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creating accurately sprung scale U/C

scale sprung struts for a Fiat CR32

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kenking-King Design24/05/2017 11:20:17
285 forum posts
826 photos

A client (in Thailand, would you believe ?) has plans for a Fiat CR32, a pretty biplane fighter, which he has scaled up 150% making it around 7 feet span. As he has trouble finding piano wire there he asked me to make all such components, including cabane struts and U/C. Each main U/C comprises a wishbone hinged under the fuselage near the centreline, with a sprung strut from the axle area attaching high up on the fuselage side and to the rear. Estimated finished model weight is 24-26lbs, and the springing has to be such that it will compress by a scale amount when on the ground to achieve a scale attitude, datum line at 12 degrees nose up.

The first step was to establish relative wheel loadings, working from a side view, and taking moments about the wheels.

fiat cr32 weights.jpeg

Sorry if that isn't clear enough to read, but the upshot was that the main wheels carry 88% of the weight, or 44% per wheel, the tailwheel 12%. Next step was to translate this into spring force in the struts, and here I confess my rusty geometry wasn't up to the task, so I decided to build an inverted test rig to allow force measurements to be taken. Firstly though I had to make up the wishbones, seen here over the plan ...wishbone

with upper ball-jointed pivots ....

At the axles is a brass plate for attaching a spat, but also carrying a triangular bracket, the strut lower attachment point. Making up the plates, and ensuring squareness to the axle is shown in the next few shots ...




Next came construction of the test rig, as you can see, a sophisticated arrangement of angle iron, studding and available scrap .......

inverted test rig

the whole lot tlted at 12 degrees tail down (or up, as we are inverted). The sprung strut is represented by a long rod passing through a spring of known characteristics, with an adjustable stop. Where the rod passes through the angle iron is the relative positon of the upper strut fixing.

To be continued after elevenses ...

Chris Freeman 324/05/2017 11:38:32
353 forum posts
525 photos

Gee that is neat, I will not show my metal and soldering skills on a public forum! I like to work with wood not metal.

kenking-King Design24/05/2017 13:37:02
285 forum posts
826 photos


Thanks Chris, my woodwork is for my eyes alone, so things balance out !

An equally sophisticated means of applying the load was constructed, the load being weighed steel lumps ....

Simulated load application pod

hanging on the axle at wheel centreline .....

Pin-jointed sprung strut

A simple rod represented the strut, passing through a spring with an adjustable stop above it. The rod passes through the angle iron at the strut's upper mounting position, relatively speaking.

Adjustable reactive spring lement

By adjusting the stop to have the spring support the axle at correct parked position relative to fuselage datum, then measuring spring compression and referring to the spring rate (i.e. lbs per inch) I could calculate spring force being exerted. It surprised me to find it equalled almost exactly 70% of aircraft weight, due I suppose to the inclined nature of the strut and it applying its thrust inboard of the wheel.

At this point I invented some acronyms for fun. The rod and spring became the Adjustable Reactive Suspension Element (let's call it A), the rig became Wheel And Ground Gear Load Extrapolation (let's call it W), whilst the bucket became the Simulated Load Application Pod (S). It is self-evident that A + W needs a good S. You work it out.

The strut design was drawn up, incorporating a spring selected from the Lee Spring range (no connection) of 38lbs per inch. From airborne dangle position, strut compression had to be 8mm (nicely mixed units here) to parked position. Some precompression is required to achieve this, and by making it adjustable a range of aircraft weights could be catered for. I aimed for a wide range, from 22lbs to 32lbs.

Construction started with brass tube cylinder, rod bearing and spring piston, with an upper end closing piece ...


further work produced spring cylinders and fittings shown dissembled ....


and partly assembled .....


Note that the extended piston rod passes through the spring into the end closure piece. I made it that way to give the best possible support to the rod-in-cylinder arrangement over the full stroke, which varies from 1" to somewhat less, according to the amont of preload applied.

Not shown is a tubular aluminium sleeve which fits below the piston to set the dangle position. It can be extracted and the length altered to fine tune appearance in the air.

Note also the set of four different preload washers, giving increments of 1,2,4 and 8 lbs. One is shown in place in the earlier photo. The completed cylinder is held together by an M4 through-bolt and nylock nut. The cylinder is at the bottom end of the strut, so carries an inclined clevis fork to marry up with the axle plate seen earlier .......


thus ......


The complete strut is shown next .....


the top end being a ball socket which connects with a ball-ended shaft running through the fuselage ...


In this construction I used conventional silver solder, but also for the first time a silver-bearing 'soft' solder which in conjunction with Baker's Soldering Fluid works beautifully on piano wire, and at a lower temperature which does not seriously affect the wire's properties. I'll certainly be using it again.

I hope you found this post interesting, and perhaps even informative here and there.

