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Does take off thrust steadily reduce for fixed N1 during take off roll

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Does take off thrust steadily reduce for fixed N1 during take off roll

Old 12th Jul 2017, 13:12
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Does take off thrust steadily reduce for fixed N1 during take off roll

In a simulator on the Instructor panel, several engine parameters are recorded such as N1, N2, EGT and actual thrust delivered. One would expect the power (thrust reading in LBS) would be approximately as advertised for engine type throughout the take off roll. For example 20,000 lbs thrust per engine at 91.5%N1 rated power at start of take off roll.

Was rather surprised to see the thrust reading steadily rolling back on both engines from 20,000 lbs at start of take off roll to 18,500 lbs at 80 knots and 17,000 lbs thrust at VR of 135 knots, thereafter further steady thrust reducing at take off N1 during climb to 3000 ft agl in ISA conditions. N1 constant at 91.5N1 throughout. Is that a normal expectation? Figures are approximate except N1 which was steady throughout until N1 reduction at 3000 ft agl (sea level take off)
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Old 13th Jul 2017, 18:04
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Completely normal for a turbofan engine. The manufactuer's have computer programs for this when calculating performance. I wasn't aware that simulator control panels displayed thrust, however.
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Old 13th Jul 2017, 21:25
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Old 14th Jul 2017, 17:36
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If you do a search for Intake Momentum Drag or Jet Engine Total Thrust you should find some good explanations of this phenomenon.
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Old 14th Jul 2017, 18:12
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I think the key point here is holding a constant N1 (fan rpm).

As you accelerate in forward speed, your increasing airspeed is ramming air into the fan/compressor, and that "helps" them to spin (think of a pinwheel in a breeze), requiring less effort from the power section, so N2 probably rolls back a bit to hold a constant N1, producing less thrust at the exhaust.

If your throttle control was holding a constant N2 - then you'd get approximately constant thrust, but your N1 would climb towards redline as you increased airspeed.

As to a thrust decrease in climb, that is a normal function of getting into thinner air: less air to push (thrust) and less air to burn (to do the pushing). Even at just 3000 feet, you are in ~8% thinner air than at sea level, with a corresponding decrease in engine output.

(That may be - in Terry Pratchett's words, "A description that is wonderfully illustrative, while being, in every technical detail, completely wrong!" )
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Old 14th Jul 2017, 18:38
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Originally Posted by pattern_is_full View Post
I think the key point here is holding a constant N1 (fan rpm).

As you accelerate in forward speed, your increasing airspeed is ramming air into the fan/compressor, and that "helps" them to spin (think of a pinwheel in a breeze), requiring less effort from the power section, so N2 probably rolls back a bit to hold a constant
Not quite correct. As A/C fwd speed increases, air pressure to the Fan increases due to ram effect. As this ram air hits the Fan, the increasing pressure means that blades are now rotating in denser air and therefore increase the load on the rotor. While the pressure ratio increases the turbine now has to work harder to maintain N1 (Fan) speed.
It really depends on what is the controlling parameter, either spool speed or turbine temperature and how that controlling parameter is scheduled during the T/O run. Some engines may either maintain or increase fuelling to overcome the increased A/C drag.
Hope this helps.
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Old 15th Jul 2017, 01:30
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It makes one wonder if all the minute tinkering with RTOW tables, and the now infamous I-Pad takeoff calculations, to the nth degree, will in fact give you all the umph you'll need when you need it. The ground school instructors and OPC SFI's would red ink you for missing a couple of 100kgs here or there, and given the payload was probably at least a 1000kgs light anyway.......
It's interesting, as I'd not heard of this effect before, nor had cause to think about it. Crunch the numbers and check the settings as the engine spool up. If the engineers have compensated for this effect in their RTOW tables they seem mighty clever. It brings the whole concept of 'ball park figure' into a new light.
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Old 15th Jul 2017, 03:44
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The engine thrust output is modelled appropriately for the AFM takeoff data information by the OEM. If the conditions on the day are as input and the piloting technique as prescribed .. then the performance observed will be very close to the model.

Tinkering with RTOW only extends to the question of how accurate we aim to be. If the data is presented by an OEM computer program, it should be considered accurate. If we are dealing with steam driven paper graphs, then there is considerable room for effort in getting the RTOW output closer to the AFM graphs .. just a matter of the time and money considered appropriate to invest in the exercise.

The pain, of course, is getting closer while not ending up being non-conservative with respect to the AFM data.
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Old 15th Jul 2017, 11:14
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Originally Posted by keith williams View Post
If you do a search for Intake Momentum Drag or Jet Engine Total Thrust you should find some good explanations of this phenomenon.
I don't suppose that you are the same K Williams (kiwi) who used to work on RB199?
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Old 15th Jul 2017, 19:27
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Sorry no. Sadly New Zealand is one place I have never been.
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