This is why nobody is seriously worried about setting up a turbine powered airplane to be super efficient. They are capable of high altitudes, fast speeds, and reasonable although not miserly fuel consumption. Fly fast, fly high, and have fun. All of this other stuff is just a fun exercise for the brain.

Those equations are based on the assumption that the engine efficiency doesn't change with power setting or altitude. The engine has been idealized.

This is particularly untrue for a single shaft turboprop like the TPE331 which runs at constant speed regardless of power.

You will learn.

Mike C.

Agreed on the first.
I haven't seen anything to suggest the second is correct.

We're still mixing up KTAS with KIAS. It doesn't go further at same KIAS.

Craig, what is your KIAS and fuel burn at FL250? Check your NMPG (no wind).

Next time you fly, level at that KIAS and power back and lean out. Check your NMPG.

Report back..

According to Cessna's chart, constant % power doesn't yield constant indicated airspeed. (I'm totally confused now.)

Other than shooting an approach a week and landing with an engine shut off for proficiency, I'm not going to be using the A* until April. I'm kind of taken with my turboprop...

Mike,

Sent you a PM with FTP credentials to upload the files.

Good example. Now figure the KIAS at FL250, fly that speed KIAS at 1,000MSL. After the power reduction required to maintain that KIAS at 1,000', what happens to range at 1,000MSL?

We are overcomplicating this. Lets say the max speed a PBaron at full power can maintain at FL230 is 145kias. What power setting is required to maintain that speed at 1,000'MSL. Yes the trip at 1,000' will take a whole lot longer, but it will be at a significantly reduced fuel flow.

The red line is constant 180 KIAS.

Plane is MU-2M at 9,000 lbs, ISA temps, as published in POM (-6 engine).

Held KIAS constant, plane specific range improves. Plane goes further on same fuel.

This is typical of turboprops, slightly more so for TPE331 than for PT6.

Mike C.

Mike, I was talking about pistons in the Carson/421/Aerostar example. It doesn't hold true for TP's as you correctly point out.

The reason that this is confusing for people is that there are three discussions going on.

One discussion is about airframe efficiency. I think it has been correctly stated that the airframes are most efficient at max L/D. The faster you go above this number, the more energy you are going to have to expend per NM to stay aloft. Correct me if I am wrong, but Carson accepts that L/D max is most efficient but seeks to find a reasonable speed that allows for a faster and somewhat less efficient cruise without going too far into the region of diminishing returns.

The second discussion is about engine efficiency. I think you will find that turboprop engines are most efficient at their max ITT. It's where the engine manufacturers publish their SFC for a reason. In practice, engines are set up to reach their max ITT at some altitude, temperature, and HP output close to where the engineers expect the airframe to be operated. That's why turbines seem to be most efficient at altitude.

The third discussion is about the efficiency of the whole package. Yes turboprop airplanes are most efficient at high altitude but that is not because the airframe need to be high to gain efficiency, it is because the engines are set up that way. If they were set up for efficiency down low they wouldn't be capable of even climbing to higher altitudes. There is also the added bonus that the airframes are closer to their max L/D up high than they are down low since IAS drops with altitude.

I would wager that the reason range increases at constant power as altitude increases is that:
1. The engine efficiency stays the same
2. The aircraft is slowing down in terms of IAS and is flying closer to the ideal L/D max.
3. If the graph were continued far enough to get the IAS below L/D max, then you would see a reversal of the trend and range would begin to decrease

Likewise, if you drew a graph for that airframe that showed range at a specific (static) altitude and varied IAS, with an engine capable of developing the same HP per pound of fuel across its range, you should see range increase until L/D max is reached, then begin to decrease as IAS increases.

The reason that this is confusing for people is that there are three discussions going on.

One discussion is about airframe efficiency. I think it has been correctly stated that the airframes are most efficient at max L/D. The faster you go above this number, the more energy you are going to have to expend per NM to stay aloft. Correct me if I am wrong, but Carson accepts that L/D max is most efficient but seeks to find a reasonable speed that allows for a faster and somewhat less efficient cruise without going too far into the region of diminishing returns.

The second discussion is about engine efficiency. I think you will find that turboprop engines are most efficient at their max ITT. It's where the engine manufacturers publish their SFC for a reason. In practice, engines are set up to reach their max ITT at some altitude, temperature, and HP output close to where the engineers expect the airframe to be operated. That's why turbines seem to be most efficient at altitude.

The third discussion is about the efficiency of the whole package. Yes turboprop airplanes are most efficient at high altitude but that is not because the airframe need to be high to gain efficiency, it is because the engines are set up that way. If they were set up for efficiency down low they wouldn't be capable of even climbing to higher altitudes. There is also the added bonus that the airframes are closer to their max L/D up high than they are down low since IAS drops with altitude.