That is dramatic and must be some engine/ECU factor, not airframe.
I wonder if ISA-20 blue curve for max cruise will follow the same pattern after FL310 as others, it looks so? The inflection is mainly from decay in propeller traction due to limits on RPM for max cruise?
In the long range cruise page 366, one can see that MPG (=TAS/FF) changes the same way as does TAS changes with altitude and temperature bellow FL270, so probably this answers the initial question (which was probably about “long range cruise vs altitude” = max MPG vs altitude)
This graph shows that on a hot day, you will hit the ITT (internal turbine temp) limit at a lower altitude and will have to reduce TRQ below 100.
Also, this graph is for normal cruise, not max cruise. (before anyone says the plane doesn’t fly its advertised speed at ISA).
If you look at the following table for holding in the link Loco posted you can see the substantial variation in fuel consumption for a constant power setting (low power).
You have a 45% increase in fuel consumption for the same lower power output between 20K and SL.
Caveat, I am assuming the RPM is set constant so at a constant TRQ you should have roughly the same power output.
Yes, prop RPM is constant in this plane. Nice find.
You have a 45% increase in fuel consumption for the same lower power output between 20K and S
Actually at altitude the plane will need a bit more power in the holding. So the difference in fuel / power is even a bit bigger than that.
Sebastian_G wrote:
Actually at altitude the plane will need a bit more power in the holding.
That table is generated on constant IAS = 120kts for all altitudes, so not much energy required from aerodynamics, where that “extra power required for the hold at high altitudes” come from?
Ibra wrote:
That table is generated on constant IAS = 120kts for all altitudes, so not much energy required from aerodynamics, where that “extra power required for the hold at high altitudes” come from?
I did learn about this the hard way trying to hold around FL180 in a piston plane with absolute minimum fuel usage for over an hour. The POH values given for the hold have not been enough to keep the plane flying. This POH did only specify one holding power setting regardless of altitude. Then realized:
For the PA46-350P I was flying on contant power for every 1000ft you go up IAS will decrease by about 1kt and TAS will increase about 2kt. So when holding higher up you will have to add power to keep the IAS constant and prevent a stall. So in a piston plane if you want to strecht the maximum holding time try to hold as low as possible.
Sebastian_G wrote:
For the PA46-350P I was flying on contant power for every 1000ft you go up IAS will decrease by about 1kt and TAS will increase about 2kt. So when holding higher up you will have to add power to keep the IAS constant and prevent a stall. So in a piston plane if you want to strecht the maximum holding time try to hold as low as possible.
It is strange that the IAS should drop at altitude with constant power. Drag depends on IAS (well, CAS), not TAS, so constant IAS should give constant drag. Of course the pressurisation has to work harder. Could that be the reason?
I agree holding at high altitudes at FL180 in a SEP will not help as it means slower IAS from low engine power not covered by TAS gain from hight, nominal % power decays on SEPs mainly from weak combustion engine and propeller decay
Tough you could argue the same will happen for TPs, bit higher than that that table numbers though (probably FL300?) mainly from propeller decay but less on engine side and will practically never happen on faster jets (FL1200?) but other things will break first…
My understanding, in a piston all constant power lines (65%, 75%, 100%) will hit a wall with hight from eother full trottle or from max rpm, at that point they are the lest fuel efficient, if you keep the same rpm, then power decreases with hight as external manifold pressure decay with altitude, so for cruise, the sweet altitude spot for a SEPs is when that power decay from low external pressure matches TAS speed gain from hight as tou mentioned, for avgas SEP this seems at 9000ft for cruise IAS (diesel SEP probably higher? What will be this on a Malibu?)
However, I don’t think there is a “best altitude for economical hold”, one can’t just fly IAS for min sink rate? (10kts higher than stall is a good rule of thumb, if I can’t fly it then I am close to “coffin corner”)
