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On December 13, 1905, the Wright brothers sent a letter to Georges Besançon, the editor of L’Aérophile, a leading French aeronautical journal. L'Auto, a newspaper of Paris, published a part of the text (see the French article below - it can be read with Google Translator).

The last paragraph, which I already rendered in English, contains a claim, of the two Daytonians, saying that their 1905 apparatus was so well designed that even engines built as far back in the past as 1855 could make it fly:

Ordinary landings do not damage the apparatus at all, and flight after flight are performed without any repairs. The success results from the high scientific efficiency (sic) of the airplane and propellers and especially from the recent improved methods of balancing and steering. It has often been said during the nineteenth century that it was above all the lack of sufficiently light engines that prevented man from conquering the air. This opinion is strongly contradicted at ordinary bird speed, a well-studied airplane can currently support a weight of 30 kilograms per horse, which gives ample room for engines built even fifty years ago ...

Question: Can anybody offer an example of such an old motor, made say between 1855 and 1865?

The full article in French:

1905-12-29, “Nouvelle lettre des frères Wright”, L’Auto, Paris, December 29, 1905, col. 5-6, p. 3.

Nouvelle lettre des frères Wright

Les célèbres inventeurs donnent de nouveaux détails

Les méthodes de mesure. — Vitesse par rapport à l’air et parcours mesuré au sol. Avec un aéroplane bien combiné et bien manœuvré les résultats actuels auraient pu être obtenus depuis cinquante ans?

Le débat engagé autour des affirmations des frères Wright à la suite de l’article de l’Auto du 30 novembre n’est sans doute pas près de se clore. L’enquête de notre collaborateur Robert Coquelle, faite sur les lieux mêmes, nous a révéle d’intéressents détails. Nous sommes heureux d’apporter encore aujourd’hui de nouveaux éléments, d’appréciation en mettant sous les yeux de nos lecteurs les principaux passages d’une nouvelle lettre en date du 13 décembre, adressée par les frères Wright à l’excellente revue d’aéronautique L’Aérophile, qui leur avait demandé des renseignements complémentaires sur leurs expériences. Cette fois encore nous sommes en présence des déclarations des intéressés eux-mêmes, et certaines, d’entre elles seront de nature à raviver le débat:

Dayton, 13 décembre 1905.

Monsieur le Directeur, ...

Voici les nouveaux détails que nous croyons devoir vous donner:

Le terrain où ont en lieu nos expériences les plus récentes est une prairie plane ayant à peu près huit dixièmes de kilomètre de longueur et quatre dixièmes de kilomètre de largeur, mais d’une forme si irrégulière que les cercles décrits avaient à peu près 1 kil. 500 mètres.

La longueur des vois était mesuré avec un anémomètre Richard monté sur l’appareil, de façon à commencer à enregistrer quand l’aéroplane partait et à s’arrêter quand le moteur stoppait. Comme le moteur était arrêté un instant avant l’atterrissage, les vols se trouvaient en réalité légèrement plus longs que la distance enregistrée.

Lorsque les vols se faisaient en ligne droite par temps calme, les indications de l’anémomètre correspondaient très bien avec le parcours réel mesuré sur le terrain. Quand, au contraire, les vols étaient exécutés en cercle autour du champ, il n’était pas pratique de mesurer exactement le trajet sur le sol. Les distances sont celles enregistrées par l’anémomètre. Le vol de 39 kilomètres comportait 29 circuits sept dixièmes autour de la prairie. Le temps était pris par trois chronographes dont un fixe à l’anémomètre.

La vitesse en l’air, à peu près la même pour toutes les vols, était d’environ 1 kilomètre par minute. La vitesse par rapport au sol a varié naturellement, en des parties différentes de la course, selon la fonce et la direction du vent. Dans les vols des 3 et 4 octobre 1905, l’aeroplane fut constamment maintenu à 25 mètres au-dessus du sol. Mais dans les expériences antérieures, nous avons parcouru milles sur milles sans monter une seule fois à plus de 3 ou 4 mètres de hauteur. Quand nous volions bas la vitesse semblait beaucoup plus grande que lorsque nous tenions moins près de terre.

