In general, what are the RPM ranges/differences between the various compression stages on a 2 or 3 spool (LP,IP,HP) turbojet/fan engine?

I have a general interest in jet engines and am pretty new to the topic so I wanted to toss this question into the wind as I don't have any previous experience or knowledge to pull from and haven't really found a similarly posted question anywhere else.

In the small amount of reading that I've done when looking at the RPM ranges for the various compressors (LP,IP,HP) generally the LPC spins the slowest and the HPC usually spins the quickest and, if the engine has one, the IPC has an RPM range that falls between the two.

So my question is what sort of speeds or fluctuation in speeds for each shaft could we expect during takeoff/flight/landing? For example does the LPC fluctuate more or less than the HPC/IPC or do they remain fairly constant while in flight?

• Dec 28, 2020 at 20:11

RPM does not generally fluctuate in flight, or on the ground for that matter.

For 'fan' RPM, 100% N1, is somewhere between 2000 to 4000.(aka LP stage)
For 'core' RPM, 100% N2, is somewhere between 9000 to 14000.(HP stage)
The 3rd spool (IP stage) RPM would be in between the LP and HP nominally around 7000.

These are ballpark figures.

• Thank you for the response, I appreciate the ranges even if they are ballpark figures. For airliners I definitely understand why there wouldn't be much if any fluctuation in engine speed for fairly obvious reasons but what if we were to look at a fighter aircraft? Shouldn't we expect there to be more fluctuation? Dec 29, 2020 at 14:35

As a rough rule of thumb, the speed of a compressor/fan is such that when at high speed (i.e. takeoff), the tip of the blade is going close to Mach 1 (i.e. the circumferential velocity is close to the local speed of sound). Thus, large compressors/fan will spin slowly, and small compressors/fans will spin quickly.

If you spin way above Mach 1 (say Mach 2), then you are going to have many problems with shockwaves developing. But if you are way below Mach 1 (say Mach 0.1), then your compressor is going to be less efficient. So somewhere around Mach 1 is the sweet spot. Of course, this will vary with altitude, temperature, forward velocity, etc. Its just a very rough estimate.

For example, a giant engine like a GE9X has a fan speed at takeoff in the 2200 - 2300 rpm range, whereas a medium one like the LEAP is in the 3500-3600 rpm range, and a really tiny one like the Honda HF120 could be 17,000 - 18,000 rpm.

Further, on a given engine the fan is always bigger than the HPC, and thus will spin slower

• Mach 1 being the sweet spot makes perfect sense along with smaller diameter stages having higher RPM ranges as the smaller the diameter the smaller the tangential speed is at the blade tip. I assume this is why fighter aircraft in general have smaller size engines so that they can spin faster and provide more thrust if I understand correctly. Dec 29, 2020 at 15:34
• fighter engines vs commerical engines are driven by the fact that fighter engines need to go supersonic (here we mean the forward mach number of the plane, not the local circumferential mach number of the blade). high bypass turbofan do great subsonic but they are not good supersonic. low bypass turbo is what you want for supersonic speeds. Low bypass fan have lower diameters than high bypass, and thus spin faster. Dec 29, 2020 at 21:46