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A Rambling Thread On Aircraft Engines  
User currently offlineLightsaber From United States of America, joined Jan 2005, 12899 posts, RR: 100
Posted (8 years 2 months 3 days 9 hours ago) and read 6833 times:
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This theread, at the end became a discussion on aircraft engines.
Since a new thread was started, I'll let that one go back to the original topic,
PIA Claims B777-200LR Not Upto Specifications... (by Blrsea May 22 2006 in Civil Aviation)#259 (old)

IF you want to still talk PIA 777's:
PIA Claims B777-200LR Not Upto Specifications V2 (by LTU932 May 28 2006 in Civil Aviation)
(new)
This one is on engines.

From the last posts: (my text):

Quote:

I'll do a quick summary.

A geared turbo fan has a fixed gearbox between the turbine that powers the fan and the fan? Now, that same turbine powers the first half of the compressor (called the low compressor). All airfoils operate at their peak efficiency at the same mach number. Rather than explain the speed of sound, think of it as RPM times diameter with a temperature correction. ok?

Now the fan is a really big diameter, so it needs to spin at a low RPM. Overspeeding the fan quickly has huge noise and fuel efficiency penalties. But... fans are usually run 7% to 10% overspeed! About 1400 RPM when you might really want 1300 RPM for the fan.

The fan must be powered by the relatively small diameter turbine (low turbine). That turbine is powered by hot gases coming out the high turbine.
So hot gases mean the turbine needs a higher RPM to be efficient. The smaller diameter and hot gasses mean that the optimum RPM for the turbine is about 4,500 RPM.
The low compressor is most efficient in the 3,500 to 4,000 RPM range.

Ok, so you have one part that is most efficient at 4,500 RPM powering a part that would do best at 1,300 RPM. Simple solution, put in a gear box! Now, since the low compressor is happiest at 4,000 RPM (or less), that's the speed you run the low turbine. Because the fan is slowed down, you gain about 3% improvement in fuel efficiency. Because the turbine is sped up, you gain about 5% in fuel efficiency. We engineers say TSFC rather than fuel efficiency as its a number we can put into our equations. spin

Now TSFC is "Thrust Specific Fuel Consumption."
It the amount of fuel burned per hour (in lbm) divided by the engines thrust (at that setting). Values are dropping (better fuel efficiency) toward a TSFC of 0.510 for cruise. The harder you run a jet engine, the more thrust you get for each pound of fuel (about 7.8 lbm per gallon) so engine makers prefer to quote takeoff TSFC, but for long haul planes cruise is what we discuss. Climb is important too... but one actually looks at dozens of design points... so on a.net we usually just talk cruise to simplify things. spin

A link on TSFC:
http://www.grc.nasa.gov/WWW/K-12/airplane/sfc.html

Now, if you haven't realized, there is concern over the gearbox durability and reliability. Thus, Rolls Royce has chose another alternative. Their fan is powered by its own turbine, but the fan still runs at the inefficient 1,300 ish RPM.

But, their fix is instead of running the low compressor and its turbine at a very inefficient 1,300 RPM, they put those two on their own shaft and bearings at about 4,000 RPM. This improves fuel efficiency by about 2% (or lowers TSFC by 2%, a good thing).

What it also does is reduce engine weight! By about a ton per engine! Why? Engines are made from lots of nickel and titanium (some other stuff too). Nickel is rather heavy. Well, a more efficient compressor means that less compressor is needed as well as less turbine. This makes the engine shorter. That shorter engine, called a "triple spool" has quite a bit fewer parts in it. Really, the only downside is the extra set of bearings added which can be a bear to service. We call this new shaft the "intermediate spool" even though it has the low compressor on it. RR calls the "low compressor" the "intermediate compressor" so have fun calling it what you want. Smile

Why aren't triple spools the only engines offered? There very heavy for *small* engines. But at 60,000 lbf and above ... they're very light. Thus why the only narrow body with a triple spool is the 757. All the other triple spool engines go on widebodies.

Also, GE and RR have been able to make their double spools as efficient or even more efficient than RR's triple spools by spending money on optimizing the other parts of an engine. On a Boeing 777, if you're flying > 4,000nm get the GE-90. For flights less than 4,000nm get the Trent triple spool. I'm ex-Pratt, but since Pratt blew the Pw4098's fuel efficiency... they're out of the game effectively. cry And the pw4168 is the most efficient engine on the Airbus A330... but one needs 72k of thrust for the latest A330's and its a bit of a story why the Pratt's are stuck at 68k thrust... cry (68k is 68,000 lbf of thrust)

Since engines have to be started, takeoff, climb, cruise, land, be restarted in flight (rare, but important), produce minimum emissions, burn minimum fuel, weigh as little as possible, survive for thousands of takeoffs and hours of operation, not spit out fireballs (awwww...) etc. they are very complicated. Some added cooling to increase cruise fuel efficiency must be turned down or off for large changes in engine thrust. I alluded to that earlier. Some engines are more sensitive to wear and tear, etc.

Please take all RPM's as approximate. I was too lazy to look up exact numbers.

My background: I designed gas turbine engines for almost 5 years at Pratt. I also have some experience at Solar gas turbines. My current job is propulsion related, but my employer strickly forbids me from discussing my job in online forums. What I can say is I'm much more involved on the fuel aircraft system side than before, so I'm getting a different perspective. spin

This thread has been discussing a situation where the better climb durability and increased climb thrust of a triple spool compared to a double spool has been important. Also, all airplanes must fly safely such that if they suddenly lose an engine's thrust everyone is ok. Smile That means you do not fly a 777 or any other two engine plane directly over K2 or everest (unless they're "light") but you're allowed to fly a four engine plane. It happens that the A340 does very well in an engine out situation. Your seeing a bit of argument over how much of a diversion is required by the 777 due to those tall mountains... And some argument on the weights PIA is flying their 777's at and if they're light enough or not to safely fly near K2/Everest. But I know the kind of work Oldaeroguy and Widebodyphotog do, so I'm going to believe them.

