The fan draws air into the engine, compressing the bypass stream to produce 80 per cent of the engine’s thrust, and feeding air to the gas turbine core. The hollow, titanium wide-chord fan blade, pioneered by Rolls-Royce and introduced into airline service in the 1980s, set new standards in aerodynamic efficiency and resistance to foreign object damage. Since that time we have continued to innovate and improve on our design of wide-chord fan blades. Designed specifically for high-bypass turbofans, the breadth of these blades sets them apart from the narrow and less efficient earlier equivalents.
The largest Rolls-Royce fan blade in service is the 116 inch diameter blade in the Trent 900 for the Airbus A380.
Fan efficiency is an increasingly important contributor to improvements in overall engine efficiencies. The biggest improvements in fan efficiency have resulted from the analytical techniques that have evolved over the last 30 years as a result of the revolution in computing power. Analytical models, using techniques such as computational fluid dynamics, now cover more detail, giving better efficiency gains and more predictable designs in reduced timescales.
The size and position of the fan can make it particularly prone to vibration, which may come from any one of many sources.
Vibration can be driven by numerous influences such as aircraft/ground vortices, distortion of inlet airflow resulting from flight manoeuvres or cross wind, speed and altitude effects, and thrust reverser operation. A number of tests are performed to confirm the fan’s satisfactory reaction to these.
This is the interaction between the fan’s dynamic response and aerodynamic behaviour and is caused by distortions in air flow from cross-wind or ground vortex effects, or sometimes by the natural forcing response of the blade assembly (known as ‘flutter’). During development, an engine also undergoes a series of tests to demonstrate its mechanical integrity.
Engines are tested extensively to confirm satisfactory behaviour under all conditions, as flutter can lead to unacceptable vibration, metal fatigue and, potentially, failure. Continuing research seeks to improve understanding in this area in order to improve cost, weight and performance while achieving higher reliability.
Tests to prove an engine’s capability to cope with the ingestion of birds of different sizes and quantities, are carried out on full engines or large rigs. Advances in finite element modelling allow these complex events to be modelled with greater confidence, as the fan design can be optimised for weight and efficiency, with reduced overall cost and timescales.
Aviation authorities require evidence that if a fan blade is released while the engine is running, it is contained within the engine structure. This is usually achieved by a major test on a full engine, in which the engine has to demonstrate its ability contain a failed fan blade and to run down safely after such an event.
Rolls-Royce has designed wide-chord fan blades to be hollow in order to save weight. This employs a unique manufacturing process developed by the Group.
First generation wide-chord fan blades, used on the RB211 and V2500 engine families, use a titanium honeycomb sandwich structure, where the honeycomb is diffusion bonded between two solid sheets. The next generation of fan blades, for the Trent range of engines, features an internal structure that is created during a process that diffusion bonds and super-plastically forms three sheets of titanium.
Hollow design allows significant weight savings to be made in the fan blade, especially at larger sizes, and a follow-on weight saving in the fan disc, structure and containment features.
The Rolls-Royce wide chord fan blade is perhaps the best example of the application of titanium, coupled with advanced processing techniques, to give a significant service advantage. Modern blades are manufactured from three sheets of titanium representing the two outer skins and the internal corrugated structure. An inhibitor is applied, to define the internal structure, and then the three pieces are bonded in a high temperature pressure vessel. The blade is twisted and the cavity inflated at very high temperature using an inert gas in a shaped die to yield its final aerofoil shape.
The total process results in bonds with properties equivalent to the parent material and an internal stiffening structure which bears its share of the centrifugal load. Compared to the original solid clapper design this gives a fan module which is 24 per cent lighter, an overall engine weight benefit of 7 per cent, with a significant increase in foreign object damage (FOD) resistance over competitor designs.
A wide variety of high-technology and innovative manufacturing techniques are applied to the fan system. Laser shock peening, used on the contact surfaces of fan blade roots that are subject to high levels of stress, imparts a compressive layer which increases fatigue resistance. Linear friction welding can be used to create high-integrity joins between fan blades and discs in the creation of a fan blisk (or bladed disc). A blisk has advantages in weight and performance, although they are currently confined to use on military fans or in core compressors. Filament winding is a means of manufacturing composite components rather than by conventional lay-up. Its advantages are speed, strength, reduced weight and cost.
New concepts and technology in fans are constantly being considered as the benefits can be very powerful. Examples of new fan concepts include fan blades made from composite material, where the move to ever larger and slower fans together with advances in manufacturing technology could provide significant benefits for some engine designs. Open rotor concepts can offer a step change in efficiency. The larger diameter unducted fans improve propulsive efficiency, resulting in a potential to reduce fuel burn by up to 30 per cent.