Props

[REVENUEMAN]

Can anyone please explain what Negative torque feedback is ?

Thanks

[MkVIII]

Could this answer what you seek to know?

http://www.biozentrum.uni-wuerzburg.de/genetics/behavior/learning/thrust.html

They use a fly as the subject, but perhaps it all applies in the physics sense anyway?

[Daysleeper]

The fly article is very interesting.

however I think you mean in turboprops where negative torque is where the prop is driving the engine rather than the other way round. This can be due to high airspeed with a low engine power setting or an engine failure. The system senses negative torque and changes the blade angle (coarsens) to alleviate the situation. Also acts as a backup to Auto feather although it tends to have a more limited blade angle range to work in.

On the right track?

[Specnut727]

I have read of instances of engine 'run away', where the pitch control failed and pitch went fine at high airspeed, causing engine over-speed. In extreme cases, props and engines were lost. This was on piston engines.

Can this happen, or has it happened, on modern turboprops, or are there sufficient sensors and safety systems to prevent it ?

Thanks.

[Daysleeper]

Well this isnt modern but in 1956 a Trans Canada Viscount had an uncontained prop overspeed. Unfortunately the crew carried out an emergancy descent at high speed (rather than low speed) and the prop hub failed, the departing blades destroyed the number 3 engine and killed a pax as they passed through the airframe. The crew were able to carry out a safe landing without further loss of life.
In aerodynamic terms while its certainly possible I imagine the various prop manufacturers have spent a lot of time working out failsafes.
Any -8 Q pilots out there want to comment on a truely modern aircraft?

[Genghis the Engineer]

I think that you've got the emphasis in the wrong place.

It's negative feedback, of torque.


The basic principle is straightforward. There will be a device (doesn't really matter how it's done) that measures torque. This is then subtracted (negative!) from the reference torque that you should have.

The difference is "negative feedback". This is then added to the control input into the machine (prop / engine / whatever). This can be mechanical or digital, and simplistically looks like this...

http://www.det.bhtafe.edu.au/amps1/feedback.gif

The basic system, known as a negative feedback loop is the fundamental building block for controlling virtually any stable system - whether it's an engine running sped, you trying to stand upright, your heart rate....

G