Chapter 16

 

Chapter 16

Carving Airscrews: Making blanks, cutting from blanks, balancing.

CARVING airscrews is by no means as difficult as a good many aero-modellers seem to think. It is necessary to have patience and take care, but apart from that there is little in it.

We have already seen how to mark out a blank, and this is then cut out carefully to the outlines. For this job we can use a bow-saw, or, on small propellers, it can be done with a knife. We can, of course, buy propeller blanks, but these are usually made to shape to suit duration type models, which require a large diameter, and consequently large pitch, and they are therefore unlikely to be suitable. That does not mean they will not fly the model, but that the model-will not fly at its best. To get the best will probably mean trying two or three different pitches, and to get the most suitable one we shall almost certainly have to carve it ourselves.

Fig.48Fig.48

It will be most convenient if we can have a large wood block clamped to the table or bench, on which to work, as shown in Fig. 48, which also shows useful tools for the job. Now in Fig. 49 we see on the left a blank with two lines marked A, which we draw about 1/16in. from the corners. We want to be careful which corners we choose, as they make a difference to direction in which the propeller revolves. Using the corners shown, we get a propeller that revolves anti-clockwise when viewed from the front, and this is the way that is easiest to wind up. Across the blank, from one line to the other, we make a series of saw cuts, as shown in the second view, and carve away the shaded part.

Fig.49Fig.49

If you have no chisel, do not worry, a pocket-knife is quite good. In fact the writer rather likes a pocket-knife for the job, in spite of the fact that he has been told that pocket-knife carpenters never go to heaven, and at one time he was employed for six months as a woodworker! Well, having cut away the corner, we smooth it down with a rasp or sandpaper. We turn the block over now and mark the lines B. On the top face (or widest) the line should be about one-quarter the width of the block. This gives the triangular blade section seen in the end view. We make more saw cuts and cut away the shaded part again, and smooth off. This time we do not make it flat across, but rounded, as seen in the end view, and also at AX in Fig. 50. We can, if we like, make the rear face hollow, as at BX, but it is doubtful if there is any advantage in this. Note the similarity to wing sections. In fact a wing section here is very good if we can manage it. The next job is to balance the airscrew, which should be done to the best of our ability.

Fig.50Fig.50

A good way to get the balance right is to drive a strong needle into our block of wood, to use it as a spindle, with the propeller free to revolve on it. Now if we mark the blades A and B and revolve the airscrew slowly by hand, we can note which blade stops at the bottom. This blade, then, is the heavier, and must be lightened by sanding as near the tip as possible. Try for balance again and keep on till both blades are the same weight, and come to rest horizontally. The airscrew may not be properly balanced even now, so let's see what we can do about it. Rub off-the old marks A and B and put on new marks-AA and BB. See Fig. 50 again, and note that AA is for one whole side and BB for the other. If side AA always comes to the bottom we shall know it is the heavier, and we must sand all'along that side from tip to tip. If side BB always comes to the bottom we shall have to sand that all along. We keep on like this until, when the .airscrew is perfectly balanced, it will come to rest in any position. We can finish the airscrew by lightly sanding all over with very fine glasspaper arid polishing or painting as desired.

This photo shows another of Mr. TownerThis photo shows another of Mr. Towner's models in flight - a De Havilland Hornet Moth.

On a lot of aircraft nowadays we find three-bladed airscrews are fitted, so we may wish to make one of these. The best way is to make each blade separately, and all as nearly alike as possible, and mount them in a boss or hub. Each blade should be marked out and carved as before, but the inside end should be made rounded. For the hub we can use two discs of wood about 1/4in. thick, grooved to take the blades. These grooves can be cut square at first, and then rounded with a piece of glass-paper wrapped round a nail or thin pencil. We want to be able to turn the blades to get the pitch right, but they must not be loose in the holes. To get them properly in place, with the angles correct, we shall find a chock handy. This can be cut from 1/16in. or 1/8in. sheet balsa and should give the angle at about two-thirds from boss to tip. We then lightly glue it to a board and mount the airscrew on a pin, as shown in Fig. 51. We can set the angle of each blade by revolving the airscrew so that each blade in turn is brought on to the chock. For glueing this the slow drying type is, of course, the stuff to use.

Fig.51Fig.51

Another method by which we can make a three-bladed airscrew would be to leave the hub end of each blade rectangular. For two of the blades we could use a two-blader cut across the middle. These blades could then be fixed between two discs of 1/16in. sheet balsa or ply, in which case the blade angle would be right straight off. The spaces between the blades should be filled with sectors of balsa and the boss carved to the desired shape. The blades should, of course, be spaced accurately. We can do this easily by drawing a circle the same diameter as the airscrew, and, using the radius, divide the circle into six equal parts. Three of these points will now be the position of the blade tips.

 
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