The stability of a model is greater when its course is downward than when in a horizontal flight. In this position, the center of pressure is much less likely to shift, and the pole of gravity has rather a steadying effect. For this reason a glider thrown from an elevation travels much more evenly than a power-driven machine. Try reversing your planes. Why is it that my aeroplane sails all right, but when I increase the power of my motor it falls off and darts about until it reaches the ground? It has been found in experiments of man-carrying machines that an increase in the speed of a flight would often render an ordinarily steady machine unstable. It is argued by some authorities that the air is churned up, as it were, by the forward planes, and that the rear wing, therefore, rocks just as will a boat in rough water. If your model flies well at a certain speed, you had better stick to the motor, and try a larger plane. How far apart shall I place the two wings of my monoplane? There is no general rule possible. Some designers believe that the two planes should be separated by a distance equal to four times the width of the main plane. An excellent model which makes long flights is illustrated herewith, in which the distance is equal to nearly twenty times the width of the main plane, while another successful model has an open space of only twice the width of the main plane. Much depends, of course, upon the speed at which the aeroplane flies. The wings may be much further apart on a high-speed than on a slow-flying model. Are there any simple equations for working out the relation of the proportion of the size and weight of model aeroplanes? There are many such equations, but none of them are simple. The formula which obliges one to solve a complicated problem in algebra or calculus to know where to cut off a stick is obviously absurd in the case of small models. These formulas which may be found in technical books on aviation seem to savor too much of certain school books to be brought in for the builder of model aeroplanes. A good rule to remember is that the thrust should equal the weight of the machine. Some of the large aeroplanes are driven by propellers made up of four blades. Would you try this arrangement on a model aeroplane? By no means. As explained in the chapter on propellers, the work performed by your propeller blades is entirely different from that of an electric fan. It is true that fairly efficient propellers made up of four blades have been used on some passenger-carrying aeroplanes. This form might prove effective on a model aeroplane, but increased weight is prohibitive. By carefully designing your two blades, you can get equally as much work out of them and at the same time reduce the weight by one-half. It is argued by some authorities that the single blades act upon undisturbed air and are therefore more efficient than a four-blade propeller, as the air is always churned up.