The Focke-Achgelis FA 330 Bachstelze (English: Wagtail) was a type of rotary-wing kite, known as a gyroglider or rotor kite. They were towed behind German U-boats during World War II to allow a lookout to see farther.

Because of their low profile in the water, the submarines could not see more than a few miles over the ocean. To solve this, the German admiralty considered a number of different options, including a folding seaplane (Arado Ar 231). In the end, they chose the FA 330, a simple, single-seater autogyro kite with a three-bladed rotor.

The FA 330 could be deployed to the deck of the submarine by two people and was tethered to the U-boat via a 150 m (500 ft) cable. The airflow on the rotors as the boat motored along on the surface would spin them up. The kite would then be deployed behind the U-boat with its observer/pilot aboard, raising him approximately 120 m above the surface where he could see much farther; 25 nautical miles (46 km) compared to the 5 nautical miles (9 km) visible from the conning tower of the U-boat. If the U-boat captain was forced to abandon them on the surface, the tether was released and the FA 330 descended slowly to the surface.

When not in use, the FA 330 was stowed in two watertight compartments aft of the conning tower. Recovering, dismantling, and stowing the FA 330 took approximately 20 minutes and was a difficult operation.

As Allied air cover in other theatres of the war was considered too much of a threat, only U-boats operating in the far southern parts of the Atlantic and the Indian Ocean used the FA 330. Despite its advantages, the use of the Fa 330 only resulted in a single sinking when U-177 used one to spot, intercept and sink the Greek steamer Eithalia Mari on 6 August 1943.

The Allies came into possession of an FA 330 in May 1944 when they captured the submarine U-852 intact. After the war, the British government did successful experiments towing Fa 330s behind ships and jeeps, but the development of the helicopter quickly occupied the attention of the military.

U-boats that deployed FA 330 kites included at least U-177, U-181, and U-852

Specifications:
Type: Gyroglider
Manufacturer: Focke-Achgelis
Crew: 2
Number Built: N/A

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Dimensions:
Weight: 180 lbs.
Height: N/A
Length: 14'6"
Rotor Diameter: 24 ft.
Disc Area: 450 Sq. Ft.
Disc Loading with 150 lb. pilot: .73 lb/sq. ft.
Typical Rotor Speed: 205 rpm

Performance:
Typical Airspeed: 25 mph
Minimum Airspeed To Obtain Flight: 17 mph
Ceiling: N/A
Rate Of Climb: N/A
Roll Rate: N/A
Payload: N/A

Focke Achgelis Fa 330
[Source: Unknown]

Sources:
Brown, Eric, Captain - Wings of the Luftwaffe , 1979, Airlife Publishing Ltd., Shrewsbury
Gunston, Bill & Wood, Tony - Hitler's Luftwaffe , 1977, Salamander Books Ltd., London
The Complete Encyclodepia of World Aircraft, 1997, Barnes & Nobles Books, ISBN: 0 7607 0592 5