Since the power available for driving the model is very limited it is
obvious that every precaution should be taken to enable the propeller to
absorb every last fraction of energy stored in the motor. With this end
in view the bearing or thrust block in which the shaft of the propeller
revolves should receive careful attention in order to remove as far as
possible all causes which would result in friction.
[Illustration: FIG. 48. Simple bearings.]
The simplest form of bearing is a simple piece of sheet brass or
aluminum having a hole drilled through it and bent up at right angles so
that it may be lasted to the frame as shown in Fig. 48 by A.
Single bearings of this type are employed on the model aeroplanes
manufactured by toy makers whose only desire is to flood the department
stores and toy shops around Christmas time with their impossible
machines. Such single bearings are a decidedly poor and unsatisfactory
construction. Unless the elastic is very short it soon begins to vibrate
in unwinding. Since the rubber is directly connected to the propeller
shaft, the propeller is set into vibration as shown by the dotted lines
in the second part of Fig. 48. The long blades of the propeller
considerably magnify the motion and there is a very appreciable loss of
power due to the erratic path of the propeller and the increased
friction at the bearing. The rubber skein also offers considerably more
resistance to the forward travel of the machine than if it were not in
vibration.
[Illustration: FIG. 49. Double bearings.]
The advantage of a double bearing more than offsets the added weight.
Such a bearing is formed out of a piece of sheet brass bent up at right
angles at both ends as in Fig. 49. The third method of construction in
the accompanying figure makes it possible to employ lighter sheet metal
in the construction of the bearing and still resist the pull of the
rubber and the thrust of the propeller successfully.
[Illustration: FIG. 50. Simple thrust bearing.]
*Friction is reduced* and the thrust taken up by placing one or two
glass beads between the propeller and the bearing. Only those beads
which are flat, with parallel sides and have a round hole in the centre,
should be used.
Four or five copper washers or rings may be made to serve the same
purpose with equally good results.
Another method is to employ two washers separated by a small spiral
spring. Such an arrangement is employed on some of the French models and
might be called a "friction thrust." That is, when the rubber skein is
wound up tight and the propeller is released the friction acts as a
brake and reduces the speed, preventing the propeller from "racing." As
the elastic unwinds, the tension, and with it also, the friction becomes
less so that the propeller revolves more rapidly and maintains a
somewhat even speed. The importance of preserving as far as possible an
even propeller speed can hardly be overestimated and that is why such
emphasis has been laid in several places upon the desirability of a
model whose propeller is driven by a long skein of rubber composed of
the fewest possible number of strands.
[Illustration: FIG. 51. Ball thrust bearing.]
*Ball bearing* thrusts are by all means the most desirable, but not all
models are large enough to accommodate their size and weight. Wherever
it is possible, however, to use them it should be done. The increased
amount of energy available for turning the propeller will make it
possible to employ less rubber and so increase the number of turns and
consequently the length of the flight.
[Illustration: FIG. 52. Hooks.]
*The hooks* at either end of the rubber skein are apt to cut the rubber
unless some precautions are taken to prevent it. This can be done by
binding with cotton thread or slipping a piece of rubber tubing or
aluminum over the hook as shown in Fig. 52.