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kitplane01
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Derating a turboprop engine

Wed Feb 12, 2020 4:26 am

VSMUT wrote:

Expanding on T4thH:
The ATR 42-600 and 72-600 have the same engines, but due to VMC, the 42 has them limited to a lower power setting. The 42 essentially lacks to tail surface to laterally control the aircraft with one engine failed and the other running at full power. That is the purpose of the bigger rudder, it allows them to unlock the greater power available in the engines. They are not actually putting different engines on the aircraft, hence why a 72-600S won't be likely. The engines don't really have the growth left in them to add anything significant.

The 72-600 is rated at up to 2750 hp in reserve take off power and 2475 hp in takeoff. The 42-600 is rated at only 2400 hp in RTO and 2160 hp in takeoff. Both lie at 2132 hp in cruise. Figures are per engine, with the same engines and propellers mounted on both types.



In the case of the ATR-42/72, the engines run at the same power during cruise. They seem to run at 2132/2750 = 77%, which seems quite standard. But suppose some other plane had engines that ran at a much lower percent of power during cruise, say 50% of max possible power. How much might this hurt fuel efficiency, and how much might this help reduce maintenance?

How bad in economics is significant derating of turbine engines?
 
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Starlionblue
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Re: Derating a turboprop engine

Wed Feb 12, 2020 6:44 am

Is 2132/2750 the prop RPM? This does not indicate engine power. The props are variable pitch, so the RPM would remain pretty much the same in the cruise regardless of power settings.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
VSMUT
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Re: Derating a turboprop engine

Wed Feb 12, 2020 9:51 pm

Starlionblue wrote:
Is 2132/2750 the prop RPM? This does not indicate engine power. The props are variable pitch, so the RPM would remain pretty much the same in the cruise regardless of power settings.


It is shaft horse power.
 
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Re: Derating a turboprop engine

Wed Feb 12, 2020 10:14 pm

VSMUT wrote:
Starlionblue wrote:
Is 2132/2750 the prop RPM? This does not indicate engine power. The props are variable pitch, so the RPM would remain pretty much the same in the cruise regardless of power settings.


It is shaft horse power.


Ah, I see.
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744SPX
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Re: Derating a turboprop engine

Wed Feb 12, 2020 10:58 pm

77% seems pretty high, at least compared to turbofans which operate at what, maybe 10% in cruise?
 
hitower3
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Re: Derating a turboprop engine

Thu Feb 13, 2020 10:52 am

Starlionblue wrote:
Is 2132/2750 the prop RPM? This does not indicate engine power. The props are variable pitch, so the RPM would remain pretty much the same in the cruise regardless of power settings.


FYI, as far as I remember, the props of modern Turboprop aircraft revolve at significantly lower RPM, like 800-900. Unlike Turbofan engines, the blade tips must not exceed the speed of sound.

BR Hendric
 
gloom
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Re: Derating a turboprop engine

Thu Feb 13, 2020 12:40 pm

hitower3 wrote:
FYI, as far as I remember, the props of modern Turboprop aircraft revolve at significantly lower RPM, like 800-900. Unlike Turbofan engines, the blade tips must not exceed the speed of sound.


Both ATR and Dash use 1200rpm for takeoff. In cruise, Dash climbs/cruises at 900rpm (75% max; option of 850rpm, at least on Q400, for cruise) while ATRs I've been working with (300s) used 86/77 % on climb/cruise, if I remember correctly. Since 500s (auto mode) they use auto, but I can't recall what setting it uses for cl/cr. 84%? 82%? Something like that, I believe.

Cheers,
Adam
 
nmcalba
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Re: Derating a turboprop engine

Thu Feb 13, 2020 1:57 pm

744SPX wrote:
77% seems pretty high, at least compared to turbofans which operate at what, maybe 10% in cruise?


Turbofans operate at fairly high power settings in cruise - way more than 10%

Think about it - even a 4 engined aircraft like a 747 will drift down if it loses an engine at cruise altitudes.

The actual figure depends on so many questions - including what what you mean by power setting - a typical turbofan which uses N1 as a measure of power (RR use EPR instead) will cruise at between 70 & 85% N1, although there is not a linear relationship between power and N1. Another way of expressing it is fuel flow - a typical aircraft like a A320 might burn 1200kg per hour per engine in the cruise - whereas at take off power fuel burn might be about 2500kg per hour per engine. At cruise altitudes engines produce a lot less power than they do at sea level - but they work more efficiently.
 