Pour l’instant, nous, préférons ne pas donner plus de détails sur notre machine. Notons seulement que la légèreté n’a pas été cherchée. L’appareil ne comporte aucun dispositif aérostatique, ni quoi que se soit de plus léger que l’air. Les organes du moteur, tel que cylindres, pistons, bielles, etc., etc., sont du poids ordinairement usité en construction automobile. Les plans porteurs et le bâti de l’appareil sont établis beaucoup plus lourd qu’on n’a l’habitude de le faire dans les machines volantes.

Les atterrissages ordinaires n’abîment pas du tout l’appareil, et vols après vols sont exécute sans aucune réparation. Le succès résulte de la haute efficacité scientifique (sic) de l’aéroplane et des hélices et spécialement des méthodes perfectionnées récentes de balancement et de direction. On a souvent dit, pendant le dix-neuvième siècle que c’était surtout le manque de moteurs suffisamment légers qui empêchait l’homme de conquérir de l’air. Cette opinion est tout à fait controuvée a la vitesse ordinaire de l’oiseau, un aeroplane bien étudier peut actuellement soutenir un poids de 30 kilos par cheval, ce qui donne amplement de la marge pour le moteurs construits même il y cinquante ans …

Signé: Wilbur et Orville WRIGHT.

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    $\begingroup$ The only engine I'm aware of is that made in 1848 by John Stringfellow, a steam engine that successfully powered a 10-foot wingspan model, which achieved the first powered flight. The payload was negligible, however. Perhaps the Wright brothers were either misinformed or purposefully overstated the fact? $\endgroup$ – Digital Dracula Apr 25 at 7:58
  • $\begingroup$ Why is it OK to Ask that with no relevant research… ie, here's a wing with X-much lift, so it needs Y-power, provided by a propellor driven by a gear-set powered by the engine you're asking about? $\endgroup$ – Robbie Goodwin Apr 25 at 20:57
  • $\begingroup$ related-- new chat room -- chat.stackexchange.com/rooms/123457/… $\endgroup$ – quiet flyer Apr 26 at 14:27
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First, I would like to mention a free book, that I skimmed a few days ago, which reached the conclusion that the Wrights committed one of the greatest technical frauds of the twentieth century. The text concentrates on the 105 flights of 1904. The work is based only on primary sources, mainly documents of the period 1903-1905, in majority letters of the two brothers and the answers received by them, plus newspaper articles (all quoted in full).

After reading the letters and articles you start to ask yourself how is it possible that so many authors credit Wilbur and Orville Wright with building the first heavier-than-air man carrying plane that ever flew when, in fact, the two inventors just tried to fool the newspapers (especially those of Dayton), Octave Chanute (a personality of the aeronautic world of the time), Georges Spratt (a fellow aviation enthusiast), Carl Diesentbach (the New York correspondent of the German journal "Illustrierte Aeronautische Mitteilungen") and both the US War Department and British War Office, by pretending they had performed no less than 105 flights in 1904 and, in many instances, describing aerial trips that are physically impossible, like the ones of August 13, 1904, when the plane, Flyer II, got energy from the headwind, which accelerated the apparatus.

"The Press", the only newspaper that, on May 26, 1904, furnished a list of witnesses (friends of the Wright family and an unnamed reporter) who saw the alleged flight of the same day, later in the year, on December 17, 1904, acknowledged that nobody had ever seen the two inventors flying powered planes.

The Wrights and their impossible 1904 flights

 "The Wrights and their impossible 1904 flights", by Bogdan Lazar, April 5, 2021.


Now, to give an answer to the question of whether about 50 years before 1905 there existed engines capable of powering a plane, I would say that (see the clipping from the Scientific American of March 13, 1869) John Stringfellow won (after June 1868) a prize of 100 pounds for a steam motor that developed an estimated max power of 1.14 hp and weighted 5.89 kg but without water.

The Wright brothers might have heard about this engine, form Octave Chanute, for example, and scaled it up 20 times reaching the conclusion that a 22.8 hp motor with a mass of 117.9 kg was possible in 1868. However, this is pure math.

