Fig. 57.—Looping the loop.
And there is the peril, when near the ground, of a down-rush of wind caused by hills or woods. More than once, when making a landing near trees, a pilot has found his machine swept in a descending current. What happens, in such a case, is that the wind curls over the wood or hill that may form the obstruction, and sends a strong eddy downward; should a machine be caught in such a trend, it may be carried to earth, pell-mell, before the pilot can check its dive. Again—as another unseen danger—there are “holes in the air.” The atmosphere is in constant movement, it must be remembered; currents of warm air ascending and currents of cold air descending; and sometimes, between a layer of warm air and a layer of cold, there may be violent swirls and eddies. An aeroplane may fly into such a disturbed area and fall suddenly a distance of many feet, through being caught and swept earthward in a rapidly descending current.
But such dangers, like others which threaten, are being surely but steadily lessened. Machines are now built and flown which, owing to the shape and angle of their planes, have a stability which is almost automatic. This means that, when flying in a wind, the pilot need not be working incessantly at his levers. His machine, once a suitable height has been reached, will fly with no more control than a touch upon the rudder-bar. Apart from questions of safety, such stability enables long flights to be made without fatigue. Should he be flying a machine which needs constant work at the controls, and should the wind be high, a pilot becomes often so exhausted that he is compelled to descend. An automatic stable biplane, called the D.F.W., is illustrated in [Fig. 58]. Its main-planes are swept back in the shape of an arrow-head; the rear extremities of the top-plane, forming large ailerons that are locked in position or movable at the pilot’s will, are uptilted at their ends; while the lower main-plane is given a dihedral or sloped-up stabilising angle. The uptilted ailerons, which bring a negative or top pressure to bear upon each wing-tip, and the dihedral angle of the lower plane, which acts in conjunction with them, produce an effect which checks automatically any tendency to a sideway roll.
Fig. 58.—D.F.W. (German-designed) Biplane.
A. Hull, which is steel-built, containing pilot and passenger; B.B. Main-planes—the lower at a dihedral angle; C.C. Uptilted stabilising ailerons, which may be locked in position; D. Stabilising fin; E. Rudder; F. Elevating-plane; G. 100-h.p. motor (which is enclosed) and propeller.
The arrow formation of the planes preserves the stability of the machine in a fore-and-aft direction. Should its pilot force it up at too steep an angle, it will merely come to a halt in the air and then dive forward and resume its normal flight; while, if it is made to plunge too steeply downward, its bow will rise automatically and the machine resume an even keel.
Another machine which is stable in flight, owing to the peculiar formation of its wings, which resist a diving or plunging movement, or a lateral swing, is the Dunne biplane—as designed by Lieutenant J. W. Dunne. This craft is seen in [Fig. 59]. Using such a machine, pilots have flown for long distances with the control levers locked, the biplane adapting itself automatically to the wind-gusts and preserving its equilibrium without aid of any kind. It has neither fore-plane nor tail; it is made to ascend by elevators which are in the form of hinged flaps, or ailerons, and is steered by two rudders at the extremities of the main-planes.