Frazer Nash reduces design risk

Citation

(1998), "Frazer Nash reduces design risk", Aircraft Engineering and Aerospace Technology, Vol. 70 No. 2. https://doi.org/10.1108/aeat.1998.12770baf.002

Frazer Nash reduces design risk

Software solutions for aerospace

Frazer Nash reduces design risk

"The ability to drive the assembly and visualize its motion was absolutely essential. If we had not been able to fully exercise the model in the software, we could have produced a system that looked very good, but that had performance problems we'd have discovered only when we had already made various parts and built the whole system. I-DEAS software saved us having to repeat the whole design and manufacturing process" (Peter Entwistle, Principal Consultant, Frazer-Nash Consultancy Limited).

Situation

The aerodynamics of modern aircraft are very complex, yet the facilities currently available for testing aircraft models in wind tunnels normally generate only one or two directions of movement, i.e. up and down and/or side to side. This can limit the effectiveness of the tests. The Defence Research Agency (DRA) in Bedford, UK, which provides a comprehensive range of wind tunnel facilities, identified the need for a flexible facility to carry out more realistic and demanding tests. The new design would be similar to the Stewart Platform used for flight simulators and would enable larger movements, within six degrees of freedom, the maximum number of directions in which objects can move: surge, heave, sway, roll, pitch and yaw.

DRA recognised that the development of this facility presented a number of design challenges. Since these were beyond the scope of its own expertise and resources, it contracted with Frazer-Nash Consultancy Ltd (FNC), a UK-based mechanical engineering consultancy, to develop the initial concept and produce a working design. To configure a mechanism of this complexity within the constraints of the existing wind tunnel FNC used the power and flexibility of a 3D modelling and analysis tool.

Objectives

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  Create a novel test rig for the existing wind tunnel that would provide static positioning and dynamic motion of an aircraft model in six degrees of freedom.

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  Maximize the range of motions and the operating speed of the rig that would be mounted on an existing turntable in the floor of the wind tunnel.

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  Design the rig so that it could easily be removed from the wind tunnel when not required.

Process vision

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  Identify whether the proposed geometry would result in the predicted motion by creating a dynamic 3D assembly of the test rig mechanism.

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  Check the integrity of the motion and identify potential collisions with the existing wind tunnel structure by driving the assembly through realistic manoeuvres.

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  Visualize the developing design and demonstrate design issues to the client.

Actions

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  The FNC project team used I-DEAS Master Series software to develop a 3D assembly of the test rig positioned in the wind tunnel. The assembly was created so that the model plane could be manoeuvred by dictating the length of the six legs (actuators) supporting the platform on which it was mounted. This simulated the action of the real control system.

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  Using a simple software program, similar to the one that DRA would use to operate the finished test rig, the FNC engineers calculated the actuator motions necessary to produce typical model plane trajectories. These motions were then used to drive the actuators in the software assembly, producing dynamic visualizations of the test rig in operation.

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  The initial simulations of the proposed geometry showed that the motion of the model plane did not result in the anticipated trajectory. Derailed investigations revealed the existence of previously undocumented singular positions in the Stewart Platform geometry. This was a critical discovery since a Stewart Platform is unstable at a singular position and so the motion becomes unpredictable. To maintain control of the mechanism, it is important to avoid singular positions.

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  Based on the simulations, the team usedI-DEAS software to optimize the size and position of the actuators and their mountings to maximize the motions that could be produced, while fitting within the existing structure of the wind tunnel and avoiding singular positions.

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  Following a design review with DRA, the test rig configuration was approved and FNC produced detail design drawings of the mechanism components.

Results

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  DRA acquired a unique international test facility that will provide realistic flight trajectories within a wind tunnel.

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  Because the FNC engineers were confident that the assembled test rig would respond to the control system inputs and generate the required range of motions, there was no need to produce a physical prototype.

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  A previously undocumented family of singular positions in Stewart Platform geometry was identified. In the context of the project, this discovery was important because without it the FNC team would have designed a test rig that did not work. In a wider context, there is improved knowledge of this class of mechanism which is increasingly being used for devices such as flight simulators and machine tools.

Plans

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  The test rig is currently being procured. Actuators are being made and tested to ensure that they meet the performance criteria for this demanding application.

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  The I-DEAS assembly is being used to make design enhancements during the development process and will be available if DRA wishes to illustrate the motions of a proposed test before an expensive and time-consuming physical test programme takes place.

Details from Frazer-Nash Consultancy Ltd. Tel: +44 (0) 1306 885050; Fax: +44 (0) 1306 886464.