Rated Thrust, At Which RPM?
 

I have a question about published thrust figures for turbojet engines. Apologies if this has been asked before, i did search, but didn't find anything conclusive.

I am interested to know whether rated thrust is given at 100% RPM or max RPM?

For example, under ISA conditions, the RR Adour might have a rated thrust of 5115lb, and N1 limitations of 99% N1 for maximum continuous operation, and 104% N1, maximum dry, for 30 minutes.

Would that rated thrust figure be valid only for 100% N1, with any RPM increase on that 100% providing an increase in thrust?

Thanks for any assistance.

e) None of the above :O

100% N1 is just a number, it doesn't represent any limit or any specific value - I'm sure you can find more about this on the flight testing thread. Maximum N1/N2 values are usually determined by bearing loading - e.g. how fast can the shafts turn without overheating/destroying the bearings.

However, the takeoff/go-around/full rated thrust N1 setting depends strongly on outside air temperature. If we simplify by saying that the engine thrust is linearly corelated with fuel flow and the fuel-to-air ratio remains constant, we can say that the more air you get into the engine, the more thrust the engine will give you. With high temperatures (let's say 35°C), the air density is low and the volume airflow (and thus N1) has to be increased to maintain the same mass airflow as we have for example if the temperature is -40°C.

For example, the CFM56-3C1 installed on B737 Classic aircraft is flat-rated at 32°C (so the maximum thrust is constant from 32°C downwards). In order to achieve full rated (takeoff) thrust the N1 at 30°C is 96%, while at -40°C the takeoff N1 is only 84%.

Thanks FlyingStone, but the question was more related to the thrust rating given by the manufacturer. Using N1 was a mistake on my part.

The limits are based on % HP RPM in the document i am looking at, not N1.

Using the limits above, i take 100% HP RPM to be the design speed, and the speed used in the thrust rating measurements, with 104% HP RPM being the maximum available for a short time only.

Is this correct, or would the thrust figure published come from testing at maximum RPM?

Quick answer on the run.

Rarely would it be maximum RPM, that usually doesn't limit thrust until quite high Altitudes.

More typically, Flat Rated Thrust will be Pressure limited, until such time as maximum EGT is reached, whereupon maximum thrust will be Temperature limited. RPM, as stated, usually comes much later.

People like Barit 1 will give you the full serve:ok:

Pre EEC/FADEC, you would obtain a reference for max thrust from tables as part of TO data calculation.

This would usually reference a target N1, or a target EPR depending on engine/aircraft/customer preference, but would vary depending on ambient conditions. There was no single N1 figure that represented max thrust.

These days, the a PFM does the calculations and puts a nice little red arc on the appropriate gauge, and this USUALLY represents fire-wall thrust- levers.

Rated thrust is usually given as max takeoff thrust at ISA, sealevel, 15C etc
(whatever N1 that is)

Thanks Morten, that answers my question perfectly.

Cheers all.

Rated thrust for most turbojet/turbofan engines means flat-rated thrust - holding a constant thrust over a wide range of OAT. Over this temperature range, the air density varies quite a bit, so the rotor speed required also varies, holding the same rated thrust. Higher OAT, lower density, higher N1 for the fan. Holding a constant thrust means the static structures (bearing housings, engine mounts, pressure vessels etc.) have stable design criteria.

This all works great up to the flat-rate (or corner point) OAT. Beyond this point, we start worrying about temperature levels, so thrust (and N1) start reducing to keep from cooking the turbine. So the max N1 occurs at the flat-rate temperature - typically ISA+15 (i.e. 30C at sea level).

But as mentioned earlier, the displayed N1 percentage is purely an arbitrary number. Every engine model has 100% defined differently. BTW, I've never heard of bearing speed being a speed limitation, and I've been in the game over 45 years. The limiting speed is always determined by stress loads on the blades and rotor discs.

Correct and to add, vibratory stress levels often go hand in hand with speed excursions above 100% of the rated number whatever that is.

The basic stresses are designed around a mechanical rated 100% with a 25-30 % safety level for discs above that and a 5-10% safety level for blade vibration in a running engine.

I once did a study on the variations between flat rated 100% speeds as published in the operating specs and the mechancial limit speed of 100% on the type data sheets and found that the differences were all over the lot between engine models.

The engine manufacturer is not too worried about the performance rated speed going above 100% on a deterioated engine or under some outside temp-pressure variations but would take a dim view if the mechanical limit speed of 100% were exceeded by more than 5%

Of course the newer engines with Fadecs make this a moot point to worry about today.

Today it's mostly turbine life that self corrects operating engines on the edge

Yet there are still lessons learned out there in the data base behind all this.