monorail
Posts: 588
Joined: Mon Sep 13, 2004 2:49 pm

Turbofan RPM

Sat Sep 18, 2004 12:58 pm

Sorry if this has been discussed before but I couldn't find anything

How many RPM would a turbofan, like on a 737 cruising at 450 knots, be doing? How much does RPM at cruising speed vary from commercial jets, from a 717 to a 777?
Playoffs? Don't talk about playoffs!
 
Alessandro
Posts: 4962
Joined: Wed Sep 12, 2001 3:13 am

RE: Turbofan RPM

Sat Sep 18, 2004 7:38 pm

Maybe you could contact these guys, http://www.geae.com/feedback.html
and ask. Can´t find anything on their homepage..
From New Yorqatar to Califarbia...
 
air2gxs
Posts: 1443
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RE: Turbofan RPM

Sat Sep 18, 2004 10:06 pm

CF680C2B6F max N1 rotor speed 3854 rpm, N2 speed 11055 rpm

PW4000 Max N1 rotor speed 4012 rpm, N2 speed 10,450 rpm

CFM 100% N1 rotor speed 5175 rpm, N2 speed @ 100% 14460

As you can see these are max numbers (except for the CFM). Cruise power rpm is dependant on power setting.
 
SlamClick
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RE: Turbofan RPM

Sat Sep 18, 2004 11:14 pm

Also worthy of note, 100% is not necessarily the highest allowable RPM. It is a designation made somewhere along the development of the engine. It is not uncommon to see a power setting of 104% or so being called for. It is, however, a useful benchmark.

Happiness is not seeing another trite Ste. Maarten photo all week long.
 
air2gxs
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RE: Turbofan RPM

Sun Sep 19, 2004 12:10 am

Slamclick is correct, the max allowable on a CF6 (as we operate it) is: 117.5%. That is why I made the distinction in my text.
 
timz
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RE: Turbofan RPM

Sun Sep 19, 2004 7:04 am

So PW4000s fans (?) can turn at 4000 rpm-- so the tips are supersonic?
 
air2gxs
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RE: Turbofan RPM

Sun Sep 19, 2004 8:02 pm

As far as I know the tips never go supersonic. Sets up all kinds of nasty vibrations. Vibrations are very bad to jets. We do everything we can to minimize them.
 
Yikes!
Posts: 284
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RE: Turbofan RPM

Sun Sep 19, 2004 9:40 pm

The fan is usually turned through a reduction gearbox. Don't know the ratio from type to type...
 
air2gxs
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RE: Turbofan RPM

Sun Sep 19, 2004 10:19 pm

The fan is not turned by a reduction gearbox. The fan is a directly connected to the N1 shaft, which is connected to the low pressure turbine. This is direct drive.

Propellers on turbo-props are driven by a reduction gearbox.
 
gaut
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RE: Turbofan RPM

Mon Sep 20, 2004 4:43 am

The Fan tip never go supersonic; It's why bigger engines turn slower.

«Horum omnium fortissimi sunt Belgae.»
 
timz
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RE: Turbofan RPM

Mon Sep 20, 2004 7:26 am

So does the PW4000 fan reach 4000 RPM or not?

Correct me if I'm wrong, but isn't the fan at least 94 inches across? If so, even 3000 RPM would make the tips supersonic.
 
Yikes!
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RE: Turbofan RPM

Mon Sep 20, 2004 8:03 am

Can't argue, Air2gxs. Was thinking turboprop. My apology. Bottom line though, the lowest energised turbine wheel(s) turn(s) the fan and, through engine design, the fan turns at an optimum RPM. Hence the advantage during the takeoff run.
 
prebennorholm
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RE: Turbofan RPM

Mon Sep 20, 2004 8:12 am

A 94 inches fan rotating at 4,000 rpm has a tip speed of almost exactly 500 m/s which is Mach 1.5 to 1.65 depending on altitude (Mach 1 speed varies according to air density).

To get the true airspeed at the tips we must add the forward speed component, so the airspeed at the fan tips at max N1 is closer to Mach 2 than Mach 1.