Unfortunately, you will find a lot of A vs. B on these forums. I will vouch that neither OldAeroguy nor Widebodyphotog take sides; they both will present their data and tell you the conclusions they make. Most of the time we're making assumptions off incomplete data... so expect some small errors. But this is discussion. I'm certainly not in the market for a 777 or an A340. duck

I hope this helps,
Lightsaber
[quote]

and...

[quote] Oh, I missed a few definitions:

Oh, if it helps, engines work by "Squeeze-bang-blow" You squeeze the air, "bang" fuel (add heat energy) and then "blow" the air/fuel through the turbine to extract work (more work out due to volumetric expansion)

Engine terms:
GTF=geared turbo fan
LPC=Low pressure compressor
LPT=Low pressure turbine, usually drives the LPC and fan
HPC=High pressure compressor
HPT=High pressure turbine (only drives the HPC, on the GeNx also a generator)
IPC=Intermediate Pressure compressor. An RR term for the LPC on its own turbine
IPT=In a Triple spool, the turbine that drives the LPC (aka IPC).
RPM=Revolutions per minute (in my posts, there are other uses for this)
Cassing=the pressure vessal that holds the engine together.

Quoting Lumberton (Reply 242):
But doesn't the gear box itself add weight? Are you saying that the weight savings elsewhere compensate for the gearbox weight?

IIRC you posted elsewhere that many crashes win WWII were from gearbox failures? How will the OEM's sell gearbox reliability?

The gearbox more than compensates. By having a slow turning (or more precisely, low mach #) LPC, extra stages must be added to it and the LPT (since it too is turning too slowly for the optimal blade shapes). This stretches out the engine. The pressure vessal (aka casing) of the engine must be made from Inconel 715 (ok, in low pressure designs Inconel 625 is possible, but 715 casts easier in spin casting). That's a really heavy nickel alloy. The longer the engine, the thicker the nickel must be to keep the engine "stiff" (adding weight) *and* the longer the casing must be (adding weight). The gearbox adds a few hundred pounds yet in total on a 60k engine would weigh in like a triple spool. (or net 2,000 lbm weight savings).


You recall correctly on the WWII engines. About half of the engine failures were due to gearbox failures. Why?

1. No temperature sensors. We now have cheap reliable temperature sensors.

2. No synthetic oil. This makes a huge difference!

3. New gear tooth designs (this really helps)

4. Testing, testing, and more testing.

To my knowlege, only Pratt has taken the time to characterize high horsepower gearboxes into the detail required to predict (via oil pressure and temperature) a gearbox failure 100+ cycles before the failure.

...snip of some comment...

IAE is gearing up to launch a GTF for their next engine. (Finally, the "superfan!"). Due to contracts, that locks both RR and Pratt into IAE for the A320NG and 737RS. Thus, that keeps the triple spool out of the < 35k market for a generation. I do belive Boeing will allow IAE to offer an engine on the 737RS. Why? Its either that or Airbus will be the only one with a GTF and you know GE will get its engine on the Airbus too... Lets put it this way, a GTF offers the same drop in fuel burn as composites and bleedless combined! I expect a solution like the 787 where IAE and CFM (or will it be GE solo sans SNECMA) share a nacelle. With bleedless designs, that's quite possible.

ok, I was asked to say if the 787 truly shares a nacelle or not... I cannot answer that. However, Boeing tried to keep control the nacelle to keep it a common part... does anyone know if they did?

Also, the thread ended on why the triple spool doesn't have better efficiency:
my start at my speculation:



Quote:


************************************
***warning, intering speculation zone***
************************************

It could also be the turbine technology. While shrouded turbines have theoretical advantages (blade tip sealing) its a bit wasted with the required added cooling for the added metal. I know one reason GE and Pratt avoid shrouded turbines is that with optical measurements minimizing turbine blade tip clearences via active cooling, the real world benifit of shrouded turbines is believed to be negative by their numbers. But then again, by GE's numbers every Pratt PW4090 would have snapped its low spool shaft by now and every GE-90 by Pratt design methodology would have fan surges. So each engine maker has a few tricks they just don't share with *anyone*. Smile

I also think its because RR just doesn't put on a 2nd row of high turbine blades. This can dramatically help the engine efficiency (if paired with one or two more HPC rows of blades) by increasing the engine pressure ratio (higher pressure=more efficient energy extraction of the fuel's heating value). But ohh... it would add weight big time and cost... a lot of cost. Sad

Also, look at the older RR cross sections. e.g., the Trent 800
http://www.rolls-royce.com/civil_aer...ucts/airlines/trent800/default.jsp

I see two issues:
1. A very old design pre-diffuser concept (design of the exit of the HPC). This adds 1/2% to fuel burn. Since RR now owns Allison and they have the sweetest pre-diffuser designs I've ever seen... I bet the Trent 1000 fixes this (I can't help but notice this part of the engine is missing on the RR online pages for the future trents... scratchchin so something is changing that they don't want GE to know about...)

2. That long distance between the high turbine and intermediate turbine. That's a killer for engine efficiency. Unfortunately, this distance is to get around the extra set of bearings and this seems to be still a long stretch in the Trent 1000 (but not as extreme). Well... thats *really* hot gas flowing at an high subsonic mach number. So a bunch of added cooling air must be dumped in and the pressure that should be turning a turbine is thrown away to the considerable losses always present in high mach # flow environments. This could increase fuel burn by 2%!!!

