FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1909
1909 - 0315.PDF
MAY 29, 1909. a point is reached on the descending arc of their stroke when the reaction is equal to the weight, and the machine no longer presses upon the ground. The air has, in other words, become solid in respect to that particular machine, and may be represented diagrammatically (Fig. 2) as taking the place of the ground in supporting the machine. As the downward beat of the wings continues, their effective area and their effective velocity both rapidly increase, and consequently the surplus force thus gene- rated is available for levering the body of the machine upwards. At first the change which takes place may be likened to the air dividing itself into two solid fulcrums, which place themselves close up on either side of the body of the machine immediately beneath the wings (Fig. 3). The machine may now be regarded as a pair of hinged levers resting across a pair of parallel bars. At this moment the leverage of the wings is at a < maximum—it is assumed for convenience that external pressure is being applied to the wing tips—but the capacity for upward movement in the body of the machine is obviously very small indeed. The surplus force generated by the increase in the effective area and downward velocity of the wings produces an effect, however, which may be likened to a rapid change in the position of the air fulcrums which move outward and slightly upward in con- tact with the wings (Fig. 4). In such a position as that represented, the leverage is reduced, but their capacity for raising the body of the machine is much increased. At a certain point the reaction which the wings are able to obtain from the air reaches a maximum (Fig. 5), and thereafter decreases until the ...... . air f u 1 - T n\ I c r u m s i • Y 9 1 are once more brought back to their initial position beneath the body of the machine (Fig. 6). The Idle Stroke. Hereafter they vanish, for the wings have ceased to be effective, and the machine, now left unsupported in space, commences to fall. In the first second of time which elapses, the machine will, neglecting air resis- tance, fall through about 16 ft. in altitude, for all bodies which are heavier than air have the same downward acceleration (32 ft. per sec. per sec.) due to gravity. Suppose, for example, that the down stroke of the wings was effective in raising the body of the machine through a height of 4 ft., then the time which would be occupied by the machine as a whole falling bodily to the ground would be approximately, say, ^th of a second. If, how- ever, the wings can make their upstroke and return to their effective angle (Fig. 2) on the subsequent down stroke before that ^th of a second has elapsed, then they will have regained their grip upon the air before the machine touches the ground. The problem is not perhaps quite as simple as this, for the machine having once commenced to fall, must first be brought to rest, and as its momentum is proportional to the square of its velocity as well as in direct ratio to its weight, this is by no means an easy task to accomplish, in spite of the fact that the act of falling through the air would increase the relative velocity of the wings, and thereby make them more effective. Although not perhaps strictly accurate in all its bearings, our mechanical analogy as a means of facilitating the fixing of ideas while dealing with such an intangible subject as air, is particularly useful in this instance as a means of bringing to light certain points which are otherwise apt to be overlooked. A very little consideration of the accompanying diagrams will show, for instance, that the vertical lifting effect of a pair of flat wings can only be a comparatively small fraction of their total stroke. ;, An Extreme Case. To prove this it is only necessary to assume the opposite by taking the extreme case in which it is imagined that the fulcrums could be placed beneath the wing tips throughout their downward stroke (Fig. 7), so as to render their entire move- /»\ ment effective in raising the body. Tf ™""+ '' be quite obvious to any- _ one who has followed our • : It must •1 /• 1 \ I arguments thus far that fulcrums could not \ / be created in these positions in still air, \ / because so far from having a downward \u motion—which is essential for the creation (V, of an upward reaction—the wings are virtu- ally moving upwards, and would therefore create a downward reaction from the air, assuming that the operation of the valves allowed any reaction on this side of the wing to take place at all. What happens in this extreme case must still take place in a less degree when the fulcrums are established at some intermediate point along the wings. Suppose, for instance, that the fulcrums are situated midway between each wing tip and the body when the wings are fully out- .- : t stretched (Fig. 8). The r * r~\ t -\ body of the machine is t • J ^ t • I assumed to have been lifted into its present position by the wings, and therefore to be still travelling upwards. If such is the case, then that part of the wing surface between the body and the fulcrum is detrimental to the lift, so far as it can have any effect at all. If the valves in the wings are not automatic, but are opened mechanically only on the upward stroke, the areas of the inner and outer parts of the two wings will therefore exactly neutralise each other. In any case, it will be evident that the rise of the body can only be a fraction of the stroke, and in most cases only a small fraction. We do not purpose to make any attempt at a proper, mathematical analysis of this problem or the preceding one, because sufficient has been said already to show that the valvular wing orthopter is not quite such a simple problem even on paper, as some people seem to imagine. When it comes to the practical side of the question the difficulties of making an effective valvular wing machine capable of ascending straight up in the air appear to us to be of a still more difficult character. 317
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