If the power setting is 50%, 100% or 117.5%, it doesn't change the rpm that much. Fan tips are always supersonic at anything but idle.
Always keep your number of landings equal to your number of take-offs
 
air2gxs
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RE: Turbofan RPM

Mon Sep 20, 2004 10:38 am

I never did the math, I just assumed. A quick internet search shows that this is possible. I even saw a reference to the "buzz saw" noise being the tips going super-sonic. Now I can assure you, from experience, the "buzz saw" noise occurs nowhere near 50% N1 on a CF6, it seems to appear somewhere between 85% and 90% (haven't been on a run in a while).

I got a powerplant engineer buddy. I'll contact him and see what he says.
 
IndianGuy
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RE: Turbofan RPM

Mon Sep 20, 2004 9:51 pm

Whats the difference between N1 and N2?
 
air2gxs
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RE: Turbofan RPM

Mon Sep 20, 2004 10:13 pm

N1 is the low pressure compressor, including the fan. N2 is the high pressure compressor.

Unless you're talking Rolls Royce, they use a 3 spool engine:
N1 - Fan
N2 - Intermediate Pressure
N3 - High Pressure
 
broke
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RE: Turbofan RPM

Mon Sep 20, 2004 11:01 pm

The larger the diameter of the fan, the slower its maximum RPM. If you are watching an airliner taking off and you are to the front of the airplane, you will hear a sound similar to a large rotary blade saw cutting wood. That sound is the noise caused by the blade tip shock waves as they just exceed Mach 1.

Turbomeca, some years ago, did build a turbofan engine for the corporate aircraft market that used a reduction gearbox to drive the fan. I do not think it ever went into production. Most turbofan engines use a direct drive from the low speed (or low pressure turbine). This combined fan and low pressure turbine is usually (very little is absolute) called the N1 rotor.

The next rotor, which is the high pressure rotor on all but Rolls-Royce engines, is the N2. On RR turbofan engines, this is called the intermediate pressure rotor. They have added a third rotor, starting with the RB-211-22C engine, and that is the high pressure rotor or N3.

Free turbine engines, used on mostly helicopters but are on some airplanes, throw a whole other set of designations for the free turbine section.
 
TimT
Posts: 168
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RE: Turbofan RPM

Sat Oct 02, 2004 1:26 pm

Another knot in the rope-- fan speed is also measured at the tip with magnetic sensor. I've still got the set of gauges for a P&W JT-9. The CF-6 also measured in that location.
 
dakotasport
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RE: Turbofan RPM

Sat Oct 02, 2004 1:51 pm

Guys, i remeber in school that 2 spool engines (GE and Pratt as examples) DID have reduction gearing for N1 compressor (fan). Any insight??
 
air2gxs
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RE: Turbofan RPM

Sat Oct 02, 2004 8:36 pm

Dakotasport,

I know of no turbofan engines where N1 is driven through a reduction gearbox. The 2 GE's I've worked with (CFM56 & CF6) are direct drive and the PW's (JT3, JT8, JT9, PW2000 & PW4000) are also direct drive.

The low pressure turbine drives the low pressure compressor/fan.

Are there any smaller engines that use a reduction gearbox? I don't know, but it certainly seems a waste. Un-necessarily complex and there is an associated loss of power through the transmission.
 
MD11Engineer
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RE: Turbofan RPM

Sun Oct 03, 2004 12:27 am

There was some talk about using a reduction planetary gearbox on one of the future really big fan designs, to keep the low pressure compressor speed close to an ideal value, while at the same time driving the fan at a much lower speed. I don´t know what happened to this design though. I think it was from P&W.

Jan
Je Suis Charlie et je suis Ahmet aussi
 
Santhosh
Posts: 543
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RE: Turbofan RPM

Sun Oct 03, 2004 1:37 am

Are there Transducers attached to the engine to measure vibrations on N1 and N2?If so what are the normal Vibration readings during cruise for Airbus a/c .Does vibration measurement has some units?

George
Happy Landings :)
 
air2gxs
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Joined: Mon Jun 18, 2001 1:29 pm

RE: Turbofan RPM

Sun Oct 03, 2004 2:50 am

Different engines have different vibration limits. These limits are also different between the high pressure shaft and the low pressure shaft. Some aircraft also have an overall vibration limit (broadband). The units are inches per second.

I don't believe I've ever seen an indicator that depicts in/sec, typically the manufacturer will assign a unit to a value and depict the indication as units. I know that the limit on a B767 (CF6) fan is 4 units, but I don't know what 4 units correspond to in in/sec.

