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Thrust during descents with throttles at idle

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Thrust during descents with throttles at idle

Old 23rd Sep 2000, 21:34
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Post Thrust during descents with throttles at idle

I have been told that aerodynamically speaking, a turbofan with its power lever set to idle produces negative thrust during a normal descent at high altitude.

I find that hard to believe, considering the idle thrust output produced on the ground, where an airliner can taxi with almost no power at all other than idle power.

Is that true?
Old 24th Sep 2000, 02:09
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I find it hard to believe, also. The idle RPM at altitude is signifigantly higher than at ground idle.

Old 24th Sep 2000, 03:25
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Just a guess, but I think the reason the idle RPM is so high, is due to the engine `windmilling` hence energy required, and drag produced.
When we shut one down on the 340, it will usually spin at around 20% N1 from memory (@300 kt)

<response to below post>techman, if you ehh? is re my post, then to answer your question, Yes I have heard of it, however flight idle, or approach idle as I think Airbus refer to it, is only commanded when the gear is down, or two stages of flap are selected (from memory).

[This message has been edited by jtr (edited 26 September 2000).]
Old 25th Sep 2000, 02:55
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Ehh, have you heard of flight idle??.
Old 25th Sep 2000, 11:25
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It APPARENTLY is true on the RB-211-535 series when thrust is reduced to flight idle at crz altitudes..the epr (measured across the fan only on the e-4 engines) shows less than "1.00" which would indicate that the fan is simply windmilling..however there is still a small amount of tailpipe thrust being produced...so negative thrust doubt it..coldstream fan negative thrust/windmilling drag perhaps...comments anyone?
Old 26th Sep 2000, 21:50
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On some airplanes flight idle comes on as soon as the aircraft is airborne, on others, like e.g. the jurassic, it comes on at a certain landingflap setting. On the jurassic the fan speed is above normal, low idle during flight at higher IAS. This, like a lower than one EPR, might be an indication of negative thrust.
Of course if you shutdown the engine completely the negative thrust will be even lower.
Old 27th Sep 2000, 00:54
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however flight idle, or approach idle as I think Airbus refer to it, is only commanded when the gear is down, or two stages of flap are selected (from memory).
So when up at FL330 and descending with throttles at idle, there is no flight idle active?
Old 27th Sep 2000, 01:49
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I also find the negative thrust theory hard to believe.

On the A320, and I guess on other types too, the minimum flight idle is also a function of bleed air requirements among other things, so that will have some air to tap from the engine compressor.

Flight idle on the 'Bus comes with the first notch of flaps.
Old 27th Sep 2000, 02:35
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On all engines with EPR indication, you will find that during descent with engines at idle, the EPR indicated is less than 1.00.
It can be as low as 0.80.

And with regards to negative thrust, I would rather call it drag.
And since the pressure at the intake is greater than at the exhaust there must be "negative thrust".

Even on the good old JT9D, where the PT7 probes are located in the core engine exhaust.
Old 27th Sep 2000, 10:43
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Stamatis, I am just saying there is four modes of idle, flight, approach, ground, and reverse.

The 340 burns 1250-1500 kg/hr on the ground w/o APU running. (i.e. not taxiing)

In a 30 minute idle from 410, (not including appch. man.) it will burn 1000kg = 2000 kg/hr

When you consider the added demand of the pressurisation from 410 to approach point, my guesstimation is that the two (ground idle and flight idle) are pretty similar modes, with the only exception being that `flight` idle has the function of becoming approach idle when the appropriate config is set.

With regard to the high idle RPM at altitude, I would think this is due to the compressibility of the air at alt. If you took two engines, pumped in exactly the same idle fuel kg/hr, and put one static at sea level, and one at FL350 static, which one would spin faster? Why?

In answer to your initial post Stamatis, if your definition of negative thrust, is `less thrust than a windmilling engine` then no I do not think this is the case.

If your defn. is less thrust than the drag it produces, the I do think this is the case.

<phew> wipes brow...
Any more of this and I am going to have to find my FCOM`s. I think they`re under that big pile of ammendments in the corner.

Final disclaimer: By no means am I saying this is fact, it is just my opinion, and please treat it as such. Hell, I can`t even spell engenear.

[This message has been edited by jtr (edited 27 September 2000).]

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