But I think the fact that RR tends to go for lower pressure ratios than GE or Pratt is a real part of what is hurting their TSFC. But adding that 2nd row of high turbine blades will add 1,500+ lbm to each engine... so their goes RR's weight advantage. So my guess is that the system engineering folks just went for a design better optimized for the shot hops and lower manufacturing costs. But note, we're in the speculation zone as to why RR made decisions. I've never worked for RR, so I cannot tell you for certain.

Oh, just in case you weren't familiar, the higher the pressure ratio in an engine, the more efficient it is. But with Pratt and GE, they're getting up to the temperature limits of Inconel 715 at the exit of the HPC. wideeyed As Oldaeroguy noted, its a conservative industry. No one is quite ready to make the main casting out of anything more exotic... yet. But the engine after the GenX will have to be made from something new. Are their materials out there? Sure! But they aren't proven to case as nicely as Inconel 715 in *large* spin casting applications. Sad So its possible a 2nd row in the high turbine of the trents would increase the HPC exit temperature beyound what Inconel 715 can handle. But I haven't done the math... I admit this would be a very interesting design study to look into. spin

***Exiting speculation zone***

Then one final unanswered question by Baroque:

Quote:

Hmm, that had not occurred to me, although the inverse is clearly going to be true - good TSFC will be rather useful on long range aircraft. Can we soon have a thread on high BPR high thrust engines, or are they getting taken out as collateral damage in other threads?

I was intending to have this more of a give and take thread on engines.

Oh, just to be clear, when talking about the weight added to a triple spool to increase its overall pressure ratio, its not just the added row of turbine blades that adds the weight (or its rotor). Its the added HPC rows, added length and required thickness of 3 rotor shafts (all three must get stiffer if they are longer), and the weight of the engine casing (also, this must get stiffer as it gets longer). That is why I *speculate* RR has only gone for a single row high turbine.

So if there are any more questions, I'll answer.

What I would request is more input on the engine selection's impact from OldAeroguy or Widebodyphotog. Most of my career has been "oh, you need more thrust here... WILCO!. At better TSFC? Yep!" In particular, any information on how the Trent 1000's climb performance versus the GenX is effecting customer selection.

Lightsaber


Societies that achieve a critical mass of ideas achieve self sustaining growth; others stagnate.
22 replies: All unread, jump to last
 
User currently offlineBaroque From Australia, joined Apr 2006, 15380 posts, RR: 59
Reply 1, posted (8 years 2 months 3 days 8 hours ago) and read 6784 times:

Thank you Lightsaber. The combination of the effects of various aspects of engine design and the realities of flight with them should be fascinating.

At least it is clear there IS a reason for all these things, even if the details turn out to be complex.

I guess the weight problem of adding a few blades is much like the problem of weight growth in structures. I seem to remember Barnes Wallis had a few things to say on that account. Just as Frank Whittle had a few to say about RRs early efforts at making jet engines. Still they were better than Rover it seems.


User currently offlineRheinbote From Germany, joined May 2006, 1968 posts, RR: 52
Reply 2, posted (8 years 2 months 3 days 7 hours ago) and read 6742 times:

Okay, so let's fill this new thread with life  Smile

Lightsaber, I'd like to get back to that 150,000hp gearbox. What's the concept for heat dissipation? What can we expect in terms of design life (or what's the current target)?


User currently offlineSunriseValley From Canada, joined Jul 2004, 4869 posts, RR: 5
Reply 3, posted (8 years 2 months 3 days 4 hours ago) and read 6683 times:

Quoting Rheinbote (Reply 2):
Okay, so let's fill this new thread with life Smile

I have some qoestions..

1. can this gearbox be changed on the wing? If so about how many manhours?
2. you stated that PW had got the reliability prediction down to about 100 cycles. Are PW still working on this device ,if so I wonder what new materials and manufacturing methods have been developed for it.
2. Is it something a specialty gearbox manufacturer could customise for different engine manufacturers? Seems to me Sundstrand was in this field some years ago.


User currently offlineSunriseValley From Canada, joined Jul 2004, 4869 posts, RR: 5
Reply 4, posted (8 years 2 months 3 days 4 hours ago) and read 6681 times:

I have a question; what are the indicators that tell an operator under ETOPS that they should be changing an engine.

User currently offlineGeo772 From United Kingdom, joined Jul 2004, 519 posts, RR: 0
Reply 5, posted (8 years 2 months 3 days 3 hours ago) and read 6602 times:

Quoting SunriseValley (Reply 4):
I have a question; what are the indicators that tell an operator under ETOPS that they should be changing an engine.

Magnetic Chip detectors, vibration monitoring, engine parameter moniotring, eg EGT variation, Oil pressure and temp, boroscope inspections, life.

Point of note RR Trents have Low, intermediate and high pressure sections. The actual weight difference between the trent 800 and the GE90 is nearer 2 tonnes each than 1 tonne.

The trent 1000 will have the following major differences over previous trent engines: Bleedless and the accessory gearbox will be driven by the Intermediate stage turbine.



Flown on A300B4/600,A319/20/21,A332/3,A343,B727,B732/3/4/5/6/7/8,B741/2/4,B752/3,B762/3,B772/3,DC10,L1011-200,VC10,MD80,
User currently offlineClassicLover From Ireland, joined Mar 2004, 4627 posts, RR: 23
Reply 6, posted (8 years 2 months 3 days 2 hours ago) and read 6590 times:

For me, regardless, Rolls Royce have the best engines.

GE sell by exclusivity, which they are entitled to due to the amount of money they have from other businesses. If i had to choose between GE or RR, it would be RR all the way. It's a shame P&W aren't so much in the running now...



