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skywalker92
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Pratt & Whitney PW1000G

Thu Nov 17, 2016 2:32 am

I have gone through the internet searching for much deeper information about Pratt & Whitney PW1000G and ended up with out any valuable information. Can some one explain about Pratt & Whitney PW1000G engine? I know that it consists of an additional gear box that permits to rotate low pressure compressor and low pressure turbine individually. I want to go more deeper. :smile: :smile:
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akiss20
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Re: Pratt & Whitney PW1000G

Thu Nov 17, 2016 3:18 am

If you have any specific questions I can try and answer them. It helps to have specific ideas of what you would like to know.
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skywalker92
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Re: Pratt & Whitney PW1000G

Thu Nov 17, 2016 5:58 am

I got a PDF file and went through it.. Found answers for many of my questions. This is how I understood.. In normal aircraft engines the fan,low pressure compressor and low pressure turbine is in the same shaft. In order to be more efficient we have to rotate this shaft in a considerable speed. When it is rotating in such a speed fan creates a much more noise and it is bit inefficient. So in this Geared Turbo Fan engines what they have done is they have introduces a gear box in between low pressure compressor and fan in the low pressure shaft. Now low pressure shaft consisting of low pressure compressor and turbine can rotate in their optimum speeds and fan can be rotated in a lower speed to consume more mass and accelerating them to a lower speed. Another advantage is we can rotate low pressure shaft in a faster speed and achieve more compression before the high pressure compressor. This will lead to a smaller(shorter) high pressure compressor and reduces the number of parts and weight by making the engine more lighter.

Am I correct? If not please correct me.
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kurtverbose
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Re: Pratt & Whitney PW1000G

Fri Nov 18, 2016 10:29 am

skywalker92 wrote:
Am I correct? If not please correct me.


Almost. The fan tip speed is what governs the rpm of the low pressure shaft. It's the low pressure compressor and turbine that is running too slowly, not the fan running too fast. Allowing them to speed up makes the compressor and turbine much smaller. Compare the sizes and stages of the PW1000G low pressure system with the LEAP, even though the LEAP has a slightly smaller, and so potentially faster rpm fan.
 
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skywalker92
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Re: Pratt & Whitney PW1000G

Sat Nov 19, 2016 5:19 am

Got it... kurtverbose thanks a lot...
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akiss20
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Re: Pratt & Whitney PW1000G

Sat Nov 19, 2016 7:06 pm

kurtverbose wrote:
skywalker92 wrote:
Am I correct? If not please correct me.


Almost. The fan tip speed is what governs the rpm of the low pressure shaft. It's the low pressure compressor and turbine that is running too slowly, not the fan running too fast. Allowing them to speed up makes the compressor and turbine much smaller. Compare the sizes and stages of the PW1000G low pressure system with the LEAP, even though the LEAP has a slightly smaller, and so potentially faster rpm fan.


In general I would agree, although I would phrase it more that both the fan and the LPT are operating away from their ideal speeds. As with everything in engineering, it is a tradeoff. The gear allows both to operate nearer to the tip Mach numbers they want to.
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skywalker92
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Re: Pratt & Whitney PW1000G

Sat Nov 19, 2016 9:19 pm

akiss20 thanks a lot for the guidance..
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Re: Pratt & Whitney PW1000G

Fri Nov 25, 2016 9:40 pm

The EASA certification information has much of the details. If the link doesn't work, Google:
EASA certification pw1100G

https://www.google.com/url?sa=t&source= ... _YW4HW_k6Q

Now, there is a lot of information in that document.
1. High RPM high spool. 22,300RPM maximum. What that does is a very efficient high compressor and even better for the high turbine.
2. Contra rotating. This is the root cause of the EIS issues. For a very high speed high spool going the opposite direction of the low spool creates a very fast bearing speed. The LEAP is corotating to avoid the risk. About a 3% fuel burn advantage going counter rotating.
3. 3:1 gearbox on the 10,047 rpm low spool means the low turbine is much simpler and more efficient. Most of why the pw1100G is lighter than the LEAP is the reduction in low turbine stages.
4. I started a new number to talk the low compressor. By having it at a decent Mach number not only makes it more efficient, but much higher pressure ratio per stage. That helps engine efficiency.
5. Low RPM fan. That cuts shock wave losses. Oh, the engine would have done better with a 3.5:1 gearbox, but the gearbox is the longest lead item. Hence why the MRJ gets the C-series box instead of one built for half the horsepower as was needed.


