Scott Manley | How To Design A Plane To Fly At 100,000 Feet? @scottmanley | Uploaded 1 year ago | Updated 1 day ago
Aircraft service ceilings are generally limited by their engine's ability to generate the thrust needed to sustain the airspeeds required to maintain lift at altitude. Roughly speaking the required thrust to exceed stall speed stays the same as altitude changes, but the engines get less powerful as that air gets thinner.
But in many regular planes they would be further limited by the point where the speed of sound converges with their stall speed, this is the case with the U2 which rides in a region of its performance envelope known as Coffin Corner.

Lots of these concepts are covered in the Pilot's Handbook of Aeronautical Knowledge, a free publication by the FAA
https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak

Lol at that intro.

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