Sabenapilot From Belgium, joined Feb 2000, 2712 posts, RR: 48 Reply 3, posted (13 years 3 months 1 week 1 day 13 hours ago) and read 9099 times:
Hi Bruce -
I'll try to explain you what "flat rating" is in a logical way, because what ExPratt wrote to you is not really helping you, isn't it?
First of all you have to know and understand this: There is a relation between the ambient temperature and the thrust produced by an engine; this relation is a linear relation, where the thrust increases with a decreasing ambient temperature.
So, if you draw a diagram with thrust on the verical axis and ambient temperature on the horizontal axis, you'd see more or less a straight line from the top left to the bottom right corner.
Something like this:
L- - - - - - - -- - - - - - - - - - - - - - - - amb. temp.
This means that at very high ambient temperatures an engine is not producing a lot of thrust. You can see this in Arizona for instance, where a plane has a far lower performance due to the high temperatures.
For instance a fully loaded B727 on a take-off from Phoenix will hardly climb at all after take-off during the summer.
The opposite is true as well; in very low temperatures, the engine will produce a huge ammount of thrust.
The same B727 taking off from Anchorage in the winter will now look a much more performant plane!
However, you've probably heared of Newton, haven't you? How about his famous law: ACTION = REACTION ?
Thrust is definately an action, isn't it? But where is the reaction? Well, it is in the plane's structure: the internal strenght of the engine, the screws connecting the engine to the wing, the wing itself, etc., etc.
If the action is huge, the reaction equally has to be huge, resulting in the need for very strong and thus heavy components! However, once installed these components are there to stay, even if the ambient conditions are such that they do not require such strong components, simply because the action is not that big (Phoenix in the summer). In that case they represent nothing more then a weight penalty, decreasing the plane's performances even further!
That's why the manufacturer introduces a flat rating. The engine is limited to produce a certain maximum thrust even if the temperature is very low, so there is no need for those very strong and heavy components. This is also the thrust the manifacturer is talking about when he's talking about for instance his 100,000 lds. engine. If this engine would not be flat rated, he would not be able to tell you this figure; remember the relation between thrust and temperature I've told you about?
On the diagram, this maximum thrust can be represented by a horizontal line cutting off the upper left part of the diagram.
I ------------------------------------ maximum rated thrust
L- - - - - - - - - - - - - - - - - - - - - - - - amb. temp.
If the ambient temperature is high, no influence is sensed on the thrust produced.
If the temperature is low, the thrust produced is reduced to the maximum rated thrust
The actual thrust vs. temperature diagram looks therefore something like this:
L - - - - - - - - - - - - - - - - - - - - - - amb. temp.
The point on the temperature scale corresponding with the intersection (X) of the thrust line with the max rated thrust line is called the flat rated temperature of the engine.
Ambient temperatures above the flat rated temperature result in a thrust reduction due to natural ambient limitations, temperatures below the flat rated temperature result in a constant thrust due to mechanical limitation of the engine.
(actually, the thrust slightly reduces as well, but the reason for that is very complicated, since I have to start talking about 4 other parameters infuencing thrust as well)
I hope this helped you somewhat.
If you still have questions on this, please let me know.
VC-10 From United Kingdom, joined Oct 1999, 3693 posts, RR: 35 Reply 4, posted (13 years 3 months 1 week 1 day 9 hours ago) and read 9094 times:
The Flat Rated Temp is the temp up to which the engine manufacturer guarantees the eng will produce the rated thrust. That is how it was explained to me at the CFMI training school.
The engine is not physically limited to produce the flat rated thrust. At low temps the max thrust level is controlled by limiting either the N1 (Fan Speed) or the E.P.R (Engine Pressure Ratio). This is acheived either by the Pilots or Engineers monitoring the gauges or by the Engine Control Computer (if fitted). Ignore the gauge or switch off the computer and nothing will stop you overboosting the engine.
Flat Rated Temp has nothing to do with limiting the thrust at low tmp.
Sabenapilot From Belgium, joined Feb 2000, 2712 posts, RR: 48 Reply 5, posted (13 years 3 months 1 week 1 day 4 hours ago) and read 9078 times:
Ever heard of a FADEC - Full Authority Digital Engine Control?
Do you really think you can overboost a modern multi-milion jet engine if you wanted it to? No way!
Besides, if Flat Rating has noting to do with limiting the thrust as from a specific temperature, then why is it called FLAT RATING anyway?
You, just have to look at the horizontal line in the last diagram I gave you to see that it has everything to do with limiting the thrust.