I'm not sure I even understand what I just wrote.
 
air2gxs
Posts: 1443
Joined: Mon Jun 18, 2001 1:29 pm

RE: Turbofan RPM

Sun Oct 03, 2004 2:55 am

Here, I'll let Boeing explain it, from the B767 AMM:

With engine operating, the engine accelerometers generate signals proportional to engine motion in a radial direction. These signals are received by the AVM signal conditioner, where they are converted to signals suitable for the EICAS computer. Signals are then sent to the display unit from the computer where vibration displacement or velocity is read in units.
 
G4Doc2004
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RE: Turbofan RPM

Sun Oct 03, 2004 6:35 am

I know for a fact that the Honeywell TFE-731 family of engines use planetary gear reduction for the N1 fan. The smaller engines spin faster. If I recall my TFE-731 schooling, the N2 rotor spins at around 20,000 RPM. The fan must be geared or it would be not only inefficient, but it would not survive the high rotational speed.
"Failure to prepare is preparing to fail"--Benjamin Franklin
 
FinningleyMech
Posts: 26
Joined: Fri Sep 10, 2004 4:02 am

RE: Turbofan RPM

Sun Oct 03, 2004 8:15 am

This is a pretty interesting flash animation, for the trent 800, shows details on each stages, fan, combustor, HP turbine etc, temperatures, rpm, and pressure.

http://www.rolls-royce.com/education/schools/journey/index.html

Its actually pretty interesting to see.

Hope you enjoy it, and that it answes your question (for the RR option anyway  Smile )
Unofficial A.net Finningley Rep
 
widebodyphotog
Posts: 885
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RE: Turbofan RPM

Sat Oct 16, 2004 12:42 am

PW4462 94in Fan (MD-11)
100% N1 rotor rpm is 3600rpm redline = 111.4% 4100rpm
100% N2 rotor rpm is 9900rpm redline = 105.5% 10450rpm

Typical tip velocity at 100% rpm for commercial turbofans is 1450-1500ft/sec. Wide chord fan engines will turn a little slower in that range compared to narrow blade "snubbered" fans.

There are no large commercial geared turbofans in service for airliners.

-widebodyphotog



If you know what's really going on then you'll know what to do
 
BD5JDave
Posts: 5
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RE: Turbofan RPM

Sun Oct 17, 2004 9:41 am

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:

GG- 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.

PT- 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.

 
timz
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Joined: Fri Sep 17, 1999 7:43 am

RE: Turbofan RPM

Sun Oct 17, 2004 12:26 pm

So you're saying a PW4000 with a 94-inch fan never turns faster than 3000 rpm? Or are you saying the speed of sound is higher than normal-atmospheric even for the fan?
 
BD5JDave
Posts: 5
Joined: Sun Oct 17, 2004 7:40 am

RE: Turbofan RPM

Tue Oct 19, 2004 10:27 pm

Slightly higher, yes. And yes, I doubt that it spins much more than 3000 rpm. But that's not my engine, so I can't be sure. It uses supercritical wing sections on its fan blades, which allows for a higher mach number, but I can't believe they could get 4000 rpm out of it...
 
timz
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Joined: Fri Sep 17, 1999 7:43 am

RE: Turbofan RPM

Tue Oct 19, 2004 11:43 pm

More questions:

You said almost all turbofan engines use free turbines. By that you mean a turbine that powers the fan and nothing else? On the CF6 for instance?

"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."

This is a two-shaft engine? So the fan can turn at 3850 RPM?

You can see what's puzzling me. The fan must be what, at least 84 inches diameter? So 3850 RPM would be a tip speed of at least 1411.1 ft/sec (rotational speed only). The standard-atmosphere formula figures the speed of sound to be 65.77 ft/sec times the square root of absolute temperature in degrees Kelvin, which makes speed of sound 1411.1 ft/sec at 187 degrees C.

So which assumption is wrong?
 
BD5JDave
Posts: 5
Joined: Sun Oct 17, 2004 7:40 am

RE: Turbofan RPM

Sat Oct 23, 2004 6:44 am

Yes, most turbofan engines have two shafts, one inside the other.

