Technically there was a viable aeroplane pre-1900, it was Percy Pilcher's triplane, constructed in 1899. He had spent years working on gliders and had slowly built himself up (thanks to small investment) to construct a motorised aircraft. Unfortunately before he was set to demonstrate it there was a problem with the engine so, not to disappoint the crowd, he went up in one of his gliders and suffered an accident that led to his death a few days later. His design was fogotten for a very long time before enthusiasts built a replica for a BBC documentary and tested it. It was a rickety rubber-band engined death trap made of matchsticks (as was the style at the time) but it was able to outperform the Wright Brothers' initial test flights.
Solid rockets could get people into the air.
Some time ago,I was workiong on a timeline where a meteorite hit Lincoln, NH in 1876, removing the town, toppling the Old Man of the Mountain, breaking windows in Concord and Manchester, and being heard in Boston. Among many other results, the United States went space mad. The first heavier than air flight happened only a year or two later, but it was a brute force method: A semi-aerodynamic airframe and solid rockets, fired in sequence. Range was short, but people flew (and oft made a big hole in the ground at the end of the flight.) With time, means of controlling the vehicle evolved, but witin the timeframe that I worked on, it was never safe.
Trying to have too many viewpoint characters killed the timeline.)
I had always sort of thought that the problem was in finding a light enough, yet powerful enough engine to propel a heavier-than-air aircraft. All of the aerodynamic principles were already fairly well-established (Lilienthal gliders, etc)...
I know there were several different engines other than steam developed in the 1800's - but none with an acceptable power-to-weight ratio until very late in the century. Some unknown or little-known inventor stumbling upon the Otto cycle (but doing it better) or Diesel cycle (but doing it lighter) earlier in the 19th Century would be a decent POD... or you could go full on ASB and have someone stumble onto a hitherto-unknown means of propulsion
A light weight engine is needed AND the ability to control direction vertically and laterally. Clement Ader may have gotten his steam powered Eole off the ground in 1886, but it was just a short hop. He needed a lighter powered plant and a means for controlling direction
Da Vinci attempted to build a pedal powered motor (?); could that have actually been light enough but strong enough to get the Da Vinci aircraft off the ground with more support (so Da Vinci could experiment more with it?). Many of the Da Vinci machines failed for lack of support (the rulers of the time did not take the flying machines seriously).
Yeah, good point. Flight is one thing, but controlled flight is another. Now, if someone could mate a lightweight engine early AND get away from the weight-shifting means of control as with the early gliders, and away from the wing-warping control the Wrights and others were so attached to early on, now that would be something...