Now the document lacks details of fan materials, turbine materials, hot coatings, compressor blade shape, or advanced hot section cooling.

I like to contrast to the other modern engine, the LEAP. The leap has slower spools which implies lower Mach number and thus less efficiency as well as giving up contra rotating efficiency for reliability. The LEAP does have a much more advanced fan. This is required due to it having a fan operated at unprecedented Mach numbers. The low turbine has amazingly advanced blades that cut weight, but the added turbine stages still make the engine heavier.

It is the hot section where the LEAP shines. Variable cooling if the turbine is a long overdue variable cycle technology and the LEAP is the first with this tech. The materials also allow less cool and this combination of efficiency gains are why the LEAP can compete with the pw1100G.

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Re: Pratt & Whitney PW1000G

Fri Nov 25, 2016 10:25 pm

lightsaber wrote:
1. High RPM high spool. 22,300RPM maximum. What that does is a very efficient high compressor and even better for the high turbine.

I am sure I don't need to tell you, but for others: RPM in of itself is relatively meaningless. What dictates the pressure rise per stage (really work per stage) is the tip Mach number, which is a function of RPM, blade tip radius, as well as the temperatures coming out of the LPC. As the fan pressure ratios have dropped and bypass ratios consequently increased, the physical size of the cores has gotten smaller and smaller, pulling in the HPC tip radius - forcing HPCs to spin faster to maintain the same tip Mach numbers. Furthermore, as the LPC is more heavily loaded (as noted below) the temperature at HPC inlet is increased, further demanding greater physical speed to maintain tip mach number.

2. Contra rotating. This is the root cause of the EIS issues. For a very high speed high spool going the opposite direction of the low spool creates a very fast bearing speed. The LEAP is corotating to avoid the risk. About a 3% fuel burn advantage going counter rotating.

Your statement about bearing speeds is only relevant if there is an inter-shaft bearing, rather than all bearings being to the static structure. Historically P&W have not used inter-shaft bearings, e.g. on the JT9D and PW4000 (see Jack Kerrebrock, Aircraft Engines). Do you have any source showing that the GTF uses intershaft bearings (I say this not hostilely but earnestly)

Most of the sources I am seeing attribute the GTF startup problems to thermal rotor bow rather than a bearing issue (e.g. http://aviationweek.com/commercial-aviation/new-pw-president-has-nothing-hide-gtf-starting-issue). This makes sense to me as the low speed shafts especially are becoming smaller and less stiff as the overall core size shrinks. Rotordynamics is becoming an evermore present issue with these designs.

3. 3:1 gearbox on the 10,047 rpm low spool means the low turbine is much simpler and more efficient. Most of why the pw1100G is lighter than the LEAP is the reduction in low turbine stages.

Correct. The higher rotational speeds of the LPT due to the gearbox allow higher tip Mach numbers, thus more work extraction per stage and fewer overall stages. It also allows them to not dramatically increase the LPT radius to maintain tip Mach numbers. Compare the GTF cross-section (http://356007295890291112.weebly.com/up ... ig.jpg?250) and LEAP-X (https://theflyingengineer.files.wordpre ... leap-x.jpg) cross sections and it is immediately obvious that the number of stages and radii are very different.

4. I started a new number to talk the low compressor. By having it at a decent Mach number not only makes it more efficient, but much higher pressure ratio per stage. That helps engine efficiency.


Agreed. I wouldn't be surprised if the OPR of the LPC was on the order of 3-4 for the GTF.

