Negative Torque Sensing

[uwochris]

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.

 

[pilotyip]

With a fixed a fixed shaft T-prop engine, if the engine failed and the prop remained in flat pitch the aircraft would be uncontrollable. The need for NTS goes up with speed, the higher the speed more energy is transferred from the air stream in an attempt to keep the engine on speed. The NTS automatically takes the prop to low drag configuration, thus allowing the airplane to be controlled. NTS is a back up for when you have the auto feather armed, or if it fails.

 

[avbug]

Uwochris,

Long reply warning...

The NTS system isn't a feathering system, though it's easy to confusing feathering with NTS. Yip was referring to engines that are driven through a shaft from the power section, or gearing from a shaft from the power section, when he referred to a fixed shaft engine...which differs from free turbine egiens such as the PT6A. Shaft driven engines such as the Allison or Garret turborops utilize the NTS system.

When your engine is producing power, any drag on the blade, holding it back against it's plane of rotation is torque. The twisting force on the drive shaft. Increase power produced by increasing the power lever, and the engine attempts to speed up. Through a complex arrangement between the speed sense functions of the propeller governor and fuel controller and throttle position indicator, the pilot is both scheduling fuel and commanding a propeller blade increase. This is similiar to what a constant speed propeller does in effect, when you increase throttle in a piston powered airplane. So far, familiar.

Increase power, the blade angle increases. You're used to this; the blade angle increases to maintain a constant RPM, and the more power you apply, the faster the engine wants to go, so the greater the blade angle must be to create enough drag to keep the propeller speed in check. When the angle increases, more drag is created on the propeller blade, which means more resistance, or twisting force, on the drive shaft and gearing. This twisting force is, of course, torque.

Increase power, blade angle increases, and you see a torque increase. Now imagine retarding the power lever to idle. As power decreases, the blade angle decreases. The blade flattens out, and the torque drops off. As the torque drops to a low value, you eventually reach a point where the engine is no longer driving he propeller, and it's being driven by the slipstream. As it's being driven by the slipstream and is flat pitched, it's speed wants to increase. The propeller governor in the turboprop engine does two things; it changes blade angle, and tells the fuel controller through a mechanical linkage to reduce fuel flow. With the fuel flow reduced to idle, it can't take away any more, and it won't try because the engine isn't driving the propeller. It's ony choice is to increase blade angle to prevent a speed increase. However, the governor only knows how to increase blade angle to prevent a speed increase when torque is being applied, because it does so as a function of engine RPM and a number of other factors...it's job is to maintain RPM when the power is pushed up, by increasing blade angle using oil pressure. It needs a helping hand.

In comes the negative torque sensing system. It isn't there to feather the propeller. It's there to help prevent an overspeed by moving the propeller to feather in one sense...but it's real job is to prevent the engine from being driven by the slipstream in a situation in which the engine isn't producing enough torque to drive the propeller. If the slipstream drives the propeller, the prop is absorbing an enormous amount of drag and energy, which acts to retard the propeller disc...it's like throwing out a big parachute. When the power lever is retarded to flight idle, depending on the engine settings, the drag can be significant in a turboprop (especially shaft driven engines) in a negative torque situation if the slipstream is driving the propeller.

Normally the engine is using the propeller to drive the airplane forward; it's pulling the airplane through the air...but in a negative torque situation, it's doing exactly the opposite. Instead of imparting energy to the air around it and propelling the air rearward and pulling the airplane forward, it's absorbing energy and holding the airplane back. It's like throwing out a parachute, or putting on the brakes. In a multi engine airplane, it's much more serious than just having an engine fail. When the engine fails, it should go to feather by itself, and drag isn't an issue...but when it's an negative torque state, it's working against the ariplane. The faster the slipstream, or in other words, the faster the airspeed, the greater the drag. Double the airspeed, quadruple the drag, and so on...the situation compounds exponentially as airspeed is increased.

To prevent this situation, the NTS system drives the propeller blade to a greater angle to load the propeller back up, reduce drag, and keep the RPM in check when the engine isn't driving the propeller any more. It does this through a separate system than the propeller governor speeder spring...the propller governor and it's primary control, the speeder spring and speed sense unit, is concerned with only one thing...how fast the propeller is turning. The NTS system is concerned with torque, however, and it's job is to increase the blade angle to load the engine when it learns the torque is approaching a preset value close to zero.

