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cp35
10th Apr 2011, 13:00
I have always wondered, but never been able to find out what 100% means regarding N1,N2,N3.

100% means maximum, yet rotor speeds are often 118% etc, so what do they base 100% on, what does it mean 118% N1, 118% of what?

Shawn Coyle
10th Apr 2011, 13:25
It's a number. That's all. Too difficult to make a gauge that shows the actual RPM (numbers are too large - 36,000, etc.) and there would have to be a different gauge for every engine and every engine installation.
The system works, and has for a long time.

Machdiamond
10th Apr 2011, 16:09
The ratio is often totally arbitrary and determined by the engine manufacturer.

At one point they decide, 28250rpm = 100% and then the engine evolves and its limit goes higher but the original ratio is retained. So it is typical to have actual limit a few % above 100%.

100% does not mean maximum here.

Pilot DAR
10th Apr 2011, 18:00
It makes it easier for the boss to expect everyone to give 110%...

TheChitterneFlyer
10th Apr 2011, 19:54
If you were to compare the RPM gauge of your car to that of an aircraft engine it would be appropriate to say that where the red-line commences it would relate to 100% rpm; or where the manufacturer might give an appropriate guarantee that all will be well. A temporary overshoot into the red wouldn't do your car engine too much harm; the same applies to an aircraft engine. A Take-Off limit of, say, 110% for X minutes, would be much the same as saying that such a temporary overshoot is acceptable for the defined period.

Though, I'm dumbfounded to understand why the absolute maximum couldn't allways be 100%... wouldn't that make more sense?

TCF

Mad (Flt) Scientist
11th Apr 2011, 05:26
Though, I'm dumbfounded to understand why the absolute maximum couldn't allways be 100%... wouldn't that make more sense?

Not really.

Even assuming you were to choose the reference n1 value such that the maximum N1 allowed was 100% of the reference, you'd still have conditions where that was above the allowable maximum. Engines may be limited by temperature considerations, for example, and when these are more limiting the maximum N1 must be reduced.

Of course, you could have a more complex engine control scheme, so that instead of % N1 you were actually seeing "thrust %". But that would be far more complex to implement, and recall that this is a control numbering philosophy which predates FADECs. Also, any complexity is one more chance for something to be wrong - maintaining as simple a control scheme for the engines as practical makes sense in terms of minimizing exposure to errors in design implementation.

And finally, even if we did use "thrust %" instead of N1 (or EPR, for that matter) - which thrust shall we use? MTO? With APR/ATTCS boost? MCT? There's no simple, clean, answer.

aviatorhi
11th Apr 2011, 07:06
It also has to do with the certification of the engine.

The PT6 line for instance has been constantly improved/upgraded over time, when the engines originally came out the Ng limit was 100%. Over time, due to the improvments and upgrades many modern variants can easily go ver the 100% Ng limit, however setting their new max as 100% would mean getting the engine recertified (as a new engine rather than a variant), if Pratt and Whitney simply say the newer variant is capable of 106% then no new engine certification is required.

TheChitterneFlyer
11th Apr 2011, 07:36
Even assuming you were to choose the reference n1 value such that the maximum N1 allowed was 100% of the reference, you'd still have conditions where that was above the allowable maximum. Engines may be limited by temperature considerations, for example, and when these are more limiting the maximum N1 must be reduced.



Good point

cp35
11th Apr 2011, 11:44
Thanks for the replies.

How does the engine manufacturer decides that X rpm is 100%, is it metallurgy, temp, press or thrust?

cp35
11th Apr 2011, 11:48
Another question.
How can you calculate the thrust produced by the engine parameters, ie N1, N2, egt, ff.

aviatorhi
11th Apr 2011, 12:59
100% (if the manufacturer decides to go with 100% as the limit) will (typically) be the design/certified limit for a particular engine. Things like derating, the "real" red line (may be more than 100%), etc. etc. come into place.

As far as determining thrust, use the chart. All the indications you mentioned are interrelated to each other in an indirect manner (ie nonlinear) and vary with temperature and altitude.

Shawn Coyle
11th Apr 2011, 13:10
100% does not imply a limit any more than say, 26,850 represents a limit. It's a number, nothing more. The limits are what are in the flight manual.

Mad (Flt) Scientist
11th Apr 2011, 18:23
Thanks for the replies.

How does the engine manufacturer decides that X rpm is 100%, is it metallurgy, temp, press or thrust?

Agree with Shawn C - the 100% N1 nukmber is nothing special and certain not a limit for most engines.

To answer "how did the manufacturer pick the value of X?" - often it may have been the initial expected value for max N1, before development and testing of the engine revealed the true limit, or allowed it to be increased with improved design.

as aviastorm says, determining thrust from those parameters is best done by reference to the supplied charts, or the supplied "engine deck" (computerized model of engine behaviour - called a deck because they were once upon a time on a deck of punchcards. There are some rough and ready rules of thumb, and a good engine thermodynamics book may have some of them, but they are just approximations and not really applicable to any given engine. Incidentally, often these characterizations include parameters which are even less meaningful on their own than N1% - N1 divided by the square root of a Temperature is a common parameter used to characterize the turbomachinery, for example.

David Horn
12th Apr 2011, 10:37
Of course, you could have a more complex engine control scheme, so that instead of % N1 you were actually seeing "thrust %".

