A response to a previous question indicated, “In a helicopter, aft cyclic (“pitching up” in a plane) will only serve to make the helicopter climb and slow down until it reaches equilibrium in the new attitude.” I understand that is not the same thing as a “retreating blade stall.” My question is: what will happen if you pull aft cyclic until the helicopter is vertical to the ground, that is, pointing straight up to the sky and you hold that attitude and power setting? Will it simply “slide” backwards vertically when it can climb no further (like a hammerhead stall in a fixed wing)? How do you recover?
If you pull a helicopter into a vertical attitude, you'll have to unload the rotor to maintain the vertical path, since the rotor's down thrust is also part of your forward thrust component, unlike an airplane with a separate propeller than can continue pulling the plane up while the wings are unloaded.
You can't maintain a vertical climb on rotor thrust like you would an airplane, because the rotor thrust component acting on the helicopter's vertical axis will pull the machine sideways, toward the rotor. You have to effectively become a ballistic object with the rotor disc more or less a pinwheel doing nothing at that point, and the height of the hammerhead would be limited to the inertial energy in the machine once vertical.
Once vertical, when it stops going up, you would kick the thing around with the tail rotor to get it pointed down and accelerate down and pull out.
Of course, you'd better be in a helicopter with a semi-rigid rotor system that is designed to tolerate aerobatic maneuvers and 0-G flight without danger of exceeding blade flapping limits and chopping the tail boom off.
If you were in a machine with a 2 blade teetering rotor, forget about it; you'd become a falling object from mast bumping breaking off the rotor head as soon as you got zero G.