Besides, the explanation I gave you comes directly from the thermodynamics course on jet engines of the Belgian Royal Military Academy, one of the top institutes on thermodynamics in Europe! The theory is for obvious reasons somewhat simplified here, but still is 100% correct.
The defenition you give, namely: The Flat Rated Temp is the temp up to which the engine manufacturer guarantees the engine will produce the rated thrust
is fully correct, but does not show the mechanism of flat rating, it just tells you what it is, not why.
According to you Flat Rated Temp has nothing to do with limiting the thrust at low temperatures.
Then explain me what flat rating is, if it isn't what I say about it.
Sabenapilot From Belgium, joined Feb 2000, 2712 posts, RR: 48 Reply 6, posted (13 years 3 months 1 week 1 day 4 hours ago) and read 9072 times:
Also, you say:
The engine is not physically limited to produce the flat rated thrust.
I have never said that.
The engine is electronically prevented from exceeding a certain maximum thrust which comes available as from ambient temperatures below a specific temperature (i.e. the flat rated temp) in order to avoid structural internal damage to the engine components which were never designed to resist thrusts higher then the maximum rated thrust.
Sabenapilot From Belgium, joined Feb 2000, 2712 posts, RR: 48 Reply 7, posted (13 years 3 months 1 week 1 day 4 hours ago) and read 9071 times:
If you want to write a reply on these 2 notes, please include your own personal answer to this simple question:
Why does with a steadily increasing ambient temperature the maximum thrust of a jet engine stays constant up to a certain ambient temperature (i.e. the now famous flat rated temperature) and then drops off?
VC-10 From United Kingdom, joined Oct 1999, 3693 posts, RR: 35 Reply 9, posted (13 years 3 months 1 week 8 hours ago) and read 9057 times:
Having re-read your original post I assume you are only referring to FADEC engines. I was talking about engines in general i.e. non FADEC aswell.
Yes a FADEC engine will control the max thrust possible but, the only thing controlling the max thrust on a JT9-7 is the hand on the thrust lever. An example of what I mean is when trimming a JT9 at below 0 Deg C you have to carry out the run with the compressor bleeds locked open to ensure you don't overboost the engine.
ExPratt From United States of America, joined Mar 2000, 311 posts, RR: 0 Reply 10, posted (13 years 3 months 1 week 5 hours ago) and read 9049 times:
If one looks at a graph of an engine's thrust (y-axis) vs temperature (x-axis), the line for maximum thrust will go from upper left to lower right. (Cold days, more available thrust. Hot days, less available thrust.) The thrust loss as the temperature increases is essentially linear until the temperatures get into the high 80s and low 90s F, when the thrust really drops. In the early days of turbine engines (JT3Cs,J57s), the rated thrust was referenced against standard day conditions: 59 deg F and 29.92 inches of mercury. If the temperature was warmer than 59 deg F, the engine would produce less than rated thrust. If the temperature was less than 59 deg F, the engine could produce more than rated thrust. One might think that excess thrust is a good, but extra thrust may not necessarily be a good thing (I know all the pilots out there will argue with me on that point, but bear with me.) As the higher thrust JT3Ds and subsequent engines came into service, the problem of asymetrical thrust and effects on aircraft control at the minimum control speed (Vmc) came about. If a 4-engined airplane is at Vmc in standard day conditions and loses an outboard engine, the airplane is still controllable. However, if the airplane loses an outboard engine at less than standard day conditions, then the airplane may not be controllable. So, the flat-rated thrust concept is born. The engine will be capable of generating the rated thrust up to the specified temperature. Going back to thrust vs temperature chart, the engine's thrust line goes across from the left at the rated thrust level until it intersects the flat-rated temperature line, and then falls off to the right as the thrust on the early engines did. The specified temperature is determined by the gas flow temperatures entering the turbine. As the air temperature inceases and the air becomes less dense, more fuel has to added to maintain the thrust level. As more fuel is added, the added fuel plus the higher inlet temperatures causes the temperatures of the gases entering the turbines to increase. But even with the superalloys and advanced turbine airfoil cooling schemes, there is a limit on how hot the gases entering the turbines can be before the blades and vanes are damaged. The flat-rated temperature is that temperature where the engine can still produce the rated thrust with a safety factor margin so the turbine blades and vanes are not damaged. For hydromechanical control (non FADEC) engines, the engine's thrust is controlled by the crew's positioning of the power levers. So when the air temperature is less than the flat-rated temperature, it is possible for an engine to produce greater than rated thrust, or to be overboosted. An engine should never be overboosted. But over the years, there have been countless events where an aircrew found themselves in a situation, usually from windshear, where they had to push power levers to the stops and overboost the engines to save the airplane. An engine should never be overboosted, but in an emergency anything is fair game to save the plane. It is a lot easier to replace an airplane full of engines than an airplane full of people.
