Ludwig Föppl

Karl Ludwig Föppl (27 February 1887 – 13 May 1976) was a German mechanical engineer who succeeded his father, August Föppl as professor of technical mechanics at the Technical University of Munich. During World War I, Föppl worked as a cryptanalyst, initially in Inspectorate 7/VI, and later in the war within General der Nachrichtenaufklärung.

By 1940, he was a full member of the Bavarian Academy of Sciences. Föppl was one of the earliest cryptoanalysts in the German Army, working at this profession during both the first and second world wars, eventually becoming Chief of Sixth Army's Evaluation Office. His work was kept secret from both his family and his colleagues, even his later university assistant Friedrich L. Bauer, who would also become a well known cryptologist in his own right, never knew. In 2005, the work of Hilmar-Detlef Brückner of the Bavarian State Archive brought this aspect of Föppl's career to prominence. Brückner's work was subsequently fleshed out from information contained in Föppl's unpublished autobiography, still retained by his family, several chapters of which provided details of his work during the two world wars. His brother-in-law was the physicist Ludwig Prandtl. Föppl completed his Abitur in 1906 and studied mechanical engineering for two years at the Polytechnical Institute. He then spent a year at the University of Göttingen studying the theoretical aspects of engineering. He was promoted in 1912 to Dr Phil in mathematics, with a thesis titled Stable arrangements of electrons in the atom . He worked as an assistant to Felix Klein, a leading mathematician on Group theory, complex analysis and non-Euclidean geometry. In March 1914, Föppl habilitated in mathematics. He started teaching as a Privatdozent at the Physics Institute at the University of Würzburg, working with Wilhelm Wien. and the cipher was a variant of the Gronsfeld cipher. Föppl suggested it was used as it enabled the cipher clerk to encrypt messages very quickly and could be easily enciphered by ordinary sailors, with a key that was changed every few weeks, that could be broken within a single day. It was his photoelasticity laboratory, which gave him the possibility to continue working in the first years of the Second World War.

Föppl made an essential contribution to contact mechanics, even when this part of his work became acknowledged only in the last years. The key contribution is contained in his 8-page paper "Elastic stress in the ground under foundations" of 1941. In this paper, Föppl determined the deformation of the surface of an elastic half-space under the action of arbitrary plane as well as arbitrary axis-symmetric pressure distribution, in a form that allowed a simple inversion and thus solution of an arbitrary contact problem. This inversion was made in the dissertation of his doctoral student Gerhard Schubert, published in a shortened form in 1942. of his father August Föppl (with Otto Föppl) and co-author of Drang und Zwang.

Awards and honours Iron Cross, Second Class - 4 July 1915 Iron Cross, First Class - 16 April 1916 Military Merit Order, Fourth Class With Swords - 16 April 1916 Military Merit Order, Third Class With Swords - 12 September 1916