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HYPNOS
13th Oct 2008, 04:53
Im puzzled by the term "climb thrust" while reading a book, for an airliner to climb, throttle would be set to a specific climb thrust, when it gets higher, air is less dense, the thrust producing by the engines should be less. (Coz im flying a warrior and trying to appply same principles to a turbofan engine) Now, what Im asking is, do the engines actually spin faster to produce same "climb thrust" as lower altitude or just let it decrease? :confused:

And during a climb, IAS would be kept the same but TAS is increasing, is it becasue you can fly faster when LESS DRAG is experienced even though your engines are giving less thrust/same thrust?
I really dont get the idea when you can fly faster with less thrust but I also doubt that the engines will keep up the thrust while climbing coz there must be an end that the engines cant work any harder. :ugh:

correct me if im wrong......

clonemole
13th Oct 2008, 05:06
Well flying the 737 my take on your question is your commanded engine RPM tends to increase as you go higher till it reaches a max value.. generally reaches about 99-100%(104% being the max).

And yes you fly a const IAS till your mach no. catches up with you either way you keep seeing your TAS increase. finally at cruise of 350-400 you will be doing an approx IAS of 240, mach of .77 and TAS of around 450.

The difference between the warrior and the NG here is that the NG engines should be rev'ed to higher RPM only when the air gets rare enough otherwise under sea level density setting a climb thrust of 100% may not be good for the engines.

Tight Slot
13th Oct 2008, 05:07
Why do you think jet aircraft fly higher? Less drag perhaps.....

Most modern jet engines work at high RPM, so with the wing design, work best at high alt and a good rpm as per max speed/mach no

regards

Check the RR web sites or the GE ones if one comes push to shove....

SNS3Guppy
13th Oct 2008, 05:50
Hypnos,

Some principles you will read about regarding high performance airplanes and turbojets will apply to the flying you're doing with your warrior, but some won't. You ask some great questions, and you're to be congratulations on expanding your understanding...that's a excellent habit as a pilot.

Climb thrust has several connotations, especially in turbine equipment; it has to be put in context. During takeoff, the thrust that's set depends on a number of factors. Thrust can be limited by several factors, depending on the type of engine and the operating conditions, too. It's a little more complicated than a piston engine where takeoff is nearly always done at maximum RPM and throttle setting, on a fixed-pitch, normally aspirated (nonturbocharged) engine...such as the warrior.

In a turbine engine, reducing thrust for takeoff is a common practice. This is done for a number of reasons ranging from noise abatement to increasing engine life...so long as the takeoff can be made safely and all the necessary climb performance can be met. Takeoff profiles typically involve a couple of different profiles, but are similiar in that at some point after takeoff, climb thrust is set. Takeoff may be made with maximum thrust, or at a reduced thrust setting, and setting climb thrust after takeoff may mean a thrust increase, or decrease in thrust.

A typical takeoff involves a climb to 1,000' where climb thrust is set, using a speed about ten knots above the takeoff safety speed (known as V2). The climb is then continued to 3,000' above the departure field elevation at that speed and that power setting. Keeping that power setting, the nose is lowered slightly, vertical speed reduced to five hundred or a thousand feet per minute, and the airplane allowed to accelerate and the flaps to be retracted.

During the climb, several types of climb thrust may be used. Maximum climb thrust will be calculated for the climb, and often a reduced number somewhat below that will be used. Just like on takeoff, an engine may be limited by the maximum allowable temperatures internally, or it may be limited by other power settings such as torque, EPR (engine pressure ratio), etc. The maximum value that limits the engine may be used for the climb, or a lower number may be calculated for the climb. This is sometimes called climb thrust (the power setting to be used for the climb, as opposed to the maximum value that could be used for the climb...which would be maximum climb thrust.

To further complicate the matter, several values for the climb thrust may be available, limited by time. The maximum thrust is often called maximum continuous thrust, whereas other values may be allotted to the engine for shorter durations, such as five, twenty, or thirty minutes. Additionally, emergency thrust is available for situations such as a windshear encounter, and go-around thrust...all having different values, all being maximum thrust situations for their intended use...and all of them being climb thrust values. A go-around or missed approach, for example, will often involve a reduction of thrust as the airplane climbs out, from go around thrust to climb thrust...even though both are valid climb values.

Reducing thrust just a little has a very large influence on the longevity of a powerplant...just a few degrees or a few percent reduction can increase engine longevity by ten percent or more...or in economic terms, millions of dollars. To say nothing of increasing the safety margins by which the engines operate by decreasing thermal stresses and damage to the internal components.

Turbine engines are most efficient at high altitude because they must spin faster to produce the same thrust. As a turbojet climbs, the engine speed increases, and with an increase in engine speed, up to a point, engine efficiency increases. At lower altitudes, the engine produces too much thrust with the power pushed up that far...at 6,000' for example the airplane is limited to 250 knots, and pushing the power up too far will easily push the airplane faster than that. Accordingly, the engine must be operated at a much lower power setting to keep the speed in check (and to respect other airspeed limitations such as gear, flap, or even Vmo limitations)...the engine isn't being operated nearly as efficiently.

At higher altitudes, true airspeed increases while indicated airspeed decreases. The airplane can fly faster and faster with less and less drag, but the engine must turn faster to do the same job...to compress the thinner air and to produce the same thrust...the engine operates more efficiently.

