September 12, 2016
So people say that turbine engines, compared to recips, put out lot of power, but use more fuel.
That’s sort of like saying that the Titanic did not complete its’ journey.
That’s technically true, but not really beginning to cover the, um, tip of the iceberg.
I’m flying a PT-6a-42 engine these days on my Evolution, and have learned that fuel flow is simply voracious: A turbine simply does an incredible job of turning huge amounts of fuel and air into huge amounts of power, and with the PT-6 especially, the pilot is always operating a fascinating balance between fuel and air to keep the whole thing going. And further fascinating me was the discovery that the specific fuel consumption of the PT-6 (the fuel burn per horsepower delivered by the engine) was actually BETTER than the reciprocating Continental TSIO-550 from my previous airplane in some cases! So why was I going through fuel at such an absurd rate???
First, what is it like to operate the PT-6?
Well here is how it works:
Engine-off, you hit the starter to spin up the compressor. That gets air flowing through the engine.
That air then flows through a windmill that is geared down through a transmission to the propeller.
So, as you hit the starter and the compressor spins up a flow of high-pressure air through the engine, the propeller begins to turn slowly as the air races through the windmill inside the engine that is geared to the propeller.
What is interesting here is that there is literally no connection between the compressor and the prop!
The only transmission is… AIR!
It is AIR that compressed by the compressor and expands out through the windmill that is hooked to the prop, turning it.
Anyhoo, once the compressor is spun up to at least 13% rpm by the starter, you introduce fuel. POOF! Now you have fuel burning, and that causes the air in the compressor system to REALLY want to expand, which it does on its’ way through the little high-speed windmill inside the engine on its’ way to the exhaust stacks, and the prop really starts screaming about that time.
The turbine inlet temperature inside the engine goes to almost 2,000 degrees F within 30 seconds of start.
You are burning about 26 gallons per hour here, at idle.
NOW, here is the interesting thing: What happens if you add fuel BEFORE you spin up the compressor to to 13% rpm?
Well, you just melt you engine is all.
Here is the answer: The fuel combustion in a PT-6 is so hot that it would MELT the internal engine components if they were not protected by an insulating layer of.. AIR! There has to be enough AIR moving through the engine that the AIR acts as an insulating wall to keep the temperature of the flame away from the walls of the engine! For this reason, you MUST keep the engine spinning at a minimum compressor RPM to keep it from over-heating and destroying itself! (exactly 13% to start fuel input, about 55% once it is fully running and the fuel flow stabilized).
As all this is happening, you hear and feel the compressor spinning up, the propeller spinning up AFTER that, and see the engine temperatures approaching 2,000 degrees F… it really is quite something to see!
Now, once the engine is running, at least in a Lancair Evolution, which is a very light-weight, un-insulated carbon fiber shell with a turbine bolted to it, you hear the usual turbine scream right in front of you, combined with the sound and feel of a huge rush of air through the engine. So you taxi out and the engine IDLES so fast (to keep that wall of air in place that insulates the engine!) that the airplane wants to taxi at 40 or 50 knots. You have to keep putting the propeller into reverse mode (called ‘beta’) to keep the airplane from over-speeding on the taxi!! Once on the runway, as you advance the throttle, the temperature SKYROCKETS due to throttle advance! Why? Well it’s because we are dumping in more FUEL, but have not yet built up enough compressor SPEED to build up enough of a CUSHION OF AIR to insulate against the bigger FLAME! So the temperature shoots up as you add throttle, and then proceeds to DROP as the engine speeds up and the air insulation wall build up! You see, when the engine is turning SLOWLY, that is what it is by far at its’ most vulnerable: There is just barely enough air to insulate the engine from its’ flame, and very little dynamic pressure moving through the turbine, so the turbine is really slow to spin up! You have a heavy metal turbine versus AIR, of COURSE it spins up very slowly due changing air dynamics inside the engine as you add power.
So the engine is very sluggish and hot and finicky at idle: It’s already running hot due to minimum insulation, and so additional fuel will spell TROUBLE. But you HAVE to add fuel to speed the engine up! So that’s why you are in a tenuous situation at idle and must advance the throttle very slowly and carefully: Only add fuel as fast as the engine can spin up and add air!