I do quite enjoy a spot of flying - more so when it's not in Economy!
User currently offlineContnlEliteCMH From United States of America, joined Mar 2005, 1455 posts, RR: 44
Reply 7, posted (8 years 2 months 3 days ago) and read 6538 times:

Quoting Rheinbote (Reply 2):
Okay, so let's fill this new thread with life

Lightsaber, I'd like to get back to that 150,000hp gearbox. What's the concept for heat dissipation? What can we expect in terms of design life (or what's the current target)?

I don't know what to expect in terms of design life. However, I've been able to run a few figures for the heat dissipation question.

Assumptions:
1 hp = 0.746 kW
specific heat of water = 4.160 J/g-K (or 4.16 kJ/Kg-K)
specific heat of oil = ~2 J/g-k (or 2 kJ/Kg-K)

Assuming a 90% transmission efficiency, the losses through the transmission at full load would be approximately 11,190 kW, or about 11 mW. I don't know what the transmission efficiency would actually be, but I expect it would be at least this high. I hope it would be much higher, and some cursory research tells me to expect at least a 95% efficiency, and possibly 98%. Let's take 98% efficiency. That gives a dissipation requirement of 2238 kW.

I did some quick math. (If any of you find errors, please correct it.) If you used 100 Kg of oil as coolant, one minute at 150,000 SHP and 98% efficiency would result in a temperature rise of 671 K. That value assumes absolutely no heat transfer *out* of the oil. Obviously, you'd use a heat exchanger and some mass flow rate to effect a heat transfer rate. Furthermore, the altitudes at which this kind of heat transfer is required are also the altitudes at which heat transfer in an oil-to-air exchanger is most efficient. Perhaps an oil-to-fuel cooler would be a better choice, as the capability of liquid-to-liquid heat transfer tends be quite good.

Consider as well that it is not difficult to pump 100 Kg of oil in one minute. There is no exact correlation between the 100 Kg figure I picked as a heat sink, and the 100 Kg of mass I cite in the circulation figure. I picked them only as examples, but if you consider a working temperature in the oil of 100 to 150 C, I don't think cooling out of the gearbox will be problematic -- even if the efficiency isn't 98%.

I would also assume two additional items. First, the system will surely be designed to cool the gearbox at indefinite maximum thrust. Second, such a cooling requirement would only be required at takeoff thrust. If climb thrust is only 20% to 40% that of maximum thrust, then cooling will surely not be an issue.

Perhaps Lightsaber or somebody else can provide figures on the working temperatures, volumes, and flow rates of the oil in a modern turbofan. Will the engine oil also be used for the gearbox?



Christianity. Islam. Hinduism. Anthropogenic Global Warming. All are matters of faith!
User currently offlineLightsaber From United States of America, joined Jan 2005, 12899 posts, RR: 100
Reply 8, posted (8 years 2 months 2 days 16 hours ago) and read 6414 times:
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Quoting ContnlEliteCMH (Reply 7):
Assuming a 90% transmission efficiency

There would never be a GTF at 90% efficiency.  Wink

98% is a better starting point for gear efficiency.

Quoting ContnlEliteCMH (Reply 7):
Obviously, you'd use a heat exchanger and some mass flow rate to effect a heat transfer rate. Furthermore, the altitudes at which this kind of heat transfer is required are also the altitudes at which heat transfer in an oil-to-air exchanger is most efficient. Perhaps an oil-to-fuel cooler would be a better choice, as the capability of liquid-to-liquid heat transfer tends be quite good.

Actually, you do an oil to air cooler with the cooler in the *fan duct*. Yes, there is a tremendous propulsion loss due to the drag of having an oil cooler in that fast of an airstream. Also, realize that the oil would be completely recirculted within seconds as is engine oil. So yes, the oil heats up a bunch, but then its cooled by an incredibly fast blast of air (near sonic speed). Convienient... the harder the engine is working, the faster the cooling air.  spin 



Quoting ClassicLover (Reply 6):
For me, regardless, Rolls Royce have the best engines.

GE sell by exclusivity, which they are entitled to due to the amount of money they have from other businesses. If i had to choose between GE or RR, it would be RR all the way. It's a shame P&W aren't so much in the running now...

??? GE right now has the lowest fuel burn. The JT8D remains the lowest cost engine to maintain (and cheapest to operate when oil < $25/bbl). Yes, RR makes good engines, but realize all engine decisions are economic. Each engine family is a compromise tailored toward their primary customer base.

The GE-90 easily has the lowest fuel burn on the 777, The Trent has by far the highest! But... the trent is far lighter and thus in < 4000nm missions ends up with the best efficiency. I've already gone over why a triple spool has a climb advantage. The Pw4090 is the least expensive 777 engine to manufacture. IF the Pw4098 has met promise (which it missed on TSFC by 4%!), it would be besting the GE-90A on fuel burn. The Pw4168 is the most fuel efficient A330 engine. IF Pratt hadn't been hamstrung by a darn nacelle contract or had realized the compressor bleeds would fry the 68k nacelle when the engine is pushed to 72k... Pratt would still rule the A330...

Yes, couda, shouda woulda... but understand that no engine maker is invulnerable. RR is a bit weak in the single isle market as is Pratt. Only GE is strong there (CFM-56 on the 737/A320 and the CF-34 family con the CR7/CR9/E-jets). Yes, I disount the Allisons and CR2... sorry, but I don't think they have much of a future sales life. Pratt somehow has remained very strong on business jets (thanks to the parent company letting P&W Canada have a bit more say in their strategy). Yes,the BR 710 is the high end business jet engine...

The point is that the engine market is a very fractured market. For example, I bet that Honda/GE will do very well in the VLJ market. Why? Honda has a passion for mass production that the other engine companies are tyring to copy, but haven't acheived.

