Once he can make a short, straight flight and land neatly, the pupil is ready to attempt a turn while in the air. A half-turn to the left is first essayed, and for the reason that it is more easily accomplished than a swing to the right. This is due to the fact that the engine and propeller, revolving to the left, tend naturally to incline a craft in that direction. The pupil finds as he turns that he must--in addition to putting over the rudder--move the ailerons at his wing-tips; and he needs to make this movement because the biplane, while it is in the act of swinging, begins to heel inwards. The outer plane-ends of a machine, when it is forced thus upon a turn, have indeed a natural inclination to “bank,” or rise. This is because, as the craft swings, the outer wing-tips move more rapidly through the air than do those upon the inside of the turn; they exercise, for the moment, therefore, a greater lifting power; and the result is that, with this sudden and extra “lift,” they force up the outer side of the machine. On a very rapid turn, if this movement went unchecked, a machine might heel to such an angle that it slipped inwards and fell to the ground; but there is no fear of this if the pilot, moving over his hand-lever, draws at the ailerons upon the side of the craft that is tilting down. This causes the plane-ends to rise, and the machine swings upon a normal path. Fig. 96 shows a school biplane making a “banked” turn. A certain amount of “banking” is necessary when turning; it helps a craft to incline smoothly and prevents it from skidding outwards, as it might do were its planes not heeling to the swing. Any pronounced degree of “banking” must not, however, be indulged in by a pupil. In even an acute “bank,” to a skilled pilot, there is no element of risk; experience has told him, in a way nothing else could, just what angle may be safe, or what not; but the beginner, remembering that any steep inclination of his planes may cause a side-slip, must not venture so to heel his craft. Sometimes, becoming over-bold, a novice may “bank” steeply once or twice without mishap; but then perhaps, while attempting to repeat the manœuvre he had made before, he may pass without knowing it over the danger-line, and find his craft slipping helplessly beneath him. If flying quite near the ground, as probably he will be, the mishap may mean a crumpled wing and nothing worse; and it will serve the purpose of teaching him a greater caution. [Illustration: FIG. 96.--Banked turn on a biplane. A.A. Represent the ailerons, which are drawn down by the pilot to prevent the machine from heeling too far inwards.]