Fan (turbofan)
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The low pressure compressor in a turbofan engine is often called a fan. Turbofans are a type of airplane engine that have evolved from the axial-flow turbojet engine, essentially by increasing the relative size of the Low Pressure (LP) Compressor to the point where some (or in many cases, most) of the air exiting the unit actually bypasses the core (or gas generator). This bypass air either expands through a separate propelling nozzle, or is mixed with the hot gases leaving the Low Pressure (LP) Turbine, before expanding through a Mixed Stream Propelling Nozzle.
Typically, these fan units absorb thousands of horsepower, the power being provided by the expansion of hot combustion gases through the low pressure turbine.
- Front fan
The fan is normally located at the front of the engine, directly behind the air intake system. A shaft connects the fan to the low pressure (LP) turbine, which drives the unit.
- Aft fan
Several turbofans feature an aft fan, where the fan rotor blades are mounted radially outwards of the (LP) turbine rotor blades. This dispenses with the need for an (LP) shaft. In an early example, General Electric bolted a fan/turbine unit to the rear of a J79 turbojet, to convert it into the CJ805 turbofan.
The unusual General Electric CF700 turbofan engine was also developed as an aft-fan engine with a 2.0 bypass ratio. This was derived from the T-38 Talon and the Learjet General Electric J85/CJ610 turbojet (2,850 lbf or 12,650 N) to power the larger Rockwell Sabre 75/80 version of the Sabreliner aircraft, as well as the Dassault Falcon 20 with about a 50% increase in thrust (4,200 lbf or 18,700 N). The CF700 was the first small turbofan in the world to be certificated by the Federal Aviation Administration. There are now over 400 CF700 aircraft in operation around the world, with an experience base of over 10 million service hours. The CF700 turbofan engine was also used to train Moon-bound astronauts in the Apollo Project as the powerplant for the Lunar Landing Research Vehicle.
The GE36 UDF Demonstrator used a similar arrangement to convert an F404 mixed exhaust turbofan into a propfan.
- Supersonic fan
Early gas turbine fans rotated at subsonic tip speeds, to avoid the generation of shock waves in the airflow. Modern fans, however, often rotate at supersonic tip speeds, and exploit the shock waves. Some advanced designs can generate a pressure ratio of more than 2.2:1 in a single stage, although 1.8:1 is more typical.
- Supersonic through-flow fan
Although supersonic fans rotate at a supersonic tip speed, the axial flow is subsonic. However, some experimental devices have demonstrated supersonic axial flow. All the speed lines on the resulting fan map (or characteristic) are virtually horizontal, unlike those of more conventional units.
- Variable pitch fan
Several ultra-high bypass ratio turbofan demonstrator engines (e.g. Rolls-Royce/SNECMA M45SD-02) have incorporated variable pitch fans, much like the variable pitch propellers on a turboprop engine. Varying the pitch of the rotor blades improves the low flight speed handling of the low pressure ratio fan unit, without the need to resort to a variable area cold or mixed flow nozzle. Reverse thrust down to zero aircraft speed is also practical.
- Variable geometry fan
Some multi-stage, high pressure ratio, fans on military turbofan engines incorporate variable geometry, (e.g. F404). The variability is usually confined to the inlet guide vanes. Although the leading edge of the vane is static, a piano-type hinge allows the trailing edge to be adjusted in pitch, to redirect the airflow onto the first rotor. VIGV's enhance the surge margin of the fan in the mid-flow region.
- Propfan
Some ultra-high bypass ratio turbofans dispense with the fan nacelle and have an unducted fan rotor. The fan blades, which resemble scimitars, are especially shaped to work efficiently at flight speeds up to about Mach 0.75. General Electric demonstrated a propfan engine, called the GE36 UDF, in the 1980s.
- Overhung fan
Turbojets and early turbofans used the inlet guide vanes to support the front bearing of the (LP) compressor/fan rotor assembly. Today, the fans used in turbofan engines are often to an overhung design, where the fan rotor is cantilevered out forward, beyond the front bearing. This facilitates the removal of the inlet guide vanes. Consequently, the fan rotor blades are the first aerofoils encountered by the engine airflow.
- Snubbered fan
Prior to the introduction of wide chord fan blades, fan blades fitted to turbofan engines often featured snubbers. These are protuberances that stick-out at right angles to the fan aerofoil, somewhere between mid-span and blade tip. The snubbers on adjacent fan blades butt-up against each other, in a peripheral sense, and improve the vibration characteristics of the blade.
Wire lacing (e.g. as in early Rolls-Royce Pegasus engines) is an alternative approach.
- Wide chord fan
As might be expected, snubbers reduce the aerodynamic efficiency of fan aerofoils. Rolls-Royce pioneered a more efficient alternative: wide chord fan blades. The increased blade chord (i.e. width) is used to enhance the vibration characteristics.
Wide chord first went into service in the RB211-535E4 for the Boeing 757 in 1984 and have been a feature of the RB211/Trent/V2500 engine family ever since. Potential weight increases are usually offset by making the blades hollow. Other engine manufacturers have now introduced wide chord fans.
- Swept fan
Engine manufacturers are beginning to introduce so-called swept fan blades, which should yield benefits in aerodynamic efficiency and noise.
