Short introduction into Turboprops


What is it?

Basically a turboprop or turboshaft engine is nothing more than a turbojet engine. Only where a jet engine gets it's propulsion from it's exhaust thrust, the turboshaft gets it's power via a shaft connected to the turbine. Connect a prop to that shaft and we have a jet propelling a prop. Naturally, the turbine shaft turns WAY faster than what we would want a prop to turn, as a prop turning supersonic makes the prop tips inefficient and would soon rip itself apart. Both types thus use reduction gearboxes to convert the high speed low torque power of the turbine shaft to the low speed high torque power needed on the propeller shaft.

What types are there?

Basically there are 2 types: free turbine turboshafts and geared turboshafts.

A free-turbine turboshaft is a form of turboshaft or turboprop gas turbine engine where the power is extracted from the exhaust stream of a gas turbine by a separate turbine, downstream of the gas turbine and is not connected to the gas turbine. This is opposed to the power being extracted from the power spool via a gear box. The advantage of the free turbine is that the two turbines can operate at different speeds, and that these speeds can vary relative to each other. Since the turbine will spin much faster than the prop (a supersonic moving prop is very inefficient), a reduction gear is normally built in between the prop shaft and the turbine shaft. A much used free turbine turboprop is is the P&W PT6 engine.

PT6 Cutaway. Note that, for negating the need of a long propshaft, most of these engines are mounted "backward" in the aircraft.

The other type is the geared turboshaft engine.

In this version, the propeller is coupled directly to the turbine through a reduction gear that converts the high RPM, low torque output of the gas turbine to low RPM, high torque on the prop(shaft). A much used geared turboshaft engine is the Garrett/Honeywell TPE331 series.

Comparing both.

Both engines deliver a way better power to weight when compared to internal combustion engines, with less moving parts and thus a very high TBO.

Both turbines spin between 20000 to 40000 rpm, way too fast for a prop to be efficient (or even survive the forces). Normally the props would turn to 1200rpm.

On a free running turbine, the engine can run but the prop motionless, allowing the engine to provide power to the generators without turning props, making an additional APU unnecessary. And since the starting mechanism only has to start the relatively small gas turbine, not much power is needed, meaning it can start on battery power alone, thus also not needing a GPU for startup (although using one would prolong battery life).

It also means it can start with feathered blades on the prop. Since the variable pitch mechanism on the props is maintained via engine oil pressure, shutting down an engine and thus cutting pressure pushes the blades into feather mode. This is also as a precaution for engine failures, as a fine pitched prop turning in the airstream acts as a very effective airbrake, thus reducing the chances of a succesful emergency landing. Because the prop shaft is connected to a free running turbine, the engine is already running and thus oil pressure already present to unfeather the blades, thus not causing excessive strain on the engine when starting. With direct geared engines, this poses an issue, the blades first needing to be unfeathered before starting, otherwise the strain on putting power on the shaft onto which feathered props are turning will kill the shaft and thus the engine. As an extra precaution, most of these engines have start locks, preventing the blades to come out of full fine pitch after start until the pilot "unlocks" the props.

Free turbine design allows the compressor to regulate its own rpm, and permits operation of the propeller at lower rpm in climb and cruise, reducing vibration, noise, and fuel flow(less power required to run the gearbox at lower rpm). Direct drive engines can only be pulled back about 4% in flight, while most free turbine engines can reduce prop rpm 10-15%. Also, engine core temperature is inversely related to power turbine rpm, which makes it easier to inadvertently over-temp a direct drive, since that rpm can be controlled by the pilot via the condition lever.

Direct drive engines do have the advantages of simpler design, and no spool up time during rapid power applications, but do need extra attention during shutdown. In a direct drive the prop and gearbox provide additional drag on the engine, and during shutdown slow the core to a stop earlier than it would be in a free turbine. This prevents the hot section from expelling as much heat as a free turbine engine, and can lead to shaft bow which can seize the engine, and is clearly undesirable. This is why you'll often see pilots of direct drive turboprops outside spinning the propellers by hand after they've arrived, it's to push the hot stagnant air out of the engine and rotate the engine shaft some to prevent a bow from developing.

Free turbine are also much more forgiving by their very nature. You can go from max rated power to flight idle and not hurt the engine or prop. Where as on a direct drive you may do damage to the gear box. Free turbines are typically more efficient at low altitudes than direct drives. But direct drives perform better at high altitudes, and become more efficient than free turbines. Naturally, on the down side of that the free spinning turbine has a lag compared to sudden thrust variations from the gas turbine, which the direct driven turbine does not have.

Note that the gas turbine itself also has a spooling lag of it's self, just like any other jet turbine.

A direct drive TPE-331 burns less than a similar size PT-6 in cruise flight, which makes them a little more efficient, but decreases engine life considerably.

Finally, another problem with early direct drives is the ability, at higher speed, of the prop to drive the engine at low power settings. This causes a negative torque condition on the shaft, the negative torque sensing system will assume the engine has failed and try to feather the prop. That only lasts until the prop pitch increases to the point that the engine starts to drive the prop again, and it unfeathers and goes back to negative torque, which starts the whole thing over again. It makes for a really fun ride, until the shaft gets too much strain from the prop driving the engine instead of the engine driving the prop, making the engine fail. To prevent this, throttle idle is normally set to about 20% power instead of 0% to prevent this from happening. This naturally has the consequence that at throttles idle there is still 20% forward thrust, making descents, approaches and landings something to think extra about compared to other engined aircraft.

What does this mean for me in the sim?

Well, noting that MS made a bit of a mess of the TP replication in FSX (and continued in P3D afaik), it comes down to the model.

The TPE331 is VERY(!) well done in the PMDG J41, in which the burning of an engine is easily done by overstressing the engine (too low RPM on high power, starting with unfeathered prop) or approach speeds are too high (throttle idle = throttle 22% --> first slow down, then go down!).

The same engine is moderately modelled in the Aerosoft OV10 Bronco, but poorly in the Carenado DO228, with only the start locks simulated but not the power vs condition lever settings.

The free turbine ones seem a bit better to program into a sim because of the lesser limitations. Good replications are found in the Aerosoft Twin Otter (enhanced version) and Lionheart Kodiak, and excellent ones in the Majestic Dash 8 q400. Less but adequate ones are found in for instance the Carenado C208 (pax and cargomaster).