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SheikhDjibouti
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Re: Derating a turboprop engine

Fri Feb 14, 2020 1:54 am

nmcalba wrote:
744SPX wrote:
77% seems pretty high, at least compared to turbofans which operate at what, maybe 10% in cruise?
Turbofans operate at fairly high power settings in cruise - way more than 10%

Think about it - even a 4 engined aircraft like a 747 will drift down if it loses an engine at cruise altitudes.

The actual figure depends on so many questions - including what what you mean by power setting - a typical turbofan which uses N1 as a measure of power (RR use EPR instead) will cruise at between 70 & 85% N1, although there is not a linear relationship between power and N1. Another way of expressing it is fuel flow - a typical aircraft like a A320 might burn 1200kg per hour per engine in the cruise - whereas at take off power fuel burn might be about 2500kg per hour per engine. At cruise altitudes engines produce a lot less power than they do at sea level - but they work more efficiently.

Whilst I have known this general fact for some time (years, decades...), it was literally just a few hours ago I stumbled across a perfect example of this in Wikipedia.
The specifications for the 737 NG include a line in the performance table specifying "Cruise Max Thrust" - I have never noticed this before.
The actual figures are 5,960 lbf (at FL350, mach 0.8) which is only 22% of the rated take-off thrust for a CFM56-7B.
However, it is still "100% power", and the throttles in the cockpit will be pushed forward against the stops (in order to effect a climb to even higher altitudes)
One presumes that despite being at "full throttle", the engine is only consuming 22% of the fuel it would use at the same setting at sea level. Or more likely, something less than 22%.
Consequently I believe your numbers for the A320 at cruise are exceedingly generous; I would guess closer to 15% fuel rate versus take off at sea level. Certainly a lot less than 1200kg/hr in your example (not that I am suggesting your figures are correct per se...)

If you could ever get an ATR-72 up to 35,000 feet, you might find a similar drop-off in available hp (and likewise fuel consumption)
Last edited by SheikhDjibouti on Fri Feb 14, 2020 2:09 am, edited 1 time in total.
Nothing to see here; move along please.
 
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Re: Derating a turboprop engine

Fri Feb 14, 2020 2:04 am

gloom wrote:
hitower3 wrote:
FYI, as far as I remember, the props of modern Turboprop aircraft revolve at significantly lower RPM, like 800-900. Unlike Turbofan engines, the blade tips must not exceed the speed of sound.

Both ATR and Dash use 1200rpm for takeoff. In cruise, Dash climbs/cruises at 900rpm (75% max; option of 850rpm, at least on Q400, for cruise)...

The PW150 used on the Q400 is limited to 1,020 rpm max, suggesting it requires a significantly larger prop optimised for slower rpm (despite the higher airspeeds!)
Nothing to see here; move along please.
 
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Starlionblue
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Re: Derating a turboprop engine

Fri Feb 14, 2020 3:17 am

SheikhDjibouti wrote:
nmcalba wrote:
744SPX wrote:
77% seems pretty high, at least compared to turbofans which operate at what, maybe 10% in cruise?
Turbofans operate at fairly high power settings in cruise - way more than 10%

Think about it - even a 4 engined aircraft like a 747 will drift down if it loses an engine at cruise altitudes.

The actual figure depends on so many questions - including what what you mean by power setting - a typical turbofan which uses N1 as a measure of power (RR use EPR instead) will cruise at between 70 & 85% N1, although there is not a linear relationship between power and N1. Another way of expressing it is fuel flow - a typical aircraft like a A320 might burn 1200kg per hour per engine in the cruise - whereas at take off power fuel burn might be about 2500kg per hour per engine. At cruise altitudes engines produce a lot less power than they do at sea level - but they work more efficiently.

