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I've read that a U-2's takeoff speed is around 115 mph (or kts?). Given those glider wings, how slow could one fly, lightly loaded, at low altitude? Say, less than 10,000 feet?

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A copy of the flight manual was released under a FOIA request and can be found here. It actually does not say a whole lot about stalls but it does reference some minimum speeds in the emergency engine out procedure section under "COMPLETE POWER FAILURE IMMEDIATELY AFTER TAKEOFF"

  1. Maintain a safe flying speed and control until contact is made

NOTE: The following minimum safe speeds (10kts above stall) are recommended for the accompanying take off fuel loads

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Fuel | Min Speed

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685 | 90

So at ~runway altitude lightly loaded the plane could fly as low as 90Kts which is 10 Kts above the stall speed which is in line with other answers here. (Operations Manual Page: 3-3). At full fuel the stall speed rose to ~105Kts.

The manual devotes a whopping 2 paragraphs to stalls and lists no performance figures for them in section 6.

Strangely (or perhaps not so strangely) the the document lists an "Appendix I" which should contain the Operating Data (performance figures) but is absent from the document and regrettably where you would find the true answer.

Keep in mind this is the Handbook from March 1st 1959 and there have been several variants since so the numbers may vary.

Note the takeoff speed for a lightly loaded U2 was indeed 95-100Kts (Operations Manual Page: 2-12)

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    $\begingroup$ Thanx, Dave. Close enough: I will assume that the swing-wing wing-extension space planes I'm designing as backstory for Sci-fi could loiter at 90 Kts. lightly loaded, 120 or so with a full load. JO $\endgroup$ – J.O. Nov 18 '20 at 23:28

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