Scientific American. / New Series, Volume 20, Issue 11, p. 169; Mar 13, 1869, New York

Source: Scientific American. / New Series, Volume 20, Issue 11, p. 169; Mar 13, 1869, New York

(As a note: An earlier article which appeared in the September 23, 1848, issue of the Sci. Am. and mentioned some experiments performed by a certain Mr. Stringfellow, is vague. The text contains no clear description of what the inventor did.)

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    $\begingroup$ Interesting to read that in 1868 the lack of proper materials was cited as the main reason preventing the construction of flying machines. Wooden gliders show that even with the technology of the late Middle Ages gliding flight would had been possible. What was really missing was a lightweight, powerful and reliable engine. +1 $\endgroup$ – Peter Kämpf Apr 26 at 8:44
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    $\begingroup$ This Wright hoax idea is barking mad conspiracy stuff. I have read much contemporary UK source material, including accounts by Col. Capper, Superintendent at Farnborough, who visited the Wrights with his wife and became personal friends. They discussed aeronautics on level terms. The Wrights clearly knew what they were doing and this imagined hoax would have been more work than the effort put in to making it actually happen. The Flyer III was way ahead of the world when it appeared, and that did not evolve out of Machiavellian timewasting. $\endgroup$ – Guy Inchbald May 2 at 6:27
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    $\begingroup$ @duofilm Other contemporary correspondence in various UK archives shows that Col. Capper's personal influence and knowledge overcame this obstacle. The War Office insisted that the planes be demonstrated to them and flying lessons given to a few soldiers, before they paid up. The Wrights agreed. The deal only fell through because the Wrights demanded a thousand dollars a plane and the War Office were not prepared to pay that; they argued that Capper's in-house pioneer, Lt. J.W. Dunne, could make a better plane for a lot less. You have been fooled by a clever cherry-picker. $\endgroup$ – Guy Inchbald May 2 at 13:10
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    $\begingroup$ @duofilm Keep your crazy-logic speculation to yourself. I prefer documented fact. But you might like to consider that most contracts are for the supply of deliverables and not for proof of pre-existence. Over and out. $\endgroup$ – Guy Inchbald May 3 at 13:26
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50 years before, in 1855, not even the Lenoir engine, a two-stroke combustion engine which ran on natural gas (saving Lenoir from inventing the carburettor after having invented the spark plug already) had been invented yet.

This only leaves steam or compressed air engines - ignition engines needed inventions which only occurred in (induction coil by Rühmkorff) or after 1855. The lightest ones would be fireless and rely on a reservoir of high pressure steam or air, allowing only a very short operation. However, even the earliest fireless locomotives were only invented after 1855.

Carbonic acid engines were indeed used to power very early airplane designs and were initially developed in the 1820s, so they would had been technically feasible. But nobody tried them for aviation before the 1890s, probably because the first such machines were unfavorably compared to a ticking time bomb.

I think the Wright brothers claim is preposterous and reeks of arrogance. No wonder they were titled "the lying brothers" at this time.

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    $\begingroup$ There may be a cultural aspect here if you are assigning the year1855 is a hard truth-or-lie value. Nothwithstanding whether or not one of Lenoir's engines from the 1860s was capable of making the power to weight, Wright's mention of "50 years ago" wouldn't be taken as a literally, exactly at least 50 years ago by American or Canadian ears. If you say you were were doing some activity 50 years ago, I will just assume some time in the early 70s. I wouldn't accuse you of lying if I found out the actual year was 1975. The Wright statement of 50 years I take as "mid 1800s", not exactly 1855. $\endgroup$ – John K Apr 25 at 13:52
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    $\begingroup$ @JohnK Thank you for the hint, I should rephrase the answer. However, it took at least until the last decade of the 1800s for power to weight ratio to become feasible for flight. Even Hiram Maxim preferred a steam engine as late as 1894. Lenoir's engine of 1863 could only produce 1,5 HP from a displacement of 2.5 liters. $\endgroup$ – Peter Kämpf Apr 25 at 14:05
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    $\begingroup$ Agreed. He probably threw that line in without really researching it, or was doing what is known in sales as "puffery" or exaggerating for effect (the letter could be thought of as a marketing document). The Wrights' own motor was pretty interesting, considering how crudely it was fashioned. The crank started out as a steel slab with the crank profile roughed out with a drill press! The EAA built an exact copy of their motor and runs it at Oshkosh. Sounds like a 4 cyl farm tractor. $\endgroup$ – John K Apr 25 at 16:10
  • $\begingroup$ I don't understand the hostile tone of this answer. How is it relevant that from 1903 to the spectacular public demonstrations of 1908 reports of the Wright brothers' growing success were widely disbelieved? This answer is also weak because it ignores the quantified claim made in the letter: that the Wright machine required only one horsepower per 30 kg. I think you need to identify the first engine which would have worked in their machine. If it was available decades earlier to the previous generation of aviation pioneers, then their claim is true in substance, though perhaps not in detail. $\endgroup$ – David42 Apr 26 at 14:23
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The aviation enthusiasts to whom this letter was address in 1905 would have understood the Wright brothers to be referring primarily to special light-weigh steam engines specifically designed for use in aircraft. Enthusiasts had been building such engines since at least the 1840’s and by the 1870's you could have one built for you if you could afford it.

