I have a few problems with what he has to say in this article.
I work on P&W and RR engines. They both use N1 speeds to gauge output or thrust.
He says this:
"EPR is the measure of a quantity that relates to the performance of the engine. N1 relates to a parameter which is responsible for the performance of the engine. As such, N1 does not take into account the other variables which may affect thrust, such as engine performance degradation after several years. If, for example, 50,000lbs of thrust demanded a hypothetical EPR of 1.27, no matter what the status of the engine, an EPR of 1.27 in the same atmospheric conditions is guaranteed to deliver the same amount of thrust."
In the same atmospheric conditions, N1 will produce the same output, also. Atmospheric conditions are what dictate N1 speed. Atmosphere changes, N1 and EPR will change, IAW parameters established within the aircraft flight manual and the Air Data Computer, if equipped.
"However, if this thrust required around 90%N1, and the engine gradually degraded over time (time period being many months or years), the N1 required to deliver the same thrust in the same environmental conditions will now be higher."
Not true. Since we're talking turbofan engines, take your pick of 2 or 3 spool engines, but for clarity, well assume a 2 spool:
Standard day. If a certain N1 speed is needed to make TO power, then it will always be that speed at that standard. The part of the engine that will need to increase to compensate for wear/dirt is N2. If it takes more "power " to turn N1, it comes from N2/T1 (the actual engine, HP section). N1 on a 2 spool engine is driven by T2, a LP free turbine, so it can hunt and wander a little to find its own "best speed". N2/T1 is the actual engine, responsible for making power to drive the rest of the engine.
"In this way, N1 is not a reliable parameter for thrust setting over very long periods of time, while it is the presence of an N1 indication that enables crew to recognize performance degradation."
One of many. The main indicator of engine "condition" is ITT/EGT. Remember, N1 is a variable, but dependent on others. Sure atmospheric conditions change, and there's a chart for that stuff in the AFM, but simply put, if it takes more power to drive N1, ITT (Intermediate Turbine Temperature) goes up. More fuel and air=higher ITT.
Running an engine to any power setting above idle should have planned output. If you are going to find max POD (Power Of the Day), the run qualified person (Pilot/A&P, Crew chief, whoever) must take local ambient temperature, humidity, and/or pressure altitude and a few other variables into account. This calibrated/corrected N1 speed (the fan speed) is an expected output that the engine should meet. If an engine can't make POD (by not making N1 speed, the variable responsible for making 70-80% of the engine's thrust) it is thereby deemed un-airworthy, and must be diagnosed.
Almost all modern engine manufacturers have gone to some sort of electronic engine control, be it an EEC or FADEC, but almost all will use data from a Air Data Computer, and will tell you what N1 speed is needed for POD.
I have done too much work on this particular post to throw it away, but the more I read that article, the more problems I find with it. So, take this post as you will, but that article has some serious issues. It's an interesting read, for sure, but I would be cautious. For example, the "pitot" tube at the spinner, and the one in the LP turbine connected to N1, are Aneroid Bellows, both used to measure air pressure. Used how they are in that picture, they could sense differences in air pressure, as he says, but surely to the Aviation God(s), no one calls them "Pitot tubes" if they're in the engine.
That guy may be a great pilot, but a questionable engineer...
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