Читать книгу The Rise of the Flying Machine - Hugo Byttebier - Страница 39

S. P. Langley (1890/1896)

In 1890, when Langley had finished his experiments with aerodynamics, he arrived at the conclusion that: “I have not asserted that mechanical flight is practically possible since this involves questions as to the methods of constructing the mechanism, of securing its safe ascent and descent, and also of securing the indispensable conditions for the economic use of the power I have shown to be at our disposal — the condition, I mean, of our ability to guide it in the desired horizontal direction during transport — questions which, in my opinion, are only to be answered by further experiment.”

Langley’s experiments had shown the theoretical possibility of sustaining 200 lbs in the air for each horsepower “rightly applied”. Steam engines which gave one horsepower for every ten lbs of weight were then available and Langley thought that it was not necessary to defer further experiments, but that the building of flying models should be started at the earliest opportunity. He was already aware that a higher power to weight ratio would be needed than his earlier experiments had indicated.

To quote again from Langley’s report: “At the time the experiments with flying models were begun, Pénaud’s ingenious but toy-like model was the only thing that could sustain itself in the air for even a few seconds and calculations founded upon its performance led to the conclusion that the amount of power that was required in actual free flight was far greater than that demanded by theoretical enunciation.”

Langley thereupon built over 30 modifications of Pénaud’s rubber-driven model and spent many months in ascertaining the laws of stability during horizontal flight. He also experimented with several wing-shapes, some of which were biplane constructions, as well as studying how many screw propellers would be necessary to convert the power available into thrust and where they should be situated on the plane.

But in spite of his efforts he was not at first able to obtain flights of a duration equal to those Pénaud had achieved and as the use of twisted rubber as a power source limited his possibilities, Langley finally decided to resort to steam as the power source for a model aeroplane.

Therefore in 1891, Langley with great enthusiasm, set about building his first model, driven by steam, which was later called “Aerodrome No. 0” as well as building the steam engine that was to power it. The resulting machine was a monoplane of 50 sq ft with a fixed tail at the rear, which was a sort of second wing with a surface 25 sq ft. The total weight of the machine was calculated as not exceeding 25 lbs.

The experiment turned out to be “a hopeless case” because Langley obtained neither the power he had intended nor the strength and stiffness necessary to oppose the aerodynamic forces at play and when the plane was finished it weighed nearly twice as much as originally estimated.

In the meantime, Langley was invited by Maxim to see his great flying machine in England. He did so but did not agree with the “general design” of Maxim’s aeroplane, and upon his return (in 1892) he built a second “Aerodrome” which was named No. 1. The dimensions of this second model were on a more modest scale, as it weighed only 4.5 lbs with wings of 6.5 sq ft surface. But in this case the results were again negative as the engines were unable to provide the power necessary to lifting the plane.

A third plane (No. 2) was no more successful than the previous ones and the year 1892 “closed with disappointment”. Aerodrome No. 3 was then built. It was structurally stronger and boilers of the Serpollet type were installed “at great cost of labour, but again no results” were obtained.

For Aerodrome No. 4, which was built at the end of 1893, a tandem-wing form was adopted in order to improve the longitudinal stability. Hargrave had publicized this type of construction in his paper at the international air conference and had mentioned the experiments made by an Englishman, D. S. Brown, in 1873 and 1874 when his unpowered models had flown fairly successfully. Brown had used wings of equal size placed one behind the other at the same angle of incidence, quite unlike Pénaud’s system of 1871.

Langley then made a new effort to obtain more power by using steam pressures of between 70 and 100 lbs/sq in. He discontinued the Serpollet type of boiler and used a continuous helix of copper tubing but he did not really make much progress.

“The great difficulty”, wrote Langley, “was in keeping down the weight because every time one of the flying models was completed the weight had so increased beyond the original estimate that the aerodrome was too heavy to fly”. But in the latest one the weight was kept below 10 lbs, and seemed about right so that the time had arrived for the first launchings.