Go here: http://www.geae.com/education/engines101

You'll learn what you want to know, I think. Enjoy!
 
widebodyphotog
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Joined: Wed Jun 30, 1999 9:23 am

RE: Turbofan RPM

Thu Oct 28, 2004 11:19 pm

Timz

The fan on a CF6-80a or CF6-50 is 86.4in diameter. I'll assume that it's not a -80c or 80e. That fan, spinning at an angular speed of 3850rpm is supersonic, but that is only at the end of the fan blade. The velocity of the blade decreases from the tip to the root, so only part of the blade is at sonic or near sonic velocities. The blade airfoil profile also changes from root to tip from a subsonic to supersonic airfoil.

Also on that CF6 the 4 stage LP turbine spins not only the fan, but a 3 stage "core booster", which is a 3 stage LP compressor directly behind the fan that flows into the HP core compressor. So the LP turbine spins the fan and the 3 stage LP compressor all at the same angular speed or RPM of 3850 max.

-widebodyphotog
If you know what's really going on then you'll know what to do
 
A/c train
Posts: 674
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RE: Turbofan RPM

Fri Oct 29, 2004 12:06 am

Santhosh,
Vibration indication works on the piezoelectric crystal method, were if you press down it, it builds up a charge goes back to original shape which produces a current, this is measured as a frequency.
The way its displayed to the flight crew is so they can understand, its measured in 'units' of vibration, the limits vary from engine to engine.
1 unit of vibration is 3.3" per/sec.
for example, on airbus 321, the information is sent from a Vibartion accelerometer to the EVMU (engine vibration monitoring unit).
- I have never seen a reduction gearbox between the LP turbine and LPcomp/fan.
- I have never seen a Free Turbine on a Turbofan engine.
 
widebodyphotog
Posts: 885
Joined: Wed Jun 30, 1999 9:23 am

RE: Turbofan RPM

Fri Oct 29, 2004 12:10 am

OK, now for the more complicated stuff...

Supersonic velocities at fan or compressor blade tips are really not a problem at all. As long as you have the right aerodynamics there is nothing really alarming about it and most large turbine engines have hot things moving at sonic velocity all the time. What you can not have is sonic velocities THROUGH a turbine engine or supersonic airflows. In the case of the CF6 Timz is working on, the 86.4 in diameter fan at an angular velocity at max RPM of 3850 has a tip velocity of 1451.4 fps. But the velocity of the air flowing past those blades is considerably less than that. The angle of attack of the blades is what's important here. the angle of attack of the fan blades changes from root to tip as does the aero profile. What you have to do is take the cosine of the angle of attack of the blades with the plane of rotation and multiply by the tip velocity of the fan. You will get a velocity well under the speed of sound, as the angle of attack at the tips of the fan is small, That is the speed of the airfow through that part of the eingine. Sorry I don't have the blade profiles for a CF6 on hand at the moment to tell you what those aoa's are...

Also the rotational speed limit of a turbine engine is based on the strength of the rotor disks, blades, and temperature tolerance of the engine. The danger of spinning too fast is that something will break and fly out of the engine, or compressor stall in a "choked" inlet which may cause the same, not that you'll get a calculated supersonic velocity somwhere.

-widebodyphotog
If you know what's really going on then you'll know what to do
 
widebodyphotog
Posts: 885
Joined: Wed Jun 30, 1999 9:23 am

RE: Turbofan RPM

Fri Oct 29, 2004 12:30 am

There are some notable examples of geared turbofan engines:

Allied Signal TFE-731 (Bombardier learjet, Lockheed Jetstar)
Allison ALF-502/507 (Avro RJ, Bombardier Challenger)

Both are two shaft turbofans with the LP turbine turning the fan through a planetary reduction gearbox. Both of these engines are less than 7,000lb thrust and not the large commercial engines we're talking about in this thread.

-widebodyphotog
If you know what's really going on then you'll know what to do
 
Klaus
Posts: 20594
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Widebodyphotog

Mon Nov 01, 2004 7:51 pm

WIDEBODYPHOTOG: Supersonic velocities at fan or compressor blade tips are really not a problem at all. As long as you have the right aerodynamics there is nothing really alarming about it and most large turbine engines have hot things moving at sonic velocity all the time. What you can not have is sonic velocities THROUGH a turbine engine or supersonic airflows.

Thanks. That was the crucial information in this whole debate!  Big thumbs up

Now things are getting consistent.  Smile

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