Langley's Law

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According to Langley's Law an airplane requires less power to sustain its flight at higher speeds.

Samuel Pierpont Langley wrote on aviation in his 1891 work Experiments in Aerodynamics. James Means quoted him in the 1895 Aeronautical Annual and dubbed "Langley's Law" his statement on the relationship between speed and propulsion. Langley's statement with emphasis added:

[...] these researches have led to the result that mechanical sustenation of heavy bodies in the air, combined with very great speeds, is not only possible, but within the reach of mechanical means we actually possess, and that while these researches are, as I have said, not meant to demonstrate the art of guiding such heavy bodies in flight, they do show that we now have the power to sustain and propel them.
Further than this, these new experiments (and theory, also, when reviewed in their light) show that if in such aerial motion, there by given a plane of fixed size and weight, inclined at such an angle, and moved forward at such a speed, that it shall be sustained in horizontal flight, then the more rapid the motion is, the less will be the power required to support and advance it. [...] To make the meaning quite indubitable, let me repeat it in another form, and say that these experiments show that a definite amount of power so expended at any constant rate, will attain more economical results at high speeds than at low ones, e.g., one horse-power thus employed will transport a larger weight at twenty miles an hour than ten, a still larger at forty miles than at twenty, and so on, with an increasing economy of power with each higher speed, up to some remote limit not yet attained in experiment, but probably represented by higher speeds than have as yet been reached in any other mode of transport—a statement which demands and will receive the amplest confirmation later in these pages.

"Langley's Law" was controversial when first promulgated—criticized by the Wright Brothers, Lord Kelvin, and others—and is now generally considered wrong.[1][2]

Publications referring to Langley's Law