Roadburner426 wrote:That whole negative torque thing is a huge deal on the C-130's. A few props separated from the shafts and/or disintegrated so now they have pretty defined limits on what they can do. Scary to think stuff like that can happen on aircraft. Although a year or so back we did have an incident in the Navy H-60 community where a tail rotor separated from the tail gear box of an aircraft in flight. That was down to maintenance error though and not over torqueing on the pilots part. Still scary stuff like that can happen when there are all kinds of procedures and safety devices in place to stop it.
Negative torque issues mainly do with turboprops with direct drive, or single spool in other words. The issue comes that if, on a high-speed descend for instance, we pulled back the power lever all the way to the flight idle, the propeller governing function essentially attempts to keep the prop and the engine speed at somewhere around 100...96 % or so, where it is usually set at. The issue here is that it would need to drive the compressor as well. While much of the energy going into gas compressed is extracted in the turbine even if we had a flame-out, this is not even nearly 100 % efficient. And the power required to turn a compressor is...well, astonishingly high: as a rough rule-of-thumb, it makes up some two-thirds roughly of the power extracted by the turbines (in case of 500 net shaft hp engine, some 1000 hp is roughly required to run the compressor).
The first issue would be maintaining control of the airplane, not so the engine. In TPE331 series this is temporarily dealt with NTS system (reducing the need to
instantly feather if an engine went out for instance - it is not there to protect the engine!), that automatically moves the prop a bit towards feather by dumping the oil from the prop control system when triggered by the negative torque sensing feeling above the threshold. This "NTS'ing" can be felt as slight lateral jerks while the power is reduced. Any engine should be able to deal with a flameout condition without sustaining damage making the situation unrecoverable. In case of TPE331, the NTS system is in place to keep the blade angles from racing towards flat due to regular governing function in case the engine fails, or if the pilot reduces the power below what is needed to turn the prop.
Prop braking doesn't always equal to negative torque - this positive torque is actually the hazardous condition in case of PT6. The problem rises precisely from the fact the engine is producing significant amounts of "positive" torque whereas the prop is at negative angles. Any attempt to regularly govern the engine in that situation causes an immediate, and sustained, loop to overspeed the engine, as the blades naturally tend towards feather, and there exists an area of significantly lower engine load (the aerodynamically flat pitch range) in between, and there is no further governing system to back up the entering on this range from happening.
-Esa
