104 % N1

[Sonia767]

Hi,

How is it possible to get more than 100 % in engine parameters i.e N1 and N2 etc ????

I always thought 100 % was the maximum you could get in or at anything :D



Sonia

[OBK!]

The maximum of 'something' is infinity...if you think about it. 100% just means "everything" or "total"....110% means "a bit more than everything" or "more than the total".

Engines can run at more than 100% because they are very powerful. Engines are however seldom run above 100% except when an extra bit of "umf" is required, say in takeoff (tho the engines are derated most of the time), go around, recover from stall etc.

[Sonia767]

So OBK,

When you are given a thrust rating of an engine is 56000 lbs for e.g. is that

-100 % or

-the 110 % etc

so in reality that engine may give you 61600 lbs

So in that case then why say the engine is rated at 56000 lbs

And on the other hand if the 110 % is 56000lbs then..............oh why dont the manufacturers be more specific................:O

Sonia

[Techman]

An engine will have a limit to the RPMs allowed. This is a limit set by the mechanical stress on the Fan/compressor/turbine blades etc. This is the maximum allowed N1/N2.

An engine will also a have rating eg. 56000 lbs. This rating will not be reached at a fixed N1. The N1 will vary with the atmospheric conditions, such as pressure and temperature.
The lower the temperature the lower the N1 and vice versa.
Although at one point you will reach the egt limits, if the outside air temperature is high enough, and then this will then limit your N1 and your thrust. This is also known as a flat-rating temperature. Up to this temperature you will have the rated thrust available.

The percentage used by the various engine manufacturers can have various origins. It could be that the 100% is where the engine is most efficient, or it could be that it really is the max N1 or it could be that it is just an arbitrary value picked during the design process.

[ATPMBA]

Whether is 100% or 104% I just think of it as an arbitrary number.

100% may represent 36,000 rpm, 101% a little more. It’s whatever the manufacturer determined what the percentages are equal to.

On some small jets I flew years ago, 101.5% was about the top limit, many times we used 96%-98% for takeoff.

[mustafagander]

% speeds for engine rotors are just numbers.
The manufacturer very carefully establishes limits in "real" rpm during testing/certification. When it comes to production, they often just look up the catalogue and buy in a tacho system which will do the job, then calibrate it to establish the operating limits as will be presented to the crews.
Really it matters little if the limits are calibrated in bananas as long as all the documentation is consistent. Think also of derivative engines e.g. GE CF6 series - those on our B744s have N1 117.5 and N2 112.5 as limits, likely because the engine got more grunt and there was no real need to certify a new tacho system.

[dusk2dawn]

I have been told, that the 100% relates to the "grandfather" engine certification. When a derivative engine is certified the ratings are given as relative to the original.

[Sonia767]

Hi and thanks to you all.I get it now ;)

Sonia