Quoting Geo772 (Reply 5):

The trent 1000 will have the following major differences over previous trent engines: Bleedless and the accessory gearbox will be driven by the Intermediate stage turbine.

Taking all of the power off of the intermediate turbine will help the Trent 1000 have nice climb and cruise stall characteristics. This will allow the engine to push the LPC and HPC stall margins. Why is this good? This allows for a more efficient compressor without having the engine risk hocking out a fireball out the front.  Wink

But again, that long legth between the high and intermediate turbine has got to be killing the Trent's efficiency. Don't get me wrong, I think the Trent 1000 is a great concept. Just always understand every design is a compromise, even whichever is your favorite.  spin 

Quoting SunriseValley (Reply 3):
1. can this gearbox be changed on the wing? If so about how many manhours?
2. you stated that PW had got the reliability prediction down to about 100 cycles. Are PW still working on this device ,if so I wonder what new materials and manufacturing methods have been developed for it.
2. Is it something a specialty gearbox manufacturer could customise for different engine manufacturers? Seems to me Sundstrand was in this field some years ago.

Good questions.
1. The gearbox is a shop change. The engine would be pulled and the gearbox would require 4+ shop hours to change out.
2. The reliability is more in mapping out the gearbox cooling, etc. As a conservative industry, I'm not aware of the exact materials... so I will avoid speaking where I don't know enough.
3. A gearbox manufacturer could customize for the different manufacturers if they bothered to know their failure modes that well. FYI, Sunstrand is now merged into Hamulton-Sunstrand (ex Hamulton-Standard), a division of United Technologies, the parent company of Pratt.  scratchchin  Note: I believe Textron did work too on GTF gearboxes.

Quoting ContnlEliteCMH (Reply 7):

I would also assume two additional items. First, the system will surely be designed to cool the gearbox at indefinite maximum thrust. Second, such a cooling requirement would only be required at takeoff thrust. If climb thrust is only 20% to 40% that of maximum thrust, then cooling will surely not be an issue.

Normally good assumptions. However:
1. Maximum thrust has a time limit. Usually just a few minutes, so the answer is that you might overheat the oil a bit at the end of this interval.
2. Takeoff must be possible in 125F weather (Pratt's standard maximum temperature takeoff, but that's *Not* at maximum thrust.) However, once you start climbing, the temperature in the fan case drops toward -55F or maybe a little warmer (yes, the air outside is -70F in the stratosphere, but the recovery of pressure and the fan will heat the air a tiny amount.) So its easy to cool the gearbox for cruise. For climb... after the first few minutes the air is much cooler than the assumed 125F, so again, cooling is easier. Also remember how fast the air is hitting the oil cooler. Imagine your car radiator being hit with 500mph+ air. I bet it would cool a bit better. Hint: your cars fan probably accelerates the air to Mach 0.1 (at best). Not mach 0.8+.  spin  Recall again, the oil coolers are put *inside* the fan duct for FOD reasons. (on the inner diameter so that the fan centerfuges any water/birds/stones/whatever away from the vulnerable oil coolers including the current engine oil coolers.)

Quoting ContnlEliteCMH (Reply 7):
Perhaps Lightsaber or somebody else can provide figures on the working temperatures, volumes, and flow rates of the oil in a modern turbofan. Will the engine oil also be used for the gearbox?

Pratt keeps this information tight to the chest. Or at least, I do not know. I do know the gear oil and engine oil would be seperate systems.  Smile

I'm excited where engines are going. We're seeing contra-rotation go from being a fantasy to a reality. I know all three major engine makers are working hard to replace nickel in their high turbine blades. Each has their own solution... the first to get it to market has a definative advantage. However, recall the pw2000 was the first commercial engine with single crystal turbine blades... it took a decade+ of service to get the durability right.  Sad I'm also excited about integrade blade/hub rotors. As simple as it sounds, this saves a lot of weight in an engine and that's always good. I'm not as certain about the high RPM high spools (instead of 15,000 RPM, how about 30,000+?!?) I think it will happen in military engines; there will almost certainly be a noise issue in civil aircraft.  Sad I also believe we are at a breakthrough in composites. All composite& titanium nacelles will probably be the future.  Smile And CFD (which used to be my bread and butter) is only getting better.

Good night,
Lightsaber



Societies that achieve a critical mass of ideas achieve self sustaining growth; others stagnate.
User currently offlineBeaucaire From Syria, joined Sep 2003, 5252 posts, RR: 25
Reply 9, posted (8 years 2 months 2 days 16 hours ago) and read 6407 times:

I'm no expert in engine design or not even remotely familiar with the constraints -but Lightsaber's posts gives an excellent introduction in the complexities of engine -design.
Thanks for sharing your expertise with "greenhorns" and allow us to understand a little better ,why those dam engines are so expensive !!!!
Welcome on my RUL....



Please respect animals - don't eat them...
User currently offlineBaroque From Australia, joined Apr 2006, 15380 posts, RR: 59
Reply 10, posted (8 years 2 months 2 days 9 hours ago) and read 6333 times:

Quoting Lightsaber (Reply 8):
how about 30,000+?!?

Eeek! must be about the right comment there. Presumably this is to get more work out of smaller and therefore lighter blades?

Quoting Lightsaber (Reply 8):
composite& titanium nacelles will probably be the future

Surprising that these improvements are so late in the curve.

Quoting Lightsaber (Reply 8):
in their high turbine blades.

Can you say what sort of range of solutions are being looked at there? Will there be an end to the nickel era? Ceramics must be tempting, but they are still brittle? Presumably there is quite a way to go with the working temperature range that the combustors can give. Any estimates of how much additional efficiency is out there to be achieved - just from basic thermodynamics?

A question that might be more for Geo772, is there much in the Trents that can be traced back to the acquisition by RR of Bristol and Armstrong Siddeley, not to mention Vickers Met, I guess?