The way power is measured or described in the cockpit really varies...in the airplane I'm flying right now, our chief power gauge is the EPR, or engine pressure ratio gauge. It tells us about the difference between the pressure at the front of the engine, and the pressure at the back...an EPR ratio of 1.0 means that the pressure at the back of the engine is the same as at the front...push the power up above that and the EPR starts to climb. An EPR of 1.5 means that the engine is pressure at the back of the engine is greater than the pressure at the front...it's producing thrust. The actual value isn't important, though it's in the neighborhood of about 60,000 lbs of thrust.

As we climb, we can calculate, or the airplane will calculate, the climb thrust setting at any given altitude...the EPR setting required increases as we climb to higher and higher altitudes. We may have to move the thrust levers to meet this value, or the value may increase all by itself...for a given thrust lever position/power setting, the EPR increases as we climb. All we need to do is tweak it...and that might mean pulling the power back slightly to keep within limitations, or pushing it up. Very often we will do a reduced thrust climb. We do this once we reach 10,000 during the climbout, and for us it's a simple process. We reduce our EPR by .04 if we're heavier than 600,000 lbs, and we reduce it by .06 if we're 600,000 lbs or less. We will maintain this reduced thrust until our climb rate drops to 500 fpm; this typically happens about 27,000' to 30,000'. At that point we increase thrust to the maximum (I usually keep it .01 or .02 low to keep from exceeding any limits), until we reach our cruising altitude. From then on, we keep the thrust where it's needed in order to maintain .84 mach. (This, incidentally is generally well below the maximum cruise settings).

If one goes too high, one is operating less efficiently. We find that cruise at the optimum altitude isn't always possible. Generally cruising just a little below that is preferable to trying to climb above it...we end up having to carry too much power and fly at too high an angle of attack (with too much drag) if we're above our optimum altitude. I mention this because there's a lot more to choosing a cruise altitude when operating efficiently than simply taking the engine as high as it can go.

Mark1234
13th Oct 2008, 06:00
when it gets higher, air is less dense, the thrust producing by the engines should be less. (Coz im flying a warrior and trying to appply same principles to a turbofan engine)

I'm not an expert, but it's not the same principles - the piston engine is taking in an amount of air, and attempting to burn as much fuel as possible through it, so as the air density reduces, you have the familiar reduction in power.

A turbofan engine however, passes far more air than it can burn fuel for (without melting), so doesn't behave the same way - my basic understanding is it will not suffer the same reduction in performance.

Wizofoz
13th Oct 2008, 07:16
Mark1234,

No, the curves will be different but the result will be the same and for the same reason- a reuction in available thrust as altitude increases due to lower air density.

It is mitigated in a jet as it has more capacity to increase it's rpm up to a limit, but then tails off in a similar fashion.

A piston gets it's power by burning fuel/ air in the cylinders, and gets it's thrust from the prop. A Turbo Fan gets it's power from burning Fuel/air in the combustion stage and MOST of its thrust from the By-pass fan, so the same basic physics apply to both.

Mark1234
13th Oct 2008, 13:25
Ok, let me phrase that slightly differently; see if it makes more sense (I'm just getting my head around jet theory myself):

The piston engine can burn as much fuel as it can grab air - or to put it another way, there's no oxygen left in the exhaust. Power output is totally dependant on mass airflow.

By my understanding, the gas turbine is limited by temperature - you can't put in enough fuel to burn all the oxygen without melting the turbine section. So there's a bunch of oxygen left over in the exhaust (that's what afterburning/reheat uses). Therefore mass airflow doesn't limit your ability to turn fuel into power - temperature rise does.

That said, I think the penny just dropped.. I'm only considering the ability to generate power - the engine / gas generator, not the reaction mass - propellor / fan. The OP wasn't asking about that was he? How am I doing :}

SNS3Guppy
13th Oct 2008, 13:51
Mark,

The piston engine and the turbine engine both burn fuel. That's about where the similarities end.

In a piston engine, the more fuel you add, the cooler the engine runs.

In a turbine engine, the more fuel you add, the hotter the engine runs.

The piston engine can burn as much fuel as it can grab air - or to put it another way, there's no oxygen left in the exhaust. Power output is totally dependant on mass airflow.


The oxygen ratio in the exhaust is determined by the mixture setting, and power output is depenne engines tend to be dent on a number of factors including mass airflow, induction air pressure (and temperature), fuel/air mixture ratio, engine RPM, engine displacement, timing, etc. It's entirely possible to have substantial unburned fuel in the engine exhaust, or the opposite.

By my understanding, the gas turbine is limited by temperature - you can't put in enough fuel to burn all the oxygen without melting the turbine section. So there's a bunch of oxygen left over in the exhaust (that's what afterburning/reheat uses). Therefore mass airflow doesn't limit your ability to release power from fuel - temperature rise does.


From a practical point of view, temperature is often not the limiting factor in the turbine engine. Various other parameters limit it first, such as torque (turboprop), EPR, or speed (N1, etc). Turbine engines tend to be limited by those factors at lower altitudes, and temperature limited at higher altitudes.

The majority of airflow through a turbine engine isn't used for the combustion process. Particularly so with a high bypass turbofan engine. Through the engine core, however, the part where the burning is taking place, most of the airflow is used to prevent flame contact with the burner walls, and only a percentage of the total mass airflow is used for combustion. Only when combustion temperature reaches a certain temperature does it become limiting, and other factors may well limit the function of the engine prior to that time.

Particularly at low operating speeds, excess fuel can produce excess temperatures and excess inernal pressure, causing compressor stalls and damage to turbine inlet guide vanes and blades. This isn't a problem in a piston engine, where thermal damage usually only occurs at high power settings and with mixtures close to the ideal (peak) setting.