Little is known of Karl Rapp's childhood and adolescent years. However, it is known that Rapp learned the engineering profession and was employed by Züst automotive company from approx. 1908 to 1911. It is believed he was active as a technical designer with Daimler Benz until 1912. Shortly after, he left Daimler-Benz for a higher position in Flugwerk Deutschland GmbH where he headed a department that built airplanes.
In October of 1913, Rapp and his partner, Julius Auspitzer, finally set up Rapp Motorenwerke GmbH, which served as a replacement for the Flugwerk Deutschland GmbH department of aeronautical design and engineering. This new company had a new aim – to build engines for all kinds of air and land vehicles. The company expanded rapidly and employed 370 coworkers by 1915.
Rapp Motorenwerke GmbH supplied engines to the Prussian military during World War I. Contracts were terminated and in 1917, a new name had to be selected to take the place of Rapp Motorenwerke GmbH. It was decided that Bayerische Motoren Werke would be the name the engine company would adopt from then on. Until the end of the war, aero-engines remain the company's only product. The BBE aero-engine was a big success under the designation BMW IIIa.
After Rapp left the company (immediately it was renamed BMW) he became chief engineer and head of the Aeroengine Department of the L.A. Riedlinger Machine Factory where he was probably employed until October 1923. Rapp lived in Switzerland from 1934, running a small observatory making solar observations. Karl Friedrich Rapp died in 1962 in Locarno.
According to en.wikipedia and danleimel.wordpress.com