the magazine PROFILE

ISSUE 144 25 the magazine

When it comes to the future of the jet engine, Dr Paysan has the big picture. And usually in 3D.

A modelengineerDR GERALD PAYSAN ASSOCIATE FELLOW, WHOLE ENGINE MODELLING

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aysan is Global Chief of Structural Systems Design and an Associate Fellow for Whole Engine Modelling.

“When we design an engine we often think about the aerodynamic design first and how to design the individual elements to go with it. For example how many turbine stages do we need? But it is equally important to think about how we design the whole engine structurally and how the individual elements interact.”

Whole engine modelling looks at the behaviour of the product during the aircraft’s operation allowing the design of a robust, quiet and efficient engine. Complex mathematical and computer models are used to define how engine components must work together to meet customers’ requirements. This informs detailed design work at component and sub system level.

At the heart of the model is a finite element analysis of the whole product.

GeometryFinite element analysis is a way of predicting the mechanical behaviour of a complex structure, such as an engine, by splitting it into small parts of known geometry, such as quadrilaterals, triangles, hexaeders or tetraeders. The mechanical equations which describe the behaviour of these individual parts and the interfaces with their neighbours can be combined to create a system of equations that describes the engine as a whole. The number of equations and calculations needed depends on how detailed the model needs to be. For a very high fidelity model millions of calculations are performed.

Banks of high performing computers are needed for these types of analyses. Models can run for several days or even weeks. “Which is why,” according to Paysan “it’s important to have models of different complexity or ‘fidelity’ so you have the right methods at the right time; quick methods for the early stages of development and really detailed models, which represent complex behaviour, later for certification and in-service support. Beyond this, a key element of our improvement strategy is to use the knowledge gained from these very detailed models to inform our faster design methods, so we can maximise the value.”

Whole engine modelling supports the lifecycle of the product.

“A critical source of competitive advantage, particular in the corporate jet market, is cabin

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Finite element analysis is a way of predicting

structure, such as an engine, by splitting it into

needed for these types of analyses. Models can run for several days or even weeks. “Which is

have models of different complexity or ‘fidelity’ so you have the right methods at the right time;

Career• Dr Gerald Paysan graduated from TU Berlin in 1993 with a Diploma in Engineering. After that he spent six years researching on structural component design and lifing and on exploring a new multi-physics based approach to simulating fretting wear and fretting fatigue within joints, leading to his PhD.

• His first industrial job, in 1999, was with Daimler Chrysler Rail Systems.

• In 2001, he joined the whole engine mechanics team in Rolls-Royce in Dahlewitz. He led both the whole engine mechanics and the intermediate casing design and make activities for the TP400 and then became

Chief of Whole Engine Thermo-Mechanical Design, firstly for Rolls-Royce in Dahlewitz and then globally.

• Since 2012 he has been Global Chief of Structural Systems Design where he is responsible for the development of Rolls-Royce capabilities in this area. He describes the best part of his job as: “Working with great people across the world, I work in a key area for the company offering great opportunity to make an impact. Involvement in the entire lifecycle of the product from cradle to grave and the close work with universities and research partners on the development of technology that matters.”

ISSUE 144 27 the magazine

comfort. Airframers are striving to achieve noise levels at room level. Engines are an integral part of the aircraft system and one of the sources which can lead to noise in the cabin. We therefore make great efforts to ensure smooth running engines. Vibration is very much a system problem well suited to whole engine modelling.”

Vibration is increased if the centre of gravity of any one of the rotating parts is slightly off centre. Paysan’s team put huge effort into reducing the imbalances of each individual component as well as defining specific ways of assembling these parts to reduce the remaining imbalances in the assembled engine. “We achieve balancing qualities which are far better than industrial standards – the remaining module imbalance on corporate jet engines is equal to a shift on centre of gravity of less than two microns, less than the diameter of a human hair.”

The daily operation of an aircraft places a variety of mechanical and thermal loads on an aircraft engine. For example, a large airliner making a hard landing can place forces of up to 5G on the engines. This causes components throughout the engine to bend and flex in response.

Designing to withstand this level of load will have a profound effect on the product as a whole in terms of weight, reliability, fuel efficiency and cost. “A whole engine model

allows the behaviour of the engine in these circumstances to be calculated and decisions as to the optimum design of individual components and sub systems to be arrived at.”

Whole engine modelling can also be used to help solve problems once an engine has entered service. The model can simulate the engine’s

situation at the time the problem occurred. This can help service engineers understand what happened and recommend improvements.

A major focus for Paysan’s team is to develop detailed whole engine models which accurately represent the physics of very unlikely extreme

events, such as the loss of a fan blade or

foreign object damage, which engines have to be designed to withstand. These are non-linear events which happen very fast. According to Paysan: “It is vital to have an understanding of the loads the engine is exposed to during the event and as it runs down.”

Looking to the future, Paysan sees “more engine design and validation by computer reducing the need for doing some of the large and expensive tests that we do today. This doesn’t mean we wouldn’t do any testing but doing more tests that help us develop and

validate our methods and then apply these to different products and product variants, reducing costs and the time taken to develop new products.”

More multi-physics based system level optimisation combining key analysis types such as mechanical, thermo-mechanical and

CFD, virtual reality techniques as well as a systematic approach to model validation is needed to make this a reality. It is all about finding the right balance between testing and analysis. “When you have differences between the test data and the model how do you know what to adjust?”

The answer is again to break the problem into smaller parts. Initially starting with an individual component such as an intermediate casing. For instance, to validate the mechanical behaviour the component is hit with a modal impact hammer to excite the structure and measure its resonance

behaviour. The behaviour can then be compared to the finite element model of

that component and the model adjusted to reflect the test data. Once this has been done at a component level a sub system can be assembled and the process repeated eventually reaching a whole engine level.

“This is a very systematic way to get a good representation of the dynamic behaviour of the engine which has been very well received by customers. This world-class approach allows us to compete globally.”

Author: Simon Kirby consults and lectures in marketing communications with a particular interest in technology. He has worked in communications roles extensively in both the public and private sector.

A key element of our improvement strategy is to use the knowledge gained from these very detailed models to inform our faster design methods.

less than two microns, less than the diameter of a human hair.”

represent the physics of very unlikely extreme

events, such as the loss of a fan blade or

foreign object damage, which engines have to

behaviour. The behaviour can then be compared to the finite element model of

that component and the model adjusted to reflect the test data. Once this has been done at a component level a sub system can be assembled and the process repeated eventually