Chapter 17

 

Chapter 17

Nose-Pieces: Gearboxes, simple noses, free-wheels.

ONE of the most easily constructed types of gearbox is that shown in the drawing of Mr. Towner's Miles Kestrel Trainer M9. That drawing is rather small, but a similar type is shown in Fig. 52. We have a very fine photograph and sketches of a first-class gearbox designed and built by Mr. C. J. Burchell. It is the sort that would be very suitable for a model with "Gipsy" engine. Four gears are shown, but we can make this type with two or more.

Fig.52Fig.52

The back plate is the most difficult, and most important, so let us do that first. Fig. 53 shows the shape, and it is cut from 3/16in. ply. Lay a rule on the ply and mark a line along the centre with a strong pin, needle or compass point. Near the top we drill a hole 1/16in. dia. for the spindle. We shall be using 16 s.w.g. wire for the shafts, as this is the size that suits the holes in the gear wheels. To get the position of the next hole, we put the first gear wheel in place, and push the second up to it so that the hole is over the centre line and the teeth are in mesh. Through the hole in the second gear we drill a 1/16in. hole through the ply.

Fig.53Fig.53

Holes for more gears are drilled in the same way. We also drill holes for the wood screws for fixing the back plate to the front or nose-block. The holes for the spindles are now drilled out large enough to take screwed bushes, which are pushed through and fixed with nuts. The bushes can now be cut short and glue put round the nuts to stop them unscrewing.

Before going any further we had better see if the holes are in the right place. Put the gears on the spindles with the spindles through the bushes, and see if the gears will turn fairly easily. It is best for them to be a little bit tight, and if they are very tight, or if there is much play between the teeth, it is advisable to start again with a new piece of ply. If they are all right - as they will be if we have worked carefully - we can carry on. The gear wheels are soldered to the spindles, and the spindles put through the bushes. On the end of the spindles we can solder cup washers or small brass nuts, except on the % shaft that takes the propeller. This is left free for the time being to allow us to fix the top wood screw.

The next thing to receive our attention is the nose-block, which can be made of some hard, or medium hard, wood like walnut, bass or spruce. Fig. 55 shows how we start with a rectangular block and cut away the outside and hollow out the back. We can best find the position for the hole for the propeller shaft by putting the back plate on first and drilling through the top bush with a 1/16in. drill. At the same time we can try the gears to see that the shafts are not catching on the inside of the nose-block. Now we take off the back plate and drill out the nose-block, and fit another bush. You will notice that a step has been left in the hollow part for the nut, fixing the bush, and the bush has been drilled slightly larger nearly all through to decrease friction on the shaft. Also we glue a disc of ply to the front to come flush with the face of the bush. A spot of glue on the nut will prevent it unscrewing. Now we can glue and screw the back plate to the nose-block and put in the top shaft. On this we thread a cup washer and solder on a driving disc, wire loop, or whatever we intend using to drive the propeller. On page 24 is a photograph of the finished gearbox [see chapter XX].

To fit this gearbox to the nose of the fuselage, we can use a piece of 1/16in. or 1/8in. thick ply cut to the outline of the nose, with a hole in it to fit the back plate. This is glued on to the nose of the fuselage.

The simpler type of gearbox is made in much the same way as the one just described, but instead of a back plate of 3/16in. ply fixed to a nose-block we use two pieces of 1/8in. ply, glued and held together by the bushes. The front piece is the same size and shape as the front of the fuselage, and the back piece is a bit smaller, to fit in a piece of ply glued to the front of the fuselage. We drill the two pieces of ply as before, and put the bushes in with the nuts on the back, and leave them long. These bushes can be held in place quite well by covering the nuts and faces of the ply with Durofix. The gears are put on the shafts or spindles as before.

Fig.55Fig.55

Another method of building gearboxes is shown in Fig. 56. It is the type that never seems to wear out or go wrong, and the writer has had a number of years' service from one box so constructed. The two plates are of brass about 20 or 22 s.w.g., and we drill them with 1/16in. holes to suit the gears. We mark the positions in the same way as we did for the first type, and when drilled 1/16in. in the right place we open out the holes to 1/8in. Into the two plates we solder two short lengths of 1/8in. outside diameter brass tube of a fairly thick gauge. We can usually get this more easily from ironmongers or model engineer shops than from model aeroplane shops nowadays. The tubes then are soldered into the plates, and should project about 1/32in. at each end. . The smaller plate is to keep the tubes the correct distance apart near the gears, and so need not be very big. About 1/4in. wide and about 1/2in. longer than the distance between the shafts is all right. We can either use ready-made shafts or make them from 16 s.w.g. steel wire, .and solder the gears on. We drill the tubes with a 1/16in. drill and see that the shafts run freely. The shafts are put in place and cup washers soldered on to leave just a little end play, but must be adjusted so that the gear wheels line up with each other. One shaft is cut short and the other left long for the propeller.

Fig.56Fig.56

We might mention here a method put forward by Mr. S. E. Capps for getting the spindle holes in the right place without a lot of bother and experiment. We put the gearbox back-plate on a smooth, hardwood block, and lay the gears on top. Where the teeth mesh we put two or three thicknesses of Jap tissue paper, and hold the gears in place with pins driven into the wood. Now we melt sealing wax all round the gears and the plate to hold everything nicely in place while we drill the holes. We must use a sharp drill that just fits the holes in the gears, and drill slowly and carefully, so that we do not make the plate hot, or it will soften the wax and allow the gears to move. See Fig. 57.

Here is a tip. When soldering on cup washers, put a small piece of paper with a spot of oil on it on the shaft first. It prevents the solder sticking the shaft to the brass plate.

