loco wrote:
before anyone says the plane doesn’t fly its advertised speed at ISA
and the speeds are great. Of course there’s price in fuel for that but the speed and range are remarkable.
Nice article @Xtophe, there are more speeds listed there than the dots you can have on a single PFD !
@Peter, yes external pressure/manifold pressure only give you a proxy for the torque (as you will try max torque allowed by ambiant pressure anyway), are there SEPs where torque or power is actually measued on engine or propeller instead of picked from some POH (+ASI+OAT+ALT) formulas?
On TPs, my understanding this is effectively measured on propellers and pilots will get it displayed and easily do power = torque×rpm…
Off-topic but interesting Airbus article following the discussion at the end of page 1 and start of page 2 about the limiting factor for max altitude. Obviously, some of the safety factor are for large a/c (FAR/CS 23) and smaller a/c regs might not be exactly the same. But the physic is the same.
On a piston, torque is proportional to fuel flow, if you are at peak EGT, and then power = torque x rpm x (some constant).
MP is a proxy for torque if everything else is constant.
Indeed the holding power settings won’t work at altitude. For the TB20 it is 18" and 2200rpm, which will give you some low speed, probably about 100-110kt IAS but this won’t work at say FL100, let alone FL200.
I think Sebastian meant constant RPM, there is no instrument that give you %power on a SEP (or maybe on those MFDs that crunch live data against POH numbers, othetwise one has to think %power = %MP×%RPM×%speed but then RPM is linked to speed with some room for decoupling if you have a VP propeller)
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”)
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?
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:
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?
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.