Happy modelling,


Paul Blakeborough24/05/2017 14:16:12
20 forum posts
26 photos

A lovely engineering job Ken... and great to see my plans getting used somewhere.. and in Thailand!?!!?? Original shot attached.420213_109465199183717_1579372881_n.jpg

Edited By Paul Blakeborough on 24/05/2017 14:17:20

Edited By Paul Blakeborough on 24/05/2017 14:21:30

kenking-King Design24/05/2017 16:14:16
285 forum posts
826 photos

How nice to hear from you Paul, and I'm very sorry I didn't give you the credit you deserve for the original plans. I love your model, any more pics ?

Bucksboy24/05/2017 16:27:03
575 forum posts
109 photos

I'm in awe, thanks for posting. I'm always very interested to read your posts so please, don't stop!

Denis Watkins24/05/2017 17:26:01
4325 forum posts
104 photos

Same here Ken, these posts are an inspiration, and must generate orders too, myself included, I will be in touch when I can finalise drawings.


Stevo24/05/2017 20:17:41
2699 forum posts
419 photos

Engineering at its stunning best.

Advise where you got the Silver bearing soft-solder?


Geoff Jackson24/05/2017 20:24:43
160 forum posts
3 photos

I was just about to ask if this was from Paul's planfrom RCM n E a few years back. Shamefully mine is still on the project bench ! I must get my finger out!

Paul Blakeborough25/05/2017 08:02:11
20 forum posts
26 photos

I think Ken may confirm Stay Brite....

There are other websites selling it ! ... great stuff to work with.

Regards, Paul.

Paul Blakeborough25/05/2017 08:35:54
20 forum posts
26 photos

As requested Ken...


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Edited By Paul Blakeborough on 25/05/2017 08:36:40

Norman Ledoyen25/05/2017 08:48:34
2 forum posts

Hello to all. I am the lucky fellow in Thailand that Ken has manufactured the material for. I will state that he has done one hell of a great job. Makes me want to have that undercarriage and cabane in my hands right now. I must also say a big thank you for the great set of plans by Paul Blakeborough. I will try to add to this thread as construction starts and progress starts..

kenking-King Design25/05/2017 09:12:28
285 forum posts
826 photos

Stunning model Paul, thank you for these additional photos. I can quite understand why the pilots were so proud to fly them.

Steve, regarding silver bearing solder, Paul is perfectly correct to name 'Staybrite', but when I started looking for the original supplier I ran into some initial difficulty, can't remember exactly what, but I found the same product specification elsewhere and bought from two separate sources. When I'm next in my shed I'll look out the names to add to the source Paul has quoted. It wasn't all that expensive, and is lovely to work with. In my first trial on piano wire, well cleaned of course, I brushed the surface with Baker's, heated gently and got some solder to wet the surface. At that point I brushed on a little more Baker's, and the solder film spread like wildfire, a joy to behold. My nerdynis is showing so I'll stop there,



Paul Blakeborough25/05/2017 09:27:23
20 forum posts
26 photos

Used it for this light scale Waco YMF-5 U/C .... the flux that comes with it is excellent, and the solder flows superbly.img_4051.jpgYes a stylish machine to say the least....

Staybrite in stock at

Edited By Paul Blakeborough on 25/05/2017 09:33:29

Geoff Jackson25/05/2017 09:39:07
160 forum posts
3 photos

Thanks Ken and Paul for the pics and all that info. Now to get my half finished Cirri back on the build bench!

kenking-King Design25/05/2017 14:01:00
285 forum posts
826 photos

Ref. silver-bearing solder suppliers:-

Staybrite is shipped from Australia and carries a £20 shipping charge, many others come from the USA and likewise cost a lot to ship. THAT was the problem I came up against after reading about Staybrite and trying to find it. My current solder is U.K. sourced, either free delivery or £0.99, highly preferable !

Anyway, I've just had a terrible time trying to rediscover my two suppliers on eBay. All sensible references to 'silver-bearing' etc got me nowhere, but I finally struck gold, in a silver sense.

If you go to Ebay enter the search title 'soft wire lead free' and up will pop a few suppliers. One of them, mr. bead, offers a 3 metre length of 1mm dia solder for £4.99. It is type 96S meaning 96% tin and 4% silver, with a melting point of 221C. This is what I have been using.

My second supplier is h.m.comp. You'll see them on the same screen. their product is 95% tin, 4% silver, 1% copper, melting point 217C. I haven't used it yet so can't comment, but don't suppose it will perform much differently. He is offering 3 for 2.

Hope this helps, we all need to operate as economically as possible, don't we ?



Manish Chandrayan25/05/2017 14:14:32
630 forum posts
73 photos

Lovely work on the U/c Kenthumbs up

And what a beautiful model and photography Paul

For those using Stabrite solder and the acid flux that comes with it, just a small tip, the joints scrubbed in caustic soda solution will take away all the residue from flux and ensures the joints will not corrode. But I guess most of you would already know that wink 2

Norman Ledoyen16/06/2017 14:03:55
2 forum posts

I have had the material that Ken built for me and I am tremendously pleased with the quality of the workmanship and craftsmanship that went into getting the project completed for . Make this your man to go to for quality manufacturing. Many many thanks Ken.

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