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Leading Edge Actuation Topology Design and Demonstration

Final Report Summary - LEATOP (Leading Edge Actuation Topology Design and Demonstration)

Executive Summary:

The Leading Edge Topology Optimised Design and Demonstrator (LeaTop) project is an EU Clean Sky project. The aim of the research is to design and demonstrate the construction of a seamless morphing leading edge device that is capable of withstanding full aerodynamics loads.

Seamless morphing leading edge devices show potential to reduce noise and drag of a wing, however, practically designing and implementing these devices as an end solution within the structural constraints of modern materials proves to be a challenge. The focus of this research is on the development of a mechanism capable of morphing the leading edge with respect to a set target airfoil shape. The project uses the in-house aeroelastic tools developed for topology optimisation to generate, design and build a demonstrator that is fit for testing the system under equivalent aerodynamic loads.

It is demonstrated in the project how a concept for the leading edge actuation system can be created. The input to the concept generation process was provided by the Green Regional Aircraft (GRA) partner Fraunhofer Gesellschaft in terms of geometry of the leading edge, loads and requirements. Based on this input, a topology was designed for the actuation system, together with skin stiffness requirements.

The result of the concept design was a layout of an actuation system and skin stiffness distribution which allowed the prescribed morphing deformations of the leading edge under a given aerodynamic load. The layout and skin were analysed using finite elements to validate their feasibility.

The parts of the kinematic system which will carry out the prescribed deformation of the morphing leading edge were milled from aluminium. The morphing skin was also made out of aluminium and was supplied by the GRA partner Fraunhofer Gesellschaft. They were also responsible for producing the spar, a complex geometry which was milled out of aluminium as well. All these parts were assembled and mounted into a test rig. This test rig was designed such that the weights, representing the aerodynamic loads, could be applied in the correct direction.

Three types of experiments were carried out; (i) strain measurements in the skin to get an estimate of the strain distribution when the leading edge is locked in its cruise shape and when the leading edge is morphing into landing configuration, (ii) displacement measurements of the actuation system to investigate whether the actuation system is truly locked when in cruise shape, and (iii) shape measurements of the morphing leading edge to compare to the intended morphed target shape.

The experimental results showed that the objectives of the LeaTop project were met, i.e. that the cruise shape could be preserved under cruise loads, and that a morphed target shape could be achieved.
Project Context and Objectives:
The Leading Edge Topology Optimised Design and Demonstrator (LeaTop) project is carried out for the Green Regional Aircraft (GRA) ITD. The objective of GRA is the development of a low-weight, low-noise, potentially all-electric aircraft. One of the technologies that contribute to meet the aforementioned targets is a morphing leading edge. Such a leading edge ensures a seamless airfoil at the leading edge, eliminating the traditional gap one gets when using a slat. The removal of this gap has two advantages. The first advantage is an improved chance to get laminar flow: a slat, even when retracted, will create a discontinuity in the airfoil causing a transition of the boundary layer from laminar to turbulent. Such a turbulent boundary layer causes more drag than a laminar boundary layer. The second advantage is when the slat is extended, i.e. in landing configuration. The gap between the slat and the wing is a source of noise during landing, hence removal of that gap contributes to the now-noise configuration.

Implementation of a morphing leading edge comes with important considerations such as elastic skin, load introduction, actuation system, lightning strike protection, anti-icing systems, etc. A few of those considerations have been addressed in the LeaTop project:
1. What type of skin can be used for a morphing leading edge
2. What are the reactions resulting from the aerodynamic loads on the wing box
3. Can the cruise shape of the morphing leading edge be preserved under cruise loads
Project Results:
The results of the LeaTop project will be described based on the objectives and research questions of the project. Those are given item by item below.

