Chapter 19

 

Chapter 19

Landing Gears: Methods of springing, wheels.

WE have already seen that undercarriages are better left off when possible, but there is a considerable number of aeroplanes with fixed undercarts. Where we have an undercarriage, some form of springing is advisable, unless the model is very small and light. Let us turn again to the drawing of the Miles Trainer and see the neat and effective undercarriage leg there. We make this by threading a piece of steel wire through a length of spring curtain rod, soldering it at each end. The bottom end is then bent at right-angles to form the wheel axle, and the top is fixed to the aeroplane. In the case of the Trainer, a U piece is soldered on, and the two ends of the U fit into a balsa block.

Fig.71, Fig.72Fig.71, Fig.72

Another effective under-cart is shown in Figs. 71 and 72 and is that fitted to Mr. Hasting's Hawker Fury. Fig. 71 shows the inside works, and we can see how a piece of wire has been bent to shape, with brass tube threaded on. The tubes are then tound and glued to the fuselage structure, and a spring is fixed to the wire at the top. The other end of the spring is fixed to the fuselage, so that it is stretched when the undercarriage legs are pushed backwards. Note that where the spring is fastened to a cross strut two more struts have been fixed between this point and the longerons. Also struts have been fixed up from the bottom longerons to the top, just behind the brass tubes. To represent the rear struts of the undercarriage, a piece of rubber tube or cord is used. This is glued in place with only a slight tension, just enough to prevent it sagging. The front leg is faired with balsa.

Fig.73Fig.73

A few more methods of springing undercarriages are shown in Fig. 73. The main point is to have as much spring as possible. For instance, a sketch is shown of a wheel sprung inside a spat, but if it is at all possible we shall do best to have the leg sprung also. The chief trouble is that with the spatted type it is the spats that hit the ground. There is also the trousered type, that suffers from the same trouble. In this type we can have the trousers held against the under surface of the wing with rubber bands that pass over the top. The trousers will then knock off on landing.

In Fig. 74 is a really smart way of springing a cantilever undercarriage leg that would do very well for a model of the Gloster Gladiator. The sketch shows the leg for a large, heavy model, using steel tube with the wheel axle soldered on. But for a smaller model we can use two pieces of bamboo 3/16in. by 1/16in., with a piece of 3/16in. square balsa glued between them. The wheel axle could be bound and glued to the front piece of bamboo. At the top a piece of wire is put across the two pieces of bamboo and rubber bands are attached to it, the balsa being cut away. The rubber bands are fixed to formers in the fuselage at the top and bottom, using wire hooks or loops. The strength and tension of the rubber can be adjusted to suit the weight of our model.

Fig.74Fig.74

We can see from the foregoing description that steel wire plays a very large part in most undercarriages for strength of the legs themselves and for springing. The wire used varies in thickness according to the weight of the model and the design of the undercarriage. We can, however, quote a few instances as a guide. Let us look at Fig. 73 again, and take some of the examples in turn. The one at the top left is suitable for a twin-engined machine with undercart retracting into the nacelles, and about a ten-ounce model. The wire could be 16 s.w.g. throughout. The one below would do for a 14-ounce model using 14 s.w.g. wire

and an eight-ounce model using 18 s.w.g. The two below and alongside could be 20 s.w.g. for an eight-ounce model, and 16 s.w.g. for a 12-ounce one. For the top middle we could use 20 s.w.g. for a six- or eight-ounce model, and for the bottom right 20 s.w.g. for an eight-ounce machine.

It is best to bend the wire in as few pieces as possible, though there is a limit to the sharpness of any bends. This limit is best found by experiment, and it is suggested that the reader tries bending a few odd bits of wire of various sizes to see how sharp a bend he can get without it cracking. The sharpness depends to some extent on the tools used, since a sharp edge increases the tendency to crack. For bending the wire we can do with two good strong pairs of pliers, one flat-nosed and the other with one flat and one round side.

When we have wire fixed to balsa it should be bound with thread and well glued, but a better job would be made by glueing a piece of thin ply to the balsa first.

Now we come to the wheels. These can be bought or made up. Very good-looking ones can be obtained made of solid balsa or hollow celluloid. They can also be purchased made from sponge rubber, or real pneumatics, in an assortment of sizes. We can make use of the heavier varieties for the main wheels with advantage, and the lighter ones, that is, hollow celluloid, for tail wheels. We could, of course, use the celluloid ones for the main wheels, but a little extra weight is usually an advantage in this position. If we can put an aluminium or brass bush in the main wheels they will run more freely.

Fig.75Fig.75

The best way of making wheels is to start with a ply disc, as shown in Fig. 75. We-can cut away some of the ply to lighten it if desired, and then the rim is glued on. This is cut from 1/8in. sheet in circles and is glued on with the grain crossing. A. screwed bush is put in the middle of the ply disc, and this bush can be held in a hand drill for trimming the rim to shape. The drill should be held in a vice or fixed to a table or bench, and the wheel can be turned, using first coarse and then fine glasspaper to shape the rim. The middle of the wheel can be shaped in this way, but we shall probably have to do the second side on an extra piece of ply. It can then be glued to the ply disc; and, to add a more realistic look, string can be glued round between the rims and middles. On the side opposite the bush it will be a good idea to stick on a nut or washer to prevent wear.

Wheels are best held on their axles by small washers soldered on each side. Should the reader be unable to solder, the next best thing is to bind on a little bit of thread and hold it with a spot of glue. The spot of glue can be pushed to shape with a wet finger if a cellulose type is used.

The usual thing to hold the tail wheel will be a small wire fork with the ends turned in, rather like the top right undercart in Fig. 73. This fork is then bound and glued to a fuselage former or a bamboo strut. When the tail wheel is faired, i.e. a small spat, it can be fixed in with the axle glued at each end to the fairing.

"NIGHT PATROL"

Many scale model enthusiasts like to arrange their models in realistic set-ups. Here is an excellent example of what may be done with a few lead soldiers.Many scale model enthusiasts like to arrange their models in realistic set-ups. Here is an excellent example of what may be done with a few lead soldiers.

 
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