sure about only one being built: I've read somewhere that there were as many as half a dozen prototypes, and certainly there is photographic evidence of slight differences between at least two examples. Either that, or the sole example did not originally have the T-section straps down some of the plate seams on the side (amidships, below the mud chute). Also, they may have experimented with different armaments, as some sources state it was a 57mm, whilst photos sometimes show a skinny barrel (the 37mm) and others a fatter one (a 57mm/6pr, presumably). In any case it's an interesting machine and deserves to be included in any consideration of the D - it is a cousin!

As for the plans? I have not drawn any more since my last post, but I have mulled over the shape of the side profile, which I now think looked little, if any, different from the mockup. Now that you have both shown fresh interest in the plans, I must get back to work on it

PS - had a quick at John Glanfield's "The Devil's Chariots" in a bookstore recently: according to him, Medium C production was cancelled so that the Medium D could proceed, despite reservations from some about troops being unable to keep up with a faster tank. Some sources say that plans were initially made for 75 tanks, with this number being reduced and reduced again, but Glanfield states that the original desire was to order 500(!) before lack of money caused the number to be dropped to 75, then 45, then 20. His account says that the D* and D** were ordered for comparative tests of stability, suggesting that they were planned rather than a reaction to insufficient stability in previous attempts. Interestingly, he implies that the original "D" was intended to be amphibious (some sources say it was not), and says that the first example (built by Fowler's) was sent for amphibious trials before it was destroyed by a fire.

On a related subject, mentioned a few weeks ago in a different thread, the Mk V (serial 9425) that served as test-bed for the snake-track and wire-rope suspension did have a speed of about 10-12mph according to Glanfield, as someone said at the time, although it was apparently capable of going twice as fast downhill! Whilst on the subject of Johnson's experiments, Glanfield quotes 180hp and around 20mph for the sprung, Rolls-Royce-engined Whippet, rather than the 30mph and 360hp figures usually encountered; he also quotes the same horsepower for the Rolls-Royce powered prototype for the MkVIII. No idea if these figures are correct, but one of the foreign Wikipedia entries for either Medium D or Whippet (Estonian, perhaps?) gave the 20mph figure when I looked at it a few months ago, and 360hp does sound rather high when the latest production model of R-R Eagle (the VIII) had about 270hp at best (if Wikipedia has correct figures!).



-- Edited by TinCanTadpole on Friday 12th of April 2013 04:05:43 AM

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Legend

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There is another line of development of the Medium D - the U.S. M1921 and M1922 medium tanks. The Americans received Medium D plans

and built their own version. The M1922 had the snake track. The main difference between the U.S and British tanks was a turret with a 37mm

gun on the American tank. Financial constraints on the U.S. Army meant that only one example of each type was built as experimental vehicles.

The Ordnance Dept testing had endless problems with an underperforming engine and noted that although the suspension worked well the suspension cable

wore out very quickly.

Regards,

Charlie



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Legend

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All R-R Eagle engines are not the same - they varied in power output from 225 hp (Eagle I) to 360 hp (Eagle IX).

The 360 hp engine wasn't produced until 1922 so it can be ruled out as the Whippet power plant. Most likely it was a 300 hp

Eagle VIII - 3300 odd were produced 1917-22. The earlier, lower powered, marks were built in fairly small numbers (hundreds).

Regards,

Charlie



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Legend

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Yep, I realise as much: according to Wikipedia both Eagle VII and Eagle VIII were in production at the time, but both were well below 360hp. 180hp sounds rather low, although I suppose it depends what fuel was available for the flyboys and what the army got: IIRC it says in "A New Excalibur" that the tanks ran on 45 octane fuel, so perhaps the Eagles had to be de-rated somewhat to avoid pre-ignition. It also depends whether a brand-new aero engine could be requisitioned for an experimental modification, or, as I suspect is more likely, it was an older/older-spec unit that the flying corps/air force could more easily spare.

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Hero

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I came across this pic of a Medium D recently, not sure if you already have it? Not sure what variant it is, seems to be without all its panels, which is interesting.



-- Edited by MK1 Nut on Tuesday 16th of July 2013 10:32:57 AM

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Hero

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It's a lovely work. Must be very satisfying to see our own drawings taking shape in tridimensional form!

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Legend

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Helen! Thanks!

Those pics are not the Medium D itself, but the Johnson Light Infantry Tank of about 1921. The first pic, or one very much like it, is on Wikipedia, but the second is new to me.
This tank was related to the Medium D and designed to fulfill a requirement for a light tank to operate with infantry, as far as I understand. It was much shorter than the 30ft-long Med. D, measuring about 21-22ft in length and with very little slope to the top track run. Figures vary from one source to another, but the weight was about 7.5 or 8 tons, speed around 30mph, with a 100hp Hall & Scott engine.

Like the Medium D it was amphibious, and had the wire-rope suspension plus flexible tracks. The term "snake track" might actually refer to the tracks of this machine rather than the "D", from what I've read. I think there was some change in the construction of the track, possibly using ball-and-socket joints to allow extra flexibility; the tracks were intended to curve a bit in fast turns to reduce drag - a bit like the track-warping of some later designs.

