Generally, the failing of early aircraft designs is attributed to the lack of sufficient power, reasoning that working out how to control an airplane requires being in the air long enough (cf. David 1919). In designing aircraft, considerable attention was devoted to providing sufficient thrust and overcoming drag. Research on thrust focused on both power of engines and the design of propeller blades.
Screw propellers were already driving steam-powered ships late 1830s, Henson already experimented with blade propellers in his Ariel in 1842, and the idea was widely spread with Penaud’s planophores (1871). Hiram Maxim conducted systematic experiments with propeller designs and registered them accurately to ensure that they would be of great value to experimenters after him (see Hallion 2003, p. 140). Interestingly, it was up to the Wrights to realize that propeller blades were basically wings rotating on a helical path and that increasing efficiency required moving away from the crude flat blades (e.g. Thomas Moy’s Aerial Steamer) or bird feather shapes (e.g. Ader’s Eole). Again, the Wrights conducted extensive tests with a variety of blade shapes, ultimately producing highly efficient propellers (see Hallion 2003, pp. 200-204).
Arguably, the main focus of inventors during the last decades of the nineteenth century was on developing powerful engines with the lowest “pound to the horsepower” ratio. With the steam engine driving trains and buses in many countries, the first powered airplanes were also equipped with steam engines; e.g. Henson & Stringfellow’s Ariel in 1843 and triplane in 1868 in the U.K., Thomas Moy’s Aerial Steamer in 1875, Maxim’s vehicle in 1890, Ader’s Eole in 1890 in France, but also Langley’s Aerodrome models in the U.S.A. in the early 1890s. Ader developed a lightweight 20 hp steam engine with 10 lb. to the horsepower for his Eole (Hallion
2003, p. 131), and Langley met Ader for advice on how to power his own Aerodrome (Hallion 2003, p. 135).
Thrust became all the more relevant when the community became aware of the relationship between thrust and lift. Langley’s research yielded a statistical table that showed that “the cheapest and best way to raise a plane in the air is to drive it forward at a small upward inclination; and that its weight can be best countered not by applying power to raise it vertically, but by driving it fast.” (Raleigh 1922, p. 53-54).
However, the breakthroughs in the power-to-the-pound ratio crucial for thrust were made in another industry, in another country, and by inventors not even remotely concerned with heavier-than-air flight. After having seen Etienne Lenoir’s two-stroke internal combustion engine (developed in 1859) during a trade fair in Paris, Nikolaus Otto brought back the concept to Germany, patented and started producing his own atmospheric pressure gas engine in 1863. In 1879, Karl Benz got a patent granted on his mini two-stroke internal combustion engine. This culminated in invention of the first internal combustion engine automobile (Benz in 1885), motorcycle (Daimler in 1885), and boat (Daimler and Maybach in 1886). Given the small size, limited weight, and convenient fuel, it was destined to replace coal(-gas) fired steam engines, particularly for aircraft. Ultimately, by 1901, Langley’s assistant Manly had reworked a Balzer 52 hp five-cylinder radial engine with less than 5 lb. to the horsepower (Raleigh 1922; Hallion 2003). Also the Wrights constructed their own four-cylinder 12 hp water-cooled internal combustion engine (David 1919, p. 24), which proved adequate even if far inferior to Langley’s engine (Curley 2012, p. 45). Despite the concerns of Chanute about it not being able to provide enough thrust, the Wrights’ calculations were correct and it successfully drove their famous December 1903 flights.