This method of securing stability, which was invented and patented by the Wright Brothers, has been widely imitated. In their later machines, the Wrights have even abandoned the front elevating surfaces and depend upon the movement of the main plane and a small elevating plane placed just back of the rear rudder for their directional control. They have thus done away with the friction encountered by the front planes, which has resulted in giving the machine greatly increased speed. Now in the model aeroplane, it is, of course, impossible to flex the planes up or down during flight. Some adjustment must be hit upon which will give the machine automatic stability. The principle of the action of the stabilizer remains, of course, exactly the same. In designing rudders for controlling horizontal flight, it should be borne in mind that their stabilizing power varies largely in proportion to their distance from the center of gravity. In most models the further they are removed, either front or back, the greater is their leverage, and the smaller need be their surface. By placing the rudder on an outrigger carried far out, a very small plane will suffice. [Illustration: A serviceable model showing excellent workmanship built by Cecil Peoli] [Illustration: A serviceable model showing excellent workmanship built by Cecil Peoli] The vertical rudders or fins, as they are sometimes called, are, of course, intended to control the movement to right or left and keep the model from sliding sideways. They have no counterpart in the wings of birds, and are believed by some aviators to have little effect. At any rate, they can do little harm since their head resistance is practically nothing. Unlike the horizontal forward planes, these fins should not be carried too far forward. In practice it is found that they often get in the way, and a slight side gust of wind striking them, with their great leverage, will knock the aeroplane completely off its course, perhaps upset it. The best position for such rudders is either above or below the main plane, or behind it, where they are out of the way of cross currents. In last year’s models, these vertical surfaces were often very large, presenting as much surface as the planes themselves. It has been found that they may be cut down in size, thus saving weight without losing their efficiency. A long vertical fin, or keel, has the disadvantage of presenting a dangerously broad surface to any cross current of wind. The question of the position of the rudder was taken up in a previous volume. A glance at the successful model aeroplanes of the year shows that the vertical rudders have been adopted very generally. Considerable ingenuity is displayed in adjusting them.