Propeller feathering in a fixed shaft constant-speed turboprop engine is normally accomplished with the condition lever. The propeller is not on the same shaft as the basic engine turbine and compressor. Unlike the fixed shaft engine, in the split shaft engine the propeller can be feathered in flight or on the ground with the basic engine still running. A typical free power-turbine engine has two independent counter-rotating turbines.
One turbine drives the compressor, while the other drives the propeller through a reduction gearbox. Powerplant engine and propeller operation is achieved by three sets of controls for each engine: the power lever, propeller lever, and condition lever. The power lever serves to control engine power in the range from idle through takeoff power.
Forward or aft motion of the power lever increases or decreases gas generator RPM N1 and thereby increases or decreases engine power.
The propeller lever is operated conventionally and controls the constant-speed propellers through the primary governor. The propeller RPM range is normally from to The condition lever controls the flow of fuel to the engine. Like the mixture lever in a piston-powered airplane, the condition lever is located at the far right of the power quadrant. Leaning is not required in turbine engines; this function is performed automatically by a dedicated fuel control unit.
The ITT indicator gives an instantaneous reading of engine gas temperature between the compressor turbine and the power turbines. Because in the free turbine engine, the propeller is not attached physically to the shaft of the gas turbine engine, two tachometers are justified—one for the propeller and one for the gas generator. The propeller tachometer is read directly in revolutions per minute.
The N1 or gas generator is read in percent of RPM. The ITT indicator and torquemeter are used to set takeoff power. Climb and cruise power are established with the torquemeter and propeller tachometer while observing ITT limits. Gas generator N1 operation is monitored by the gas generator tachometer.
Proper observation and interpretation of these instruments provide an indication of engine performance and condition. The thrust that a propeller provides is a function of the angle of attack at which the air strikes the blades, and the speed at which this occurs. The angle of attack varies with the pitch angle of the propeller. Forward pitch produces forward thrust—higher pitch angles being required at higher airplane speeds.
The feathered position produces no forward thrust. The propeller is generally placed in feather only in case of in-flight engine failure to minimize drag and prevent the air from using the propeller as a turbine. In reverse pitch, air is pushed away from the airplane rather than being drawn over it. Reverse pitch results in braking action, rather than forward thrust of the airplane.
It is used for backing away from obstacles when taxiing, controlling taxi speed, or to aid in bringing the airplane to a stop during the landing roll. Reverse pitch does not mean reverse rotation of the engine. Turbine engines are extremely heat sensitive. They cannot tolerate an overtemperature condition for more than a very few seconds without serious damage being done. Engine temperatures get hotter during starting than at any other time. Thus, turbine engines have minimum rotational speeds for introducing fuel into the combustion chambers during startup.
Hypervigilant temperature and acceleration monitoring on the part of the pilot remain crucial until the engine is running at a stable speed. Successful engine starting depends on assuring the correct minimum battery voltage before initiating start, or employing a ground power unit GPU of adequate output. Engine temperatures may approach the maximum in a matter of 2 or 3 seconds before the engine stabilizes and temperatures fall into the normal operating range.
During this time, the pilot must watch for any tendency of the temperatures to exceed limitations and be prepared to cut off fuel to the engine. An engine tendency to exceed maximum starting temperature limits is termed a hot start. The temperature rise may be preceded by unusually high initial fuel flow, which may be the first indication the pilot has that the engine start is not proceeding normally. Serious engine damage will occur if the hot start is allowed to continue.
A condition where the engine is accelerating more slowly than normal is termed a hung start or false start. This is usually the result of low battery power or the starter not turning the engine fast enough for it to start properly. In modern turboprop engines, compressor surge is a rare event.
A surge from a turboprop engine is the result of instability of the engine's operating cycle. The operating cycle of the turbine engine consists of intake, compression, combustion, and exhaust, which occur simultaneously in different places in the engine.
The part of the cycle susceptible to instability is the compression phase. In a turbine engine, compression is accomplished aerodynamically as the air passes through the stages of the compressor. How does a turboprop engine work? There are two main parts to a turboprop propulsion system, the core engine and the propeller. The core is very similar to a basic turbojet except that instead of expanding all the hot exhaust through the nozzle to produce thrust, most of the energy of the exhaust is used to turn the turbine.
There may be an additional turbine stage present, as shown in green on the diagram, which is connected to a drive shaft. The drive shaft, also shown in green, is connected to a gear box. Turboprops are a hybrid of jet engines and the more traditional piston engine propeller that you see on smaller, lightweight airplanes. Turboprops are reliable options and were designed to fill the gap between high speed, high altitude jets and low flying light airplanes.
This does not mean turboprops are slow or fly low compared to jets, however. Both turboprops and jets are powered by turbine engines, so they are essentially the same thing and thus, are considered to be equally as safe.
The main difference is that turboprops have a propeller on the outside of the engine while jets have fan blades inside the engine housing. One area where turboprops might get the nod is on smaller runways.
Because of the drag propellers cause, they actually allow the aircraft to stop much more quickly than a jet. In this balanced field scenario, turboprops get the nod. A more legitimate safety question is the safety factors between turboprops and jets vs pistons. Turboprops and jets are considered safer, and especially those with twin engines. If safety is a concern of yours and you are able to have a say in the aircraft you charter, the redundancy of a turbine engine is about as safe as you can get.
On a flight from Palm Beach to Hilton Head, the Conquest had a guest on board who happened to own a popular light jet. A turboprop engine uses the same principles as a turbojet to produce energy, that is, it incorporates a compressor, combustor and turbine within the gas generator of the engine.
The primary difference between the turboprop and the turbojet is that additional turbines, a power shaft and a reduction gearbox have been incorporated into the design to drive the propeller.
The gearbox may be driven by the same turbines and shaft that drive the engine compressor, mechanically linking the propeller and the engine, or the turbines may be separate with the power turbine driving a concentric, mechanically isolated shaft to power the gearbox.
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