If you look at pic 2, below the front spring you'll see a cam: this was apparently added to the suspension to stop the waves formed in the wire rope from working their way towards the back of the tank, so that the wave-amplitude of the rope was even along the length of the vehicle. Apparently the Medium D, which lacked this feature, tended to end up with bigger waves in the rope at the back of the tank, and shallow waves in the forward part of the rope, causing the back of the tank to sink - you can see this in some photos. The cam acted as a tensioner, somehow, to pull against the ropeand stop it slipping backwards.

A point of interest is the flat centre panel on the turret roof; as far as I can tell, the Medium D mockup had a flat panel like this, but the prototypes instead had either a curved roof or a pent-roof with a chine up the middle.

One last point: the sides are not actually missing any panels, it was normal for those openings to be there. At a guess, I'd suggest it was to allow sideways movement of the bogies when the tracks flexed in turns.

Thanks again Helen for posting these pics; the drawings have been on hold for a while, but I did start transferring the manual drawing to Photoshop, which is adequate given the lack of really accurate dimensions.

The Tank Museum has a length of surviving suspension from this machine.

Diego - do you mean the pics in Helen's last post? Those are not drawings, they're photos.


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Hero

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Nope I was referring to the paper mock-ups made from the drawing tests!

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Legend

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Aha! Yes, I certainly enjoyed seeing the 3d model completed, because it showed that the drawings were on the right track. The models are only basic ones, with the details of the tracks, suspension and mud chutes ignored in order to focus on the profile of the track frames and shape of the turret.

As for the drawings, there are still things I can add to the side elevation, which I have two slightly different versions of - I'll pick whichever looks more convincing - but the rear view and plan view (which I have to start again, because of the changes in the side view) I can only draw part of at present.



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Hero

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More or less what happened to me when I started to draw that bleedin' Horse Ambulance on a Commercar chassis last year. Many changes on the side views led to changes in all the other views, until I got sort of mixed up on what were going on. I should start anew in 1/36 or 1/18 scale (which would be unwieldly given my drawing surface) but that's the best way to keep track of all the detail and dimension changes as one adds them.

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Legend

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Yes, it does get confusing doesn't it! I ended up with a few slightly different side views on different sheets of A4, occasionally wondering which went with another sheet on which the front or plan views were being drawn. Having transferred the work to Photoshop, it's easier in some ways because track rollers can be copied and pasted very easily, and it should be useful for the tracks when (note the optimism) I get that far.

Since you mention the Horse Ambulance, how's it doing - have you done any more recently?

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Hero

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I downloaded a software named "Inkscape" to make vector drawings which I was reccommended. So far I haven't touched the thing. I'm addict to pencil and paper... I have enough screen time with my work and my idle web browsing (and forum watching!)

About the Commercar, just last saturday I re-drew the basic outlines -to some fitting scale- and measurements of the RC chassis, choosing ONE of the several varieties. One thing tha got me thinking is that the Horse Ambulance version has the cast iron wheels and not the artillery wheels I have on my references. But I just assumed that the wheel type was interchangeable and there was no modification to the rest of the chassis when they were used.

D.



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Legend

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Long time since I gave an update on the drawings - I lost interest for a while, but Charlie's Medium D article for Landships II encouraged me to continue.

The direction has changed, because for practical reasons I have decided to draw the Mk D(M) first, which can be altered to step back to Mk D*, then finally to Mk D. I plan to draw the side views first for these three, then move to a different view and again draw one for each variant, until I have a set of four-view drawings for each tank.

The drawing is done using Photoshop Elements, which has disadvantages compared with vector drawing programs, but I'm familiar enough with it and greater accuracy is not really necessary when one can't get exact dimensions.

This first drawing is the progress so far on the left side elevation of the D(M); the drive chain case still needs a little work, plus there are more rivets to add, the cupolas, mud chute details, and of course the tracks. The colour-coding is just for convenience when drawing, it will just be black & white when finished.



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Legend

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And this is the Mk D* side view so far. The chain case needs re-done, plus mud chute details and the turret - what you see here are construction lines for the D(M) turret, showing how one drawing is altered to make another.



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Hero

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Hiya,

Good to see you've started again on your plans, I know how disheartening it can be at times when there is little information to work with. They're looking good and lets hope the Imperial War Museum will publish a bit more of there archives soon and encourage you to keep going.

Every time I go MK1 tank hunting online I have a look about for any Medium D info as well. There is a few titles for Medium D photos on the IWM site, but at the moment they are still blank.

Helen x



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Legend

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Thanks for that , Helen, I've looked there myself and found a blank for a picture described as the side view of the Medium D - hope they manage to upload more photos some time.