Whilst I have known this general fact for some time (years, decades...), it was literally just a few hours ago I stumbled across a perfect example of this in Wikipedia.
The specifications for the 737 NG include a line in the performance table specifying "Cruise Max Thrust" - I have never noticed this before.
The actual figures are 5,960 lbf (at FL350, mach 0.8) which is only 22% of the rated take-off thrust for a CFM56-7B.
However, it is still "100% power", and the throttles in the cockpit will be pushed forward against the stops (in order to effect a climb to even higher altitudes)
One presumes that despite being at "full throttle", the engine is only consuming 22% of the fuel it would use at the same setting at sea level. Or more likely, something less than 22%.
Consequently I believe your numbers for the A320 at cruise are exceedingly generous; I would guess closer to 15% fuel rate versus take off at sea level. Certainly a lot less than 1200kg/hr in your example (not that I am suggesting your figures are correct per se...)

If you could ever get an ATR-72 up to 35,000 feet, you might find a similar drop-off in available hp (and likewise fuel consumption)


The fuel consumption does not decrease linearly, but you get very roughly a third of take-off fuel flow in the cruise.

The reason for lower thrust is lower mass flow due to lower air density in the cruise. Plus losses because the difference between exhaust velocity and engine velocity is much smaller than standing still on the ground.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
nmcalba
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Re: Derating a turboprop engine

Fri Feb 14, 2020 12:05 pm

SheikhDjibouti wrote:
nmcalba wrote:
744SPX wrote:
77% seems pretty high, at least compared to turbofans which operate at what, maybe 10% in cruise?
Turbofans operate at fairly high power settings in cruise - way more than 10%

Think about it - even a 4 engined aircraft like a 747 will drift down if it loses an engine at cruise altitudes.

The actual figure depends on so many questions - including what what you mean by power setting - a typical turbofan which uses N1 as a measure of power (RR use EPR instead) will cruise at between 70 & 85% N1, although there is not a linear relationship between power and N1. Another way of expressing it is fuel flow - a typical aircraft like a A320 might burn 1200kg per hour per engine in the cruise - whereas at take off power fuel burn might be about 2500kg per hour per engine. At cruise altitudes engines produce a lot less power than they do at sea level - but they work more efficiently.

Whilst I have known this general fact for some time (years, decades...), it was literally just a few hours ago I stumbled across a perfect example of this in Wikipedia.
The specifications for the 737 NG include a line in the performance table specifying "Cruise Max Thrust" - I have never noticed this before.
The actual figures are 5,960 lbf (at FL350, mach 0.8) which is only 22% of the rated take-off thrust for a CFM56-7B.
However, it is still "100% power", and the throttles in the cockpit will be pushed forward against the stops (in order to effect a climb to even higher altitudes)
One presumes that despite being at "full throttle", the engine is only consuming 22% of the fuel it would use at the same setting at sea level. Or more likely, something less than 22%.
Consequently I believe your numbers for the A320 at cruise are exceedingly generous; I would guess closer to 15% fuel rate versus take off at sea level. Certainly a lot less than 1200kg/hr in your example (not that I am suggesting your figures are correct per se...)

If you could ever get an ATR-72 up to 35,000 feet, you might find a similar drop-off in available hp (and likewise fuel consumption)


It's getting a bit off topic - but 1200kg per engine per hour is in the ball park. An ancient A320 fully loaded would be a bit more, a brand new 320neo would be a bit less.

Your figure for the output thrust at cruise is about right - but the reduction in thrust is not matched by a similar reduction in fuel consumption - the thrust reduction is mainly due to the higher altitude meaning that the mass of the air being moved through the engine is much less. But there isn't a equivalent reduction in fuel flow to get that thrust - although overall the engine is working more efficiently in terms of work done vs fuel burnt. The figure you also need in addition to cruise thrust is SFC - I don't know about the CFM56-7B - but the elderly V2500A1 was 0.58 (lb fuel/hr/lbs thrust) - with a typical cruise thrust of 5050lb - multiply the two together and you get 2929 lb fuel per hour - or 1328kg in proper units.

Another way to do a good gross error sense check is simply look at the fuel capacity of a vanilla A320 - 27,200l - 22,000kg - if the average fuel flow was much less than 2400kg/hr you would be looking at an endurance of way over 10 hours - even allowing for TO/Climb
 
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SheikhDjibouti
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Re: Derating a turboprop engine

Fri Feb 14, 2020 2:30 pm

Starlionblue wrote:
SheikhDjibouti wrote:
nmcalba wrote:
The actual figure depends on so many questions - including what what you mean by power setting...
Another way of expressing it is fuel flow - a typical aircraft like a A320 might burn 1200kg per hour per engine in the cruise - whereas at take off power fuel burn might be about 2500kg per hour per engine. At cruise altitudes engines produce a lot less power than they do at sea level - but they work more efficiently.

