Hello folks. Since GEAP has a bad web site, and I am an GEAP engineer, maybe I can help. There's a lot of misunderstanding here, so I'll try to clear most of this up.
First, almost all turbofan engines use free trubines. A notible exception is the gear reduced TFE-731. There are many reasons why most all engines are of the free turbine type, but I'll leave that for later.
First, a few definitions:
- Gas Generator. This is the center of the turbofan engine. It is a simple jet, that is, compressor, combustor, and turbine. This is the part that burns the fuel and produces exhaust gas to drive the fan.
- Power Turbine. This is the part of the engine that creates the power (torque) to turn the compressor, propeller, or fan. There are two types of PTs.
HPT- High Pressure Turbine. This is what turns the compressor for the GG
LPT- Low Pressure Turbine. This is what turns the big fan out front, or the propeller on a turboprop (turboshaft).
Free Turbine- This is a turbine that is being turned by the gasses produced by the GG
. A free turbine is not physically connected to anything except what it is driving (LPT to Fan, for example).
Now, the rotation speed of the engine is dictated by it's diameter. Period. It's that simple. The maximum RPM of any fan or compressor stage is limited by mach. The tip speed cannot ever - EVER - exceed Mach 1. Actually, there is curvature in the blade wing section, and aerodynamic accellerations dictate that you really can't turn them past about mach .92-.95, depending on wing planform.
Now some of you are saying, hey, if you do the math, some blade tips are going way beyond mach 1. To that I say: wrong. Sorry, wrong. Remember that the speed of sound is dictated by the density of the air. At 50,000 feet, Mach 1 can be as much as 100 kts less than at sea level because of the much higher air pressure. Because, by definition, the blades are compressing the air, the velocity to reach mach speed rises. So, Mach 1 might be 1200 kts if the air is sufficiently compressed (final stages of a compressor, for example).
So, this is why the smaller GG
has a higher RPM than the PT
and fan. The GG
's diameter is much smaller in a high bypass turbofan than that of the fan. The trick is designing a PT
that will produce just the right power to turn the fan at the proper speed. I am currently working on the CF6 program. The particular engine I deal with has a max GG
RPM of 11,500 RPM. The PT
has a max RPM of 3850 RPM. The fan for this motor is about 3X
the diameter of the GG
, but they have nearly the same tip speed (actually the GG
has a much higher tip speed because the pressure inside the case allows for a much higher "mach speed".)
Now, for why Mach 1 is the limiting factor. First, vibration is a byproduct of the real reason. When someting passes Mach 1, it produces a shock wave. What that means is, the air is compresses so fast that it doesn't have time to get out of its own way. This is fine for a properly designed aircraft wing becasue aft of this shock wave the air is subsonic. But when you have so many wings (blades) stacked on top of one another, the shock waves begin to colide with on another. This makes the air misbehave, and not travel in the smooth path dictated by the engine design. Basically, you get compressor stall- kind of. Actually not, but kind of. So your compressor no longer compresses, and the engine quits working. The shock waves cause a very noticable buzzing sound, and can even cause the compressor housing to grenade. That's bad.
So, to fix this, we can do two things. First, the aircraft designer makes the inlet slightly smaller than the fan diameter. This increases the volume slowing the incoming air and increasing the pressure a little. Then the fan gobbles it up and increases its pressure even more. Whatever this resultant pressure is, this is what the engineer uses to dictate max RPM to keep the fan below Mach 1. Because the pressures vary throughout the engine, we use HP
recoup, VGVs, and bleeds to keep everything working smoothly with almost zero stall.
As far as vibrations go as pertaining the Boeing blurb, these are phisical vibrations for the most part. There are accellerometers on the engine that measure vibration. Most vibration comes from bearing deterioration, improper alignment, accessory wear, and rub. There can be aerodynamic vibration, but in today's computer controlled engines, this is rarely a problem. The fuel control computer and VGVs simply won't allow it.
Finally, 4000 RPM is high for a 94 inch fan. That's 1118 MPH tip speed. I don't think so Tim.
And for the fellow who runs the CF6 GG
at 117%, you better not!! The only way you are running at 117% is if someone reprogrammed the computer. You do that and you void the warrenty. The only way to get 117% is to over-fire the thing. What temps are you running? I've seen CF6's scatter at less than 1600 F, so watch yourself! The only reason for running at 117% is if your fan speed isn't getting it. If this is so, you have either a burnt up LPT, or a bad fan. Time for an overhaul, my friend.