Model Flying Machines

by Alfred Powell Morgan

FIG. 45. Accentricity. The effect of placing the center of gravity too low.

If the machine meets wind, the motion is somewhat increased. In fact the author has seen a small biplane turn completely over and actually "loop the loop." When the machine flies with the wind the effect is largely reduced. If the wind is of just the right strength and comes from the rear, the machine will fly quite steadily. If too strong, however, the model will dive to the ground. A tail somewhat dampens the swing while an elevator will slightly increase it.

The only other methods of partially mitigating the evils of a single screw are to ballast the machine, that is, place a weight on one side or to give one plane an increased sustaining surface. The first may be dismissed immediately because the weight will cause one side of the machine to drop as the elastic runs down and the reaction of the propeller becomes smaller. The last named method is the usual one employed. The wing on that side of the machine opposite to which the propeller is revolving is given a larger surface than the other and so exerts a greater lift on that side. This also has disadvantages, however, for by giving one wing a greater lifting power the machine is caused to fly in a long spiral path when the propeller begins to run down and when it stops completely to glide in the same manner.

FIG. 46. Simplest method of fitting two propellers to a model aeroplane.

The propeller should be placed as nearly as possible on a level with the planes. The centre of pressure on the planes and the centre of gravity should coincide if true stability is desired. The centre of pressure on a machine having the planes set at a dihedral angle is halfway between the lowest point and the highest providing the planes are the same width all the way along. If they taper towards the ends it is slightly lower while if they are wider at the extremities it is higher. The rubber skein and the propeller are usually placed on top of the fusellage of a dihedral winged machine.

The Double Propeller Machine. The best method and the only one which entirely removes the difficulty is to fit the machine with two propellers. A machine having two propellers to the author's mind is the only one worth much attention.

Fig. 46 illustrates the simplest arrangement for fitting two propellers to a machine. In the first a second propeller is attached to the other end of the skein. At first it might seem in the second arrangement that there would be difficulty in getting the screws to revolve at the same speed. However, if the propellers are similar and the same number of rubber strands employed to drive each, the difference will be so small as to be negligible.

When the first arrangement is employed the pitch of the screw in the rear must be slightly greater than that in the front because it is revolving in the slip of the latter.