Peter wrote:
whereas pistons, especially non-turbo ones, have a fairly constant fuel flow per mile versus altitude
A turbine (like the Jetprop) has a fairly constant fuel flow in lbs or gph no matter what altitude you fly at, the difference being the higher you go the less dense the air so the TAS is faster …. only to be spoilt by some raging headwinds.
As Loco says you can leave the decent until the last minute and decend at 2000fpm with no worry of shock cooling the engine.
quatrelle wrote:
A turbine (like the Jetprop) has a fairly constant fuel flow in lbs or gph no matter what altitude you fly at
That’s definitely not true for the Allison / Rolls Royce M250. Maybe that’s because of its low thermodynamic rating, that is it loses power with altitude earlier than other turbines?
They lose power with altitude. As the air thins, the internal temperature raises and that’s what sets the limit up high (low it’s torque, normally).
To give you an example – on takeoff, my power levers are only about halfway to the stops. As I climb, the ambient temp drops and if I don’t advance the levers, the temps of the engines will fall lower and lower and aircraft will eventually stop producing power. So, to keep same power, I need to inch levers forward as I climb. Eventually, they will reach either the stops, and/or temps will be at the limit set by the manufacturer and the plane will stop climbing. This is what’s called “temping out”. In my particular plane that’s 545 degrees C. I tend to reach the temp limit and the stops of my power levers around 18000ft. Doesn’t mean she doesn’t still climb, but you’re trading speed for climb rate here – you have no more fuel to introduce (as there is not enough air to be able to combust it without causing an overtemp situation that weakens the metal inside the engine).
@AdamFrish, once you reach the temp limit, doesn’t the hourly fuel flow reduce with further climb?
As I understand it, a turboprop engine becomes more efficient with altitude (more power per pound of fuel), in addition to the effect of lower density on the airspeed (higher TAS for the same power). That is until it reaches the altitude where it becomes temperature limited.
A turbocharged piston engine’s efficiency does not change much with altitude, so a turboprop gets a double penalty for flying low.
An example – at 850 hp output and in ISA conditions, the TBM850 (all figures from the POH)
above FL240, the engine output becomes temperature limited and at FL300 it only delivers just above 700 hp of the full 850, but the TAS is still 320kt
So until the turbine becomes temperature limited (or to use Adam’s words, while he can still advance the throttles in the climb) the turbine becomes more efficient. I believe that is caused by the increasing compression ratio of the power turbine.
Once it is temperature limited, the compression ratio remains constant or might even have to be reduced,
So in conclusion – the TP aircraft really “wants” to be flown at least at the altitude where it becomes temperature limited, there are huge benefits to climb to that level. Above that, the trade-off becomes more “normal”, i.e., more like what turbo piston pilots are used to.
The source data for the curious.
Note – the engine is rated 850hp max continuous output, but limited to 700hp for take-off, approach and landing. So 100% torque is 700hp, and the cruise figures are for 121.4% torque (850p).
I do not know if the max torque in the upper levels is actually achievable, there are three limits: the inter-turbine temperature (ITT), the power turbine RPM (Ng), and the torque. The latter is designed to keep ITT and Ng below the red line with a bit of margin, but that may not always work in practice.
ITT is the limit at higher levels. Whether 100% TRQ is achievable would depend on ISA deviation. Also if the inertial separator is ON or OFF. It bumps the ITT significantly.
I checked the manual for my naturally aspirated Extra. It’s only 10kts difference in TAS for same fuel flow between 2 and 10k feet. Only a theoretical argument can be made for late descents. Still, my understanding is that for max range one should burn as much fuel as possible where it is used most efficiently, not lower.
In practice I typically get cleared for descent at beginning of 3 degree slope in the TBM and wouldn’t argue with controller that I want to descend later, because it would be a mess if everyone did that. For N/A one would want to avoid rapid cooling, so again not practical to delay descent.
If I take examples from the PA46-350P, for a 500 nm trip at cruise speed at FL250…
Climb takes 35’, 90 nm, 20 USG ; cruise is at 203 kts TAS, 22.5 USG/h, and we consider three types of descent:
- LOP Vno descent : 800 fpm,165 kts IAS, 15 USG/h. It takes 111nm, 31’ and 9.6 USG. It means cruise is over 299 nm, so 88’ and 33 USG. Total fuel consumption 60.6 USG
- Power off descent (glide) at 90 kts IAS takes 60 nm and 0 USG. It means cruise is over 350 nm, so 103’ and 39 USG. Total fuel consumption 59 USG
- Turbine style descent (means gear down) but within POH limits which require TIT>1350 and MAP above 20": 1500 fpm, 165 kts IAS, 11 USG/h. Takes 16’ over 60 nm and 3 USG. Cruise is 350 nm over 103’ and 39 USG. Total fuel consumption 62 USG.
denopa wrote:
- Power off descent (glide) at 90 kts IAS takes 60 nm and 0 USG
You shut off the engine?
It’s a theoretical exercise to see the limit case. Glide speed is by definition the most fuel efficient speed.