The statement made by the Wright brothers has two parts: 1) It is a mistake to conclude that 19th-century aero-engines had insufficient power for flight simply because the flying machines in which they were mounted failed to fly, and 2) suitable engines existed 50 years ago. The first statement is almost certainly true. The second statement is true only with the major qualification that the engines actually build before the 1870's seem to have been small-scale prototypes.

In 1905 when the Wright brothers wrote this letter the only working powered airplanes are models too small to carry a pilot. As far as the Wright brothers know nobody has been able to get a man-carrying airplane more than a meter into the air, keep it there for more than a few seconds, and travel more than 100 meters along the ground.

We now now that there are at least six principal reasons why 19th century airplanes did not work: 1) The wings had a poor lift to drag ratio, 2) The propeller blades did not have the shape they would need to provide good thrust for the input power, 3) the airplanes lacked three-axis control, 4) the controls they did have were difficult to operate, 5) the operators did not know how to fly an airplane, and 6) the steam engines were not powerful enough to overcome problems one and two.

The Wright brothers are reporting that they now have an airplane which actually flies. They have achieved this by attacking problems one through five. If you solve all of them, you do not need to solve problem six.

Their core claim about airplane engines is expressed in numbers. They say that with their design improvements an airplane can fly with an engine which produces only one horsepower for every 30 kg of gross vehicle weight. This is an approximation of what they actually achieved on December 17, 1903 at Kitty Hawk with an airplane which weighted 338 kg and a 12 hp engine (28 kg/hp).

This claim is also in line with what other researchers had already discovered. The Wright brothers had read Octave Chanute’s 1894 book Progress in Flying Machines in which he reports (p. 73) tests of airplane wings (not entire airplanes). Professor Langley found that a wing could lift 200 lbs (90 kg) per horsepower while Mr. Maxim’s tests yielded a figure of 133 lbs (60kg).

enter image description here

The 1903 Wright engine as seen in 1916 after restoration

The homemade 12 hp engine used by the Wright brothers weighed 170 lbs dry. With fluids it weighted about 200 lbs. In 1903 it produced between 12 hp continuously and 16 hp peak (7.5 and 4.6 kg/hp respectively). This is very modest performance for a gasoline engine. For comparison the Clerget 9B used in the Sopwith Camel in 1917 weighted 173 kg (dry) and produced 130 hp which is 1.33 kg/hp (slightly worse with fluids of course).

While it is difficult to find reliable and detailed information about the performance of 19th century steam aero-engines, it is clear that a number of them had power to mass ratios equal to or better than that of the Wright engine.

technical drawing of engine with tubular boiler

1843: William Hensen and John Stringfellow obtained a British patent for the concept of an "Aerial Steam Carriage". This was to be a passenger airliner with a 150 foot wingspan driven by a 50 hp light-weight steam engine. They sought investors and began experiments with a model with a 20-foot wingspan and a working light-weight steam engine. They conducted tests between 1843 and 1847. Though the engine worked, making the even a large-scale model strong and stiff enough to fly outdoors proved beyond their ability.