Turning from the sublime to the ridiculous as it were, Fig. 58 illustrates about the simplest thing in nose-blocks. To make it simpler we can leave out the spring and the screw. We use two pieces of 1/8in. ply, one the size of the fuselage nose and the other to fit just inside the nose. They are glued together and held with a screwed bush and nut. For the propeller shaft we can use 16 s.w.g. wire, bent to form a motor hook at one end, threaded through the bush, a cup washer put on, and the propeller, and the shaft bent over at the end, poked into a small hole in the propeller. The shape of the hook shown is to suit a bobbin, the use of which we shall learn about when dealing with rubber motors. The spring and screw are there to stop the propeller from revolving when the power has run out, and at the same time to keep the rubber from moving about in the fuselage after it has unwound, causing the centre of gravity to alter its position. We put the screw in such a position that when the hook hits it the propeller will be horizontal, to prevent risk of damage when the model lands. We do not want the spring very strong, about 26 or 28 s.w.g. steel wire will do nicely. A suitable piece can be taken from a worn-out cable from a bicycle or motor-cycle brake.

Fig.57Fig.57

The spring is made by bending the wire round a propeller shaft. We grip the shaft and the end of the wire in a pair of pliers, and wind the wire on in a helix (spiral), leaving a gap of about 1/16in. between each coil. We must hold the pliers very tightly and keep the wire just tight all the time. When we have about four complete coils, we push them all up together, keeping them tight, and then let go. This helps to even up the gaps between die coils. The spring is finished by cutting off the odd ends of wire and closing the ends of the spring up against the next coil. The screw has to be adjusted to suit the spring, so that when the motor is wound up enough to take up the slack in the fuselage the screw still catches the motor hook; but when it is wound up a little more the pull of the rubber compresses the spring and allows the shaft to revolve.

Now let us consider free-wheel devices. Let .us first note the effects on the model. Firstly, about the only time we see a full-size machine land with the airscrew stopped is when it is done at a display to show that an aeroplane can be landed safely if the engine fails, so for looks we must use a free-wheel to let the propeller revolve after the power has run out. Against this there is much less risk of damaging the propeller in a bad landing if we arrange for it to stop horizontally when the power runs out. From a performance point of view things are interesting. When an aeroplane is gliding, the propeller acts as a brake to a more or less extent. If the pitch is equal or greater than the diameter, the braking effect will almost certainly be greater with it stopped than freewheeling, or, as it is sometimes termed, "wind-milling." If the pitch is about three-quarters or less than the diameter the braking effect will be less with it stopped than windmilling.

Fig.58Fig.58

Well, then, what happens when we put the brake on an aeroplane? The first thing is that it slows up, and in doing so loses some of its "lift." This means that if the machine is gliding it will come down more steeply, and although its speed is less it will reach the ground sooner. To see this more clearly, and to explain another point, let us draw a picture in our minds. Imagine a wall with two ladders leaning against it, with the top of each level with the top of the wall. One ladder has twenty rungs and the other has thirty. The bottom of the short ladder will be nearer the wall than the bottom of the long one. Suppose we walk down the short ladder in ten seconds, then our speed is two rungs per second, and we represent the aeroplane with the brake on. Now if we walk down the other ladder a bit faster, say two-and-a-quarter rungs per secorid, in ten seconds, we shall have got down twenty-five rungs, so we still have five rungs to go to reach the bottom. Although we have travelled faster down the long ladder, it has taken us longer to get to the ground; it works out at just over twelve seconds.

From this we can decide whether or not we want the propeller to revolve after the power has run out, and, as for some purposes it is an

advantage to have a free-wheeling airscrew, we will describe two ways of making them.

The simplest way is shown in Fig. 59. Firstly we make a loop of wire about 28 or 30 gauge. We wind the wire on the propeller shaft for about three turns in the form of a spring, then make the loop and another two or three turns. You can see this more clearly in the sketch. We solder this on to the propeller shaft so that a projection on the propeller pushing against the top of the loop tends to wind up the wire and not unwind it. From a piece of wire about 20 or 22 gauge we make a pin to go through this loop. This pin has an eye-shaped end, made by bending the wire round a nail. It is held on to the propeller by a long, thin nail or screw, and must be free to swing round. It is fastened to the propeller so that the free end will go through the loop and just catch on the shaft.

Fig.59, Fig.60Fig.59, Fig.60

Another type of free-wheel which is very neat is shown in Fig. 60. Here D represents the propeller shaft and B is a catch made from a coil of wire of about three or four turns soldered on. A is a pawl that we can make from a piece of sheet brass about 16 or 18 gauge. We drill a hole in it to take a wood screw about No. 2, then cut the outside with tin-snips and file to the shape shown. C is a small piece of spring wire about 30 gauge (a strand from a bicycle brake cable). One end is soldered to a wood screw fixed in the propeller boss. It should be adjusted so that it only just pushes the pawl into engagement with the catch. When making gearboxes and nose-blocks we want to keep the overall size as small as possible, so that in the event of a bump on the propeller the nose-block pulls out easily, lessening the risk of damage.

This 40 in. span model of the Monocoupe, built by Mr. Barry, is yet another example of the beautiful workmanship displayed by aero-modellers. The model has moveable controls, and there are over 100 parts in the scale reproduction of the five-cylinder engine!This 40 in. span model of the Monocoupe, built by Mr. Barry, is yet another example of the beautiful workmanship displayed by aero-modellers. The model has moveable controls, and there are over 100 parts in the scale reproduction of the five-cylinder engine!

 
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