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Why do turboprops and jets burn so much more fuel per mile at low level?

Ibra wrote:

many commercial can still do well in terms of fuel efficiency flying up to FL500 but CAT cruise is limited to FL450 it seems…

In case of depressurisation above FL400, you can’t survive on 100% oxygen at ambient pressure — you’d need pressure breathing which would be impractical for passengers. I suppose 100% oxygen will suffice for a short time above FL400 so if you start an emergency descent immediately after depressurisation you can go a bit higher but not all that much.

ESKC (Uppsala/Sundbro), Sweden

Airborne_Again wrote:

In case of depressurisation above FL400, you can’t survive on 100% oxygen at ambient pressure — you’d need pressure breathing which would be impractical for passengers. I suppose 100% oxygen will suffice for a short time above FL400 so if you start an emergency descent immediately after depressurisation you can go a bit higher but not all that much.

In the CJ in the event of depressurisation with only crew on board you must descend to FL410. If passengers on board you must come down to 250 as their masks aren’t able to support a higher altitude.

EGTK Oxford

I think you are asking why the fuel required per output horsepower decreases for a turboprop engine, while it is (nearly) constant for a turbocharged piston engine?

I am asking why a TP or a jet does so much more MPG at altitude.

Superficially the main factor must be the “TAS gain”. Aerodynamically, a given power through a given air density will give you a given IAS. And the higher you go, the higher the TAS gain.

Are there any numbers around for the MPG? Then one could see if the altitude improvement is entirely caused by the TAS gain, or if there are additional engine factors. I reckon there must be factors in the engine design too (inefficient at high air density).

Administrator
Shoreham EGKA, United Kingdom

Engine factors as well, a piston extracts near 100% compression even while loitering at sea level while a turbine to achieve compression efficiency has to be at full power, which is temperature limited. At height the FCU reduces fuel due to lower density but the compression is still high, also lower intake temperature improves turbo fan efficiency.

Oxford (EGTK), United Kingdom

Peter wrote:

Superficially the main factor must be the “TAS gain”.

Jetprop with -35 PT5 all at same IAS

F100 …. 200kt x 34 USG/HR
F180 …. 231kt x 34 USG/HR
F230 …. 248kt x 34 USG/HR
F270 …. 265kt x 34 USG/HR

Peter wrote:

Are there any numbers around for the MPG? Then one could see if the altitude improvement is entirely caused by the TAS gain, or if there are additional engine factors. I reckon there must be factors in the engine design too (inefficient at high air density).

So my aircraft will do 21.2nm/100lbs of fuel at 12000lbs weight at ISA temps at FL100. At FL450 it will do 56.3.

Last Edited by JasonC at 28 Nov 09:13
EGTK Oxford

On the engine side, I don’t think it matters vey much what temp/pressure outside (+1bar and -50degres at the intake, will give you max 5% improvment on thermic cycle, say hot operates at 20bar and 2000degres)

For same fuel burn, you will get highest fuel-mechanial efficiency (thermal cycle & propulsive return) at max power/mach (more or less irrespective of outside) then you are left with mach to ias to tas conversion and reduced aerodynamic drag, these will improve your MPG with altitude, but will be good to compare it to actual figures

I am still not convinced on TPs, but I think the figures will still be good up to FL300

Last Edited by Ibra at 28 Nov 09:15
Paris/Essex, France/UK, United Kingdom

Jetprop with -35 PT5 all at same IAS
F100 …. 200kt x 34 USG/HR
F180 …. 231kt x 34 USG/HR
F230 …. 248kt x 34 USG/HR
F270 …. 265kt x 34 USG/HR

At ISA that translates to 172kt IAS. So that is a 1.54 gain purely due to altitude. Interesting!

Equally interesting is the constant fuel burn. TBM owners report a 2x higher fuel burn (in gallons or pounds per hour) at low level, versus say FL300. I wonder if @loco has some numbers. Achim has a TBM now but he’s gone into hiding again

my aircraft will do 21.2nm/100lbs of fuel at 12000lbs weight at ISA temps at FL100. At FL450 it will do 56.3.

I can’t calculate that backwards (it’s not likely to be linear anyway) but that seems a lot more than 1.54.

Administrator
Shoreham EGKA, United Kingdom

quatrelle wrote:

Jetprop with -35 PT5 all at same IAS

In other words consumption is the same for same IAS regardless of altitude (in your plane)?

Noe wrote:

In other words consumption is the same for same IAS regardless of altitude (in your plane)?

Exactly, same consumption at any altitude, its just the TAS that changes.

You also have the option of bringing the fuel flow back to 28 USG/HR for more duration and I would imagine that would also give you the same IAS at any altitude.

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