Some notes from a bench test of the engine have been preserved:

June, 27th, 1845, water 50 ozs., spirit 10 ozs., lamp lit 8.45, gauge moves 8.46, engine started 8.48 (100 lb. pressure), engine stopped 8.57, worked 9 minutes, 2,288 revolutions, average 254 per minute. No priming, 40 ozs. water consumed, propulsion (thrust of propellers), 5 lbs. 4 1/2 ozs. at commencement, steady 4 lbs. 1/2 oz., 57 revolutions to 1 oz. of water, steam cut off one-third from beginning. The diameter of cylinder of engine was one-and-a-half inch, length of stroke three inches.

Note that it took three minutes to get up steam and the engine ran for nine minutes. If we estimate the average pressure in the cylinders at 50% of the boiler pressure, this engine produced about 1/6 hp.

Monoplane with 10' wingspan and two propellers

Photograph of Stringfellow's 1848 model airplane

Frame with engine cylinder, sheetmetal boiler with the conical parts

Photograph of one of the numerous surviving Stringfellow aero-engines

1848: John Stringfellow builds a smaller model with a 10-foot wingspan and a new engine. According to Chanute, Stringfellow tested it indoors in a room less than 66 yards long. It ran one third of the length of the room along a guide wire. After leaving the wire it would continue to fly to the end of the room. If would frequently rise after leaving the wire at a rate as high as one in seven. The weight of the entire model including engine, water, and fuel was less than 6 ½ lbs.

In 1995 a reproduction of Stringfellow's 1848 model was build for the BBC2 program Local Heroes. This model was tested in the same mill Stringfellow had used. A published advertisement for the program implies that the model flew. I would like to know more about this program.

Single cylinder steam engine with sheet metal boiler

According to the Smithsonian this is the Stringfellow engine which won a Royal Aeronautical Society prize for best power to mass ratio in 1868. It is unclear when the engine was built. Stringfellow wrote he had removed it from an unspecified "old model" to take it to the exhibition.

1868: The Royal Aeronautical Society held a show in the Crystal Palace in London. Due to his reputation, John Stringfellow was invited to exhibit one of his models. He got them out of storage but decided none was in sufficiently good condition and so built a new tri-plane model with a 1/3 hp steam engines. During the exhibition it ran along a wire stretched across the exhibition hall. Free flight was not allowed due to the fire risk, but the machine seemed to lift the wire.

Stringfellow also exhibited a steam engine removed from one of his old models. The cylinder was two inches in diameter, the stroke length three inches. The working pressure 100 psi. It drove two four-blade propellers, three feet in diameter, at 300 rpm. As reported in Is a Flying Machine a Mechanical Possibility? (Scientific American, March 13, 1869, volume 20, Number 11, p. 169) the power was estimated as follows:

Area of piston, 3 inches; pressure in cylinder, 80 lbs. per square inch; length of stroke, 3 inches; velocity of piston, 150 feet per minute; 8 x 80 x 150 = 36,000 foot-pounds. This makes rather more than one-horse power (which is reckoned at 33,000 foot-pounds). The weight of the engine and boiler was only 13 lbs., and it is probably the lightest stream engine that has ever been constructed.

(This calculation seems to imply that the cylinder was double-acting. It also estimates that the average pressure in the cylinder was 80% of the boiler pressure which seems generous.)

If this engine used twice as much water and fuel as the 1845 engine (which was filled with 50 oz of water and 10 oz of fuel), that would add about 1.6 kg to the weight for a total of about 7.5 kg. That works out to 6.8 kg/hp, about the same as the Wright engine, albeit at a smaller size. Mr. Stringfellow was awarded a prize for the best power to weight ratio yet achieved in a steam engine.

1874: Félix and Louis du Temple tested a monoplane powered by a steam engine of their own design. The engine was equipped with a boiler in which small pipes were tightly packed into a lightweight firebox. According to Chanute (Progress in Flying Machines, 1894, pp 90-92) the machine, which was intended to carry a man, had a wingspan of 40 feet and weighted about 160 lbs. According to Sobolyev (История самолетов мира, 2001, p. 27) the engine weighted 59 kg. If it developed the 6 hp some sources suggest, then that is 9.83 kg/hp, worse than the engine in the Wright Flyer.

