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Aerodynamic Load Estimation at Extremes of the Flight Envelope

Tags: Air

State of the Art - Background

Previously, aerodynamic aircraft data was primarily determined by using empirical data, analogies, and wind tunnel experiments. This data forms the basis for the structural dimensions of the vehicles and therefore influences their weight and fuel burning. More importantly, the layout of flight control systems and the design of control surfaces are also based on this aerodynamic data. In general, singular cases were used for limit load predictions. This approach yielded rather rough estimates of global loads over the entire flight envelope leading to serious safety issues. Finally, secure but heavy aircraft structures were designed.

Today, secure aircraft structures must be designed to be as lightweight as possible in order to come up with environmental friendly vehicles. This necessitates small or even zero margin safety risks, which in turn call for the precise prediction of aerodynamic data over the entire flight envelope, including fringe areas and beyond, as limiting cases can no longer be foreseen. In addition, more detailed information for aerodynamic data is needed for the better optimization of single components, as well as the overall aircraft. Competition in aeronautical industry also leads to quite significant reductions of design cycle times.

Over the last two decades "testing" has been increasingly complemented by tools for the numerical simulation of aerodynamics, steadily increasing their capabilities. These computational fluid dynamic (CFD) tools have now reached a sufficient level of maturity regarding the quality of their results for major parts of the 'inner area' of the flight envelope. This maturity is based on the experience gained by design simulations near the cruise point.

The challenge is now to introduce CFD as the major source for aerodynamic data prediction in the aircraft design process.

However, numerical simulation techniques for aerodynamic applications have known deficits, specifically at the extremes of the flight envelope. Complex flow phenomena, in conjunction with high configuration complexity, makes high-fidelity simulation a challenging task.

As well as this, providing aero data based on high-fidelity simulation for any flow condition and requested configuration in suitable time scales overshoots current computational resources by far: from an actual aircraft development point of view, aero data production needs to cover all combinations of deviations in configuration (cruise/high lift, control surface deflections) and/or flow condition (M, α, β) finally leading to a list of multi-dimensional requirements.


ALEF's objective is to enable the European aeronautical industry to create complete aerodynamic data sets of their aircraft based on certified numerical simulation approaches within the respective development processes. i.e. ALEF will kick-off a paradigm shift from greater confidence in experimentally-measured data to just as great confidence in computational results. Beyond the scope of ALEF this paradigm shift will essentially influence the overall aerodynamic development process.

The objective has three aspects:

- Comprehensiveness: the ability to predict aerodynamic forces, moments and their derivatives in time for any point of the flight regime;

- Quality: the accuracy of each flow simulation result used for prediction of aerodynamic data and to the coherence of aero data integrated over the complete flight envelope from tools of varying fidelity;

- Efficiency: the need to deliver aerodynamic data over the entire flight envelope for loads and handling qualities, as well as for performance within time frames dictated by multi-disciplinary industrial design processes at given costs and computational resources.

The certification of numerical simulation for aerodynamic data prediction is derived from the first two aspects of the objective. They ensure the trustworthiness and reliability of numerically predicted data over the entire flight envelope in industrial development frameworks.

Description of Work

The ALEF project is anchored in between two major work-packages which first (WP 1) define the scope of aerodynamic loads estimation, the procedures necessary (Task 1.1) and the quality of the aerodynamic loads estimation requirements (Task 1.2). The other bridgehead is the assessment (WP 4) which checks on the demonstration (Task 4.1) of test cases and requirements defined and provided in WP 1 (Subtask 1.1.1), subject to quality and efficiency goals. The lessons are learned (Task 4.2) from demonstrations together with experience, expertise, tools and processes provided by steady and unsteady aero loads simulations (WP 2 and WP 3). They will provide insight into the state-of-the-art aerodynamic tools and processes suitable for load estimations, together with their potential and capabilities to be applied for realistic industrial applications on complex aircraft configurations and their components. The outcome will be a set of tools and processes rated with regard to their capabilities, their efficiency and needs of future developments.

Expected Results

The ultimate scope of the use of simulation tools in aero data generation is to cover all flight conditions and configurations by means of a numerical toolbox. This would ensure an up-to-date and fast estimation of the most recent status of aircraft with consistent data. Unsteady behaviours and flexibility could be incorporated in the standard aero data prediction process. ALEF will essentially contribute to a 70% wind tunnel testing cost reduction by 2020, which will cut the aerodynamic development effort by about 40%.

ALEF’s impact on aero data production sequence (schematic)
ALEF’s impact on aero data production sequence (schematic)