User currently offlineLightsaber From United States of America, joined Jan 2005, 12899 posts, RR: 100
Reply 11, posted (8 years 2 months 2 days 9 hours ago) and read 6333 times:
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Quoting Beaucaire (Reply 9):
Welcome on my RUL....



Quoting Baroque (Reply 10):

Eeek! must be about the right comment there. Presumably this is to get more work out of smaller and therefore lighter blades?

Actually, its to do the work with 1/2 the number of blades. This shortens the engine, gets rid of parts, and lightens the engine that way. In actuality, each blade is bigger and heavier than the current designs. But the weight savings is still tremendous if it can be pulled off. I know just enough to say its going to be a tough design to get to work. I expect the first one to be in a military engine.  Wink

Quoting Baroque (Reply 10):

Surprising that these improvements are so late in the curve.

Not really. Believe it or not, its a very conservative industry. But... there is no reason an aluminum aircraft cannot have a composite nacelle.  Smile

As to the materials in a turbine, I have to tiptoes around an NDA  Sad So I must speculate in general and not talk specifics.

Ceramics still have hope, but it is not the focus of GE or Pratt. Both of those companies are looking more at refractory metals due to their incredibly high temperature resistance. This should suprise no one. If you know what refractory metals are, you know that they have incredible temperature capabilities. But, Tungston, Moly, Tantalium, Niobium, Rhenium, and maybe I'm forgetting a few. Platinum works well too, but is a wee bit pricey.  Wink

http://en.wikipedia.org/wiki/Refractory_metal

But none are ready for use.
Time to hit the beach.
Lightsaber



Societies that achieve a critical mass of ideas achieve self sustaining growth; others stagnate.
User currently offlineContnlEliteCMH From United States of America, joined Mar 2005, 1455 posts, RR: 44
Reply 12, posted (8 years 2 months 2 days 4 hours ago) and read 6323 times:

Quoting Lightsaber (Reply 8):
Actually, you do an oil to air cooler with the cooler in the *fan duct*. Yes, there is a tremendous propulsion loss due to the drag of having an oil cooler in that fast of an airstream. Also, realize that the oil would be completely recirculted within seconds as is engine oil. So yes, the oil heats up a bunch, but then its cooled by an incredibly fast blast of air (near sonic speed). Convienient... the harder the engine is working, the faster the cooling air.

I saw this when I toured GEAE in Evendale just before I graduated from Ohio State. I don't know what the exchanger was used for, but I noted that it was a rather expensive unit, designed for maximum transfer in a very small space. Actually, that particular unit got the attention of more than one of us in the tour group, because several of us had just taken an elective course in heat exchanger design from the head of the nuclear engineering department.

There are a couple of notes I can give you definitively on liquid-to-air heat exchangers. First, the amount of cooling on the leading edge of any fin is much, much higher than at any point along the length of the fin. In fact, adding more fin length is a waste. It adds weight but doesn't do much for cooling. So you want to stack as many rows of fins as you can, which is what the heat exchanger in the GE engine did.

Second, concerning heat transfer rates and efficiencies, air density is generally much more important than the temperature or velocity of the air moving across the exchanger. In fact, air velocity tends to reach dimishing returns because of the leading edge effect which dominates fin transfer rates. And it's been a while since I ran the numbers, but if you cut the density of the air by half, and reduce the temps quite a bit, you may actually get *less* heat transfer than when the air was dense. Please take with the proverbial grain of salt; I haven't looked at the equations in quite some time.

So, let's assume that the air temperature aft of the fan is a full 125F. Since the oil can be some exotic synthetic with a standard operating temperature of 85 to 100 C, and say up to 150 C, you'd still get substantial heat transfer, even at 125 F (which is about 52 C). And if the gearbox needs a specific minimum temperature at altitude, you can just throttle the oil flowrate to the cooler.

Interesting note about counterrotation: that was what the GE UDF did to drive the twin props on the rear of the engine. Art Adamson said he couldn't figure out how to handle the tip-to-tip velocities needed in the turbine. But he was an avid tennis player, and loved doubles. He said one day he watched his opponents converge on a ball hit down the centerline, and the both swung their racquets at the ball. He said he had a flash of insight, and that problem got a LOT easier to solve thereafter.

On the day of my tour, Art confessed I asked questions that surprised him. He pulled our group leader aside and suggested that I leave a resume. I was flattered. I went into IT and I don't regret my choice, but I have always loved turbomachinery, and always will. The engines are the best part of an airplane!



Christianity. Islam. Hinduism. Anthropogenic Global Warming. All are matters of faith!
User currently offline777WT From United States of America, joined Jan 2005, 875 posts, RR: 1
Reply 13, posted (8 years 2 months 1 day 19 hours ago) and read 6229 times:

Quoting Lightsaber (Reply 8):
Pratt somehow has remained very strong on business jets (thanks to the parent company letting P&W Canada have a bit more say in their strategy). Yes,the BR 710 is the high end business jet engine...

Doesn't Pratt have a good hold in the military engines department?
So to say that the loss to GE and RR in the commerical aviation has made up for the loss for Pratt as they're involved more in military?


User currently offlineMolykote From United States of America, joined Aug 2005, 1340 posts, RR: 29
Reply 14, posted (8 years 2 months 1 day 19 hours ago) and read 6215 times:

This is a great post that provides some insight to the engineering tradeoffs and problems in jet engine design. Engineering often takes years to catch up to the cutting edge of science. Lightsaber's post goes a long way toward illustrating why this is true.

