JasonC wrote:
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.
At ISA, density altitude as %of standard on ground:
FL000: 1
FL100: 0.904
FL450: 0.237
So FL450 is about 3.8 times less dense than at FL100, which in turn would mean that you need to go 1.95 faster (SQRT(3.8)) to get the same “IAS” (I’m just using the standard formula, ignoring complex aerodynamical effects at high speed which I don’t know about).
Assuming constant thrust (and constant fuel flow to achieve that constant thrust), I’d then expect fuel consumption at FL450 to be about 51% of what it is at FL100.
Jason’s numbers suggest it’s about 37% so I imagine that difference is due to engine performance. From what I understand (which is little), at high altitude you can be more efficient as the turbine will spin faster for the same fuel flow, and the compression will be more efficient)
Peter wrote:
I am asking why a TP or a jet does so much more MPG at altitude.
I think you need to further break it down to Jet, TP (free turbine), TP (direct drive) and even a pure jet which can be quite efficient at high speed and high altitude.
It is my understanding it is how well the engine size is matched to the operating envelope of the aircraft. i.e. you want max rpm. This is the dominant variable, because as mentioned earlier engine speed effects the compression ratio, unlike a piston engine. High speed also effects the compression ratio in a jet because of the ram effect.
This often means that at low level you may be torque limited (the turbine needs to spin slower) or you have too much thrust and you IAS would be excessive, either of those reduce the compression ratio. In the case of direct drive (where RPM is always high) the propeller blades will have an excessively fine pitch causing a reduction in efficiency.
In the case of jetprop that was mentioned earlier, which I believe is a smaller pt6, it is only torque limited over a small part of the envelope, and it should be able to maintain higher engine RPMs over its operating envelope. In the examples given do these result in similar engine speeds?
quatrelle wrote:
Jetprop with -35 PT5 all at same IASF100 …. 200kt x 34 USG/HR
F180 …. 231kt x 34 USG/HR
F230 …. 248kt x 34 USG/HR
F270 …. 265kt x 34 USG/HR
Perhaps of interest are the very large RR turbofans which have three spools, and the some of the latest turbofans that have a gearbox, this allows turbine speeds to increased relative to the fan diameter. Further increasing efficiency.
This is distinctly different to the physics of a piston engine where slower engine rpm is usually better.
Finally I think there are distinct differences between a propeller and a jet which relates to propulsive efficiency instead of engine efficiency however that is difficult to compare to a piston engine as there aren’t many piston engines that drive a jet. 
A jet has a higher propulsive efficiency at higher mach.
Noe wrote:
Assuming constant thrust (and constant fuel flow to achieve that constant thrust), I’d then expect fuel consumption at FL450 to be about 51% of what it is at FL100.
Jason’s numbers suggest it’s about 37% so I imagine that difference is due to engine performance.
It depends on which speed you will fly relative to sound, less than Mach 0.2, that approximation should be correct and you should get 51% moving from FL100 to FL450 on same engine thrust
IAS to TAS conversion, gets more elaborate at high speeds due to air compressibility
If you fly fixed mach (= same engine thrust for each 1lbs fuel on a turbofan) all the way between FL100 to FL450, you should get less than 51% on mpg
Peter wrote:
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
In the manual page 361:
SL TAS 242kts @ 88GPH
FL310 TAS 320kts @ 55GPH
That is roughly 2x fuel per mile.
Ted wrote:
which I believe is a smaller pt6, it is only torque limited over a small part of the envelope
Ted …. * Its torque limited up to around F230 due to IAS limits, unlike a TBM which has higher airspeed limits ….. and very much higher costs.
* Thats in cruise, in the climb you can use more torque and climb at the VNE, (at least for most of the climb), then your fuel burn goes up as you would expect.
After around F230 you can then get the throttle fully open …. full throttle torque at F270 is around 980 lb which gives 172kt IAS or 265kt TAS (depending on OAT etc).
Ted wrote:
In the examples given do these result in similar engine speeds
F100 ……………………. 87.9% …. thats engine RPM as a % …. all vary slightly due to OAT etc.
F180 ……………………. 93.2%
F230 ……………………. 96.5%
F270 ……………………. 99.6%
loco wrote:
In the manual page 361:
SL TAS 242kts @ 88GPH
FL310 TAS 320kts @ 55GPH
That is roughly 2x fuel per mile.
Am I correct?
What is GPH at chosen IAS (e.g. 190kts) at different altitudes and what’s corresponding TAS? I.e. the question is whether MPG efficiency comes only from TAS gain by flying higher or engine efficiency at higher altitudes plays additional role.
the question is whether MPG efficiency comes only from TAS gain by flying higher or engine efficiency at higher altitudes plays additional role.
It’s not as much the efficiency that has increased at higher alt, as it is the power that has decreased at higher alt. This enables you to fly with less power at higher efficiency at higher alt.
The gain in speed is mostly due to increase in TAS.
Emir wrote:
What is GPH at chosen IAS (e.g. 190kts) at different altitudes and what’s corresponding TAS? I.e. the question is whether MPG efficiency comes only from TAS gain by flying higher or engine efficiency at higher altitudes plays additional role.
There’s no such table in the manual but in the long-range cruise tables (p. 365, 366) you can find that 148 IAS at FL180 is 39.2 GPH and same 148 IAS at FL310 is 34.3 GPH. It wouldn’t only be TAS gain then, but it is the most important factor.
quatrelle wrote:
F100 ……………………. 87.9% …. thats engine RPM as a % …. all vary slightly due to OAT etc.
F180 ……………………. 93.2%
F230 ……………………. 96.5%
F270 ……………………. 99.6%
The performance of that engine is very well matched then It is interesting all the speeds are restricted by Vmo, which I am guessing is based on the original piston engine design limits Vno etc.
A quick bit of google foo found this for the jetprop http://www.jetag.ch/images/TrainingHBPKS/GCPA46J18.pdf local copy
What it does show, is the engine is more efficient at higher altitudes or more specifically the engine is more efficient at higher power settings where the turbine can operate at the rpm it was designed for. It still burns 26Gph at 125kts at 6000ft, according to the table.
loco wrote:
There’s no such table in the manual but in the long-range cruise tables (p. 365, 366) you can find that 148 IAS at FL180 is 39.2 GPH and same 148 IAS at FL310 is 34.3 GPH. It wouldn’t only be TAS gain then, but it is the most important factor.
I found interesting graph in POH you shared.
