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Turbofan "Negative Torque"?

Sun Oct 17, 2004 3:19 am

Do turbofan engines (especially the fan) suffer from being exposed to "negative torque" (I know those terms are never used when talking about jets...) during a high speed descent at a low or idle power setting, like the turboprops? Or does the fan always suck air in no matter its rotational speed and the speed of the a/c?

If it does occur to them, how do they cope with it? Is it a problem?

(sorry if already asked...)
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RE: Turbofan "Negative Torque"?

Sun Oct 17, 2004 4:02 am

I guess by negative torque you mean does the airflow into the engine ever drive the engine.

Without getting into great detail. The EEC uses airspeed, altitiude, throttle position, and a host of other parameters to prevent this. So normally it is not a problem. If it does occur the EEC will alter fuel scheduling and open bleed valves to keep engine parameters stable.

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RE: Turbofan "Negative Torque"?

Sun Oct 17, 2004 5:45 am

What's this about turboprops-- you're saying the airstream sometimes drives them? And that's bad for the engine?
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RE: Turbofan "Negative Torque"?

Sun Oct 17, 2004 6:11 am


Thanks! Yes I did mean the case where the air flow drives the engine instead of the opposite.
I am not familiar with the term EEC though, but I suppose it is what controls fuel flows and maybe RPM, like a FCU...?
By "alter fuel scheduling" do you mean it will increase it slightly to maintain a "zero thrust" condition?
How does the EEC sense if that condition occurs since as far as I know torque is not measured on jet engines?


On a turboprop when the power setting is backed too much during a high airspeed condition, a situation of negative torque may be encountered, which is when the ram air drives the prop (thus the engine, on single shafted turbines), and this is very bad for the gears of the gearbox apparently (or so said all my instructors and training manuals).

It is not really an issue on free turbines, though. On many single shaft engines, they use a NTS (Negative Torque Sensing) system to sense it and to mechanically coarse the blades to where they again get a "grip" on the airflow.

It also has a use sometimes when the engine fails, by reducing the drag created by the dead prop before it is manually feathered.
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RE: Turbofan "Negative Torque"?

Sun Oct 17, 2004 8:08 am

Whoa nelly.

Hello. I am an engineer with General Electric Power Systems. I work with Aero and Aero Derivative power plants every day (except most weekends). I konw a thing or two about this stuff, so I hope I can help.

First, you are absolutly right, kind of. When the pilot pulls back the throttle to decellerate from, say 400 kts to 250 in order to initiate a decent, you, the passenger, will notice a "breaking" feeling. What is happening are two things. First, the thrust from the engines go away. Second, drag takes over and pulls the jet slower. This is a simplistic explanation, but it will work.

Those big engines are very good at producing drag. Very good. Basically it's like holding a large billboard out in the airstream. The fan is driven by a free turbine, that is, a turbine that is not physically connected to the gas generator, or jet engine. The jet engine is inside the whole assembly. It's real purpose is to generate large amounts of gas to drive the multiple turbines that turn the fan. When you stop producing gas to turn the big fan, the fan will no longer produce thrust and instead generate tremendous drag. While it is true that the gas generator portion of the engine has various bleeds that will keep everything a-okay inside the GG, the air coming in will push hard against the blades of the fan and go through it and exit out the rear. Because the GG is still producing thrust, and the rotating mass of the LPT and fan are dripping with momentum, these effects are limited by the throttle setting. There is not "anti-torque", just loads of drag. Kind of...

Okay, that explanation wasn't very good.

As far as your turboprop deal, this is only true if the pilot sets a flat blade pitch on the propeller. If you feather the prop, it will produce very little drag. The pitch of a prop has the sole purpose of dictating what speed a particular aircraft will travel at a certain throttle setting. When you have the prop set for high cruise and suddenly flatten it out, the prop is suddently in a configuration of a slower speed, and will try very hard to bring the PT's RPMs up to match the airpseed. The resultant is drag, drag, drag, and drag. Maybe a grenaded PT also...

Hope that helps. Probably not...
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RE: Turbofan "Negative Torque"?

Mon Oct 18, 2004 9:45 am

"What's this about turboprops-- you're saying the airstream sometimes drives them? And that's bad for the engine?"

Timz: On older generation fixed shaft engines, yes, that is exactly the case. On aircraft such as the Viscount or HS-748, minimum propellor torque/RPM are the order-of-the-day during descent from altitude.

There is an 8 year old AD on RB211-535-E4 engines to maintain engine-anti-ice-ON during an idle descent under some circumstances to prevent "inadvertent engine run-down". Boeing's words, not mine!

Basically, in response to your question, with older engines, YES, it is a factor.
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RE: Turbofan "Negative Torque"?

Tue Oct 19, 2004 1:59 am

The Allison 501D13 series engines coupled the Aeroproducts 606 propeller have an NTS system to prevent the prop from driving the engine. It is used on the L-188 and I assume on the CV-580, among others.
When the system is activated, airplane will wiggle a bit as each engine/propeller combination may activate the NTS system at a slightly different time. The propeller blade pitch is changed to unload the engine and this can also result in a very slight variation in propeller RPM.
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RE: Turbofan "Negative Torque"?

Tue Oct 19, 2004 10:32 am

The TPE-331 also have a NTS system.

Used also to prevent torque being applied to the engine from the prop in case of high speed-low power situation.

You hear it and feel it cycling very distinctively when it comes on.


So the fan just windmills and starts to produce drag? Thanks!
Since it is a free turbine I should have thought it was no real problem on the high pressure spool anyway.

I just wanted to know wether jet engines ever stopped producing thrust in flight and started to drag at low power-high speed. So that's answered.
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