Fuel Flow Reduction With Altitude (Constant Power)
****e,
One day someone might let me loose with a modern helicopter with all the bells and whistles! When you state "I often change altitude" do you mean 'increase' altitude?
Everyone,
I tend to fly 100 to 200nm trips around the UK so fuel burn per se isn't often a significant factor whereas minimising airborne time is. When there's a tailwind component I tend to cruise climb using a constant IAS (120kts in the A109) and monitor the GPS groundspeed closely. When the G/S starts to decay, I've just passed the optimum altitude. When there's an obvious strong headwind I tend to fly as low as possible commensurate with flying neighbourly. If there are significant forecast wind direction changes then these need to be reassessed en route.
I now stand by for incoming flak for not factoring in the IAS/TAS relationship; however, it works well enough for me.
JJ
One day someone might let me loose with a modern helicopter with all the bells and whistles! When you state "I often change altitude" do you mean 'increase' altitude?
Everyone,
I tend to fly 100 to 200nm trips around the UK so fuel burn per se isn't often a significant factor whereas minimising airborne time is. When there's a tailwind component I tend to cruise climb using a constant IAS (120kts in the A109) and monitor the GPS groundspeed closely. When the G/S starts to decay, I've just passed the optimum altitude. When there's an obvious strong headwind I tend to fly as low as possible commensurate with flying neighbourly. If there are significant forecast wind direction changes then these need to be reassessed en route.
I now stand by for incoming flak for not factoring in the IAS/TAS relationship; however, it works well enough for me.
JJ
Avoid imitations
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Do I mean increase altitude? Yes... and no. I'm lucky enough to fly a bell and whistle equipped aircraft so these days I just watch the ETA readout and see if I can improve it. 
In mitigation, I do look at the Met Office's F214 before first takeoff so I have some idea of whether it's worth climbing higher. The other thing I consider is the turbulence as I get fed up with chasing the autopilot and that awful airspeed warning woman.
In mitigation, I do look at the Met Office's F214 before first takeoff so I have some idea of whether it's worth climbing higher. The other thing I consider is the turbulence as I get fed up with chasing the autopilot and that awful airspeed warning woman.
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Matthew:
to be pedantic - the power required curve is not what you need for a turbine engine helicopter. You need to use the fuel flow vs. Airspeed, and that's a different curve.
to be pedantic - the power required curve is not what you need for a turbine engine helicopter. You need to use the fuel flow vs. Airspeed, and that's a different curve.
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Mmmmmm ....
I am of the opinion that fuel flow measuring equipment shud be mandatory especially on newly certified machines!
Even the AW139 does not have have them .... instead relying on a "lookup table" to fake a f'flow indications.
I mean with all the computer power available and miniturisation of modern electronics etc AND the wealth of really useful information produced why not?
I am of the opinion that fuel flow measuring equipment shud be mandatory especially on newly certified machines!
Even the AW139 does not have have them .... instead relying on a "lookup table" to fake a f'flow indications.
I mean with all the computer power available and miniturisation of modern electronics etc AND the wealth of really useful information produced why not?
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I totally agree, a fuel flow readout is an excellent in-flight planning tool. I'm surprised to hear the 139 doesn't have them, especially as it's smaller brother does, displayed on its auxiliary "T's and P's" page of the EDU. The fuel flow in kgs/hr for each engine is shown immediately above the fuel contents. I look at my ETA, compare the fuel flows to the fuel total remaining and can very quickly work out, even in my little brain, how my fuel is working out.
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Shy Torque and spinwing:
You are both absolutely right about the requirement for fuel flow information. There is no requirement to have range or endurance information in the flight manual (it's not required by the certification standards, and would not be 'approved').
But we hang people who run out fuel on cross-country flights. Why the authorities don't require it as a minimum is beyond me. Maybe the insurance companies ought to start demanding it as part of the fit....
Put a fuel flow device in your helicopter, marry it up with your GPS and learn how to use both, and you'll be a lot better off.
You are both absolutely right about the requirement for fuel flow information. There is no requirement to have range or endurance information in the flight manual (it's not required by the certification standards, and would not be 'approved').
But we hang people who run out fuel on cross-country flights. Why the authorities don't require it as a minimum is beyond me. Maybe the insurance companies ought to start demanding it as part of the fit....
Put a fuel flow device in your helicopter, marry it up with your GPS and learn how to use both, and you'll be a lot better off.
Stan
Relating to your specific question - (What causes fuel flow to decrease with altitude for a given torque setting in a turboshaft powered helicopter?).
