Turbines + altitude

[vaqueroaero]

Can someone please explain this : why does the efficiency of a turbine engine increase with altitude?
Thanks.

[Jonasraf]

It is because of decreasing density of the air, so therefore increases the flow out of the exhaust pipe.

I think:confused:

[topcat450]

Also as the air is thinner...a/c have less drag so can fly faster and further when they're higher...doesn't work like that for heli's though..they rely on dense air to provide lift...

An other area I'd not really considered...but just have...higher up..where air is less dense and colder..that might make it more efficient as cold air when heated would expand more than half-warm air....I know what I mean in my own head...but can't get my words out..:8

[Thomas coupling]

Jonas!

For turbine engines:
The volume of air entering the front of the engine remains the same at any height. True the density changes, but for engine efficiency alone (as opposed to blade efficiency), the main factor is the temp. The colder the air the more dense the fixed volume is. Therefore the more "goodness" there is for the turbine to chew on. Hence more energy produced. However there is an optimum height for this to happen because then the air gets too thin with height and the density too light, efficiency tails off. Ultimately, with height the engine would flame out as it couldn't breathe!!
For a gazelle (for the engine only), the optimum efficient operating height for the turbine is about 16000+'

When you then discuss blade efficiency with height, the same initially happens: cold thick air - more grip for the pitch of the blade, more lift. But this dies off very quickly by comparison to the engine benefits, and the air as it thins with height dictates, causing the blades to grasp at this thin air producing less lift for the same pitch setting.
Ultimately there is an overall optimum operating height between blade and engine which I think in the gazelle is around 8000'.

I stand to be corrected. it's been a long time since P of F:eek:

[Shawn Coyle]

Turbines are more efficient at higher altitude because they are operating closer to their limits and hence have better specific fuel consumption (pounds per hour per horsepower).
The power required increases with altitude in helicopters (since we don't worry so much about drag on the fuselage - the reason why fixed wing jets fly so high), so we need more power from the engines. However the power available from the engine decreases with altitude, and so the engine ends up operating closer to the design power output (higher N1, particularly).
At the higher N1, less percentage of the power is being used to turn the compressor, so the useful power fraction is higher, and hence the specific fuel consumption for the useful power put out, increases.
For the PT-6 in the Bell 212, the fuel flow at sea level is about 600 pounds per hour, and at 10,000' it's about 500 pounds per hour for about the same True Air Speed.
If you have no wind, or a tailwind, it's worth climbing to altitude. If you have a headwind (story of my life), then it may not be worth climbing...

[BlenderPilot]

piston and jet engines work on the same principle . . .

suck and blow, suck and blow . . . . . . .

And I can also tell you the 212 here in MMTO (8445FTAMSL) well the engines are not particularly more effective, you always bite the N1 redline, with plenty of TQ and ITT available.

[NickLappos]

The efficiency of the engine (power out for fuel burned) is determined by the Carnot Cycle, where we calculate the temperature of the air in the engine vs the temperature of the ambient. as the Temp difference becomes greater, the heat and work move more easily from the hot to the cold (this might be imagines as a thremodynamic "pressure" of sorts) so that the power plant is more efficient.

For a turbine, the altitude increase causes the thinner air to need to be heated to higher temperatures for a given power. In effect, power is measured by the amount of air times its temperature rise. Less air at altitude means more temp for constant power. More temp (higher tgt or T5) means more efficiency.

A turbine might give 20% less fuel flow for constant power if you climb and drive the T5 up by 100 degrees C.

There are two implications to this fact:
1) Smaller turbine engines, with less excess power, are better for a helicopter when range or efficiency are the goal. The small engines are working harder (hotter) in normal conditions, where they are more nearly at peak efficiency.
2) Turbojets and turbo fans have gobs of excess power at sea level, because they are designed to be very efficient at altitude, where the thin air is heated very high. In thick air, they run cool and have lots of excess power, so the OEI climb rate of turbo jets is always awesome at low altitude.

[[email protected]]

TC, I think I am correct in saying the Gazelle Astazou engine is slightly different as it is a fixed spool engine compared to the free power turbine of most other helicopters. Like the fixed wing turbofan the fixed spool engines have oodles of surplus performance at sea level and the transmission torque is the limiting factor - the N1 is constant and so therefore is the mass flow through the engine, the only variable is the amount of fuel squirted into the combustion chamber.
As you go higher with the Astazou it becomes more efficient as more of the mass flow is used in combustion until eventually all of it is utilised. Since the only way to get more power out of the engine is to throw in more fuel the T4 becomes the limiting factor on this type of engine unlike a free power turbine where the N1 limit is usually the stopper.
The design is good for hot and high which is why Allouettes (also fixed spool) are still used widely in mountain ops.

[SASless]

All that jives.....with the 212/412....Torque settings determines range/endurance more than altitude but in the Jetranger or Hughes/McDonnell Doug egg birds.....altitude certainly makes a difference. The wind is always the key.....and if the customer is paying.....seems Vmrc should be the target speed. ( a point younger pilots should consider very carefully!)