Post such as this one are a welcome change from the buzz word marathons. (composite fan, geared turbofan, 7 spool, ramjet, ceramic turbine, tail-mounted GE-90, bleedless, etc etc)



Speedtape - The asprin of aviation!
User currently offlineBaroque From Australia, joined Apr 2006, 15380 posts, RR: 59
Reply 15, posted (8 years 2 months 1 day 18 hours ago) and read 6191 times:

Quoting Molykote (Reply 14):
This is a great post that provides some insight to the engineering tradeoffs and problems in jet engine design.

True indeed. Engines have not been given the prominence they deserve.

Quoting Lightsaber (Reply 11):
are looking more at refractory metals due to their incredibly high temperature resistance. This should surprise no one. If you know what refractory metals are, you know that they have incredible temperature capabilities.

I draw a slightly negative conclusion from this, ceramics must be really difficult if the refractory metals seem more promising. The subject of tungsten came up recently in the context of depleted uranium. I wondered whether using uranium for counterbalance weights was due to low cost for a byproduct or that if they were able to achieve higher densities with uranium. Tungsten has a marginally higher SG than uranium but I think it is difficult to get fabricated parts near its true SG just because it is difficult to melt.

Presumably, if it is possible to fabricate blades out of one or other of these metals in a commercially viable process, it would transform the high temperature parts of the engines.

Just the thought of gearboxes transmitting that sort of power makes me nervous, let alone wondering how you would cool them. Is there any way other than GTFs of achieving the same sorts of speed relations? I sound a bit like Pam Ayres story of the guy who fell off the cliff and clung onto a tuft of grass shouting help!

Quoting ContnlEliteCMH (Reply 12):
The engines are the best part of an airplane!

I agree. Fascinating things. There would not be much point in all that aluminium, GLARE or CFRP without engines.


User currently offlineParapente From United Kingdom, joined Mar 2006, 1548 posts, RR: 10
Reply 16, posted (8 years 2 months 1 day 12 hours ago) and read 6111 times:

What a facinating set of contributions! Very topical as the subjects of Global Warming (CO2 emissions) and Global Dimming (Carbon particle emissions) are red hot at the moment.Efficiency will be the only factor worth considering when governments start to put huge taxes on air travel -as they will. Land based pollution can be tackeled via electricity produced via nuclear generation-both the US and UK are leaning this way (France went that way 20 years ago) and just watch China and India do the same. Then the spotlight will truely be on aircraft pouring out tons of CO2 and C pollution at 30,000 feet!
My question to the learned authors is this.
To power a modern engine one has to deflagrate the fuel.This is really the most inaficent method of obtaining power.What are the chances of a Technilogical leap by the 3 big companies (together?) in developing the Pulse Detonation Engine . This has to be the way to go as far as I can tell.


User currently offlineLightsaber From United States of America, joined Jan 2005, 12899 posts, RR: 100
Reply 17, posted (8 years 2 months 1 day 10 hours ago) and read 6050 times:
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Quoting 777WT (Reply 13):
Doesn't Pratt have a good hold in the military engines department?

Pratt nailed the F119. That's helped them win airforce orders and possibly a naval order. Of all things, a PW6000 derived F-18 engine as GE's current offering isn't enough and the navy doesn't have the funds to develop a new core. (It would be unique civil to military core conversion. I'm not aware of any currently). But this is a maybe, not a definate and has been that way since 2000 (at least).

Of course, then there is the F135.  Wink A nice design too.  Smile

Quoting Baroque (Reply 15):
Is there any way other than GTFs of achieving the same sorts of speed relations?

Not at that horsepower that I know of.

Quoting Parapente (Reply 16):
This is really the most inaficent method of obtaining power.What are the chances of a Technilogical leap by the 3 big companies (together?) in developing the Pulse Detonation Engine . This has to be the way to go as far as I can tell.

Pulse detonation will have a very tough time of meeting emission requirements. To me, the next logical step is putting a couple of fuel cells in the cargo hold that power electrically driven fans!  wideeyed 

But that technology is a long way away.  Sad But as soon as Jet-A burning fuel cells are developed, the end of the aircraft gas turbine market will be neigh.  Sad One must get away from engines limited by the Carnot cycle and go to something that takes advantage of the currently wasted Gibbs free energy. Fuel cells should be able to get 50% to 100% better efficiency in the long run.

Also, deflagration isn't an inefficient way of extracting the chemical energy unless you compare to a fuel cell or as yet unknown (to me) Gibbs cycle engine. A pulse jet is still limited by Carnot cycle efficiencies and theoretically has god awful emissions that would legally keep it out of service. Sorry to nitpick, but I am a combustion engineer by trade and schooling.  spin 

I should comment, I'm a fluids guy. So as the questions go more material and structural I have asked coworkers, so please keep that in mind. I design the aerodynamic flow paths or exterior surfaces as well as doing the chemical engineering. The structure? Not my department.  Wink But alas, I've been promoted to where I can't ignore the issue. So I hire those who know their stuff in that area. Sad to think when I have a BSME... But that was so long ago.  Wink Believe it or not, I have an MS in combustion!  hyper 

Lightsaber



Societies that achieve a critical mass of ideas achieve self sustaining growth; others stagnate.
User currently offlineContnlEliteCMH From United States of America, joined Mar 2005, 1455 posts, RR: 44
Reply 18, posted (8 years 2 months 1 day 3 hours ago) and read 5957 times:

Quoting Lightsaber (Reply 17):
Believe it or not, I have an MS in combustion!

Oh, I believe you! Combustion science was the only graduate course I took in college. The professor at OSU was Robert Essenhigh, a spindly elderly gentleman from England. He was a trip, and still rather prolific even at his age, which was advanced enough that he wore sweaters even in the middle of summer, which in Columbus frequently tops 90 F with high humidity.

