Negative Torque Systems

[Macgyver]

Hey guys,

I don't fully understand the purpose of an NTS system. I was reviewing the manual for the SA226 and it says that the NTS allows the props to windmill for an air restart at about 70% of feather. Is an NTS system supposed to be an alternative to an auto-feather system? If you have an engine failure, I would imagine that in most cases an attempted air-restart would be inadvisable, so why would you want your prop windmilling when you can have an auto-feather system which causes the failed engine's prop to immediately feather?

Thanks in advance.

[MarkMcC]

The purpose of a NTS system is quite simply to prevent the propeller from driving the engine in the scenario where you have a low power setting and sufficent airspeed for the airflow to cause an increase in prop RPM. It serves to limit undesirable negative torque that would result in this situation (i.e. torque being absorbed in the opposite sense than that for which the system was designed). In certain aircraft the NTS system is an integral part of the airstart process, and you would need it to be servicable to restart an engine that you had previously shut down. Its action not only limits undesirable torque during an airstart, but helps prevent a massive overspeed resulting from a windmilling prop being driven to an overly fine pitch.

[Flight Detent]

Tester 355,

Spoken like a real...well...a real P3 flight engineer!

I agree with you entirely.

But I have a question for you....

when the negative torque get very high, way in excess of the controlling NTS system, the "NTS INOP" light illuminates, at which time the prop is to feathered immediately. note: there is only one NTS INOP light!

the question is, whilst airstarting both outboard engines at the same time, the NTS INOP light starts to flash, which engine do we shut down - shutting down both engines is not an option!

Just thought I'd ask, a P3 flight engineer would know this one!

Cheers FD :eek:

[barit1]

The Dowty prop on the Saab 340 had an "autocoarsen" system - short of a full feather, and alleged to be a lower drag condition than feathered. But that was long ago - updates please?

[ironbutt57]

The NTS system on the TPE-331 and other turboprops takes the propellor to a low drag "windmilling in the NTS mode" as the TPE-331 folks say it in very short order...the performance figures for engine out ops is based on this configuration as opposed to the prop being fully feathered...the big advantage to this system, is if the engine were to relight, the propellor quickly returns to prop governing mode, and presto...engint thrust is once again available..if the prop were fully feathered, this would not be the case... some of the larger engines... allison 501/t-56 engines use an autofeather system during takeoff and landing, and rely on the NTS system during other phases of flight..the "auto-coarsen" system while mchanically different is along the same lines of the NTS system...the beauty of an nts system is it requires no electrical power whatsoever..and allows an engine to relight and go on about it's business in the event of sudden flameout..if the engine is mchanically unable to do so, then subsequent checklist procedures call for finishing the job and feathering it manually..hope this sheds some light on the subject...

[zerozero]

I flew the Metro III (SA227) for 3000 hours.

NTS operation also includes *automatic* (on the SA227) ignition to aid in the re-light.

The point is, in case of ice/water ingestion, there might be a momentary loss of power. In the meantime, the prop moves *towards* feather to reduce drag while the ignition comes on to help the re-light when the ice/water is finally clear. It all happens very quickly and in 3000 hours of flying around Alaska I only saw it happen once.

The Garrett is quite durable and the engine inlets have sufficient heat but the SPINNERS have no ice protection whatsoever. You really need to be very cognizant of those chunks of ice and what happens to them when they depart the spinner.

I would run the ignition *constantly* until the ice on the spinner finally melted off.

[MarkMcC]

Tester 355,

Spoken like a real...well...a real P3 flight engineer!

I agree with you entirely.

But I have a question for you....

when the negative torque get very high, way in excess of the controlling NTS system, the "NTS INOP" light illuminates, at which time the prop is to feathered immediately. note: there is only one NTS INOP light!

the question is, whilst airstarting both outboard engines at the same time, the NTS INOP light starts to flash, which engine do we shut down - shutting down both engines is not an option!

Just thought I'd ask, a P3 flight engineer would know this one!

Cheers FD :eek:

Actually...I'm a fixed wing test pilot with a fair bit of C130 background. Although the engines and props of a C130 are similar to a P3, I have no specific knowledge of the warning system to which you are referring and as such will beg off from guessing.

Must admit though, sounds like a compelling argument for airstarting one at a time!