Tag Archives: turbine engine

Turbine Engines Education Series – Operational Considerations

All turbine engines have common operational considerations including engine temperature limits, hot starts, foreign object damage, compressor stall and flameout.  These potential problems are important to understand, especially in the turbine engine overhaul industry.

Engine Temperature Limitations

Turbine-Inlet

Turbine inlet temperature is where the highest temperature occurs in any turbine engine.  This makes the turbine inlet temperature the limiting factor in turbine engine operation.

Hot and Hung Starts

A hot start is when the exhaust gas temperature (EGT) exceeds the safe limit in a turbine engine.  This occurs when too much fuel vs. air enters the combustion chamber or when there is insufficient turbine r.p.m.  A hung start is when an engine fails to accelerate to proper speed or does not accelerate to a sufficient idle r.p.m.  This may be caused by insufficient battery power or fuel system component malfunction.

Foreign Object Damage

Debris ingestion is common among turbine engines due to the design and function of the air inlet.  Foreign Object Damage (FOD) normally consists of small nicks and dents when small objects on runways or taxiways are drawn into the engine by the air inlet.  This debris typically causes nicks and dents, but usually do not cause major damage.  However, bird strikes or ice ingestion can cause enough damage to a turbine engine that the entire engine can be destroyed.  For this reason, prevention of FOD is a high priority.

Compressor Stalls

The simple definition of a compressor stall is an imbalance between inlet velocity and compressor rotational speed.  When the compressor blade angle of attack exceeds the critical angle of attack, smooth airflow is interrupted and turbulence is created.  Compressor stalls can be intermittent or steady.  Recovery from a compressor stall must be accomplished quickly by reducing power, decreasing the airplane’s angle of attack and increasing airspeed.  Very subtle throttle changes are the order of the day!

Flameout

When the fire in a turbine engine goes out unintentionally, it is known as a flameout.  When an overly rich mixture causes the fuel temperature to drop below the combustion temperature, known as a rich flameout, it is normally caused by very fast engine acceleration.  Sometimes, flameout occurs due to low fuel pressure and low engine speeds especially in wet, cold weather coupled with air turbulence.

Thrust Variations

Turbine-Engine-2115119

Engine thrust varies with air density.  Turbine engines are affected by high relative humidity and as air density decreases so does thrust.  Turbine engines can experience a loss of thrust in high relative humidity.

Operational considerations must be understood when flying or working on turbine engines.  Knowing why an engine may have a hot start or what to do during a compressor stall is necessary for determining what needs to happen during aircraft maintenance.  Covington Aircraft, a leader in turbine engine overhaul, invites you to learn more about operational considerations at www.covingtonaircraft.com.

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Turbine Engines Education Series – Types of Turbine Engines

Turbine engines are classified according to whether the compressor is centrifugal flow, axial flow or a combination of both centrifugal and axial.  The type of engine is further classified by the path the air takes through the engine and how power is produced.  There are four different types of turbine engines – turbojet, turboprop, turbofan and turboshaft.

Turbojet

trubojet

A turbojet engine was first developed in Germany and England prior to World War II and is the simplest of all jet engines.  The four sections of a turbojet engine are the compressor, combustion chamber, turbine section and exhaust.  The compressor passes air at a high rate of speed to the combustion chamber which contains the fuel inlet and igniter.  Expanding air drives the turbine and accelerated exhaust gases provide thrust.  These engines are limited on range and endurance and today are mostly used in military aviation.  They are known for being slow to respond to throttle applications at slow compressor speeds.

Turboprop

RRBRent

Between 1939 and 1942, a Hungarian designer, Gyorgy Jendrassik designed the first turboprop engine.  However, the design was not implemented into an actual aircraft until Rolls Royce converted a Derwint II into the RB50 Trent which flew on September 20, 1945 as the first turboprop jet engine.  A turboprop engine drives a propeller through a reduction gear, allowing optimum propeller performance to be achieved at much slower speeds than the operating RPM.  With their ability to perform well at slow airspeeds and fuel efficiency, turboprop engines are often used in small, commuter aircraft and agricultural applications due to their greater reliability offsetting their higher initial cost.  One of the most reliable turboprop engines is the Pratt & Whitney PT6A.

Turbofan

800px-F110-GE_Turbofan_Engine

Turbofan jet engines were designed to merge the best features of the turbojet and turboprop.  By diverting a secondary airflow around the combustion chamber, additional thrust was created.  Two separate streams of air pass through a turbofan engine.  One passes through the engine core while the second bypasses the core.  The Gloster E28/39 which flew for the first time on May 15, 1941 was one of the first times a turbofan engine was used for military or commercial aircraft.

Turboshaft

turboshaft2

The fourth type of jet engine is known as the turboshaft.  Most of the energy produced by the expanding gases drives a shaft connected to a turbine through a single stage of reduction gearing rather than producing jet thrust.  Turboshaft engines are predominantly used by helicopters.  The first turboshaft engine was built by the French firm, Turbomeca in 1949.

Turbine engines have several advantages over reciprocating engines, including less vibration, increased aircraft performance and reliability.  In addition, each type of turbine engine has its own advantages and disadvantages. For more information about the types of turbine engines, visit www.covingtonaircraft.com.

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