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Article title says plane but in the text it says powered flight.

But not "non human-powered" flight? If Icarus flapped a little, that counts.

If you can’t maintain altitude without losing significant airspeed, you’re just falling.

Even if you maintain both you can still be falling (orbits!)

Gravitational orbits lose energy over time. Not really relevant on human timescales but interesting trivia if nothing else.

Due to friction? Or is it other physics?

Gravitational waves, though in the vast majority of case they are extremely weak.