Otto Lilienthal: glider pioneer and his flying machines

Every jet you photograph at the fence carries a piece of Otto Lilienthal in its wings. The engineer from Anklam was not only one of the first people to be carried through the air with documentation – before that he spent years measuring why a curved wing carries and a flat one barely does. This article tells how Lilienthal went from bird watcher to glider pilot, which flying machines he built, what led to his crash in 1896 and where you can still encounter his gliders today.

Otto Lilienthal in one paragraph

Otto Lilienthal (May 23, 1848 in Anklam – August 10, 1896 in Berlin) was a mechanical engineer, manufacturer and aviation pioneer. Together with his brother Gustav, he spent decades investigating how birds fly and what forces arise on wing surfaces. In 1889 he summarized the results in the book “Birdflight as the Basis of Aviation” (“Der Vogelflug als Grundlage der Fliegekunst”). From 1891 he put them into practice: with hang gliders he built himself, he made an estimated at least 2,000 flights by 1896. His Normalsegelapparat is considered the first aircraft built and sold in series.

Background, education, factory

Lilienthal grew up in Anklam in Western Pomerania, the eldest of eight children of a merchant family, of whom only three survived childhood. As a schoolboy he watched the storks in the area with his younger brother Gustav and tried his hand at first wing constructions. After the Gymnasium in Anklam, he attended the provincial trade school in Potsdam from 1864, worked as an intern at the Berlin machine factory Schwartzkopff and from 1867 studied on a scholarship at the Trade Academy in Berlin, among others under Franz Reuleaux. He turned down an offer to stay on as his assistant.

Lilienthal earned his living not from flying but from steam technology. In 1881 he received a patent for a coil-tube boiler and a small engine, and two years later production began in his own steam boiler and machine factory in Berlin. The business grew to as many as 60 employees, and from 1890 Lilienthal gave his workers a quarter of the net profit – an unusually social approach for the time. Of about 25 known patents, only four concern flying machines. The factory was the financial basis without which his flight experiments would hardly have been possible.

Measuring instead of flapping: the research behind the flights

Many contemporaries tried flapping wings that they wanted to move with muscle power. The Lilienthals, too, experimented with such constructions in Anklam in 1867/68. What was decisive, though, was what Otto did from 1874: he built a whirling-arm apparatus, on whose arm he moved surfaces in a circle through the air and measured the forces that arose. That way he compared flat plates with curved surfaces, varied angles of attack and curvature and collected series of figures.

The result sounds obvious today but was a breakthrough then: slightly curved wings produce clearly more lift than flat ones at the same drag. Lilienthal presented the measured values for various angles of attack in diagrams that plot lift against drag. This form of presentation lives on in aviation as the “Lilienthal polar” when it comes to evaluating wings and whole aircraft.

In 1889 his book on bird flight appeared, in which he brought together observation, measurement and conclusions. It is considered one of the most important aeronautical publications of the 19th century. Lilienthal himself concluded from it that people could learn to fly not at a desk but only by practicing in the air – that is, through glider flights in which you feel your way step by step toward wind and control.

The flying machines at a glance

Lilienthal built his gliders from willow wood and bamboo, covered with cotton fabric. He hung in the frame by his forearms and shoulders, legs free, and steered by shifting his body weight – the principle modern hang gliders still use today. None of his manned flights had an engine. The most important types:

Year	Machine	Wingspan (approx.)	Special feature
1891	Derwitz glider	about 7 m, shortened during the experiments	first successful glider, flights of about 25 m
1892	Südende glider	about 9.5 m	wings covered on both sides
1893	Maihöhe-Rhinow model	about 7 m	foldable, patented, forerunner of the Normalsegelapparat
from 1894	Normalsegelapparat	6.7 m	first production aircraft, bumper hoop against crash landings
1894	Storm-wing model	about 6 m	smaller wings for stronger wind
1895	Small and large biplane	about 5.5 to 6.5 m	two wings stacked for more area with a small wingspan
1893–1896	Flapping-wing machines	about 7 to 8.5 m	Experiments with a carbonic acid engine, without convincing success

The dimensions come from the documentation of the Otto Lilienthal Museum and are rounded. There were also test vehicles such as a machine with a movable surface on the wing’s leading edge (1895) and an articulated-wing machine that was almost ready to fly when Lilienthal died. That he also tried rudders and wing warping shows that he was already looking for control beyond pure weight shifting.

The Normalsegelapparat: the first production aircraft

The best-known type is the Normalsegelapparat. Lilienthal developed it in 1893 and had it built in small series in his factory from 1894. With a 6.7-meter wingspan and a weight of around 20 kilograms, it was handy enough to carry up a hill. The Deutsches Museum gives a glide ratio of about 5 and a flight speed of around 10 meters per second; other sources say closer to 1:4. The machine cost 500 marks, and depending on the source, eight or nine buyers are known by name, among them the Munich engineer Alois Wolfmüller. That made it the first aircraft in history sold as a product.

A detail with great effect was the bumper hoop at the front of the frame, a curved bar that absorbed the impact in hard landings and protected the pilot. Four Normalsegelapparat gliders survive to this day, some restored: at the Deutsches Museum Munich, at the National Air and Space Museum in Washington, at the Science Museum London and at the Zhukovsky Museum in Moscow. On top of that come dozens of replicas worldwide.

