(Inspired by What's the best way to get through clouds when not instrument rated? which has at its heart a good question, but isn't getting good answers because everyone is jumping on the "don't be an idiot" bandwagon. That's generally good advice, but not salient to the question.)
Non-instrument flight into IMC happens. Sometimes because people are manifestly idiots, other times because of momentary inattentiveness, sometimes because instruments fail, and yet other times because of surprising meteorological conditions[*][**]. Knowing that no amount of planning and execution can ever guarantee we will have one of either function instruments or a visible horizon, it would be a useful arrow in our quiver to know what kind of open-loop control inputs could guarantee a stable descent.
- I've oftentimes half-joked that the safest way to get through a layer of IMC in a Citabria is to enter an immediate spin. Even a flat spin is quickly recoverable in the 7ECA, so the real concern is avoiding a graveyard spiral. Obviously, the spin remedy is not universal.
- Some gliders will enter into a gentle bank when hands and feet are taken off the controls and the airbrake are extended. Some will continue to bank and enter into a graveyard spiral (the AC-5M is an example of this).
- A slip was thrown out as an idea, and I can see a lot of merit to full deflection cross-controlled rudder and aileron while maintaining an airspeed which is north of stall and south of Vne.
- What are the sets of known open-loop control inputs which can result in non-divergent descent?
- The best answer to this question is a catalog of inputs which have been shown to work on specific airplanes, not a specific control which can work on all airplanes.
- To help frame this question, let's allow that a recoverable spin is considered to be a non-divergent descent. The maneuver does not diverge without active inputs, and can be entered into without reference to the horizon.
- What is the best way to determine which, if any, of the above are applicable to a specific airplane?
- Are there general guidelines, and do these guidelines vary based on fuel and baggage loadout (e.g. tip tanks full vs empty, c.g. forward vs aft)? Does turbulence change the optimal deflections, or just make them less effective?
Note, this is a question about pure aerodynamics. Answers pointing toward operations, preflights, calling ATC, referring to instruments to try to keep wings level, etc.. will be downvoted. Those are are useful and accurate answers for a different question, but not this one.
[*] Raise your hand if you knew that highly humid air minutes after sunset is a ripe condition for clear-air icing? I sure didn't until I found my windshield coated in 1mm of ice within 60 seconds of passing through a humid area. There was-- and continued to be out my side windows-- horizon-to-horizon visibility. It was most strange and would have been a lot scarier if I hadn't been randomly lined up on a 3mi final on the airport I was flying past.
[**] The FAA considers cloudless, nighttime, moonless flight over the ocean to (potentially) be IMC. C.f. https://www.ifr-magazine.com/technique/actual-conditions/