Firstly the gas turbine engine is just a power making machine. It does not know that it is installed in a helicopter and neither is it aware that it’s driving a rotor, propeller or fan. The gas turbine engine in a helicopter acts principally to supply a high velocity gas stream in order to drive the rotor, so what we are looking at is its ability to produce power and its efficiency in doing so.
Specific to your question, it’s not the altitude, density, tail wind, day or the week or even whether there is a full moon or not that matters the most, it is the ambient temperature that is the major factory here.
Why? Because an increase in altitude results in a decrease in ambient temperature, with a decrease in ambient temperature, engine efficiency and power increase (As engine inlet temperature (T1/T2) drops, compressor discharge total pressure (Pt3) increases). Therefore for a given load, fuel flow can be reduced for power output to remain constant.
Do other factors (air density, aircraft weight, etc) have an input to your specific question, yes they do, but not to the extent that temperature does.
Relating to your specific question - (What causes fuel flow to decrease with altitude for a given torque setting in a turboshaft powered helicopter?).
Firstly the gas turbine engine is just a power making machine. It does not know that it is installed in a helicopter and neither is it aware that it’s driving a rotor, propeller or fan. The gas turbine engine in a helicopter acts principally to supply a high velocity gas stream in order to drive the rotor, so what we are looking at is its ability to produce power and its efficiency in doing so.
Specific to your question, it’s not the altitude, density, tail wind, day or the week or even whether there is a full moon or not that matters the most, it is the ambient temperature that is the major factory here.
Why? Because an increase in altitude results in a decrease in ambient temperature, with a decrease in ambient temperature, engine efficiency and power increase (As engine inlet temperature (T1/T2) drops, compressor discharge total pressure (Pt3) increases). Therefore for a given load, fuel flow can be reduced for power output to remain constant.
Do other factors (air density, aircraft weight, etc) have an input to your specific question, yes they do, but not to the extent that temperature does.
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Sorry highlife, not so.
The engine is more efficient at altitude (same power, less fuel flow) because the operating temperature of the engine must be higher at altitude to produce the same power. Power is produced by making a given package of air rise a given amount of temperature (which makes it have higher pressure) then blowing that package of air against a turbine wheel to extract the power. At altitude, the package of air is less dense, so the temperature it must be raised to is higher for the same power.
The temperature difference between the air package and the environment determines the thermodynamic efficiency of the process. Ideally, one air molecule, heated to 1,000,000,000,000 degrees could produce enough power to hover. But since that molecule would melt its way through any turbine we can build, we are stuck with raising a few pounds of air to about 900 degrees C, and then extracting the energy.
To recap, efficiency of an engine is determined by the temperature difference between the turbine inlet and the outside air. More temp, less fuel burned for a given power. The quest for lighter, more efficient and more powerful engines has been the quest for better turbine metals that aloow higher temperatures.
This is also the reason why twins burn more fuel than singles to do the same job, and three engined helos more fuel yet. It is also why a helo with two big engines that allows OEI hover on one engine will burn more fuel than a twin with two small engines.
The engine is more efficient at altitude (same power, less fuel flow) because the operating temperature of the engine must be higher at altitude to produce the same power. Power is produced by making a given package of air rise a given amount of temperature (which makes it have higher pressure) then blowing that package of air against a turbine wheel to extract the power. At altitude, the package of air is less dense, so the temperature it must be raised to is higher for the same power.
The temperature difference between the air package and the environment determines the thermodynamic efficiency of the process. Ideally, one air molecule, heated to 1,000,000,000,000 degrees could produce enough power to hover. But since that molecule would melt its way through any turbine we can build, we are stuck with raising a few pounds of air to about 900 degrees C, and then extracting the energy.
To recap, efficiency of an engine is determined by the temperature difference between the turbine inlet and the outside air. More temp, less fuel burned for a given power. The quest for lighter, more efficient and more powerful engines has been the quest for better turbine metals that aloow higher temperatures.
This is also the reason why twins burn more fuel than singles to do the same job, and three engined helos more fuel yet. It is also why a helo with two big engines that allows OEI hover on one engine will burn more fuel than a twin with two small engines.
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To all,
I was discussing this topic again the other day, and realized I hadn't thanked everyone who contributed to the thread.
Nick,
I think I've got it now. I was focusing on N1, but turbine inlet temp was rising at the same time. It's just that in the Arriel powered S-76 T4 is never the first limit we run into in our normal operations, so I wasn't paying very close attention.
-Stan-
I was discussing this topic again the other day, and realized I hadn't thanked everyone who contributed to the thread.
Nick,
I think I've got it now. I was focusing on N1, but turbine inlet temp was rising at the same time. It's just that in the Arriel powered S-76 T4 is never the first limit we run into in our normal operations, so I wasn't paying very close attention.
-Stan-