5. Low RPM fan. That cuts shock wave losses. Oh, the engine would have done better with a 3.5:1 gearbox, but the gearbox is the longest lead item. Hence why the MRJ gets the C-series box instead of one built for half the horsepower as was needed.

The reduction in shock loss is one immediate benefit of the GTF architecture, but a longer term benefit is that it enables further decreases in fan pressure ratio. One can always decrease fan pressure ratio through blading design (decrease turning), but with the higher RPMs associated with direct shafts and the required compromise between the fan and LPC/LPT, you would still incur the shock losses for essentially no reason. The GTF architecture opens up a new part of the design space of very efficient, low pressure ratio fans.

One thing I would like to note is that increased tip Mach number does not directly imply increased efficiency. Too high of a tip Mach number and you can do a lot of *work* per stage, but the efficiency of the stage will drop greatly as shock losses grow. It is rare to see tip Mach numbers above 1.4-1.5 as that is roughly where the total pressure ratio across a normal shock really starts to fall off. Too low of a tip Mach number, however, and the work per stage is low and you will need more stages to achieve the desired pressure ratio, which costs weight and complexity.
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Re: Pratt & Whitney PW1000G

Sun Nov 27, 2016 3:50 am

2. Contra rotating. This is the root cause of the EIS issues. For a very high speed high spool going the opposite direction of the low spool creates a very fast bearing speed. The LEAP is corotating to avoid the risk. About a 3% fuel burn advantage going counter rotating.

Your statement about bearing speeds is only relevant if there is an inter-shaft bearing, rather than all bearings being to the static structure. Historically P&W have not used inter-shaft bearings, e.g. on the JT9D and PW4000 (see Jack Kerrebrock, Aircraft Engines). Do you have any source showing that the GTF uses intershaft bearings (I say this not hostilely but earnestly)

Most of the sources I am seeing attribute the GTF startup problems to thermal rotor bow rather than a bearing issue (e.g. http://aviationweek.com/commercial-aviation/new-pw-president-has-nothing-hide-gtf-starting-issue). This makes sense to me as the low speed shafts especially are becoming smaller and less stiff as the overall core size shrinks. Rotordynamics is becoming an evermore present issue with these designs.
[/quote]
There is a reason that "Pratt is adopting a two-pronged attack on the issue. All production standard engines now feature a damper on the third and fourth shaft bearings to help stiffen the shaft."

http://aviationweek.com/commercial-avia ... ting-issue

Yes, fundamentally it is the differential thermal cooling. An issue easier to solve with larger bearings. But larger bearings do not work with higher bearing speeds.

It is splitting hairs on the root cause. Shaft dynamics can never be split from bearing dynamics. ;)
I've never designed a shaft. While I've *never* been on a rotor bearing team, I've designed other aerospace bearings. Roller bearings are best, but only if you can ignore shaft bowing... Once you start having shaft bowing... There is work to be done. :)

Then again, what really needs to be done is ramp production of both the PW1100G and the PW1500G. Pratt has so many bottlenecks, they have to get a better recovery plan.

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Re: Pratt & Whitney PW1000G

Sun Nov 27, 2016 4:57 am

I do not really know much about bearing design, but I do know a bit about rotordynamics. That quote from aviationweek only mentions adding damping to the bearing. Does that inherently imply increasing bearing size? Can the bearing not be kept the same size but with additional damping mechanisms added to it?
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Re: Pratt & Whitney PW1000G

Sun Nov 27, 2016 8:14 pm

akiss20 wrote:
I do not really know much about bearing design, but I do know a bit about rotordynamics. That quote from aviationweek only mentions adding damping to the bearing. Does that inherently imply increasing bearing size? Can the bearing not be kept the same size but with additional damping mechanisms added to it?


It does not necessarily mean the bearing is any bigger. The "damper" can simply be a pair of o-rings between the bearing and the housing rather than a rigid metal-to-metal interface.
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