Design drawing of a monoplane with a single rectangular lifting surface and two propellers

Plans for Mozhaychy's 1884 Monoplane

Technical drawing of an open-frame engine

Drawing of the 10 hp engine which Messrs. Ahrbecker, Son & Hamkens, of Stamford Street, S.E. made for Captain Mozhaysky

In 1884 Russian naval officer Alexander Mozhaycky tested a steam-powered monoplane with two stream engines imported from England. According to Sobolyev (История самолетов мира, 2001, p 29) one engine produced 10 hp, the other 20 hp. He gives the combined mass to power ratio of the entire propulsion system including both engines, boiler, condenser, and separator as 5.5 kg/hp which is better than the performance of the Wright engine as measured in 1903. Sobolyev (p 30) quotes a report (Военная энциклопедия. Т16, Спб., 1916. С. 377) which reads “The apparatus rose from the ground, but being unstable it tipped onto its side and broke a wing”.

The engines were described in the Scientific American Supplement for the years 1881 and 1882 as follows:

The larger of these engines has cylinders 3¾ in. and 7½ in. in diameter and 5 in. stroke, and when making 300 revolutions per minute it develops 20 actual horse power, while its weight is but 105 lbs. The smaller engine--the one illustrated--has cylinders 2½ in. and 5 in. in diameter, and 3½ in. stroke, and weighs 63 lbs., while when making 450 revolutions it develops 10 actual horse power.

The two engines are identical in design, and are constructed of forged steel with the exception of the bearings, connecting-rods, crossheads, slide valves and pumps, which are of phosphor-bronze. The cylinders, with the steam passages, etc., are shaped out of the solid. The standards, as will be seen, are of very light T steel, the crankshafts and pins are hollow, as are also the crosshead bolts and piston rods. The small engine drives a single-acting air pump of the ordinary type by a crank, not shown in the drawing. The condenser is formed of a series of hollow gratings.

Steam is supplied to the two engines by one boiler of the Herreshoff steam generator type, with certain modifications, introduced by the designers, to insure the utmost certainty in working. It is of steel, the outside dimensions being 22 in. in diameter, 25 in. high, and weighs 142 lb. The fuel used is petroleum, and the working pressure 190 lb. per square inch.

1890: Clement Ader tested a steam-powered monoplane with bat-like wings. It was equipped with a 20 hp engine weighing 51 kg (2.55 kg/hp). With a gross weight of 330 kg the machine weighed the same as the 1903 Wright Flyer, but had 67% more engine power. Ader’s machine rose about 25 cm off the ground and flew for about 50 meters.

Machine with large sail-like wings and two propellers sits on a track in a grassy field

Sir Henry Maxim's test rig which was damaged on July 31, 1894 when the captive flying machine lifted and tore itself off of the track before the dual 180 hp steam engines could be stopped

Man in a long coat and hat, seated, holds a two-cylinder engine as big as he is on his lap

Sir Henry Maxim holds one of his 180 hp steam aero-engines

thin metal tubes side-by-side bent as a sheet into a tent-like shape

1894: Sir Henry Maxim operated a test rig for experiments with various wing configurations. In the April 1910 issue of the Journal of the Western Society of Engineers Octave Chanute described the experiments as follows:

Mr. Maxim built an enormous apparatus, weighting 8,000 pounds and spreading 4,000 feet of surface, moved by a steam engine of 360 horsepower. That machine was run upon a track of 9 feet gauge a good many times, and on one occasion it undertook a vagabond flight on its own account; its equilibrium was bad, however, and the steam was shut off; the machine alighted somewhat broken.

According to Sobolyev (История самолетов мира, 2001, p. 34) the engines, boiler, and condensers together weighted about a tonne for a mass to power ration of 3.1 kg/hp. At 8,000 lbs gross weight and 360 horsepower the Maxim test rig had one horsepower for every 10 kg of gross weight. That is nearly three times as much engine power per unit of vehicle mass as the 1903 Wright Flyer had.

