On New Year’s Eve 1934, engineers in the machine laboratory of ETH Zurich switched on a new wind tunnel for the first time. Next door stood something even more unusual: a tunnel built to blow air past models at twice the speed of sound, in a closed circuit, for as long as the experimenters liked.

Behind both was Jakob Ackeret, a locksmith’s son from Zurich who had learned his trade under Ludwig Prandtl in Göttingen and who gave the world a term every pilot now knows: the Mach number. For the next half century, ETH’s Institute of Aerodynamics was where Swiss military aircraft went to be tested before they ever flew.

A Swiss in Göttingen

Ackeret was born in Zurich on 17 March 1898, studied mechanical engineering at ETH and graduated in 1920. In 1921 he went to Göttingen to work with Ludwig Prandtl, the father of modern aerodynamics, and helped expand Prandtl’s Kaiser Wilhelm Institute for Flow Research. In 1925 he published a theory of the forces on wings moving faster than sound, translated by the American NACA the same year. It was one of the first practical theories of supersonic lift and drag, written at a time when no aircraft could get anywhere near Mach 1.

In 1929, back in Zurich, he gave a lecture on air resistance at very high speeds and proposed a name for the ratio of an object’s speed to the speed of sound.

The Army Wanted a Wind Tunnel

Switzerland’s military had been asking for an aerodynamic test facility since the late 1920s, and in 1931 ETH decided to build one in its machine laboratory. Ackeret, an associate professor from 1931, became full professor and head of the new Institute of Aerodynamics in 1934. Construction of the main low-speed tunnel began in February 1934 and, by his own account, it ran for the first time on the last evening of the year.

Its companion was the remarkable one. ETH describes its supersonic tunnel as the world’s first to run in a closed circuit, recirculating the same air instead of blowing a short burst from a pressure tank. A 13-stage axial compressor from Brown Boveri, driven by a 700-kilowatt electric motor, pushed air through a test section about 40 centimetres square at up to twice the speed of sound. When Ackeret presented it at the Volta Congress on high-speed flight in Rome in 1935, the Italians were already building their own supersonic tunnel at Guidonia, which ETH’s archive records as modelled on the Zurich design.

Every Military Aircraft Through the Tunnel

The new facility went to work for the Swiss Army almost immediately. A model of the C-35 biplane was tested before the tunnel had even been fully calibrated, and models of the C-36 family followed, photographed hanging upside down in the test section with captions noting the work was done on behalf of the army. According to the curators of ETH’s collection, from the 1940s onwards every Swiss military aircraft was tested in the ETH tunnel. Buildings, road tunnels and cars went through it too.

Ackeret’s influence reached beyond the test section. He sat on the federal commission for military aircraft procurement from 1943 and chaired it from 1950. When Switzerland tried to build its own jet fighter, the FFA P-16 from Altenrhein, the theoretical basis for its thin, straight wing came from Ackeret and his colleagues Nikolaus Rott and Max Degen at the institute in 1950. After the P-16 was cancelled in 1958, Ackeret resigned from the commission.

From Zurich to Emmen

ETH’s tunnels became the template for the federal aircraft factory at Emmen, which built its own wind tunnels and research facilities on the ETH model. The Large Wind Tunnel Emmen, begun in 1944 and fully working by 1946, has a test section seven metres wide and five high and is still in use 80 years later under RUAG, now testing electric vertical-takeoff aircraft as well as conventional designs.

An Engineer Beyond the Wind Tunnel

Ackeret’s work went well beyond aircraft. With Curt Keller he patented a closed-cycle gas turbine in 1935, and the first two-megawatt plant of the type was built in Switzerland in 1939. In Göttingen he had worked on the rotor ships of Anton Flettner, and at Escher Wyss he studied cavitation in pumps and turbines. Among his students were several who went on to shape aerodynamics, including Nikolaus Rott and Werner Pfenninger. He gave his farewell lecture at ETH in 1967 on “the road to the supersonic airliner” and died in Küsnacht in 1981.

What Survives

The Institute of Aerodynamics closed in 1988, and the supersonic tunnel was demolished in the 1980s. The large low-speed tunnel survived. It is now a protected monument and is still used by ETH’s Institute of Fluid Dynamics. In 2019 and 2020, around 430 objects from the institute, including wind tunnel models of Swiss military aircraft, passed to the ETH Library’s collection of scientific instruments, and some are on show in the exhibition “Modelle – Wissen formen” at ETH until June 2027.

Sources: ETH-Bibliothek, short portrait of Jakob Ackeret; Swiss National Museum blog, “Im Windkanal der ETH” (15 September 2026); ETHeritage blog (2017 and 2020); ETHistory, Forschungspfade; J. Ackeret, NACA TM-317 (1925) and NACA TM-808 (1935); Schweizerische Bauzeitung 94 (1929); NDB-online, Jakob Ackeret (2023); higgs.ch; G. Bridel, SVFW presentation on the P-16 (2025); RUAG Aerodynamics; Aerospace Testing International (21 September 2026); ETH-Bibliothek Zürich Bildarchiv