The NTS system increases the propeller blade angle, and in so doing, loads the drive shaft with positive torque, and at the same time serves to reduce drag on the propeller disc. Drag on the individual propeller blade is increased, but the drag of the overall propeller disc, which is absorbing the enormous energy of being driven by the slipstream, decreases because the slipstream is no longer driving the propeller. If the situation isn't alleviated, the propeller will pulse in and out of NTS...as positive torque returns with the blade angle increase, the NTS system stops it's efforts to increase the blade angle. Blade angle may stay the same or decrease. If it decreases, then torque decreases, and the propeller begins to absorb more energy from the slipstream gain...and the NTS system must once more increase blade angle.

NTS isn't feathering the propeller, nor is that the purpose or intent of the NTS. It's there to increase torque to a positive value on the drive shaft such that the propeller and engine is no longer absorbing all that energy from the slipstream. You can think of the autofeather system, or any feathering system, as an after-the-fact system that cleans up the airplane when things have gone awry. The NTS system, on the other hand, is an operational system designed to keep things chugging along, and to allevaite a temporary unwanted condition during operations. Autofeather is for engine failure, NTS is for engine operation. Autofeather functions are for when things have quit, and NTS is designed to keep things going.

Continued...

 

[avbug]

...deunitnoc

The airplane I was flying this summer used a Garrett TPE331-10/11U engine. (we had airplanes with either the -10 or -11, depending on how recently it was converted). It was set up to produce a significant amount of drag at idle...enough that retarding the power lever to idle in cruise could throw you forward against the shoulder straps and harnes, and even with the nose pointed very steeply down hill, the airplane would or could stall. LOTS of drag. We had them set up that way because the drag was a big asset when making steep downhill runs, especially ones with long descents.

In a very steep descent, the aircraft would pulse in and out of NTS, sometimes in a very pronounced manner, like a surging. A little disconcerting at first, it was also reassuring because it meant everything was working properly. When my engine failed it occured due to failure of the rear turbine bearing seal, which allowed all the oil save two pints to be ported overboard through the exhaust. My engine was producing normal EGT temperature, but without any torque. Additionally, no oil was available for propeller operation, and when I pushed the power up, the temps came up because the gas sction was still burning...the engine was producing power, but I couldn't use it because I had no control over the propeller.

My first clue that something wasn't right occured on the descent. I was doing a formation 3,500' descent in a canyon with another aircraft, and I couldn't slow down to stay behind him. At flight idle, I didn't get the drag out of the engine that I expected, and the NTS system didn't pulse. When I reached the bottom of the descent and pushed up the power, temps came up, but no torque, because there was no oil left to increase the blade angle...none for the NTS system to work it's magic on the way down, nothing to hold the propeller in place for the governor to do it's work on the descent, and nothing for the governor to use to increase propeller blade angle as I increased power with the power lever as I reached the bottom of the canyon.

It was a very unusual situation. Had I realized what was occuring (I was pretty focused on the airplane ahead and trying to stay out of the canyon walls on the descent...the windscreen was covered in ash and I was flying through smoke into the sun), I would have probably tried something differently and opened up more options...but the lack of the NTS operation was probably the biggest clue, and I missed it.

In this case, the propeller didn't really feather. It tried to move in that general direction, but I couldn't have feathered it because I had no oil pressure to dump, and it was trying to move in response to aerodynamic twisting force and it's own RPM...it was still being driven by the engine shaft, but there was no way to get torque out of it because I couldn't increase or decrease the propeller blade angle...neither could the governor...it was on it's own. It sought the position of least resistance, or itself. Not for the engine, nor for the airplane, but for itself, which was a compromise that neither myself, nor the engine, nor the airframe really wanted.

The actual propeller speed was a function of engine speed...the faster the engine tried to turn, the faster the propeller tried to turn, because it's attached through gearing to the engine driveshaft. At the same time, because it was free to act on it's own according to the various forces in flight, it wanted to align itself toward the feather or coarse position to alleviate the drag of the slipstream. It was producing more drag than I wanted, certainly more than if it were feathered, but no useful torque...sort of a perpetual state of NTS.