For FADEC-equipped engines, the FADEC will automatically limit the engine's thrust to the rated thrust level even if the crew pushed the power levers to the stops.
Sabenapilot From Belgium, joined Feb 2000, 2712 posts, RR: 48 Reply 11, posted (13 years 3 months 1 week 5 hours ago) and read 9046 times:
I think the last expanation combined with my first mail gives a full overview on what flat rating is all about.
BTW- rereading my later entries, I find them to be somewhat aggressive, but I can assure you that this was not how I ment them... Sorry for that...
One remark however:
Speaking purely for myself as a young airline pilot, I have flown B737-200 for a very short time and there is no FADEC on their JT-8D engines, but I would never deliberately overboost my engines, not even on a dramatic go-around, since you might burn them out completely before you can reduce your thrust! If EPR max on go-around is limited to 2.07 then that's the limit. No discussion about it.
I am not going to experiment and invent flying techniques in those critical stages of a flight. Remember that nobody can blaim you if it goes wrong when flying according to the book, but you will get fired when flying out of the limits, even if no dammage was made.
Fortunately nowadays we only fly modern planes at Sabena, so we don't have to worry about these limits, unless of course the FADEC is inoperative (happens about once every month to me...)
VC-10 From United Kingdom, joined Oct 1999, 3693 posts, RR: 35 Reply 12, posted (13 years 3 months 6 days 14 hours ago) and read 9026 times:
Having had time to consider your various post here are my conclusions.
With all due respect, I consider your view of the reasons behind the flat top of thrust graph is wrong. It has nothing to do with the structural strength of the airframe & engine. It is all to do with with making your job easier. By flat topping the graph it enables the crew to set a particular throttle lever angle and, providing you are below the corner point OAT of the thrust graph, the engine delivering the same thrust, regardless of whether the temp is +1 or +25.
An interesting point to back this up is the fact that the CFM56-5C2 delivering 31 200 Lbs of thrust is structurally identical to the CFM56-5C4 delivering 34 000 Lbs of thrust. The thrust limit being set by a rating plug permananty attached to the engine. This plug is plugged into the ECU and tells it what the max thrust setting is on that engine. If the operator wants to convert a C2 to a C4 all he has to do is pay CFMI a few thousand dollars, get a new rating plug, plug it into the ECU and you have a C4 engine ! No structural modification are required to the airframe. This was the same when converting a CF6-5C to a 5C2, the engine required different profiled fan blades, and new HMU and a new exhaust nozzle. No modifications were required to the airframe. Incidentally the A320 can be delivered with a variety of engines with different thriust ratings but structurally the a/c are all identical.
Your third post:-
You say you never said that the engine is pysically limted to produce flat rate thrust, I'm afraid you did. You said ".....flat rated temperature result in a constant thrust due to mechanical limitation of the engine". I agree that it is electronically controlled.
Your forth post:-
I believe I have already answered this question.
In you first post you also say that the manufacturers introduced flat rating to be able to say what thrust a engine will deliver. This is untrue. Every engine ever produced comes with a statement specifying how much thrust it will produce a particular temperature at sea level.
Another interesting point is if two operators buy the same engine type they may be delivered with different Flat Rated Temps, depending on what the predominant operating conditions are e.g press alt & temp. By this I mean if most of the operators routes are at sea level temperate conditions you will get one flat rating but if you are a 'hot & high' operator you will get a different flat rateing.
Sabenapilot From Belgium, joined Feb 2000, 2712 posts, RR: 48 Reply 13, posted (13 years 3 months 6 days 5 hours ago) and read 9012 times:
You are fully correct on the mechanism you are describing, but you are not taking about
This is yet another horizontal line introduced below the flat rated line in the diagram. Because of this similar engines can be used on various planes like the A319 and A321.
The derating mechanism consists of reducing the maximum thrust, not augmenting it like you suggest in your latest post. You might say that this depends on the way you look at it, but this is not correct!
You see, the manufacturer designs its engine to fit various present and future planes. Let's assume a manufacturer designes a theoretical engine for a certain flat rated thrust (let's say 100.000lbs.). Since at present no plane needs this ammount of thrust, he offers the production model of this engine with a derated maximum thrust of 85.000lbs. If after 5 years there is a need for a version with 95.000lbs of thust, this is no problem, since the engine has been structurally designed to do this.
