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Gas Turbine Engines

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Gas Turbine Engines

Old 9th Sep 2015, 06:16
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Question Gas Turbine Engines

Hi all. Does air density affect the performance of gas turbine engines like piston engines do? As there is a decrease in air density with altitude, there would be a decrease in performance and a loss in power as there is more fuel than air in the mixture. That is why some general aircraft have superchargers and turbochargers to boost the intake air entering the manifold.

I was reading my ATP theory book, and it is saying that the diffuser and the compressor increase the air pressure as much as possible before entering the combustion chamber. Has this got anything to do with the air density and the reduction of air density with altitude or the gas turbine engine performance are independent of air density?
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Old 9th Sep 2015, 11:47
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Anything that needs air will lose performance if there is less air to be had.
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Old 9th Sep 2015, 12:18
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The tradeoff is that less thrust is needed to push an airplane through thinner air.

Also, the big advantage to gas turbines is the [lack of] weight to produce the needed thrust or power.
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Old 9th Sep 2015, 13:19
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Yes reduced air density will reduce gas turbine performance. However the benefits of lower drag resulting from the reduced air density outweigh the penalty of lower thrust.

Also since twin engine aircraft have to be able to continue a take-off n-1 at V1, when both engines are running they have considerable available thrust.
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Old 9th Sep 2015, 19:36
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Gas turbine engines lose power as altitude increases. However, drag also drops with increasing altitude, but the power (thrust) drops faster than drag. Usually, power available exceeds drag, enabling the aircraft to climb. Once the aircraft has climbed to an altitude that power (thrust) available drops to the point that it equals drag, the aircraft can no longer climb. That is the "ceiling" of the aircraft. As fuel is burned gross weight drops and with it drag. Eventually the power (thrust) available exceeds the drag sufficiently enabling the aircraft to climb again. That is why large aircraft "step climb" as they burn fuel.

There is an error in the following statement: "there would be a decrease in performance and a loss in power as there is more fuel than air in the mixture" The gas turbine's fuel control meters fuel flow to maintain a correct fuel/air mixture at all altitudes. But any engine's power output is dependent on mass airflow. As altitude goes up, air density goes down, and with it mass airflow. A supercharger system on a piston engine can compensate, up to a point.
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Old 10th Sep 2015, 11:33
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CompressorSurge I'll try to answer all your questions here.

Yes, density altitude increase reduces thrust, just like piston engines. This applies to hot and high on the ground too, air density is reduced there as well.

Fuel/air ratio is metered by the FCU or FADEC - that's what it's there for. So fuel/air ought to be correct all the time.

Any form of forced induction, supercharging or turbocharging, simply jams more air into the induction system thereby increasing local air density and hence power available. More air = more fuel able to be burnt.

The diffuser case is placed after the last stage of compressor to expand the airmass and allow it to slow down, thereby converting kinetic energy gained through the compressor stages to pressure energy. Normal gas laws apply but be aware of the temperature of this air (hot!).

This process, diffusion, will increase the density of the air processed by the compressor, that's what it's there for. Higher density, lower density altitude, which allows more fuel to be burnt by the air processed and hence more thrust. The compressor and diffuser sections work together to maximise overall compression ratio and hence maximise thrust available.

Gas turbine engine performance, just like all other air breathing engines, varies directly as air density. Increase or decrease air density and increase or decrease thrust. Note however that increasing density altitude means reducing actual density. The reduced air density allows the aircraft to fly producing less drag, requiring less thrust, because there are less air molecules to bump into. Engines can also be designed to turn up more RPM at altitude and gain a bit of thrust. Intake air is colder so air entering the combustion section can stand a bit more fuel burning and stay below TIT limits. There is also ram recovery at the inlet increasing the pressure (hence density) at the burner section entry. Ram recovery is, simply put, that the engine inlet is "pushed" through the air and the air is "shoved" in because it simply can't get out of the way.

You asked a lot with just a few lines old mate! I hope this helps.
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Old 11th Sep 2015, 21:49
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A word or two about fuel metering on turbine engines. They are generally controlled by a governor, so as inlet pressure or temperature varies, the governor will trim fuel up or down to keep RPM where it belongs.

Multi-spool engines (turbofan e.g.) may have two governors, one on each shaft,
and either one may "take charge" to keep things within limits.

