Concept and Objectives:
In the past, aerodynamic loads on an aircraft were primarily determined by using empirical data, analogies, and wind tunnel experiments. Nowadays it is required to design a secure aircraft structure as lightweight as possible in order to come up with environmentally friendly vehicles. This necessitates the reduction of safety factors as best as possible, which can only be achieved by precise prediction of aerodynamic loads over the entire flight envelope, including fringes and areas beyond, since load limiting cases can no longer be foreseen. In addition, more detailed information for aerodynamic loads is requested, i.e. not only global loads but also local load distributions (i.e. pressure distributions) are to be delivered for better optimization of single components as well as the overall aircraft. The project will address the extremes of the flight envelope, which feature complex flows that are characterised by non-linear/unsteady aerodynamic phenomena.
The ALEF (Aerodynamic Loads Estimation at Extremes of the Flight Envelope) objective is to enable the European aeronautical industry to create complete aerodynamic models of their aircraft based on numerical simulation approaches within the respective development processes. The project aims at precise prediction of aircraft loads for entire flight envelope.
ALEF Results in brief:
Modelling plane stability in extreme flight conditions
Scientists advanced the state of aircraft modelling under extreme conditions. Coupling flow and structural models while enhancing two existing techniques enabled capturing effects that lead to unstable behaviour.
Aircraft design relies heavily on mathematical modelling and simulation. These in turn rely on accurate system definition. The flight envelope consists of the range of combinations of flight parameters such as speed, altitude and angle of attack in which the aircraft remains aerodynamically stable. The EU-funded ‘Aerodynamic loads estimation at extremes of the flight envelope’ (ALEF) project extended current models to accurately describe behaviour at the boundaries of the flight envelope.
The Reynolds-averaged Navier–Stokes (RANS) equations of fluid-flow motion are important to the modelling of air flow. Using complex computational fluid dynamics (CFD)/RANS methods, ALEF partners were able to successfully simulate steady and unsteady conditions and aircraft behaviour under extreme conditions such as transonic speed, dive speed and high load scenarios requiring deployment of control surfaces. The latter control altitude, speed and angle when moved.
In addition, by coupling advanced fluid flow models with structural models, the team made it possible to include static and aeroelastic effects. ALEF demonstrated the ability of unsteady CFD models to shift from linear to high-fidelity methods for enhanced accuracy in prediction of aeroelastic effects.
A major contribution was made regarding surrogate models, also called response surface models. Scientists extended the proper orthogonal decomposition (POD) technique to account for deployment of control surfaces and structural deformation. The resulting models were demonstrated to be a fast alternative to CFD. Researchers also highlighted the potential of another unsteady surrogate modelling tool, linear frequency domain solvers, to capture a dangerous flutter phenomenon sometimes seen with increasing air flow.
Application to industrial test cases demonstrated the superiority of ALEF CFD and surrogate modelling methods compared to the current state of the art. Thus, ALEF made a significant contribution to aerodynamic modelling for aircraft design in the extreme conditions of the flight envelope. Better risk analysis and safer planes will be the likely outcomes together with an enhanced competitive edge for the European aerospace industry.