Sensible people in 1900 knew that heavier than air flight by human beings was impossible. Man had yearned to fly for time immemorial, and ever since the Industrial Revolution, with its accompanying technological advances, attempts at powered flight had been carrying on at a frenzied pace. But all known attempts, including very well financed ones like those of Samuel Langley, director of the Smithsonian, had produced only ludicrous failures and deaths.
There were many articulate critics. Simon Newcomb, a prestigious American astronomer, wrote several engaging popular books about science. One of his topics was "The Flying Machine." Arguing with clarity and scientific rigor, he showed that the scaling laws of physics conclusively ruled out the possibility of heavier than air flight by humans.
The two big issues facing would-be flyers in 1900 were lift and structural strength; planes didn't get off the ground, and wings and other parts were always breaking. Newcomb noted that lift is proportional to surface area, which scales as the square of dimension, but weight scales with volume, as the cube of dimension. So the bigger a machine is made, the greater the ratio of weight to lift. Beyond some size, weight is greater than lift, and flight becomes impossible. Similarly, structural strength is proportional to structure cross-section, which goes up as the square, but weight and loads go up as the cube, so beyond some size the wings and other parts break under their own weight. Also, engine power to weight ratios drop with increasing engine size, so bigger planes are increasingly underpowered.
Those arguments, and the sad experience of the deluded and dying experimenters were quite convincing: small craft were easy, but ones big enough to carry a person were impossible. To further illustrate his point, Newcomb offered the example of flying animals. Very small ones like spiders don't even need wings, they can fly on threads. Insects with wings can hover, dart, and ascend vertically with ease. Very small birds, like hummingbirds and swifts, have similar aerial agility. Larger birds are more limited. Sparrows and crows can take off and fly under their own power, but have great difficulty hovering. Very large birds like eagles and herons require great effort to take off, and have difficulty powering their flight for long periods, depending instead on updrafts and passive soaring. And the very largest birds, like ostriches can't fly at all, grounded forever by the scaling laws. No animal as heavy as a man had ever flown under its own power.
(He noted that the scaling laws work in favor of lighter than air machines. Buoyancy rises with the cube, while skin weight and resistance-causing frontal area rise with the square of dimension. He was not arguing against dirigible balloons... those were a sensible idea.)
On heavier than air flight, Newcomb ends the chapter definitively:
"The demonstration that no possible combination of known substances, known forms of machinery and known forms of force, can be united in a practical machine by which man shall fly long distances through the air, seems to the writer as complete as it is possible for the demonstration of any physical fact to be."
Other sage advice on the topic included:
"We hope that Professor Langley will not put his substantial greatness as a scientist in further peril by continuing to waste his time and the money involved in further airship experiments. Life is short, and he is capable of services to humanity incomparably greater than can be expected to result from trying to fly. . . . For students and investigators of the Langley type, there are more useful employments."more... more...
- New York Times, editorial page, December 10, 1903