The specifications for the 737 NG include a line in the performance table specifying "Cruise Max Thrust" - I have never noticed this before.
The actual figures are 5,960 lbf (at FL350, mach 0.8) which is only 22% of the rated take-off thrust for a CFM56-7B....

The fuel consumption does not decrease linearly, but you get very roughly a third of take-off fuel flow in the cruise.

The reason for lower thrust is lower mass flow due to lower air density in the cruise. Plus losses because the difference between exhaust velocity and engine velocity is much smaller than standing still on the ground.

Many thx to both you and nmcalba for clarification.
In laymans terms it seems comparable to a carburettor fitted to a car engine. This is designed to mix fuel & air, with the passage of the air helping draw the fuel into the inlet (unlike fuel injection, where the fuel is "injected" into the air flow/ directly into the cylinders). If this car engine was taken to altitude, the air flowing through the carb would be thinner, and hence the "draw" on the fuel flow reduced - which is just as well because without sufficient oxygen, any "extra" fuel would remain unburnt and wasted.
(meh - it's not entirely accurate, but something I have in my head that non-av persons might relate to)
Likewise with a jet engine, there is little benefit to adding more fuel if the oxygen isn't available to combust it efficiently. Although the makers of the Convair Cv990 probably thought differently. :duck:

As regards "losses because... exhaust velocity vs engine velocity..." I am reminded that the RR Olympus engines on Concorde were reckoned to be the most efficient engines of their generation precisely because Concorde was already travelling at Mach 2 !
Indeed, nmcalba comments thus...
nmcalba wrote:
...although overall the engine is working more efficiently in terms of work done vs fuel burnt.

- which leaves me in a conundrum.

p.s. I don't doubt your numbers, or your expertise. I'm just puzzled by the contradictions.... :lol:
Maybe one day it will all drop into place, probably in the final moments of life as I drift off into oblivion. :wave:
Nothing to see here; move along please.
 
nmcalba
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Re: Derating a turboprop engine

Fri Feb 14, 2020 6:11 pm

SheikhDjibouti wrote:
Starlionblue wrote:
SheikhDjibouti wrote:
The specifications for the 737 NG include a line in the performance table specifying "Cruise Max Thrust" - I have never noticed this before.
The actual figures are 5,960 lbf (at FL350, mach 0.8) which is only 22% of the rated take-off thrust for a CFM56-7B....

The fuel consumption does not decrease linearly, but you get very roughly a third of take-off fuel flow in the cruise.

The reason for lower thrust is lower mass flow due to lower air density in the cruise. Plus losses because the difference between exhaust velocity and engine velocity is much smaller than standing still on the ground.

Many thx to both you and nmcalba for clarification.
In laymans terms it seems comparable to a carburettor fitted to a car engine. This is designed to mix fuel & air, with the passage of the air helping draw the fuel into the inlet (unlike fuel injection, where the fuel is "injected" into the air flow/ directly into the cylinders). If this car engine was taken to altitude, the air flowing through the carb would be thinner, and hence the "draw" on the fuel flow reduced - which is just as well because without sufficient oxygen, any "extra" fuel would remain unburnt and wasted.
(meh - it's not entirely accurate, but something I have in my head that non-av persons might relate to)
Likewise with a jet engine, there is little benefit to adding more fuel if the oxygen isn't available to combust it efficiently. Although the makers of the Convair Cv990 probably thought differently. :duck:

As regards "losses because... exhaust velocity vs engine velocity..." I am reminded that the RR Olympus engines on Concorde were reckoned to be the most efficient engines of their generation precisely because Concorde was already travelling at Mach 2 !
Indeed, nmcalba comments thus...
nmcalba wrote:
...although overall the engine is working more efficiently in terms of work done vs fuel burnt.

