UHBR engines

Emerging new aircraft engine solutions, such as Ultra High-Bypass Ratio turbofan engines and Open Rotor, offer the potential for significant reduction in fuel burn, noise and emissions. These technologies also allow for advances in propulsion efficiency, by increasing the bypass ratio of the air drawn-in through the fan disk and passing through the nacelle, without more energy being needed. This higher air volume provides more efficient thrust for a given fuel burn, thus allowing the fuel needed for a given thrust to be reduced.

However, this propulsion efficiency gain does not automatically lead to an improvement in fuel burn, due to several opposing factors:

  • To increase the bypass ratio, the fan diameter has to be increased, leading to bigger nacelle dimensions and an increase in nacelle drag, due to a larger frontal surface area.
  • Aerodynamic losses in the fan duct result in higher performance penalties for UHBR engines, due to the very low fan pressure ratio.
  • Fan operability issues may require some additional devices to ensure effective engine operation.

These factors make it indispensable to come up with set of major technological enablers for UHBR engines, such as operability control devices and a shorter and slimmer nacelle.

A slimmer nacelle requires to move the equipment installed within the engine and IPPS away from the fan compartment in the hotter core zone, where temperatures are up to 150°C higher than in the fan zone. At the same time, the backward installation of the equipment potentially impacts the access time for maintenance activities.

Another challenge arises from the volume available for the equipment between the nacelle and the engine, which is reduced by up to 25% for UBHR engines, compared to engines of current architecture.

The additional equipment to be included for the engine operation could add to the temperature in the core compartment, thus challenging the equipment operation even further.

Based on these challenges, today’s existing solutions need to be adapted as so

  • take up less space
  • operate within higher temperatures
  • provide enhanced reliability, whilst not degrading maintainability.

IPPS installation

Today, the installation of engine systems in the aircraft powerplant is very complex, bearing potential difficulties for routine maintenance activities and/or the replacement of equipment items. The movement of the equipment away from the fan compartment, as required by the UHBR technology, will further impact the access issue. It is, hence, crucial to provide installation solutions which do not reduce the accessibility to key equipment.

In a typical IPPS equipment package, there are some 40 different sets of equipment, and around half of them are considered key equipment and account for 80% of the weight. 90% of this key equipment are classed as Line Replaceable Units (LRU) and have to be replaceable on the field.

Apart from this complexity, the current generation of aircraft engines has a federated architecture with separate standalone sub-systems fitting each of the required functionality items. These sub-systems are developed and optimised by a variety of suppliers, following the specifications issued by the engine manufacturers, air framers and nacelle suppliers.

This approach inevitably results in a range of different sub-system solutions, which all try to address the same challenges. Hence, the development of new engine system solutions whilst continuing to use the current federated approach to engine architecture will make systems installation even more complex and potentially have a negative impact on development time, maintenance time and costs.

Without addressing these challenges, the future UHBR engines will be unable to host the equipment within the necessary aerodynamic lines and without reducing the propulsive performance. This, in turn, means extended development time and costs of the equipment solution and reduced potential benefits of the UHBR technology, due to additional drag from the nacelle.

Finally, the hot temperature conditions require appropriate cooling solutions, leading to heavier and more robust equipment. The overall result being a higher fuel consumption, more difficult access for maintenance activities, as well as a heavy cut in the overall potential strongpoints of UHBR engines.

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