The flying sites: from the sand pit to the flying hill

Lilienthal’s flying history is also a history of hills around Berlin. Each place brought him a step further.

Among those who helped with his experiments were Paul Beylich, Hugo Eulitz and Paul Schauer, who had to carry the gliders, watch flights and repair damage. Gustav Lilienthal took part in the early research but no longer in the glider experiments after the book appeared.

The flying hill is now a memorial site in the Berlin district of Lichterfelde, laid out as a monument since 1932. At the Gollenberg near Stölln, the Lilienthal-Centrum commemorates the pioneer, and there is also a special curiosity there: on October 23, 1989, an Ilyushin Il-62 of the East German airline Interflug, named “Lady Agnes” – after Lilienthal’s wife – landed on a grass runway near Stölln to stay as an exhibit.

The crash at the Gollenberg in 1896

On August 9, 1896, Lilienthal was flying at the Gollenberg again. A gust – often described in the sources as a “sun gust,” that is, a thermal updraft – pitched the machine up further, he lost airspeed and fell from a height of about 15 meters. After the accident Lilienthal was at first conscious but died the following day, August 10, 1896, in a Berlin clinic.

The autopsy found a fracture of the third neck vertebra; more recent evaluations consider a brain hemorrhage the more likely cause of death. In 2016 the German Aerospace Center (DLR) examined a faithful replica of the Normalsegelapparat in a wind tunnel. According to accounts of this investigation, no design flaw showed up; a piloting error in the critical situation is suspected instead. The replica was also on show that same year at ILA Berlin.

On his grave in the Lankwitz cemetery in Berlin is the sentence “Sacrifices must be made.” It was added only in 1940, and that these were really his last words is considered unlikely – a good example of how legends form around pioneers.

Lilienthal and the Wright brothers

Lilienthal’s death fell in a time when aviation enthusiasts worldwide followed his photos and reports. The Wright brothers in Dayton studied his publications thoroughly and long considered his tables the best published data on the subject. In their own experiments they did find that some values didn’t fit their wings and built a wind tunnel of their own – but the methodical approach, first glide and then add power, comes directly from Lilienthal’s school. How it went from there to the first powered flight is told in the article The Wright brothers and the first powered flight in 1903.

Wilbur Wright himself wrote in 1912 that Lilienthal had been the greatest of the precursors and that the world stood deeply in his debt. The Russian aerodynamicist Nikolai Zhukovsky and the French aviation pioneer Ferdinand Ferber also paid tribute to his work. The two great lines of early aviation – lighter than air with balloon and airship, heavier than air with wings – ran in parallel at the time; one led to the airships, whose path you can read about in the history of zeppelins, the other via Lilienthal to the airplane.

Where to encounter Lilienthal today

For aviation fans, Lilienthal is not an abstract name from a history book – his traces can be visited. The post aviation museums in Germany offers an overview of other venues.

• Anklam: The Otto Lilienthal Museum Anklam in his birthplace documents his life and flying machines in detail, including with replicas. A large part of his estate is held there and at the Deutsches Museum.
• Munich: The Deutsches Museum holds an original Normalsegelapparat and parts of the estate.
• Washington: At the National Air and Space Museum, a Lilienthal glider is among the oldest aircraft in the collection.
• Vienna, London, Moscow: Further originals are at the Technisches Museum Wien, the Science Museum London and the Zhukovsky Museum Moscow.
• Berlin and Brandenburg: The flying hill in Lichterfelde, the Lilienthal monument on the Teltow Canal from 1914, his honorary grave in Lankwitz and the Lilienthal-Centrum in Stölln with the Il-62 “Lady Agnes.”
• Wasserkuppe: Anyone who wants to understand how motorless flying continued in Germany after Lilienthal belongs at the German Gliding Museum on the Wasserkuppe.

Many spotters also know Lilienthal’s name from airline operations: the former Berlin Tegel Airport carried the name addition “Otto Lilienthal” from June 7, 1988. What shaped the airport until its closure is covered in the history of Tegel Airport. And if you want to experience gliding yourself, the article on the glider pilot license SPL gets you started – at one of the many glider sites you can find near you with the guide finding small airfields on maps.

What to watch out for

• Original or replica? Many Lilienthal gliders on display are replicas. The Deutsches Museum, for example, had a replica made in 1958 after an original from the estate that had become brittle; it now stands in the Flugwerft Schleißheim. Signs and museum databases clarify what you’re looking at.
• Numbers are often rounded. Flight counts, distances and sales figures are partly based on estimates and tradition. Reputable sources therefore say “at least” or “about.”
• Gliding is not powered flight. Lilienthal flew without an engine from hills. The first controlled powered flight came only in 1903, by the Wrights. Both achievements belong together but are not the same thing.
• Photography in the museum: For pictures of old fabric coverings, soft, even light helps. Flash is not allowed or unwelcome in many venues – ask if in doubt.

Conclusion

Otto Lilienthal was the engineer who turned flying from a gamble into a method. First came decades of measuring, then the 1889 book, then thousands of jumps from sand pits, slopes and his own flying hill. With the Normalsegelapparat he created the world’s first production aircraft, and with his data the foundation on which the Wright brothers built. His death in 1896 showed how dangerous flying without effective control was – and his successors closed exactly that gap. Anyone who sees an airliner on final approach with flaps extended today is seeing his insight about curved wings in action.