11/30/2019. Remarks by Johan Visschedijk: "The German company Aerotechnik Entwickelung und Apparatebau GmbH (Aerotechnic Development and Engineering Ltd.) at Frankfurt am Main, began work in the early 1960s to develop a low-cost, easy-to-fly helicopter that could be parked or stored in the minimum of space.

Designed by Wolfgang Müller, the single-seat prototype WGM 21 differed essentially from other and more familiar forms of helicopter in having four two-blade rotors, each mounted at the extremity of one of a pair of diametrically-opposed rotor support arms on top of the main rotor column. By adopting this configuration the entire output of the engine is transmitted directly to the main rotors, and it eliminates the need for tail control surfaces or a tail rotor. This permitted the flying controls to be simplified, the pilot being provided with a conventional yoke-type control column and controlling the aircraft about the vertical axis by operating foot pedals which cause the rotor support arms themselves to rotate. The support arms could also be folded, so that the helicopter could be stored within an area of only 7 ft 1.75 in by 6 ft 6 in (2.18 m by 1.98 m).

The prototype WGM 21 was built primarily to establish the basic soundness of the concept, and its construction was therefore kept as simple as possible. It had an open-work steel-tube fuselage and a tricycle landing gear with oleo-pneumatic shock-absorbers on the main units. Power plant was a 54 hp engine from a BMW 700 motor-car, which had a typical fuel consumption of ca. 2.64 gal (10 l) an hour at a cruising speed of 71 mph (115 kmh).

Completed in 1968, it was flight tested on a revolving seesaw. It showed excellent stability on all axles and could be flown by any pilot without difficulty. The LBA (Luftfahrtbundesamt, Federal Aviation Office) granted a preliminary CofA on May 17, 1968.

Following the successful testing of the single-seat WCM 21, a mockup of a two-seat version with an extensively-glazed and fully-enclosed cabin, designated WGM 22, was presented in 1968 as well.

Being a pure development company, Aerotechnik had planned from the start to sell the development with all rights to a patented manufacturer, but despite all efforts could not find a buyer, and the project was subsequently cancelled. The WGM 21 prototype is preserved at the Hubschraubermuseum (helicopter museum) at Bückeburg."