Chapter 5

 

Chapter 5

Gearboxes: Functions of gears, effects on motor.

THERE is a maxim in aircraft design that one should design a component to serve as many uses as possible in order to save further parts and cut down the weight.

Now fortunately a gearbox fulfils five very important functions, viz.:

(a) Weight in the right place. (b) Longer motor run. (c) More even torque. (d) Less torsion in the fuselage. (e) Use of smaller propeller.

The question of weight we have dwelt on at some length, but it seems it cannot be emphasised enough. People one would expect to know all about it seem to slip up on this point, or perhaps they are so concerned with the looks of the model that they can't afford to compromise! But whatever it may be, however well the model has been built and finished, if the centre of gravity is in the wrong place the model is no longer in the flying scale category and should be classed as an exhibition model only.

Now the longer motor run achieved by using a gearbox is a very great boon, especially as we propose shortening our motor to effect trim; and it comes about this way.

Suppose we require eight strands of rubber 1/4 inch wide and 20 inches long to fly our 'plane, and these are made up into one motor, the total number of safe turns we can put on is in the region of 580, depending on the freshness of the rubber, whether it is well lubricated and well stretched, etc.

When this motor is fully wound and tightly knotted it is obvious that the stretch of the rubber motor is taken up in wrapping itself around itself in the form of knots, and that if we divide our eight strands into two lots of four equal strands geared together we shall be able to get a greater number of knots on each skein, and hence a longer motor run but the

same total power output. Of course, there is added friction between the gears and shafts and bearings to account for, but taking the twin skeins only into account, where 580 turns were put on in the single skein we can now actually wind up to 817 turns on the twin skeins.

Moreover, should we use three gears and a third skein, then we can increase our total turns to 1,000. Still further, if we use a gearbox of four skeins and four gears then we can increase our number of safe turns to 1,160 ! It must be borne in mind that by using two skeins we do not double the number of turns, but it increases in accordance with the square law. That is to say, two skeins will take 1-41 times the number of turns of a single skein, three skeins will take T73 times and four skeins will take twice, and so on. The use of gears evens out the torque, and consequently the thrust or pulling power of the airscrew. In order to explain this, let us examine the single skein motor, and wind to full turns; as we are winding it becomes harder and harder to do so. Now in the reverse process, as the rubber is unwinding, the greatest power is delivered to the airscrew during the first few seconds. As the motor runs out so the power dies away, until the last few turns are of no value at all. This all means that we get an initial burst of power that takes the model rushing upwards, and then a gradual slackening off until the power is exhausted.

From the duration man's point of view this is excellent, as he is enabled to get his "ceiling" quickly, accompanied by a long, floating glide; but for the scale modellist this is all wrong.

We have already decided that the model should not only be to scale, but the style of flight should be correct. That is to say, a replica of a small light 'plane should not rush up into the sky any more than a scale version of a high-speed fighter should come floating in on a slow glide.

Fortunately, with the use of multi-skeins not only have we lengthened our motor run, but, as we still have the same amount of weight in the rubber, we still have the same amount of power. This same amount of power has to be expended over a longer period of time, and in so doing the initial burst of power is lessened, and the power more evenly distributed over the whole duration.

Fig.10.Fig.10.

By using three- or four-gear motors this effect is still more pronounced, and, in fact, with a four-skein gearbox we can utilise nearly the whole of our turns for flying and almost do away with the useless turns at the finishing end.

Some fellows who have tried gearing will tell you that they know it gives a longer power run, but they don't get the climb. This slow climb and long power flight is exactly what we want for scale flight.

The illustrations show various types of gearboxes. Fig. 10 is a four-spindle box shown applied to a Gipsy type cowling. Fig. 11 is a three-spindle box enclosed in a Kestrel or Merlin cowling. Fig. 12 is a simple two-spindle box fitted to a light 'plane with a horizontally opposed engine, and Fig. 13 is the same twin spindle with the propeller geared up to go two or three times as fast as the rubber shown fitted to a Pobjoy motor.

Fig.11, Fig12, Fig.13.Fig.11, Fig12, Fig.13.

If we refer to the chapter on aerodynamics where we discussed downthrust and sidethrust to correct for torque, we see that by reducing the initial burst of power we have helped ourselves tremendously in achieving a stable flight.

Now let us consider function number four of our gearbox. Less torsion in the fuselage. Most of us have heard of reaction. Action and reaction are equal and opposite. That is to say, the turning action of the rubber in turning the airscrew has an equal and opposite reaction in the form of the motor trying to twist the rear of the fuselage about itself in the opposite direction to the rotation'of the airscrew. This all means that the part of the airframe to which we attach the rear hook has to be strengthened to withstand this load, and in fact the whole fuselage has to be strengthened, and consequently more weight must be added to the rear.

But if, on the other hand, we use two skeins of rubber geared together, then one turns in the opposite direction to the other, and the torsion at the rear hook is cancelled out, greatly to our advantage, since the fuselage is not twisted at all.

Naturally, a combination of an even number of skeins will always produce this result, while odd numbers will only make a difference in torsion of the one odd skein.

We now come to the last, but not the least, important factor of using a gearbox, namely, the use of a smaller propeller.

Going back to the single skein arrangement as exemplified by the duration model, we have to use a large airscrew and plenty of blade area to absorb some of the initial burst of power, otherwise the motor would run out too quickly. But where we have a more or less even output of power, and spread over a longer period of time, we can use a smaller diameter airscrew, which means less blade area. A smaller diameter means a faster revving airscrew to deliver sufficient power to fly, but this means getting nearer to scale. (As a matter of fact, all S.M.A.E. records for scale models must be accomplished with a propeller of the correct scale diameter.)

While on this subject it would be as well to mention the practice of gearing up the airscrew, that is to say, arranging for the airscrew to revolve faster than the rubber motor. By this method a still smaller propeller can be used, but there is a limit to the size of propellers that can be used efficiently. Not only that, but if the airscrew is not designed correctly it will tend to go back to its old tricks again and give us a high burst of power and a long trail-away of useless turns.

So we can sum up the five uses of the gearbox and say that by its application we can achieve a long, steady output of power, maintaining a stable flight at a constant height for the longest part of the motor run, and giving us the exact effect of the prototype in flight.

 
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