As airspeed reduced, it tried to align itself more and more with the plane of rotation, which meant moving toward flat or fine pitch (high RPM position), and I had pushed the power ever all the way up as I had been instructed to do in the event the propeller didn't feather in an emergency...so the engine was trying to go great gangbusters and was only being controlled in it's desire to overspeed by the governor removing fuel. It was a very rare, unusual situation. After I came to rest on the hillside, the propeller was turning, as the engine was still turning (it ran quite some time without oil), but with the power lever retarded to the idle position the blades moved toward feather on their own...no oil pressure to keep them anywhere else, and no engine RPM or aerodynamic twisting force to move them to the high RPM position any more. When I shut it down, the propeller stopped in the feather position. I didn't pay a lot of attention to it right then, as I was busy tossing gear out of the cockpit and egressing, but when I came around the front of the airplane it was solidly feathered.

The feathering action wasn't the result of NTS, which required oil pressure...it was the natural position of the blade...when fixed shaft engines are shut down, unless they're locked in a particular pitch position ("pitch locked" is a term unique to these engines, but with multiple appliciations depending on the engine) they'll go to feather with a loss of oil pressure...or they should. Without oil pressure, they should move to feather. With oil pressure, the NTS system forces the propeller toward feather/coarse/low rpm position, but for entirely different reasons than a feathering system. The autofeather or feathering system is designed to reduce drag on the propeller disc as a whole, whereas the NTS system is designed to increase drag on each blade to create resistance against the driveshaft (increase torque), while also reducing the drag on the prop disc by moving the propeller and engine out of a condition where it's being driven by the slipstream.

Feathering and NTS sound similiar, and appear similiar, but are actually very different. I hope that explaination didn't confuse you more...the relationship between the engine and propeller in a turboshaft engine such as the Garrett or Allison is perhaps the most complex of any aircraft engine, and it can be confusing. The pilot tells the engine in broad terms what he wants, and the fuel control talks to other parts of the engine such as the governor, to make it happen. However, the pilot isn't really controlling any one particular thing in the engine...in the turboshaft engine, the pilot is only controlling a small air valve (and mechanical linkages rough fuel control functions)...it's the air valve that meters air to the fuel control unit which tells the fuel control unit what to do, and that in turn talks to the propeller governor and other system components.

From the pilot end of things, what you need to worry about mostly is pushing up the power makes things go faster and burn more fuel. In an emergency, the engine should feather, but you can't take that for granted, and must still follow through with identification, verification, and shutdown, just like you're familiar with in piston equipment (no matter w(h)eather autofeather or other similiar systems are functional, or not). In my case, I didn't do that this last time, as the failure occured at 150', and I reached a luxurious 300' before starting back down again...I just didn't have time to do much other than make a turn and put it on the ground. Once it's quit, then NTS isn't really your concern any more...if there's oil pressure for it to do it's job it will keep trying until you dump the propeller oil by feathering (or the autofeather does it for you)...but NTS won't feather the propeller for you because it's job is to increase torque to keep things running. NTS is there to put you back in the game and keep you going, whereas feather functions such as autofeather are there to pack it in for the day and end the madness. Which applies to you at any given time depends entirely on what's going on, and in general, what you tell the engine to do.

 

[uwochris]

Wow avbug, that was a great response! Very informative.

 

[Famous Dave]

Chris, are you flying yet or are you still working the ramp?

 

[uwochris]

Chris, are you flying yet or are you still working the ramp?

Hey Dave, been flying the metro for about a month now. Just finished line indoc 2 days ago

 

[avbug]

Congratulations, Chris. Your bio says under a thousand hours...you're doing well, then. Don't stop studying or asking questions. A lot of guys get through training or recurrent, and don't crack a manual until the next recurrent comes along. Understanding the systems aids you in your decision process, and it may save your life. The scenario I recounted above is a recent example (albeit a poor one).

Fly safe!

 

[Famous Dave]

Congratulations Chris. I totally agree with Avbug's advice. A lot of guys get lazy and complacent but it sounds like this wont happen to a guy like you. Safe skies.
Dave

 

[pilotyip]

uwo, another TPE tip, make sure the conditons levers are always in Hi RPM for takeoff