1. What type of skin can be used for a morphing leading edge
Morphing deformations are inherently associated with large deformations, which means that the leading edge will undergo substantial deformation when morphing into the landing configuration. The actuation system accommodating these large deformations will displace in a rigid body fashion, i.e. strain free. However, the skin will be strained when it has to accommodate the large deformations. This strain consists of a bending and axial strain component, and large strains in the skin means that the skin has to be flexible, but the skin also needs to possess sufficient stiffness to withstand the aerodynamic loads on the airfoil leading edge. Hence in order to solve this paradox, the strain in the skin should be as small as possible, meaning that the skin should only bend and not stretch. If the skin is sufficiently thin, the resulting strains from the bending will be small as well. Hence a thin metallic skin should be feasible for a morphing leading edge. Such a skin was applied to the demonstrator developed within the LeaTop project, and the feasibility of the skin was demonstrated experimentally.

2. What are the reactions resulting from the aerodynamic loads on the wing box
One of the concerns in the design of the skin and actuation system of a morphing leading edge is the connection of the skin to the front spar of the wing because potential stress concentrations at that point might cause skin damage. This was investigated in the LeaTop project, and the conclusion was that if the internal actuation system is properly designed, the aerodynamic loads are transferred to the actuation system almost entirely, which means that the applied forces are transferred to the wing front spar through the actuation system and not through the skin, avoiding the stress concentration problem.

3. Can the cruise shape of the morphing leading edge be preserved under cruise loads
The design of the actuation system is not only driven by the skin bending requirement and the force transfer requirement (see items 1 and 2 above) while morphing the leading edge, but there is also an important stiffness requirement. It is important that the actuation system, in locked condition, provides enough stiffness to the leading edge such that it does not deform under cruise loads. This is important since the cruise shape of the airfoil determines the aerodynamic performance of the aircraft wing. It was demonstrated in the LeaTop project that the cruise shape can be preserved indeed if this constraint is taken into account into the design process. The preservation of the cruise shape was measured by measuring the displacement of one point of the actuation system. This is possible because the actuation system is a statically determinate structure and hence if one point on the actuation system does not move, the entire system is locked and rigid.

The item list above were the formal objectives of the LeaTop project, however, the researchers in this project investigated one other important parameter of a morphing leading edge. Next to the preservation of the cruise shape, it was also investigated whether the experimental test setup could demonstrate that the intended target shape in landing configuration, i.e. with morphed leading edge, could be met. It was experimentally demonstrated that the intended landing configuration could be achieved under aerodynamic loads.
Potential Impact:
The LeaTop project contributes to the Green Regional Aircraft ITD. The main objective of this project is to validate and demonstrate any technology which fits ACARE Vision 2020. The development of seamless leading edge devices is an important contributor to this objective. A seamless leading edge high-lift device allows for drag reduction by removing the gaps nowadays present in conventional flap systems hence making laminar flow possible. Moreover, when designed properly, it can become less complex compared to the current implementation of high-lift devices. Such a system simplification has a direct effect on the wing mass, which can be reduced substantially. A weight reduction that leads to a reduction of fuel and the accompanied pollution. Finally a seamless leading edge will reduce the noise footprint of the aircraft as small gaps in surfaces exposed to an external air flow tend to induce additional noise.

The design of these seamless morphing high-lift devices is a tedious task. The aeroelastic design software in the LeaTop project is developed with the ability to design the actuation topology in close interaction with the skin. It is believed that this enables the design of a more coherent, better performing leading edge high-lift device. Experimental validation of this design software has been done in the LeaTop project. As such, the results from the performed calculations can be used by the GRA project members to help linearise the forced response of the leading edge. The design tool can also be used for future projects on morphing high-lift devices, and eventually can be applied by the aircraft manufacturers.

The results from the LeaTop project have been disseminated to a scientific and industrial audience via conference publications and theses. Two conference papers have been presented about the project at the International Conference of Adaptive Structures and Technologies, and a third one has been submitted and accepted to be presented in October 2013. Furthermore one MSc thesis has been written about the project and one PhD thesis addressed the project in a few chapters. The PhD thesis is accessible to the general audience and was defended in a public setting.

List of Websites:
Roeland De Breuker, r.debreuker@tudelft.nl +31152785627, Kluyverweg 1, 2629HS Delft, the Netherlands