Boeing CH-47D Chinook, the Powerplant.

The Engine

The Lycoming T55 Engine. This is actually the T55 L-11 Engine and is often utilized to represent the L-712, since they share very similar characteristics. Click-N-Go Here to view a larger image.

The little engine that could - and did...

A Lycoming T55-L712 installed as a number one engine.

The internal parts of the L712.

(1) Beginning on the left, air enters the engine past the oil reservoir and (2) flows past the accessory gearbox (AGB), where it then enters seven axial stages and one centrifugal stage of compression (3); the air then exits through a diffuser section into the combustor section (6) where it reverses direction, is mixed with jet fuel, ignited, and again reverses direction to flow across the two N1 (gas producer) turbine wheels (4), and then across the two N2 (power) turbine wheels (5). Finally, the very hot exhaust exits the rear of the engine and is expelled overboard.

L712 External Annular Combustion Chamber.

The combustion chamber for the L712 engine is a reverse flow, external annular, atomizing type located external to the turbine section. The combustion chamber consists of an annular flame tube. One end of the flame tube is closed by an annular plate which contains 28 swirl cups. Atomized fuel is introduced through 28 dual orifice fuel injectors located in the center of each swirl cup. Louvers in the swirl cups create the mixing vortex into which the fuel is sprayed. The air admission pattern in the primary zone is such that a high degree of fuel-air mixing occurs with this zone. In the dilutent zone of the liner, the hot combustion gases emanating from the primary zone are cooled with P4 air so that the combustion chamber exit profile is compatible with the turbine inlet temperature requirements. Dilution air is introduced through a hole pattern in the forward end of the inner liner assembly. The basic method of protecting the walls of the combustion chamber from the extreme temperatures of combustion (3500 degrees F) is through the use of thin films of air insulating the inner surfaces of the flame exposed areas. These protective layers are introduced through steps parallel to and in the same direction as the flow of burning gases. Approximately 30 percent of the available air is employed for cooling. This cooling film also protects the metal from carbon, fuel residue deposits, and oxidation. The 180 degree turn (left side of drawing above) of gases is accomplished in the deflector assembly passage, which is designed to produce a constantly accelerating flow due to its convergent shape. Hey, it works - trust me...

Fuel flow from the flow divider to nozzle.

An internal view of the front of the Lycoming engine.

Engine Compressor Stall or Surge

A combination of high pressure altitude, high ambient temperature, and dirty or damaged compressors tend to reduce the compressor efficiency - resulting in reduced airflow within the compressor. If these conditions become sufficiently severe, engine surge and/or stall occurs when the airflow over the compressor blades exceeds the critical angle of attack. A sharp rumble or series of loud reports emanating from the engine normally characterizes the onset of compressor surge or stall. These unusual noises are accompanied by abnormal engine vibrations, rapid fluctuations in PTIT, Torque, and N1 for the affected engine, and a noticeable loss of power and RRPM. Avoid maneuvers requiring rapid or maximum power if an engine compressor surge or stall is encountered. Possible aircrew actions include a reduction in airspeed, a reduction thrust below the point of surge or stall, and descent to a lower altitude. Aircrews must avoid conditions of repeated surging\stalling as the attendant transient torsional loads from the engine can cause damage to the engine, drive train, and associated airframe components.