- which leaves me in a conundrum.

p.s. I don't doubt your numbers, or your expertise. I'm just puzzled by the contradictions.... :lol:
Maybe one day it will all drop into place, probably in the final moments of life as I drift off into oblivion. :wave:


I know what you mean - there are a whole load of apparent contradictions and counter intuitive facts - esp as you start getting into the more exotic areas like "most of the thrust of a Concorde engine comes from the air intake", but it is a fascinating subject - and just an interest of mine not my profession.

The thing to bear in mind with your carburettor analogy is that while their may be thinner at altitude the aircraft is moving though it much faster than it does at low level - so the actual amount of air going through the engine remains high - the speed acts like a turbo charger to try and maintain the analogy - but a car engine and a turbine engine work so differently the analogy can be a bit shaky.

One of the problems is that terms get used casually in ways that are not quite technically correct - for instance power and thrust are often used interchangeably but they are actually very different things - for a jet engine Power is actually Thrust multipled by Speed, or else in an attempt to simplify things they get expressed in ways that aren't strictly speaking true - and that's before you get into the whole can of worms that is Aerodynamic Lift :banghead:

I can recommend very highly some of the education material available from NASA's Glenn Research Centre - https://www.grc.nasa.gov/WWW/K-12/airplane/index.html - it's mostly senior high school level - but very well written. It also had a very good engine simulator program which allows you to play around with all the various main parameters of a jet engine - though since it was all Java based I'm not sure if it still works.
 
mxaxai
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Re: Derating a turboprop engine

Sat Feb 15, 2020 1:30 am

SheikhDjibouti wrote:
The actual figures are 5,960 lbf (at FL350, mach 0.8) which is only 22% of the rated take-off thrust for a CFM56-7B.
However, it is still "100% power", and the throttles in the cockpit will be pushed forward against the stops (in order to effect a climb to even higher altitudes)
One presumes that despite being at "full throttle", the engine is only consuming 22% of the fuel it would use at the same setting at sea level. Or more likely, something less than 22%.

Don't the throttles give you x % of N1 or whatever your engine uses as power measurement, rather than set the fuel valves directly?

nmcalba wrote:
For a jet engine Power is actually Thrust multipled by Speed.

This is one thing that always confused me to no end - jet engines' power output is speed dependent but their thrust is not, whereas turboprop engines' (shaft) power is constant but their thrust varies with speed. For a given throttle settting, of course.
 
LabQuest
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Re: Derating a turboprop engine

Sat Feb 15, 2020 1:49 am

What about TBMs that are de-rated for takeoff but then are allowed to use full power in the climb/cruise?
 
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Starlionblue
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Re: Derating a turboprop engine

Sat Feb 15, 2020 6:35 am

mxaxai wrote:
SheikhDjibouti wrote:
The actual figures are 5,960 lbf (at FL350, mach 0.8) which is only 22% of the rated take-off thrust for a CFM56-7B.
However, it is still "100% power", and the throttles in the cockpit will be pushed forward against the stops (in order to effect a climb to even higher altitudes)
One presumes that despite being at "full throttle", the engine is only consuming 22% of the fuel it would use at the same setting at sea level. Or more likely, something less than 22%.

Don't the throttles give you x % of N1 or whatever your engine uses as power measurement, rather than set the fuel valves directly?


In manual thrust, the thrust is set by FADEC in response to thrust lever angle and environmental factors.

The thrust levers don't give a linear N1 response because power delivery for a given fuel flow is not linear. You can be on the ground at idle with a fuel flow of 800kg/h/engine and N1 is sitting at 13-14%. At takeoff with TOGA fuel flow might be 8000kg/h with N1 at 100%. In the cruise with fuel flow of 2800kg/h, N1 would be around at 80%.

In the lower range of thrust lever movement, you don't get much variation in thrust. In the upper range a small movement gives a greater change.

For that matter, thrust and N1 are not directly proportional either.

The A350 does have linear thrust delivery display, from 0 to 100%, but we still use N1 as a reference.

BTW they're not "throttles". They're thrust levers. ;)
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
N1120A
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Re: Derating a turboprop engine

Sat Feb 15, 2020 7:30 am

LabQuest wrote:
What about TBMs that are de-rated for takeoff but then are allowed to use full power in the climb/cruise?