I discussed with him the possibility of postgraduate studies in combustion engineering. He hadn't any opening at the time, but he discussed putting me in contact with a colleague at Penn State (Essenhigh used to be in PSU's employ) to study there. Fancy wearing my scarlet and gray in State College! As it was, I was not a good college student. I decided that was not in my best interests, and did something else.

FWIW, Essenhigh claimed to have built a PSR ("perfectly stirred reactor") that could burn coal powder with absolutely no smoke. I never saw it, but I have no reason to believe he was fibbing.

And boy, you're right about emissions for something other than continuous deflagration. It's tough to ensure proper mixing and predictable flame front. It's taken the automotive industry more than 50 years to figure out how to manage flame front propragation, compression ratios, and emissions control. It's only now with the advent of variable valve timing (and some other advances) they can control NOx formation without EGR. Incidentally, the air/fuel control on some of the newest engines is so good that they might be able to meet emissions requirements without the catalyst. But nobody will chance it, and the catalyst gives them a margin for error due to variations from one cylinder to the next. It also gives them a political advantage, in that post-catalytic emissions are so clean that today's emissions often meet requirements that won't be in force for years to come. Environmentalists should stop trying to make the internal combustion engine illegal by ever-increasing stringency, because the engineers will always beat them.

Of course, the two combustion environments are totally different. Reciprocating engines have a confined pressure vessel that changes size continuously, with a discrete ignition event and stoichiometric A/F ratios -- none of which a modern turbine engine sees. Here's a good question for you: has there been any research into ways to manage the A/F ratio at the combustor? It seems to me that emissions could be improved substantially if the combustor is isolated from the variable A/F ratios seen in a turbine. I just can't envision how it would be done.



Christianity. Islam. Hinduism. Anthropogenic Global Warming. All are matters of faith!
User currently offlineEMAlad From United Kingdom, joined May 2006, 444 posts, RR: 0
Reply 19, posted (8 years 2 months 6 hours ago) and read 5829 times:

I have got to say that I think the best engines are produced in my home city of Derby. Good old RR.

I flew to Australia with MH last year. Had a fairly new 747 to and from KUL 9M-MPH and 9M-MPO. I thought that the ac was fairly noisy. However we flew on a 777 to and from Adeliade. Wot a difference in noise and power on the runway. Good old Trent engines!!!!


User currently offlineParapente From United Kingdom, joined Mar 2006, 1548 posts, RR: 10
Reply 20, posted (8 years 1 month 4 weeks 1 day 17 hours ago) and read 5721 times:

Sorry clearly I got it wrong.I understood that because deflagration was inefficient compared to detonation it would force majeur create more pollution as the fuel would not be completely burnt.I also note that all the big 3 companies are looking hard at PDE and have built prototypes each following a slightly different route.Note PW trying to go for a non mechanical solution to the valves problem! (40-80 combustions a second is what is required). They are also linking this to drive fans.Are you sure they would be going to all this expence if this technology created more not less polution.

User currently offlineBaroque From Australia, joined Apr 2006, 15380 posts, RR: 59
Reply 21, posted (8 years 1 month 4 weeks 1 day 11 hours ago) and read 5657 times:

Quoting Parapente (Reply 20):
I also note that all the big 3 companies are looking hard at PDE and have built prototypes e

The mention of PDE makes me nervous, partly due to their relation to the infamous pulse jets but also a chemistry lecturer. He was a much awarded scientist who specialised in rates of reactions. He also tended to turn up under the weather for 9.00am lectures and gave truly awful lectures. He also demanded we be locked in (this was a while ago) after one student voted with his feet. But he also used to set up experiments at the front with a long induction period. He would set them going, but not tell us when the rate would accelerate. So you sat there not knowing when the large glass vessel would erupt with a loud whump! I dont suppose that would be permitted either these days.

Quoting EMAlad (Reply 19):
Good old RR

True, but see also Reply 1. Frank Whittle was not kind about them. Neither was a relative of mine who invested a part of his retirement savings in RR just before the RB211 fan debacle. Just as well they got better since then. But if the triple spool engine is now showing major advantages, you could say it is not before time. At least they have been persistent.

An interesting "what if" thread would be what if a Labour Government had not forcibly removed Power Jets and their patents from the scene.


User currently offlineLightsaber From United States of America, joined Jan 2005, 12899 posts, RR: 100
Reply 22, posted (8 years 1 month 4 weeks 1 day 8 hours ago) and read 5621 times:
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Quoting Parapente (Reply 20):
Are you sure they would be going to all this expence if this technology created more not less polution.

Pulse combustion has its applications. But I've yet to hear about its use in a large engine. Most of those looking at it are for space thrusters in the sub-Newton thrust level. heck, some are sub micro-Newton. At those scales, pulse thrust is more efficient due to the limitations on machining accuracy at small scales.

Detonation inherently produces a very lossy shockwave. Also, those shock waves, if present in a flame, enhance the production of nitric oxide (a major pollutant from all air breathing combustion sources). So... I'm pretty sure pulse jet combustion isn't the future of large aircraft engines.

I also look at it this way. Pratt has a department of hundreds of engineers working on deflagrating combustors, none on pulse jets. UTRC has a team of 3 to 5 looking into pulse jet combustion. Yes, two of those engineers are loaned from Pratt (engineers go between UTRC and Pratt as easily as Pratt engineers go from project to project), but that isn't a big effort. Oh, and UTRC is probably working on NASA money... So they might just be doing the research for the contract profit.  Sad Oh, they'll have a hope for some product... But GE is sold on lean burn combustors while Pratt has projects in lean and RQL combustion. RR does RQL's but I'm sure is looking at lean combustors. I personally favor RQL's, but that's a long discussion on its own.

Lightsaber



Societies that achieve a critical mass of ideas achieve self sustaining growth; others stagnate.
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