Anyone have any idea how noisy an unpowered, towed, autogyro would be, from the perspective of the pilot? Rotor and slipstream?
The reason I ask is that these aircraft would normally only be operated in conditions of good visibility, when altitude would provide a meaningful increase in visual detection range of targets and surface-threats, and only after a 360 deg sweep for visual and acoustic contacts, so the chances of being surprised by an enemy warship would seem to be be minimal.
The only reason for a potential crash-dive then would seem to be the threat from patrol aircraft. The conning tower of a surfaced U-Boat was brimming with lookouts, but their ability to detect the sound of approaching aircraft would have been be severely limited by the presence of the adjacent engine induction and exhaust systems. Would the autogyro pilot be able to hear an approaching aero-engine?
Also perhaps worth mentioning that a U-Boat's response to an airborne attack was not always necessarily a crash-dive. Later models were bristling with Flak, and several patrol/attack aircraft were lost to them, including a Liberator of 200 Sqn RAF, who's captain was later posthumously awarded with the VC, solely on the testimony of the U-Boat's crew.
https://en.wikipedia.org/wiki/German_submarine_U-468