And some engines - often military models - may have temperature controls that act as a governor.
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Old 11th Sep 2015, 22:53
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I agree with musthafagander, the FMU/HMU is controlled by the EEC (Trent engines), so nothing to do with governors on shafts.
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Old 29th Sep 2015, 18:37
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Modern engines do indeed have governors, but they are in the electronics - not the steam-engine style flyweight type governor. First one I worked on was the T58-10, in 1968. Not only an analog electronic governor, but load sharing in e twin-engine helo.
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Old 29th Sep 2015, 19:49
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Originally Posted by barit1 View Post
Modern engines do indeed have governors, but they are in the electronics - not the steam-engine style flyweight type governor.
Some "modern" engines still have the "steam-engine style flyweight type governor". The GE90 uses a mechanical flyweight overspeed governor that will limit N2 to slightly over redline, independent of the FADEC or the fuel metering valve position (the FADEC version of the CF6-80C2 has a similar system, as do the CFM56-5 and -7 engines). These mechanical flyweight governors are intended to protect the structural integrity of the engine should there be some failure that causes the FADEC to loose control of the engine (for example, the fuel metering valve fails wide open).
The GEnx engines on the 787 and 747-8 are the first large GE commercial engines to depend entirely on electronic overspeed protection with no mechanical backup.
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Old 29th Sep 2015, 23:43
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(for example, the fuel metering valve fails wide open)
I wouldn't trust the aircraft brakes in that failure condition either
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Old 2nd Oct 2015, 00:37
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Hi,


for example, the fuel metering valve fails wide open)
I wouldn't trust the aircraft brakes in that failure condition either
As we know there are brakes, thrust reversers, and speedbrakes to stop the plane. But before that we can still shut off the fuel by the spar fuel shut off valve and HPSOV using the start lever or the fire switch (HPSOV only).

The spar fuel valve (outside the engine) is located upstream the fuel metering valve (inside the HMU). The HPSOV (inside the HMU) is located downstream the fuel metering valve (FMV). Even if the fuel metering valve fails wide open, we can still shut off the spar fuel valve and/or the HPSOV.
The HPSOV is fuel actuated (through FMV) and electrically controlled. Fuel pressure from the FMV can not open the HPSOV if HPSOV solenoid is energized. I am talking here about the B737 NG.

If I am not mistaken, there is NNC (non normal checklist) that recommend pilots to move the start levers in the cut off position in case of N2 overspeed.
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Old 2nd Oct 2015, 05:50
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Mechanical Governors on Modern Jets...

@tdracer makes an excellent point. (First, let's understand that I know little about the most current ~~3 generations of high-pass engines, sooo.) Electronic FADECs can and do fail. Mechanical speed/fuel/air/turbo regulators can and do also fail. I do not know which type has a higher failure rate and I refuse to guess. My point is that having both types functional MAY give the pilots a second chance warning about a potential N2 over speed and allow them enough time to respond to an engine or FADEC fault, and before any serious damage is done.
In these days of mostly twins, keeping both spinners turning, even if one may be forced to spin at a slightly slower rate, seems to increase overall safety and to help protect those seriously expensive engines. Proven confidence in the current engines is extremely high and this is good. If the manufacturer believes that a good, accurately reporting FADEC is enough electronic engine management, then they will stand behind their engines and their FADECs. Apparently, most do.
One of the added benefits of an electronic and fully reporting FADEC is that operational data are easily available to the operators. If - and ONLY IF - operators use those data properly, their engines will perform better, last longer and (eventually) increase their MTBO intervals. If you've not already guessed, I'm a Huge Believer in high quality PM, performed exactly by the book. First class PM may be expensive, but making the business case to support it is easy.
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Old 2nd Oct 2015, 12:02
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First of all, tdracer is correct about a hydromechanical "steam-engine" governor in the fuel path. I had overlooked that point in my 29th Sept. post. Thank you!

