Fixed Shaft Turbines Engines

General

NOTE

The paragraphs below contain easy to read generalities and simplifications with regards to turbine engines for aircraft use. You can read about turbines in a bit more technical detail in the following AOPA article. or Skybrary PDF

Engines extract power from vibrating molecules. The hotter a molecule is, the more it vibrates. The more it vibrates, the harder it pushes surrounding molecules (pressure) . The more molecules that are vibrating together, the more force they exert as a group. In turbine engines, these hot molecules push on turbine blades continuously causing them to turn and produce torque on a shaft, which we can then use to drive propellers. The faster the engine spins, the more molecules they have to heat. Bigger engines can handle more molecules and thus make more power than smaller engines.

From the above, we note that an engine can make more power with more heat and more air molecules. When we advance the throttle in an engine, we add more fuel and thus add more heat, creating more power. When the engine is at sea level, we have more molecules available (higher density air) and can make more power there. As the engine is moved into higher elevations in the atmosphere, the number of air molecules available is reduced, thus reducing power. This can be compensated for by adding more fuel to heat and vibrate the molecules even more so each molecule "pushes harder" on the turbine blades and spins them faster.

At some operating point, after adding more fuel with the molecules being hotter and pushing harder on the turbine blades, they can either break the turbine parts, or melt them. It is these two limiting factors, along with the engine size that govern how much power it can make. This power level is called the Thermodynamic Power Rating of the engine, the absolute most power it can make at sea level under standard atmospheric conditions without melting parts from too much heat (Thermo) or breaking parts from too much (dynamic) energy. This power rating results from the engineered design of the engine.

Many times you will see a turbine engine also having a Certified Power Rating that is slightly less than its Thermodynamic rating. This power rating typically stems from its conformance to rules of certification by authorities such as the FAA, which are primarily levied for safety and reliability reasons. An engine operating at its maximum thermodynamic limits will have a limited life, so the certified power rating is one that gives reasonable confidence that the engine will perform at that level safely and reliably for an acceptable period of time.

Aircraft designs are usually optimized for a mission and for aircraft using turbo-props, that mission is commonly high-altitude cruise. If a designer calculates that his airframe structure can handle 700hp while in cruise at say, 30,000 feet, and he selects a turbo-prop design that can produce 700hp at that altitude, then due to the reasons we just discussed above, that same engine would produce a lot more power on the ground with cooler temperatures and denser air (more molecules). This situation gives rise to yet another power rating concept.

We call the Engine Power the aircraft can safely handle the Flat-Rated Power, and this power rating is selected by the aircraft designer and not the engine manufacturer. The means by which the flat-rated power level is determined is extremely involved and includes a large amount of engineering analysis by the designer. Whether or not a turbine engine can be controlled to exceed its flat-rated power and achieve its thermodynamic power is dependent upon the design of the engine control system, which is different for differing aircraft. In some simple designs, the pilot has to use less than full throttle and keep an eye on engine temperature and torque values to avoid breaking or burning up the engine and in other designs, a FADEC computer may keep the engine near the flat-rating power level automatically.

Controls

PUT IN "COMMON" markdown file under powerplants? * auto-start sequence.

Single Spool (Fixed)

Standard > Engine Specs > Locations Tab

Standard > Engine Specs > Engines Tab

Options:

Two Ways to Configure Turbine Engine Power in X-Plane

Depending on how the engine power is stated on its spec sheet, there are two ways to enter the data into X-Plane to achieve the proper turbine power behavior.

Fixed Turbine Engine Start

Simulating a Hot Start ??