Model Flying Machines

by Alfred Powell Morgan

One of the best means of determining the efficiency of a propeller is to connect it to a small electric motor which will drive it at high speed and by blowing tobacco smoke around it or holding a piece of burning rag nearby and test whether or not the air is thrown out from the sides by centrifugal force. A correctly designed propeller will pull air in from the sides instead of throwing it out.

FIG. 43. All carving finished. Sandpapering to secure a smooth surface.

Calculation in fitting a model with a propeller is almost useless. The experimental error is so large that the empirical or "cut and try" method is the only reliable one. It is best to make a number of propellers of varying pitch and diameter and give to each a thorough tryout on the machine before making a decision.

The Single Screw Machine. A propeller placed in the rear of a machine is usually more efficient than a "tractor" screw placed in front. A machine drags along considerable air with it (due to skin friction of the planes, etc.), and so a screw placed in the rear revolves in air which is really traveling with the machine itself and so the effect is somewhat as though it were traveling with the wind. A further advantage of placing the propeller in the rear of the machine lies in the fact that there is less likelihood of damage in landing.

FIG. 44. Varnishing. The propeller finished.

An aeroplane having a single screw always betrays a marked tendency to turn completely over in a direction opposite to that in which the screw is rotating. Action and reaction are always equal and opposite in their effects and so the motor has a tendency to rotate the machine against the resistance of the screw as well as to rotate the screw against the resistance of the machine.

One way in overcoming this difficulty is to set the two halves of the plane at a slight angle to one another or at a dihedral angle as it is called. Then if the machine tends to twist and turn over the lifting power of the lower wing becomes greater as it approaches the horizontal while that of the other wing grows less. Accordingly the machine resists and tends to turn back to its normal position.

Another method is to keep the weight or centre of gravity as low as possible so that the machine will automatically right itself as soon as it begins to turn. The objection to this, however, is that the machine will fly very unsteadily on a gusty day (and most days are more or less gusty). The effect of placing the centre of gravity low is shown in Fig. 45. The dotted line represents the centre of pressure acting against a plane P. The weight of the machine is centred at W. Imagine the machine in flight. Then the resistance of the plane P acting along the dotted line will tend to stop the machine while W tends to still go forward because of its inertia. As a result, the front of the machine tilts upwards and increases the angle of P, which in turn increases the resistance. The machine therefore slows down but W tends to still move forward and tilt the machine further until the thrust of the screw is unable to support the weight and so W swings back down and beyond the position shown at B. The angle of P decreases, the machine travels forward quickly and gathers sufficient speed for W to swing up again. Thus the performance is repeated and the machine will have a flight path very much like the dotted line shown in the lower part of the illustration. The motion is slight but is sufficient to considerably shorten the length of the flight.