Build your first flying scale model - Chapter 4 - The undercarriage

The wire used on models for undercarriages and the like is generally known as piano wire, which is hard and springy. It should be stocked by most good model and hobby shops.

The most common sizes used for models up to about 30 inch span are 24, 22, 20 and 18 S.W.G. The old “Standard Wire Gauge” still seems to be the commonest method of describing wire size in the UK, but is unlikely to mean anything to people in other countries. So, to give you an idea of the diameters in proper units:

24 SWG = 0.022 inches = 0.56 mm
22 SWG = 0.028 inches = 0.71 mm
20 SWG = 0.036 inches = 0.91 mm
18 SWG = 0.048 inches = 1.22 mm

You can find a comprehensive conversion guide here

Looking inside a couple of original Veron Tru-Flite kits in my collection, the wire supplied is typically 20 SWG. I thought it should be possible to get away with something a bit thinner (saving a bit of weight), and found a spare piece which measured 0.030" diameter. I guess I must have bought this as 22 SWG, though it is a little thicker than listed above.

I admit that wire work is not one of my strong points, and I struggle to bend a completely true and symmetrical undercarriage, so let’s see how I get on with this one.

It helps to mark the bend points on the wire before you start, and I find a fine tipped permanent marker pen useful for this. So, place the wire over the patterns shown on the plan and mark off the position of each bend.



The front leg is the easier to make - here are the first bends.



And here it is finished. There was quite a bit of tweaking mecessary to get the angles looking equal, and some twisting to get the wheel axles in line and pointing in the same direction. You can probably see from this photo that I was not 100% accurate in my leg length - my undercarriage came out a fraction long.



The rear legs are much harder to get right, because they have tricky little loops on the end and some nasty double angles that are difficult to work out in 3D space! You can see in this photo that I found the best way to do the loops was to leave excess wire as you bend them round the tips of the pliers. You can tighten the loops by squeezing them with the pliers.

My first attempt ended up in the bin, because I got the angles completely wrong, and second time round I discovered that it was much easier to just bend it to the plan view shape first, as shown above.



In this photo I have snipped off the excess wire from the loops, and put a length of spare wire through the loops to help stabilise it while I put in the final bends.



The first wire part is tack-glued to the keels K1 with balsa cement. View the fuselage carefully from the side, and compare the angle of the wire legs to the plan. You may need to tweak the wire slightly until the legs hang vertically. You might as well also check it from the front for symmetry while you are at it.



Find some cotton from your wife's sewing box, or wherever, cut a length, and glue one end inside K1 as shown with balsa cement.



Next smear some balsa cement on K1 in the area you will be binding, and wrap the thread around the wire as shown. When finished, cut off any excess and smear some more balsa cement over the thread on both sides of K1 to strengthen it.



Well, I did promise to show you everything, warts and all, and this view clearly shows that I got the rear legs a bit wrong. With the loops sprung over the front legs, the rear legs join the fuselage at different points, so the cross piece is definitely not at right angles to the direction of flight. Even with my perfectionist tendencies, I decided I could live with this, as I did not want to have to bend a replacement part. In fact, if you view the finished undercarriage from the side, there is a slight difference in the angles of the rear struts, but it is not very noticable.

You may notice also that I cut small recesses in the keels K1 to sit the wire in. This was to make the wire cross piece sit under the tissue on the fuselage underside, and hence make it invisible after covering.



Here is the final undercarriage all securely bound into place. Note the extra gussets I added above the front undercarriage wire - as much to help with the covering as to reinforce the join. I confess I did use a spot of cyano adhesive to tack the rear legs onto the keels before I bound them with balsa cement. It does have its uses!



The tailskid is also made from wire and bound in place using balsa cement and cotton - compared to the main undercarriage, this was a piece of cake.



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