However, he will never be able to offer a version of 105.000lbs. since this is exceding the maximum value set by the manufacturer, thus exceding the structural limits of the engine. If he wants to do this anyway, he will need more then just a new plug.
Realy, the main reason for the flat rating is the integrity of the structure. I can assure you a non-flat rated engine on a very cold day, would takeoff without the plane when setting the maximum thrust, simply because the parts keeping the engine in place are not strong enough! They were never designed to do so, since the manufacturer makes them to resist only the predetermined flat rated thrust or lower.
with mechanical I mean that there is something like a logical mechanism; but not automatically a physical object. Software is a mechanism too, I suppose. If not, I'm sorry, but English in not my mothertongue you see.
jetlife2 From United States of America, joined Jul 2006, 217 posts, RR: 25 Reply 19, posted (11 months 2 weeks 1 day 7 hours ago) and read 4762 times:
Without getting into a long description that would take forever to write...
Most of what is said about engine control above is correct. However it is not the case that all modern engine controls prohibit the pilot from achieving something above flat rated thrust.
In the normal course of business, the flight management computer will demand thrust defined as "Takeoff" or "Climb" etc. via the power management tables. The FADEC will dutifully schedule fuel and other engine parameters to achieve this condition exactly as the power management tables dictate. But: In many applications, if the pilot firewalls the throttle, which in normal "economic" operation he would never do: then depending on the specific engine/airframe combination, there are many examples where the FADEC will permit more. Up to whatever limit applies (be it EGT; N1; N2 or an airframe thrust limit set for controllability). This may well be more than "Rated" thrust depending on ambient temp and flight condition.
Why do we do this? The limits are set to protect the engine and airframe; but those limits are certainly above the values in the power management for rated thrust. If the pilot wants thrust within these limits, we as designers are going to give it to him. And as a passenger, I want him to have it.
MD11Engineer From Germany, joined Oct 2003, 13618 posts, RR: 63 Reply 21, posted (11 months 1 week 6 days 8 hours ago) and read 4583 times:
From what I´ve learned years ago:
When the ambient temperature increases, the thrust at a given throttle setting (if we would have direct acting throttles like on the first generation jet engines) falls off due to the reduced density of the air.
You can compensate for it by adding more fuel, letting the engine run faster and hotter. On the first generation engines the pilot had to do this using the throttle. Later engines do it automatically, either by using a mechanical FCU or the FADEC (for modern engines). Now adding more fuel means a higher EGT, which can exceed the materials limits of the turbine.
Military engines are often FULL RATED, this means the pilot has to watch for himself that he won´t exceed the maximum EGT when pushing the throttle forward (in an emergency he would rather dramatically reduce the life of his engine by exceeding the max EGT to e.g. escape than getting shot down).
Most civilian engine civilian engines are FLAT RATED. This means there exists a limiter, which limits the engine power if the ambient temperature is too high. This can be done mechanically (in pre-FADEC engines) or electronically.
Full thrust is available until a certain ambient temperature is reached, beyond this the engine will only deliver as much thrust so that the max EGT is not being exceeded. This means you can push the throttle all the way forward, but the actual maximum thrust will be limited by the engine control via the ambient temperature.
The ambient temperature up to which the engine´s maxiumum thrust is available is called the "Flat Rate Temperature".
dhawald3 From India, joined Jun 2011, 17 posts, RR: 0 Reply 22, posted (11 months 1 week 5 days 3 hours ago) and read 4483 times:
Quoting MD11Engineer (Reply 21): This means you can push the throttle all the way forward, but the actual maximum thrust will be limited by the engine control via the ambient temperature.
The ambient temperature up to which the engine´s maxiumum thrust is available is called the "Flat Rate Temperature".
For a given amount of fuel burn how closely is the exhaust EGT related to the ambient temperature?
will a rise of 20 deg in ambient cause exhaust EGT to rise by 20??
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 25, posted (11 months 5 days 1 hour ago) and read 4203 times:
Quoting dhawald3 (Reply 24): a) Is it limiting of thrust at lower ambient temperatures considering the structural limits and aircraft requirements??
Below the flat rated temperature, yes.
Quoting dhawald3 (Reply 24): b) Limiting of the EGT above a certain ambient temperature so as to maintain the EGT margin.
Above the flat rated temperature, yes.
If you didn't flat rate the engine, it would have a different thrust capability at every temperature (higher thrust as you get colder up until you hit the stress limits of the engine structure). For a bunch of reasons, this would be an enourmous pain in the rear for aircraft design. So they limit the thrust to a constant value (the flat rate) below the flat rated temperature. Above the flat rated temperature thrust falls off with increasing temperature.