Drawing: Cobra Drawing: Cobra

T-53-L-703 Turbine Engine

The engine used on the AH-1F helicopter is the T53-L-703. It is a free power turbine. What is a free power turbine engine? It is an engine that is made up of two rotating assemblies that are mechanically free of each other. What are the advantages of the free power turbine engines? It eliminates the use of heavy clutches and prevents rpm control problems that would exist if the engine engaged the transmission directly. The two rotating assemblies are known as the gas producer, sometimes referred to as the n1 or first stage, and the power turbine n2 or second stage. The symbol "n" refers to rotation (rpm) of the two independent stages. Actually the n1 and n2 components rotate in opposite directions and at different rpm.

What is the purpose of the gas producer? It produces all the gases and fuel for the engine operation. The power producer is driven by the hot gases as does the n1 assembly, but it supplies the power to drive the transmission, main and tail rotor.

In the designation, the T is for turbine, 53 for the military number, L for Lycoming (manufacture) and 703 stands for the modification series. The 703 is capable of producing 1485 shp (shaft horsepower). However, it has been limited to 1290 shp at 100% torque because of power train limitations.

Drawing: Engine left side

The power turbine (n2) tachometer generator operates engine scale on the dual tachometer.

Linear Actuator
The linear actuator located on the left side of engine is connected between the droop compensator and the overspeed governor. It is powered by a 28 VDC motor which either extends or retracts the actuator to control the n2 speed. Controlled by the increase / decrease switch on the collective head or on the gunner's miscellaneous panel.

Fuel Filter
The fuel filter is located on the left hand side of the engine deck. Its purpose is to filter fuel prior to entering the fuel control. A caution light on the instrument panel informs the pilot when the filter becomes partially obstructed.

The fuel drain control is used by the pilot on preflight inspection to drain a few ounces of fuel in a clear jar. The pilot can then check to see for any contamination or water.

Fire Detection System
Provides visual indication of a fire in the engine compartment via the fire detector light on the pilot's console. Sensing elements are attached to the tail rotor driveshaft tunnel, fire wall and heatshield.

Drawing: Engine right side

Combustion Section
The combustor section is constructed of welded titanium and steel and consists of an outer housing and an inner liner which make up the reverse annular (ring-shaped) flow combustion chamber. The outer housing is bolted to the diffuser section with 72 steel (heat resistant) bolts. It also houses the combustor drain valve and has provisions for attaching various fuel and oil lines and electrical wiring. The liner is of stamped and welded construction and has 22 swirl cups into which the fuel nozzels extend. Fuel is mixed with the compressed air at different points in the combustion chamber and burned. The flame continually develops the hot gases which escape out across the n1 and n2 turbines, turning them at a high rpm. The combustion liner contains louvers and drilled passageways which control the incoming air for mixing with the fuel, cooling and flame control. About two-thirds of the compressed air is used in flame control and cooling and the remaining one-third for mixing with fuel for combustion.

Engine Mounts
The engine is rigidly mounted to the fuselage aft of the cabin on the work deck platform. A free power turbine engine can be rigidly mounted because it lacks power stroke vibrations and friction torque normally found in reciprocating engines. The engine mount consist of a tripod attached to the left side of the diffuser section, a bipod attached to the right side of the diffuser, and a support tube attached to the left side of the air inlet section.

Interstage Air Bleed System (bleed band)
The interstage air bleed system is used to open and close air bleed holes located at the aft end of the axial compressor housing thereby improving compressor acceleration characteristics. The band is automatically open and closed by the actuator on a differential of air pressure signal or a set rpm. The band is normally open below a set rpm and during acceleration to prevent a back pressure buildup across the compressor rotor blades. The back pressure, if allowed to develop, would decrease airflow velocity across the airfoil shaped blades and cause them to stall, resulting in engine surges.

Starter Generator
The starter generator provides the turning force to start the engine and provides 28 VDC power source.

Variable Inlet Guide Vane
The air inlet guide vanes are mounted between the inner and outer housings in the rear of the air inlet section. They function to provide a surge (compressor stall) margin in the engine. Engine surge is recognized as a loud popping and is caused by a stalling condition across the airfoil surfaces of the compressor rotor blades or stator vanes. To prevent this and to improve engine efficiency, the guide vanes control the angle of incidence of the incoming air on the first stage of the axial compressor blades as the compressor rpm varies. By this action the air's volume and velocity is controlled which prevents a stalling condition to develop across the airfoil surfaces. At low N1 speeds, the vanes are in the closed position. Opening of the vanes depends on ambient temperature and N1 speed. The guide vanes are automatically opened and closed by fuel pressure from the fuel control unit.

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Updated: 12 January 2008
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Born on 13 October 1998