NOW, once that compressor spins up to speed, the amount of air moving through the engine becomes so extreme that the temperatures DROP, the compressor reacts QUICKLY to throttle changes thanks to all that airflow, and the engine is finally doing what it is designed to do: Putting out lots and lots of power. But you have to spin it up to max RPM for that to WORK.
In flight, at really LOW power settings, the engine is rather quiet. You are just loping along gently and smoothly. All seems nice. Until you look at the fuel flow. WHUUUUUUUUUUT!?!?!? The fuel flow is almost the same as it is at HIGH power! But we are at LOW power! How did that happen? You then advance the throttle to see what happens at high power, and the fuel flow only comes up a little bit… but the turbine changes from a soft purr to a SCREAM! You are pushed back in the seat, a screaming racing sound of fuel and air radiates from the front of the airplane, and off you go at 300 knots. All of it is totally smooth, there is no vibration at all. You are just racing through prodigious amounts of fuel and air to create huge amount of power that the prop dumps right back into the airstream. You can really feel that you are using fuel to be an air-pumping maniac. Now, this engine puts out 850 horsepower. If you look at the propeller blades, they are only each 3 feet long and several inches wide. Four of them, so they each absorb over 210 horsepower. How can a blade that is only 3 feet long and several inches wide absorb the power of 210 horses?!?! The answer lies in the equation for power: FORCE TIMES SPEED. Power is the MULTIPLE of force and speed. The prop is absorbing some pretty good force, and doing so at a pretty terrific speed. MULTIPLY those two together and you get the total package: 210 horses per little wooden blade. (Yep! Wood cores on those MT-Prop blades!)
So, what about the efficiency? Well, the specific fuel consumption, which is pounds of fuel per hour per horsepower, runs about 0.50 or so for the PT-6 in my airplane, running at full power. This compares to 0.84 or so in the Continental TSIO-550 in my old Columbia-400. YAY! The PT-6 is MORE efficient! YAY!
Oh wait: Except not. And here is why: The Continental, running at full power, risks cylinder-head overheat. In a desperate bid to mitigate this, it cools the engine by dumping in EXTRA, UN-BURNED, FUEL. ARGH!
The Continental cools itself with FUEL, the PT-6 cools itself with AIR!
At full power, the PT-6 has a HUGE advantage!
BUT, we almost NEVER run at full power!
The efficiency of the Continental is so terrible at full power due to it’s cooling with fuel that we almost never run IT at full power, and the power output of the PT-6 is so crazy that we almost never run IT at full power… the torque would flip the airplane upside down a low speeds, and run into an aerodynamic brick wall at high speeds! In both cases, the engines SIMPLY NEVER SEE FULL POWER!
Instead, we operate at CRUISE power.
So what happens there?
Two things: The Continental no longer needs fuel to cool, so its fuel burn PLUMMETS to just 0.46 gallons per hour per horsepower… TWICE the efficiency of full-power operation! In lean-of-peak cruise, the Continental is in the zone at huge efficiency, and tiny fuel burn. The Continental is doing GREAT there.
With the PT-6? A different story entirely. Due to the lower speeds of the compressor and windmill, the efficiency of the engine goes to ONE THIRD of what you get at full power. That’s right: The PT-6 at lower powers burns THREE TIMES the fuel per horsepower as it does at high altitude.
Here are the numbers for fuel efficiency, lower is better since this is fuel burned per horsepower:
low cruise 0.46 1.50
medium cruise 0.46 1.00
full power 0.84 0.50
See what’s happening?
The Continental can run with an incredible 0.46 fuel burn at most power settings. The PT-6 cannot match it, and has to run at FULL POWER to get even CLOSE. And when that PT-6 is at full power, the airplane is going so darn fast that it is running into an aerodynamic brick wall and you lose efficiency due to the AIRFRAME being dragged through the air so fast.
So you operate most of your flight at medium cruise, and the REAL efficiency of the two engines becomes clear:
Recip: Fuel burn 0.46
Turbine: Fuel burn 1.00
Over twice the fuel per horsepower.
AND, at idle, which DOES take up the beginning and end of every flight, the Continental idles at about 2 gallons per hour, the PT-6 idles at 26 gallons per hour.. WELL OVER TEN TIMES MORE.
So, the turbine engine is smooth, reliable, always starts, is simply amazing to operate with the incredible sound and feel and performance, but since we do NOT operate it at full power, the fuel consumption is simply huge.
OK, and now to see an engine-start and fly-by, click here!