What is the base of N1 measurement?
 

Does anyone know what is the "base line" against which N1 is expressed as a percentage of?
In other words, how is "100%" defined, what does it signify? It obviously is not a design "maximum" because we wouldn't be seeing values higher than 100% N1

I've asked the same question to a few different people and everyone seemed to think that the 100% was just given to a nice round number RPM somewhere near the maximum the engine could operate at.
It might be slightly more technical than that, but from a pilots point of view there is no significance in the 100%.

To be honest I can't remember either but I have heard the definition. When I'm home again I'll blow the dust off a book or two and see what I can find. I believe however that 100% N1 is the speed at which the engine produces it rated thrust. The engine may be capable of more and thus thats why we see max N1 values of say 104 or 108%.

My recollection is thst Sprucegoose is right.
When Royces did the 211 G/H series 100% N1 was that which produced the rated thrust. They have since re-cored with a Trent derivation which produces its rated thrust at a slightly higher speed, hence the 104% or so values. Incidentally, the engines controllers lie to the airframe computers about exactly what speed they are doing. This means that the FMC still thinks 100% is the rated thrust rather than the 104% or so !
It saves having to reprogram the whole bloody lot.

VnV...

"When Royces did the 211 G/H series, 100% N1 was that which produced the rated thrust."

"Rated thrust" being..... another "nice round number" perhaps???

Still not definitive, is it?

I think the explanation by HPSOV makes more sense.
If what sprucegoose and VnV2178B are saying is correct, why would one be using less than 100% for TOGA, when not using a Derate or Assumed Temp Reduced Thrust?
The B737-700 uses 88.3% Maximum TakeOff N1 for a full power Takeoff (same for Max N1% for Go-Around). If its rated power is at 100% N1, what's the whole point? The B737-700 barely reaches 100% N1, even in Max. CLIMB Power at high altitudes.

I forgot to say that the above is valid for ISA and MSL.

Like I said I've heard the explanation but can't remember what it is. I'm home tonight and I will find the answer if someone doesn't get there before me.

As far as my cranky old memory recalls,
it's just a NUMBER and is represented as a percentage of a fan rpm (the number of which is interesting but academic) nominated by the engine manufacturer as HPSOV and VnV suggest.

You could just as easily use numbers 1-10 to three decimal places like EPR, fruit machine pictures, coloured bands, or whatever turns you on. The actual thrust related to the number at any given point is whatever it is, given temp, and ambient presure and provided by the airframe/engine manufacturers.

The new FADEC systems have simple thrust lever settings like T/O, climb, crz etc.

What the engine is ACTUALLY doing, other than producing the commanded thrust and operating inside its parameters, is reasonably academic to the pilot.
Given that it has fuel availalable it's either operating correctly or you shut it down. There is little else you can do.

Whether the cockpit indicator uses USD's, zlotys or %age, it is simply that a relative indicator.

We are getting too close for my liking to the pilot and dog, flight crew scenario. :)

As Stamatis points out T/O thrust at ISA sea level at is delivered at 83% and as the aircraft climbs to less dense air the engine can accelerate towards a nominal limit of 100%. I guess a form of derate.
Either way all of the manufacturers have different ways of presenting and dealing with it.

[This message has been edited by gaunty (edited 24 September 2000).]

With all the FADEC, EICAS, epr and n1 limiter systems it hardly seems fitting that all these engine indications are even presented to the pilot...maybe we should only have an EICAS caution/warning when a parameter is being approached/exceeded..

Hear Hear.

<James T>`Make your speed two breasts and one nipple Mr Sulu`

<Sulu>`Aye Captain, two breasts and one nipple`

{Brizzz} `Engineering to Bridge`

<JT>`Grow a head Scotty`

<Scotty> `Fer Gad`s sake Capp`n, she canny take any moore! One moore nepple and she`sa gunna blow`

<JT> `Standby Scotty, I may just have to get you to tweak that last nipple outta her!` http://smilecwm.tripod.com/dvv/nuke24.gif

jtr
ROTFLMAO

Love the nuke gif

Ironbutt57
Always wondered if it was possible to programme one of those glasss thingys in the cockpit to play slot machines, QuakeIII, Pinball or somesuch when it gets boring?



[This message has been edited by gaunty (edited 26 September 2000).]

As I recall from working at 'Royces in the 70s and 80s, the reference speeds for 100%N1 were nominal figures for the 'fan design point' (TOC) and anyway were rounded off to a convenient number (3900 on -535 and 3600 on -22 and -524 if I remember right). Speeds above 100% may thus be within operating limits in some conditions and may vary between different dash no variants of the same engine.
So I think the answer to the question is that there isn't a strict definition of 100%N1 its just a convenient way of standardising dial presentations.

Anyone care to shoot me down in flames?

Yes I must agree. I've blown a fair bit of dust off a lot of books at home and much to my surprise I cannot find the information I thought I had about the relevance of % speeds of N1. The only close statements I can find simply refer to the %N1 as a reference for setting thrust...which is stating the obvious and doesn't answer the question.

Stamatis

My understanding (which means I’m not sure but its what I always believed) is that there is no significance to the 100% value at all. ie “There is no datum implied or actual that is associated with the figure”

Whenever I asked Mr Pegasus Designer (who I got to know quite well) he said “We have a sender that pushes out a signal proportional to N1 which drives the cockpit gauge. We arrange via the associated gear ratios to make the thing read close to 100% at full throttle and accept that you will use the ACTUAL numbers we quote for the various limits.

Tattooed on the back of my left hand it says Max thrust 97, Normal Lift 100.5, Short lift dry 102.5, Short lift wet 107.

JF