A small houseboat was procured and moored in the Potomac River about 30 miles from the place where the “Aerodromes” were built and modified. This meant that a trip of 30 miles and back was necessary every time an attempt at flight was to be made.

An additional difficulty now presented itself because, as in every aeroplane before or since Langley’s models, flight could not start before initial velocity (take-off speed) was attained. With very light model aeroplanes, the plane was simply thrust forward by hand but when the planes became too heavy another system had to be devised. Langley solved the problem by using a catapult, which was moved by a spring.

The first attempt at launching was made on 18 November 1893 but the troubles were not yet over and it was not until October 1894, or nearly a year later, that a successful launch was finally made.

“The aerodromes were now put fairly in the air,” wrote Langley, but new troubles arose because not on two occasions did the model behave in a similar way during flight. It was finally discovered that, when at speed, the wings were irregularly deflected by the upward pressure of the air.

Once this problem had been diagnosed, static tests were undertaken and it was found that the wings distorted “beyond all anticipation”. Langley’s models suffered from a lack of structural rigidity — a weakness that would continue to plague every Aerodrome he built, including the last one.

Later in 1894, another bigger Aerodrome (No. 5) was built weighing 2 lbs and having a wing surface of 68 sq ft this appeared to be an improvement on the former ones but even so no results were obtained at first but then quite suddenly, on 24 October 1894, a successful flight of some six or seven seconds was made.

At the end of that year (1894) Langley was introduced to a young man who was strongly bitten by the aeronautical bug. The introduction was made by James Means who later published the prestigious Aeronautical Annual for three years after the demise of the Aeronautical Journal at the end of 1894.

The young man in question was Augustus Moore Herring, who at that time was experimenting in the Bronx with a Lilienthal-type of glider. Langley engaged him to run the aerodynamics department and shop as from 20 May 1895.

During his stay at the Smithsonian Institution, Herring appears to have persuaded Langley of the efficiency of curved wing sections, rather than the flat sections which Langley had favoured so far. Herring also designed a new burner for the boilers of Aerodromes Nos. 4 and 5, in replacement of the original ones which had given such a lot of trouble.

There can be little doubt that Herring was a very talented craftsman and that he assisted Langley materially at a moment when the whole project still seemed a hopeless undertaking.

But Herring was also arrogant, and he soon began to complain to his friends and acquaintances about what he called Langley’s interference with his work. As Herring was at that time only a 27 year-old employee, the least that could have been expected of him was a modicum of patience with his employer, who was the secretary of a highly regarded institution. But that was asking too much of Herring and he resigned at the end of 1895. Langley, who was not an easy man to work with, was not sorry to see him go. But Herring thereby lost the opportunity of being associated with the success of the “Aerodromes” which came quite suddenly in 1896.

At the time that Herring left the Smithsonian Institution, the Aerodrome on test (No. 5) had been improved in the aspects in which the former ones had been found wanting and it had been possible to raise the power output of the steam engine to 1 hp or slightly more and, as Langley wrote, “the outlook for further improvement seemed to be almost hopeless”. Even so, final success was drawing near because early in 1896 the Aerodrome began to fly successfully. On May 6 1896 an official flight was made in the presence of a delighted Alexander Graham Bell when a distance of about 3,000 ft, over half a mile, was covered in flight.

A new Aerodrome (No. 6) had meanwhile been built which was similar to No. 5 in aspect and in weight but had the wing area reduced from 68 to 54 sq ft, so that No. 6 was loaded at 0.5 lbs/sq ft, a very low figure. On 28 November 1896 No. 6 made an officially recorded flight of three-quarters of a mile and stayed in the air for over one minute.

Having proved successfully that a powered heavier-than-air model flying machine was capable of stable and continuous flight, Langley was not keen on building bigger and much more costly machines but politics interfered in favour of continuation of the experiments towards a full-size man-carrying aeroplane as will be related in a following chapter.