Turbine Pilot

Steps one and two of a smooth operation

There are things in life that appear to be more complicated than they truly are: riding a bike, tying a necktie, mixing a martini, and programming the VCR. Jet engines are sort of like that: General aviation pilots tend to believe that the engines on a jet are complicated. The reality is that the jet engine's fewer moving parts result in smoother, less complicated operation. Piston engines work hard; jet engines work smooth. But the same principles apply to the operation of both.

Last month we took an overall look at turbine engines and talked about the different types (see " Turbine Pilot: Suck, Squeeze, Bang, and Blow," August Pilot). Now we take a closer look at the sections and the workings of the engine.

The Pratt & Whitney JT15D is a typical high-bypass (meaning that the engine gets most of its thrust from the bypass air), twin-spool (the engine contains two concentric shafts — one connecting the low-pressure turbine with the low-pressure compressor and one connecting the high-pressure turbine to the high-pressure compressor), axial-flow (air flows through the engine along the fore-aft axis of the engine) turbofan engine, and a good example for discussion. It is found on the Cessna Citation 500-series aircraft and the Beechjet 400 (see diagram, p. 102).

Inlet and fan — suck

Although the engine inlet seems to be nothing more than a hole allowing air to enter, it's a little more complicated than that. The inlet can't cause any turbulence that would disrupt the airflow to the compressor, since the compressor section requires a uniform supply of air to maintain stall-free performance. Even a small interruption or turbulence in the airflow supplied to the compressor can cause significant losses in propulsive efficiency and many engine performance problems.

Although sometimes referred to as the low-pressure compressor, the fan stage of a turbofan engine is part of the inlet section. What you have here is your basic fixed-pitch propeller with a compression ratio of about 1.5:1. Most of the airflow — about 75 percent for this engine — passes over the outer portion of the fan blades, through the bypass duct, and back to the atmosphere. The majority of the engine's thrust — especially at low altitudes and low airspeeds — comes from this air. The remainder of the air passes over the inner portion of the fan blades, through a set of stators, and into the compressor stage of the engine. So far, for every pound of air that enters the inlet, 12 ounces go through the bypass duct and four ounces enter the primary airstream to the compressor.

Compressor — squeeze

Right behind the fan is the compressor section of the engine and this does exactly what it says it does — it compresses the air fed to it from the fan stage to supply the engine's combustion section with sufficient air — in terms of mass, not volume. This is turbocharging to the max. The exhaust of the engine's burner section runs the compressor, which compresses the air to get the most mass for a given volume.

Two main types of compressors are found in turbine engines — axial compressors, which accelerate the airflow along the longitudinal axis of the engine, and centrifugal compressors, which accelerate the air outward from the center of the engine. The engine in the illustration below is an axial-flow engine, but it has a centrifugal compressor. Notice that the main flow of air is straight through the engine along the longitudinal axis.

So, what are the advantages to each type of compressor and why use one over the other? Like most other engineering problems, this is a study in compromises. The centrifugal-flow compressor is the oldest and simplest of the two compressor de-signs. This design, which resembles a washing-machine agitator, receives its airflow at the center of the impeller and accelerates it outward by centrifugal reaction to its rotational speed. The air is then sent to the diffuser section, which reduces its speed and increases its pressure. The compressor manifold then delivers the turbulence-free air to the combustion chamber.

Although modern designs are capable of compression ratios of 10:1, centrifugal-flow compressors are not practical in more than two stages because of energy loss in the airflow when making the turns from one impeller to the next and the added weight penalty of multiple stages. Their use, therefore, is limited to the smaller engines found on small corporate jets, rotorcraft, turboprops, and used as auxiliary power units. Centrifugal compressors are commonly used in conjunction with axial-flow compressors and are positioned as the final compressor stage. All larger engines today are of the axial-flow compressor type.

Axial-flow compressors are so named because the direction of the mass airflow through the compressor is parallel to the rotational axis of the compressor. Axial-flow compressors resemble the fan stage of the engine. They are normally of several stages, with each stage comprising two components — the rotor and the stator. Although there are three main types of axial-flow compressors — the single spool, dual- or twin spool, and triple spool — the twin-spool design is by far the most popular in current production engines. An additional variation of the twin-spool engine uses a reduction gearbox to convert turbine speeds into torque to turn the fan. The Garrett TFE-731 is this type of engine.

Among the many advantages to the use of axial compressors are the following: a smaller frontal area and consequent lower drag than centrifugal compressors, higher pressures and higher peak efficiencies attainable by addition of compressor stages, and high compressor pressure ratios created by straight-through design.

These advantages are somewhat offset by disadvantages such as good compression only in the cruise-to takeoff power settings, many stages needed because of low compression rise per stage, difficulty and high cost of manufacture, high weight because of multiple stages needed, and high starting power requirements to accelerate the large mass of compressor.

Compressor stall

Most pilots have heard talk of compressor stalls, but what in the world is a compressor stall and how does it happen? It happens for the very same reason that wings stall — the critical angle of attack has been exceeded. The angle of attack of a compressor blade is the result of the combination of inlet velocity and compressor speed. When the critical angle of attack is exceeded, airflow through the compressor slows down, stagnates, or even reverses direction. The stall condition can range from the transient and hardly noticeable to the attention-getting "hung stall," which manifests itself by loud backfire-type noises and severe fluctuation in cockpit gauges for engine rpm, exhaust gas temperature, and fuel flow.

A compressor stall can occur during acceleration for takeoff down a runway with a rapidly shifting and gusting wind of high velocity. An abrupt shift to a strong crosswind coupled with engine acceleration can cause a stalled condition in the compressor. Abrupt pitch-up and power application for a balked landing or missed approach can also provoke a stalled compressor(s). Practice maneuvers such as recoveries from approaches to stalls can lead to abrupt pitching maneuvers and compressor stalls. Any foreign object damage increases the likelihood of compressor stall conditions.

Next in line in the engine are the bang and blow — the combustion section and the turbine and exhaust. Stay tuned.

Linda Pendleton, AOPA 525616, is an art director and author for King Schools. She has accumulated more than 10,000 hours in her 27 years of flying and has given more than 4,000 hours of jet instruction.