I noticed a few weeks ago on the Classic Military Vehicles magazine website that the next issue is due to have an article by David Fletcher comparing the tank designers Christie and Johnson; the issue comes out tomorrow (20th Dec), so I'm hoping to get a copy and hoping that there are some new (and good!) photos in it. I'm not getting my hopes up too high, because any article comparing two things means that there's not much page space available to go into detail or have many photos on a single subject.

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Hero

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Just thought I would bump this page up with a few photos from the Imperial War Museum site. I expect these photos have been seen before, but they're pretty clear so I thought I'd post them in case the Medium D page could benefit from them.

 



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Legend

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It's been months since I last did any drawing on the plans, and I've yet to progress beyond the left side, but the last version I started was one I was happier with and can continue with (at last!).

The rear of the tank has always been a problem area, as I've not seen a sharp or well-lit photo of that area, but seeing the rear of the mock up has prompted me to reconsider my thoughts; I now think there's a good chance that the actual prototypes stayed close to the simple vertical bulkhead of the mock up - even if that's wrong, it will do for now, because little about the Medium D is as certain as the main ww1 tanks. Even David Fletcher is a bit sketchy when he writes about it, with width stated in different articles to be 7ft 3in, and 7ft 5in. On that score, I've tried measuring the front view photo of the mock up, having read (Chamberlain & Ellis?) that track width was 21", but the results were ambiguous.

Anyway, 'Santa' brought a copy of the Osprey Whippet book, which covers all four medium tanks, so perhaps it's time to start drawing again - maybe leaving the side details for now and sketching out the plan view and ends.

One little question, if anyone knows: David Fletcher consistently states that the tracks were based around a steel cable core, but not having seen a close pic of any track joints, I'm not certain. What I have seen suggests separate track links forming a chain, possibly with conventional pinned joints; a steel cable would seem superfluous and add extra weight. Any thoughts?

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Hero

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Glad you've not given up, it does take for ever and producing plans is always it seems this balancing act between getting it wright and accepting they will only ever be part guess work.

As for the track, I don't think they were ever wire cored accept maybe in a mockup or principle test piece. I have come across some experimental track for a Light D Tank, which combines the centre pivot with wires running down both sides of the links. As far as I can see it is an attempt to stop the links twisting too much, maybe there was a wear issue with the links twisting all over the place like as seen in the photos.

It could of course just be a repeated mix up with the wire suspension that then in time becomes almost impossible to untangle from fact.

 



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Legend

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Thanks for your input, Helen, that supports my own thoughts on the matter.

I did some more measuring of the mock up photos, and concluded that if the tracks were 21" wide, as I read somewhere, then the proportions of the mock up happily support a vehicle width of 7' 5" over the tracks.

However, the same proportions would also work for a tank 7' 3" wide, with 20.5" tracks - just like the rhomboids. My conclusions on some other dimensions - track pitch, idler diameter, perhaps also sprocket diameter - are that they seem to be about the same sizes as the pitch, idler and sprocket on the rhomboid heavy tanks, suggesting that Johnson stuck to familiar dimensions where possible. If that is so, is it reasonable to think that he would have kept the 20.5" track width (despite the bizarreness of the half-inch) as well?

One other problem with the width is that it's not easy to tell whether the quoted width is measured over the tracks, or the chain cases, or the sprocket bearing caps (which are the widest part of the tank, but in a rather technical sense). The chain cases did not feature on the mock up, possibly suggesting that as design work progressed, it became necessary to find more space inside the narrow hull for the three epicyclic gearsets per side (maybe also for track-brake drums, as on the Mk V); so perhaps width over the tracks is most likely? Any thoughts are welcome.

Two related bits of info: the aviarmor website article on the Medium D quotes the length in millimetres with perfect accuracy, at 9144mm, but gives a width of 2145mm - which is just under 7' 0.5", and would require a track width of about 19.9".
The March 1971 issue of Military Modelling has an article on the Medium D that is interesting, but contains inaccuracies - notably the scale drawing of the tank, which gets the shape wrong, adds in a part that wasn't there, and mixes up some features from different models in the series; despite this, the fact that the front view shows a width of 7' 7", possibly over the bearing caps, or at least the chain cases, makes me wonder whether the variety of widths quoted might be because they refer to different parts of the vehicle? Just a possibility, because I don't think there is a large enough difference for both 7' 3" & 7' 5" to be correct, as the bearing caps and chain cases seem to stick out too far beyond the tracks.

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Hero

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Hi, Just thought I would give this thread a nudge.

smile

A very interesting post has started with a link to some good sized and detailed photos. 

MKV with Johnson Sprung Track. In my opinion where all the confusion over cable track on Medium D's originates from.

Medium D in 1921 with Cable 'Suspension'. Nice view of the track links

LightInfantry T,D,E Tank with cable suspension again a really nice version of an existing photo.

Did you get any further with this project?

Helen x



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Legend

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By chance came across some drawings of the Medium D which were published in 1971.

The existence of reasonable drawings might hasten the production of a cardmodel medium D.

https://kingstroopmodels.com/the-british-w-w-i-medium-d-tank-by-jack-wheldon/

Charlie

 



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