Maxim, in his thesis Natural and Artificial Flight (Excerpted in The Aeronautical Annual, 1896, published as Artificial and Natural Flight in 1908) describes his experiments in detail and concludes:

My experiments have certainly demonstrated that a steam engine and boiler may be made which will generate a horsepower for every six pounds of weight, and that the whole motor, including the gas generator, the water supply, the condenser, and the pumps may be all made to come inside of 11 lbs. to the horse-power. They also show that well made screw propellers working in the air are fairly efficient, and that they obtain a sufficient grip upon the air to drive the machine forward at a high velocity; that very large aeroplanes, if well made and placed at a proper angle, will lift as much as 2 and a half lbs. per square foot at a velocity not greater than 40 miles an hour; also that it is possible for a machine to be made so light and at the same time so powerful that it will lift not only its own weight but a considerable amount besides, with no other energy except that derived from its own engines. Therefore there can be no question but what a flying machine is now possible without the aid of a balloon in any form.

So after experiments this respected aviation pioneer concluded that it was perfectly possible to build aero-engines weighing less than 5 kg/hp and that a steam-powered airplane was definitely possible.

1897: Samuel Pierpont Langley, secretary of the Smithsonian, achieved sustained powered flight with two large models equipped with steam engines which ran for up to two minutes. Interestingly in 1889 Langley had bought at least one Stringfellow engine from the inventor's son and sent it off for study.

In conclusion, when the Wright brothers wrote this letter they could allude to a well-known history of steam aviation engines stretching back more than 50 years. The steam aero-engines actually installed in actual man-carrying airplanes build and tested in the last quarter of the 19th century likely had a power to mass ratio comparable to that of the homemade gasoline engine used by the Wright brothers in their 1903 Flyer. While one can find fault with the "50 years ago" part of the statement, the overall statement that 19th century aero-engines were sufficient for sustained flight is sound.

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    $\begingroup$ Are there flying models of Stringfellow's 1848 model plane? Had the 16 lb engine+boiler of John Stringfellow really generated 1.1 hp, continuous power, it would have become, then in 1868, an extremely successful motor for powering tools (like a lathe) in small workshops, for powering small river boats, steam cars, draisines, even bicycles. In reality, Stringfellow's motor worked in pulses of a few seconds, below 10 s, followed by at least a 4-minute pause necessary for rebuilding the steam pressure. If you do the math you will see that it needs about 1.5 L of steam at 5 atm per second. $\endgroup$ – Robert Werner Apr 30 at 23:08
  • $\begingroup$ I was unable to find information about the output power of the 1848 engine. The 1.1 hp figure is for his 1868 engine and comes from the Scientific American. I did not get into the question of runtime because I don't think they are talking about practical airplanes here. They are talking about what it takes to make an airplane stay in the air for more than ten seconds and fly more than 300 feet. I have chosen to interpret their statement charitably. As readers of Chanute they would know that a kind of engine which could do that had been invented more than 50 years ago. $\endgroup$ – David42 Apr 30 at 23:54
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    $\begingroup$ @David42 , Regarding Clement Ader, I have found an article: "L'oiseau de M. Ader", Le Temps, Paris, July 9, 1891, col. 3-4, p. 2 ( see: gallica.bnf.fr/ark:/12148/bpt6k2329887/f2.item ) The author of the text says: "Installé dans un appareil affectant la forme d'un énorme oiseau, il [Ader] parvenait à s'élever dans l'espace et à parcourir une faible distance: une centaine de mètres, disent les témoins incrédules; 400 mètres disent les enthousiastes.". However, Ader himself in the long interview that follows the introduction says that he experimented. He does not claim he flew his plane. $\endgroup$ – duofilm May 2 at 5:46
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    $\begingroup$ @PeterKämpf I think you need to qualify "Using the empty weight of a steam machine for power to weight comparisons with internal combustion machines is totally misleading, bordering on manipulation." Both have hidden loads; water and fuel on the one hand, but also lubricant, fuel and possibly coolant on the other. An efficient flash steamer such as Maxim's can have excellent power-to-weight for short flights. $\endgroup$ – Guy Inchbald May 3 at 9:52
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    $\begingroup$ @GuyInchbald I agree. The Wright brothers mention increased propeller efficiency explicitly in their statement quoted in the question. I also address it above in paragraph four. It strengthens their statement greatly. My call for the identification of plausible candidate engines was posted as a criticism of another answer which simply lists engines and engine technologies which were unsuitable or unavailable. I spent almost a week attacking the problem from the other end, by asking what 19th century aviators were actually using. This proved far more fruitful and the answer above is the result. $\endgroup$ – David42 May 3 at 15:51

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