Isn't this exactly the system Airbus came up with for the A380? Called ACUTE - "Airbus Cockpit Universal Thrust Emulator".

I only fly the dinky Airbuses so have no idea if it made it onto the production aircraft, but the briefing guide states that "THR will replace N1 (or EPR/TPR) as cockpit thrust control, [with] 100% THR = maximum available thrust in actual flight conditions without air bleed."

Jane-DoH
16th Apr 2011, 02:40
Machdiamond

At one point they decide, 28250rpm = 100% and then the engine evolves and its limit goes higher but the original ratio is retained. So it is typical to have actual limit a few % above 100%.

This is when they're developing the engine? Why don't they just recalibrate everything back to 100%?


aviatorhi

It also has to do with the certification of the engine.

The PT6 line for instance has been constantly improved/upgraded over time, when the engines originally came out the Ng limit was 100%. Over time, due to the improvments and upgrades many modern variants can easily go ver the 100% Ng limit, however setting their new max as 100% would mean getting the engine recertified (as a new engine rather than a variant), if Pratt and Whitney simply say the newer variant is capable of 106% then no new engine certification is required.

Why is this recertification not required for the newer variant which will spin faster anyway?

barit1
21st Apr 2011, 03:02
Fair questions as always, Jane, but the answer is primarily in the logistics of upgrading the engine in an established airframe. The beancounters are upset enough at the $$$ for the new engine, and if you tell them to buy new gages too, you'll get a big "thumbs down". Send the gages to the shop, scrape off the old redline, paint a new redline, problem solved. :ok:

Mad (Flt) Scientist
21st Apr 2011, 17:42
Another argument for keeping the same reference N1 is simplicity of documentation.

If every time I do a thrust growth version, or a derated version for that matter, I redefine what the reference N1 is, then I end up (if I'm sat in the engine manufacturing design office) having no clue what a report means when it says a test was conducted at "100% N1" unless I also know precisely what variant of the engine the test was conducted for (not necessarily the same as that it applied to).

Similar arguments lead to aircraft OEMs often keeping a fixed reference wing area, even when the design is changed, to avoid having to ask "which wing" every time someone quotes a CL value. Again, its an arbitrary reference, so it doesn't have to have precise physical meaning.

aviatorhi
21st Apr 2011, 22:12
:ok: What they said.

SloppyJoe
23rd Apr 2011, 08:06
If say a DHC6-100 operator is upgrading to the PT6-27 engine and they are worrying about a few $1000 at most for changing the gauges maybe they should not be spending the $500,000 on the upgrade. Somehow don't think it is so you are able to keep the old gauges.

barit1
23rd Apr 2011, 15:14
Somehow don't think it is so you are able to keep the old gauges.

That's but one of many facets of the logistics issue. Mad (Flt) Scientist did a great job covering the technical side. (Thinks: at what point did the JT9D morph into a PW4000, or a RB211 to a Trent?)

phoenixcomm
12th May 2011, 15:45
:= thats per gauge! so if you had lets say a 74x thats N1, N2, N3, egt x 4 or 16 x (big number) + tech time = $50k/$100k ouch.

blind pew
14th May 2011, 10:25
Had to deal with loads of these magical numbers in the beginning of my career and then I flew an aircraft with green, yellow and red gauges which left me lots of spare brain capacity and drinking time!
KISS

FlightSpanner
20th May 2011, 23:09
In the rotary world 100% N1 is used as max continuious power, if the engines are improved through developement this 100% value is recalibrated to the new N1 speed, higher than 100% is permitted but for limited time periods, OEI for example or max take off power.

However if the manufacturer increases certified output without modifying the component then they could authorise say 104% as then new max continuious, Sikorsky have a good example of this with the S76 Gearbox, the original S76A model flew with 100% rotor speed, the same gearbox is now certified at a higher rpm, so now spins at 107% indicated.

Bumpfoh
9th Jun 2011, 12:36
Isn't this exactly the system Airbus came up with for the A380? Called ACUTE - "Airbus Cockpit Universal Thrust Emulator".

Lifted straight from A380 course notes:

Cockpit Thrust Display (ACUTE)
ACUTE (AIRBUS Cockpit Universal Thrust Emulator) is a percentage
indication of thrust.
The ACUTE function calculates percentage parameters from engine
command and thrust feedback parameters, for transmission to the airframe
and subsequent cockpit display.
The parameters are:
- THR Limit,
- THR Actual,
- THR Command,
- THR REF (Throttle),
- THR Idle,
- THR Max.
THR WML: Thrust windmilling is the THR achieved when engine in
Wind Milling (0%).
THR 100: Thrust 100 is the THR achieved when Throttle at TOGA and
Bleed Off (100%).
THR IDLE: Low-end of grey sector, corresponds to the THR achieved
when the engine is operating at Idle.
THR MAX: High-end of grey sector agrees with the THR achieved when
throttle at TOGA detent.
THR Actual: raw engine thrust corrected by engine drag.
The parameters THR100, THR Limit, THR Actual, THR Command,
THR Throttle, THR Idle, THR MAX, are sent to the airframe CDS
through the AFDX network.
When operating in Unrated N1 mode, the EEC THR parameters output
sent to the airframe CDS are not computed