In this way, adequate use would be made of the air as a speed medium and, at the same time, it would be possible to effect satisfactory landings on fairly rough ground, as well as on smooth surfaces. For the production of such a variable-speed machine, several designs have already been prepared; but, as yet, each awaits the ordeal of a practical test.
One plan, for example, involves the altering of the angle of incidence of the planes, while a machine is in flight. The angle would, that is to say, be made steep for slow flight, and flatter when high speed was required. Another device aims at obtaining variable speed by a process of reefing the planes of a machine. This would be done by reducing, or rolling up, the rear extremities of the plane surfaces.
According to this system, a machine would have its slowest speed when its maximum amount of sustaining surface was in operation, and would fly faster as the pilot gradually brought into play the reefing process.
A third scheme which is suggested deals with the telescoping of the plane-ends of a machine, thereby reducing the lifting surface.
Of these methods, the one most favoured, having regard to its practicability, is that of slightly altering the angle of the planes; and several manufacturers are already busy with experimental machines of this type. It appears likely that actual tests will, before long, be attempted with an aeroplane thus equipped.
For military work, a variable-speed machine promises to be exceptionally useful. The high speed would be employed in weathering a gusty wind, or in moving rapidly to any desired locality, when about to carry out a reconnaissance. Then, when a detailed observation was being made, the slowest speed would be adopted, so as to give the observer plenty of opportunity of studying what lay below him.
The production of a practical machine, embodying the principle of variable speeds, is now held to be merely a question of time, and of careful experiment. Therefore, the promise of the immediate future, particularly as regards the military aspect of flying, is most hopeful—both from the point of view of wind-flying, and of reliability.
One by one, indeed, the objections against the military aeroplane are likely to be overcome, until a machine is an accomplished fact which will meet all practical requirements.
IV. Power-plant of aeroplanes—Fitting two engines to obviate involuntary descents.
While writing of aeroplane development, mention should certainly be made of engines. It has been shown, already, what a wonderful step forward was made with the introduction of the revolving "Gnome." This engine will operate with the reliability of that fitted to a motor-car; there are other types of engine now achieving thoroughly good results; and they promise to do better in the future. From the point of view of propulsion, therefore, the future of the aeroplane appears assured.