surfaces 27 on ring segment members 28 one of which is transposed to depict its operative engaging relationship with its cam follower 29 which is mounted through an arm 30 to the wheel housing 16 and the other cam ring 28 is shown in its normal fixed position mounted to structure in the floor the operation of retracting and changing the directional orientation of the powered wheel drive unit relies on the interaction of two cam systems basically the two cam systems comprise an upper driving cam surface 13 and a lower fixed cam surface 27 with the cam followers 14 and 29 respectively of the upper and lower cams being interconnected between the two cam surfaces 13 and 27 by the powered wheel housing 16 and the motion to the upper driving cam surface 13 being supplied by an external motor 25 through gearing 24 and 23 the upper driving cam system or the first cam 13 and follower 14 system is interposed between the inner surface of the flanged ring member 10 and the wheel support plate 15 which is fixed to the wheel housing 16 and the lower fixed cam system or second cam 27 and follower 29 system is interposed between the wheel support housing 16 and fixed structure in the floor of the airplane fig5 8 are schematic drawings of the basic principle of the dual cam system represented in a side elevational layout wherein like numerals represent.