The PT-6A in the TBM is derated at every setting. The engine is rated at 1825 SHP, but derated to the various numbers the TBM sees. The reasons are rudder force and engine wear. The aircraft just doesn't need that much extra power to get off the ground, given how light it is, and thus the power is reserved for lower engine stress situations.
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VSMUT
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Re: Derating a turboprop engine

Sat Feb 15, 2020 7:32 am

Starlionblue wrote:
BTW they're not "throttles". They're thrust levers. ;)


And power levers on a turboprop ;) We also set power by torque.
 
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Re: Derating a turboprop engine

Sat Feb 15, 2020 7:36 am

VSMUT wrote:
Starlionblue wrote:
BTW they're not "throttles". They're thrust levers. ;)


And power levers on a turboprop ;) We also set power by torque.


:D

The fast jet guys and girls have "throttles" on their jets for some reason lost in time.

And Boeing has "autothrottle" to set the "thrust"... :roll:
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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SheikhDjibouti
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Re: Derating a turboprop engine

Sun Feb 16, 2020 1:54 pm

Starlionblue wrote:
VSMUT wrote:
Starlionblue wrote:
BTW they're not "throttles". They're thrust levers. ;)

And power levers on a turboprop ;) We also set power by torque.

The fast jet guys and girls have "throttles" on their jets for some reason lost in time.

And Boeing has "autothrottle" to set the "thrust"... :roll:

You say potato, I say tomato. :D

Throttle is a generic term, implying a restriction of fuel (fluid) flow, or more accurately "fuel" in the context of {air + combustible material}. But across a variety of modes of transport it deviates from that original idea quite considerably.
On a normal car it is an accelerator or gas pedal, and if fitted with a carburettor it controlled the air flow more or less directly. (the fuel pump operating constantly merely to ensure the carburettor chambers were topped up, whilst the butterfly valve affected the speed by regulating the air flow)

Nowadays on modern cars, including diesels, it sends an input to the ECU that controls the fuel injectors, and we are primarily regulating the fuel flow.
Unless somebody wants to tell me I've got that wrong too.

On a steam train it is a regulator, controlling the steam output from the boiler. (actual fuel flow itself is dependent on the fireman and his shovel)

And around the neck of a human being.... throttling is somewhat restricting the air flow, but can also limit the amount of fuel (blood) reaching the brain, so..... :white:
Nothing to see here; move along please.
 
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Starlionblue
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Re: Derating a turboprop engine

Sun Feb 16, 2020 2:12 pm

There is no throttle valve on a jet engine. So you're not really "throttling". :D
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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kitplane01
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Re: Derating a turboprop engine

Mon Feb 17, 2020 5:33 am

mxaxai wrote:
SheikhDjibouti wrote:
The actual figures are 5,960 lbf (at FL350, mach 0.8) which is only 22% of the rated take-off thrust for a CFM56-7B.
However, it is still "100% power", and the throttles in the cockpit will be pushed forward against the stops (in order to effect a climb to even higher altitudes)
One presumes that despite being at "full throttle", the engine is only consuming 22% of the fuel it would use at the same setting at sea level. Or more likely, something less than 22%.

Don't the throttles give you x % of N1 or whatever your engine uses as power measurement, rather than set the fuel valves directly?

nmcalba wrote:
For a jet engine Power is actually Thrust multipled by Speed.

This is one thing that always confused me to no end - jet engines' power output is speed dependent but their thrust is not, whereas turboprop engines' (shaft) power is constant but their thrust varies with speed. For a given throttle settting, of course.


Maybe this helps.

Airplane mode:
Power = Thrust * Velocity
Car Mode:
Power = RPM * Torque

For an engine measured in horsepower, you will get a constant horsepower but as the plane speeds up the thrust will decrease. Just like in a car as you speed up the wheel RPM will increase, but torque at the wheel will decrease. (Engine torque might stay roughly constant, but the transmission ratio will change, reducing wheel torque.)

For a jet engine measured in thrust, the thrust will vary depending on airspeed and altitude. Don't think of thrust as a constant, but as something that varies a lot. Within limits, the engine will produce more thrust as air hits the inlet faster. It will produce less thrust as the air gets thinner at altitude. Max thrust at altitude will be 1/2 or 1/3 or so of thrust at sea level.

In real life the equations above have more terms, but what I wrote is about right sort of.

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