Excuse my weak RW knowledge, but what exactly happens in a helo at high altitude? Assuming the air is thin enough to prevent overtorque, can you just keep putting the collective on until you get a rotor stall?
Depends which thing limits you. Don't automatically think of the air as "thin enough to prevent overtorque" although under specific conditions that is possible. The higher you go the more torque the rotor needs provide the same amount of lift and the engine does what it can in rarefied air to meet that demand, but generally speaking...
-Your engine might hit a temperature limit- TOT/TGT/ITT/T4.5; some fuel controls automatically "top" to prevent exceeding this and some don't (you can keep on pulling, go right through it, and torch your engine). This tends to happen at hot-high-heavy ambient conditions.
-You might hit a torque limit (varies between different aircraft by whatever critical component most often in the main transmission) but I don't think I've ever heard of automatic torque limiting (except the H-60 autopilot in certain modes, but that is not part of the engine fuel control). This tends to happen high-heavy hot or cold ambient air. Also one way to think of a torque limit in your day-to-day flying and your reputation with maintenance and the front office as vaguely like an airframe G-limit.
-Your engine can hit max Ng/N1; some fuel controls "top" this one too.
-You can pull so much pitch that the blades hit critical AoA and stall but the engine and transmission are still within torque and temperature limits. The blades don't usually stall evenly as they go around and around which can be very "exciting" as a pilot (understatement). This tends to be more likely not only at hot-high-heavy but at high speeds or aggressive maneuvering. It is unheard of in a hover.
-You could possibly hit the control stops but this varies between designs.
Ground effect and just a bit of airspeed (like 10-20ish knots depending on design) help
very noticeably. Most helicopters also have a very pronounced "bucket airspeed" typically from 40-70ish knots where power required is about half of what it takes to hover or go max blast. Service ceiling is normally higher than hover-in-ground-effect (HIGE) ceiling. HIGE is always higher than hover-out-of-ground-effect (HOGE) ceiling.
Hope that makes sense in a helo aero for dummies sort of way.