But the purpose of this device on a modern engine is simply to protect the engine from destructive overspeed in the event of FADEC breakdown. Normal control (governing for power management) is done via the electronics.
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Old 3rd Oct 2015, 01:49
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Electronic FADECs can and do fail. Mechanical speed/fuel/air/turbo regulators can and do also fail. I do not know which type has a higher failure rate and I refuse to guess. My point is that having both types functional MAY give the pilots a second chance warning about a potential N2 over speed and allow them enough time to respond to an engine or FADEC fault, and before any serious damage is done.
No Fly Zone, FADEC has proved to be much more reliable than the hydromechanical control systems - roughly an order of magnitude more reliable. In fact, dual channel redundant FADEC systems are sufficiently reliable that we allow "Time Limited Dispatch" - where one can dispatch for an extended period of time (up to 2000 hours for one FADEC engine type) with "loss of redundancy" engine control faults without adversely affecting the Shutdown or "Loss of Thrust Control" rates. In fact, introduction of FADEC has been a primary contributor to the dramatic reduction in shutdown rates I've seen during my career that has allowed ETOPs. When I started, one shutdown per 10,000 hrs. was "acceptable" - today one shutdown per 100,000 hours is cause for concern, and many engine type shutdown rates are 1 or 2 per million hours.
Many of the early FADEC systems used some sort of 'flyball' overspeed protection system. A primary concern was overheat - something like a burst anti-ice duct could heat the FADEC well above its temperature limit (~100 deg. C), and the manner that a FADEC would fail due to overheat is rather unpredictable (including possibly commanding the fuel metering valve wide open) - and the designers were not confident they could design an electronic overspeed protection that would stay alive during an overheat long enough to protect from a FADEC going crazy.
However over time, FADEC designers have come up with electronic overspeed protection systems - independent from the engine control portion of the FADEC - that can be made from (relatively) simple electronic components that have much better high temperature characteristics relative to the rest of the FADEC. These have been certified to stay alive well above the temperature at which the rest of FADEC will fail.

Lomapaseo, failures which could result in what we call "UHT" - Uncontrollable High Thrust - are extremely rare. In the FADEC fleet, they are on the order of 1 per hundred million engine operating hours - again roughly an order of magnitude better than the ~10-7/hr. rate for mechanical engine controls (a leading cause for mechanical controls being broken/separated throttle cables). "Up and Away", UHT is not normally a big deal - there is enough airspeed that the aircraft is controllable, and the engine can simply be shut down if needed. However on the ground can be a different story.
The most recent Boeing engine/aircraft combinations (777-300ER/200LR, 787, and 747-8, and the upcoming 737 MAX and 777X) also have a system that - on ground only - will shutdown an engine that goes to high thrust uncommanded, or remains at high thrust after the throttle is retarded. Airbus also has a system on their latest aircraft that is intended to protect against UHT, but understandably I don't know much about it.


Oh, barit1, just razzing you a bit

Last edited by tdracer; 3rd Oct 2015 at 02:04.
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Old 3rd Oct 2015, 02:57
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Lomapaseo, failures which could result in what we call "UHT" - Uncontrollable High Thrust - are extremely rare. In the FADEC fleet, they are on the order of 1 per hundred million engine operating hours - again roughly an order of magnitude better than the ~10-7/hr. rate for mechanical engine controls (a leading cause for mechanical controls being broken/separated throttle cables). "Up and Away", UHT is not normally a big deal - there is enough airspeed that the aircraft is controllable, and the engine can simply be shut down if needed. However on the ground can be a different story.
Generally agree

I just wasn't too impressed with the CX A330 that got dirty fueled with embedded water which managed to freeze the innards of the fuel metering valves. Not a problem in continued flight until they tried to land at HKG and got one hell of a surprise during landing roll-out. Then there was that PA A310 that had a metering valve go south while trying to land on a snowy runway at DTW. Lots of swearing on the CVR.

Yup the pilots have to recognize which engine and be damn quick about shutting it down if they are trying to slow down.
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Old 3rd Oct 2015, 03:43
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Generally agree
Thanks, although I should add a disclaimer - I'm an engineer at Boeing (something I've never attempted to make a secret, although I'm still occasionally accused of being a Boeing mole ). But that means I normally only see the data for the Boeing installations. Although I know of no reason why the brand A rates would be meaningfully different, when you start talking 10-7 or 10-8, one event can make a big impact. Like the CX A330 event (a dual engine event to boot).
BTW, I spent a lot of time going over that CX report - those Cathay pilots deserve a lot of credit, they did a fantastic job.
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Old 3rd Oct 2015, 13:48
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those Cathay pilots deserve a lot of credit, they did a fantastic job.
I quite agree, any pilot faced with an engine that kicks in with too much power at low aircraft speed is gonna have their hands full way outside their training (heavy foot on the pedals). Thus the desire to ensure a very rare occurrence rate for that combination of "what-ifs"
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Old 3rd Oct 2015, 16:29
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Getting back on topic,
No one has mentioned intake design and ram effect yet!
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Old 4th Oct 2015, 02:26
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Oh, barit1, just razzing you a bit
tdracer, what a sweetheart!

But seriously, I'm quite amazed at the progress of reliability statistics in electronics.

45 years ago, my boss was an ex-field rep on REALLY early, vacuum-tube controls on a turbojet. He said that after preliminary teething issues, and with the maintenance guys gaining experience, one chronic problem remained. And that problem was cables and connectors!
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