@MichaelHall I think you hit the problem spot on the head. My original question wasn't explained well because it allowed a lot of room for misinterpretation. It looked just enough like a lay question about loops that people didn't give it much thought, and it was just hard enough that the lay answer is wildly different from the dynamics answer.

My background is controls, with a specialty in autonomous robotic flight (aka drones), so in general this stuff is highly relevant to me. The more I intuitively understand about flight, the more natural my flight algorithms are, and the less they look like a PC gamer who finds himself writing flight computer software.

And it doesn't hurt that as a glider pilot I can be obsessed with efficiency. I suspect inefficiency arises from the mismatch of the forces and torques about a pitching airframe. And since there aren't two controls to independently control PITCH rate and TURN rate, I wonder, "Is the inefficiency not that bad, or is it that it's really hard to solve the problem?"

@MichaelHall Hey, thanks a lot for this! It's great to have a conversation where someone can say, "Hey, that's not quite what I meant but I see how you saw it that way."

@KennSebesta so to fly a constant speed sinusoidal, on is skidding (in pitch) at the bottom and may be either coordinated or slipping at the top, depending on net G's.... the moon makes a coordinated orbit around earth (from tidal locking). Cheers.

@CharlesBretana a thought on "fictitious forces". Yes, that's a good name for them. They have no place on a vector diagram (even though many books have them there). With aircraft, the opposing force is either gravity or drag. An aircraft in turning flight should show an unbalanced centripetal vector because rotation never allows drag to catch up with the direction of centripetal acceleration.

A side slip can reach steady state because there is balanced rotational torque and no change in direction.

The crosswind is the other balancing force. (In a forward slip, forward motion is the "crosswind").

Michael Hall, Please, no need to apologize... I also sometimes let a bit of sarcasm into my posts and regret it afterwards. No offense taken, but I appreciate the comment.

@Robert DiGiovanni, Although I risk confusing the issue even further, I can't resist pointing out that Gravity itself is a "fictitious" force. The way you can tell you are in an accelerated frame of reference (FOR) is simple. What does a G-Meter read? If it reads anything other than zero, you are in an accelerated FOR. Einstein showed us that Gravity is NOT a force, it is just the perception we have in 3-D of the distortion of Space-Time due to Mass.

Another great clue as to whether something is a "Fictitious" force or not, is Do you only "Feel" the force when it is NOT causing an acceleration? The only time you "Feel" gravity is when we are resisting it. We really don't feel it, we feel the force of the lift on the aircraft that is stopping Gravity from accelerating us in opposition to the acceleration of the FOR. If Gravity is allowed to accelerate us, (Zero-G free-Fall) then we no longer feel anything.