Focke Achgelis Fa 330 Bachstelze (Wagtail)

Early in 1942, Focke Achgelis at Laupheim were asked to design a simple single-seat gyro kite which surfaced U-boats could tow aloft to extend the observer's range of view. At this time, the U-boats were being forced away from the dense shipping areas around the coasts of Britain and the United States to hunt further out into the Atlantic where there was greater safety, but where their low position in the water made searching for, and shadowing, the spread-out convoys a very difficult task unless a bosun's chair could be attached to the periscope. 

The gyro kite, designated Fa 330 Bachstelze, was seen as some sort of solution and ingenuity was shown in its design. The machine could be easily assembled or dismantled in a few minutes and stowed through a U-boat hatch. The body structure consisted of two main steel tubes, one horizontal and one vertical. On the horizontal tube was mounted the pilot's seat with controls and a small instrument panel, and landing skids, and, at the rear end, a simple tailplane, fin and rudder. The vertical tube, behind the pilot's seat, formed a pylon for the rotor. 

The freely-rotating rotor had three blades, each of which consisted of a tubular-steel spar with plywood ribs and thin plywood and fabric covering. Each rotor blade had flapping and dragging hinges with adjustable dampers. Blade pitch could only be adjusted, with screws, on the ground before take-off. The best results were normally obtained with the blade pitch as coarse as possible, although starting was then more difficult. In addition to the flapping and dragging dampers, there were also inter blade connecting cables and blade-droop cables, the latter being attached to the blades and to an inverted tripod extending upward from the rotor hub. The rotor axis was slightly ahead of the machine's c of G, and the towing cable attachment point was slightly ahead and below the c of G. 

Movement of the control column tilted the rotor head in the appropriate direction for longitudinal and lateral control, and operation of the rudder pedals gave directional control. The tailplane was not adjustable. The Fa 330 was launched from the deck of the surface-running U-boat by giving the machine a slight backwards tilt once the rotor was revolving. If there was a wind, a push by hand sufficed to get the rotor moving, but otherwise a pull-rope was wound around a grooved drum on the rotor hub. In case this rope did not slip off when the rotor started, an over-ride mechanism was fitted. 

Pilot training was given in a wind-tunnel at Chalais-Meudon near Paris, and the kite was very easy to operate and could be flown hands-off for up to 10 seconds. It is believed that two or three crew members of each Fa 330 equipped U-boat learned to fly it. 

Having 150m of towing cable available, it was possible to maintain an altitude of 120m thereby extending the possible range of vision very usefully to 40km compared with only 8km on the U-boat deck. In an emergency, the pilot, who had telephone contact with the U-boat, pulled a lever over his head which jettisoned the rotor and released the towing cable. As the rotor flew away and up, it pulled out a parachute mounted behind the pylon. At this stage, the pilot, attached to the parachute, unfastened his safety belt to allow the remainder of the Fa 330 to fall into the sea while he made a normal parachute descent. In a normal descent, the kite was winched in to the deck and, upon landing, the rotor brake applied. 

Although designed by Focke Achgelis, the Fa 330 was built by the Weser-Flugzeugbau at Hoykenkamp, near Bremen. This particular factory manufactured Focke-Wulf Fw 190 fuselages, a few Fa 223 helicopters and about two hundred Fa 330s. Variations made in the basic design were an increase in rotor diameter to 8.53m on late machines and the option of adding simple landing wheels to the skids. There was also a proposal, designated Fa 336, to build a powered version of the Fa 330 with landing wheels and a 60hp engine.