N1 > 100%
 

Someone asked me to explain why jet engines can produce "more than 100%" power. By that they meant that at max power, the N1 spool can read in excess of 100%. Of course power is measured in other ways, but I'd forgotten the explanation why N1 is designed to sometimes exceed 100%. Help?

Forgive my ignorance - I haven't got to the chapters on jet engines yet. But my understanding was that jets are rated to produce a certain amount of thrust, not power.

For a given amount of thrust, power = thrust x speed. So to increase the power, you simply have to get your aircraft to go faster (e.g. by lowering the nose). This would make the concept of "more than 100% power" meaningless.

Do you mean more than 100% thrust?

(And sorry, but even if that is what you mean, I can't answer your question. Like I said, I haven't got to the chapters on jet engines yet!)

FFF
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If you do a search you will find this question answered.

Briefly, 100% N1 is an arbitrary figure based on the design tip Mach no for the fan usually, but often rounded for convenience (e.g. 3900 rpm on an RB211-524). It allows some commanality of instrument presentations but does not imply a limit.

There may be RPM limits specified for stress or vibration reasons but more usually you will be limited by temperatures etc.

Thrust has a great deal to do with fan flow capacity for the big iron, which is critically tied to the fan blade tip condition and clearances over the blading in the engine. All these vary from engine to engine, so even with a fixed amount of RPM the thrust can vary.

There are also variances associated with the control logic on the engine. If the control is able to meter more fuel than the internal pressure goes up along with temp and coresponding RPM, if the blade flow doesn't break down in a stall, than more thrust will be produced.

Large margins protect you from the damages of centrifugal overstress, but if you get too high above 100% RPM you may encounter hidden vibratory stresses within the rotating components.

The 737 has an N1 limit 0f 106% and N2 limit of 105%. Hope I got those the right way round.

FlyingForFun

That's very interesting! So according to you, you can make an engine produce more power by lowering the nose of the aeroplane? :)

I think you'll find that the extra energy you get when you lower the nose comes, from gravity (potential energy), not the engines. Your error comes from not considering all the forces acting on the aeroplane. It is true that "power" (rate of energy conversion) is proportional to force multiplied by the velocity in the direction of the force, but you must take into account the sum of all forces acting on the body, not just the engine thrust.

The bottom line is that an engine operating at a given speed and air pressure/temperature eats fuel at a known rate. Subject to the efficiency at the operating conditions, there is only so much energy the engines can get out of that lump of fuel, no matter how much power you think the aeroplane is expending. If you look at the problem in these terms, you will see that for a given set of environmental conditions, constant thrust equals constant power developed by the engines (as opposed to used by the aircraft).

It may help if you consider just the engines and the air flow through them. An engine dumps out hot air behind it at a rate that is independent of aircraft speed. If you want to use your power/thrust/velocity equation, apply it to the mass of air that is being propelled through the engine.

Covenant,

But surely the faster a jet engine moves through the air, the more air passes through the intakes in a given time??? :confused:

FFF
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Just to get back to the topic, N1 can be and regularly is more than 100%.

100% is just a number. When you look at the op limit speeds on big fan engines, thay are all over the place. e.g.

CF6-80 B2 N1 117.5, N2 112.5

RB211-524 N1 110.1, N2 107.7, N3 99.2

Often the tacho generators used by the engine manufacturer are whats in the parts bin. After all as long as our limits are presented in a consistent fashion, it matters little what the actual number is and what it really equates to in RPM. We are operators, not designers.

There was a long thread on this subject some time back, perhaps a search would help you find more info.