TU Berlin, FMRA
TU Berlin, FMRA
The Department of Flight Mechanics, Flight Control and Aeroelasticity (FMRA) of TU Berlin is developing a flight demonstrator with high aspect ratio transport aircraft configuration, TU-Flex. The DLR Institute of Aeroelasticity supports the development by simulation and experimental activities. A full scale wing of the aircraft, built from fiber composite materials and designed for large deflections, was tested in the Crosswind Test Facility (SWG) Göttingen.
In addition to the wind tunnel tests, this article starts with an explanation of the theoretical background of a high aspect ratio wing and its features when used on modern transport aircraft.
The background to the project, which is being funded by the German Federal Aviation Research Programme (LuFo Klima), is the urgent need to reduce climate-damaging emissions from new aircraft, particularly for passenger transport. This reduction goes hand in hand with more efficient engines, lighter structures and, above all, significantly improved aerodynamic performance by lowering the overall drag.
These parameters go into Breguet‘s range equation, which can be used to estimate the maximum distance an aircraft can fly between takeoff and landing:
Wikipedia
A long range is equivalent to a highly efficient aircraft or reduced emissions. In this formula, V denotes the airspeed, e is the specific fuel consumption and g is the gravitational constant. The first term can therefore be increased by increasing the airspeed or decreasing the specific fuel consumption (more efficient engine with high bypass ratio). The term In (mstart / mend) denotes the (logarithmic) mass ratio between takeoff and landing (the difference is the fuel which was consumed during the flight), an increase means the empty mass of the aircraft must be kept as low as possible, a goal which is met by lightweight construction with appropriate materials (e.g. fiber composites). Finally, is the so-called glide ratio of the aircraft (Lift over Drag), which is a mere aerodynamic parameter and indicates how far an aircraft can glide from a given altitude. Its value depends on the overall aerodynamic drag of the vehicle, where the wings have the largest share. A typical value for the glide ratio of a modern jet transport aircraft is 18. The greatest increases in performance and range for transport aircraft with a classic design can be achieved almost exclusively by increasing the glide ratio. However, as the lift depends on the aircraft mass and is thus fixed, the aerodynamic drag must be reduced. The drag is made up of two main components, the frictional drag of the boundary layer, and the so-called induced drag. The first component can be reduced by enforcing a laminar flow over a large part of the wing (approximately from the leading edge to mid chord) - however, the technical realization of such a laminar wing is extremely challenging for jet transport aircraft. The second component, the induced drag, depends on the design of the wing. It is reduced if the wing has a high aspect ratio Λ, which is defined as:
Λ = b2 / F
In that equation b2 denotes the square of the wing span and F is the wing area. Consequently, increasing the aspect ratio requires a slender wing. Gliders, for example, have very high glide ratios (up to approximately 60), which is only feasible by means of their characteristic, slender wings. Older transport aircraft (e.g. Airbus A300) have values of just under 8, while modern long-haul aircraft (e.g. Airbus A350) were developed with wings with an aspect ratio of approximately 10.
The feasibility of aircraft with aspect ratios of over 15 is currently being investigated in various DLR projects as well as in cooperation with the European aircraft industry..
As highlighted in an earlyer article, the development of high aspect ratio wings comes with major technical difficulties. The problem is the significantly increased structural flexibility, which leads to much greater (bending) deflections of the wing even in cruise flight. If the stiffness of the wing is to be increased to reduce these deformations, the wing becomes heavier; looking back at the Breguet range equation introduced above it becomes evident that a tradeoff between permissible deformations of the wing in flight and mass increase is to be found, a process that requires sophisticated aeroelastic analyses in the preliminary design of the aircraft. In particular, the aeroelastic characteristics of high aspect ratio wings have been the subject of extensive research for several years.
Another issue of aircraft with high aspect ratio wings is their specific flight mechanical properties. In particular, the mutual interactions of the involved disciplines – aerodynamics, structural dynamics and flight mechanics – must be taken into account from the beginning of the aircraft design process. For example, applying the classical flight mechanics approach which uses linearized coefficients to assess the flight mechanical stability and controllability of the aircraft can lead to erroneous results because the structural flexibility of the wing shows a strong impact on these coefficients. Novel and more sophisticated methods for flight mechanics and aeroelasticity, which will be necessary for future generations of aircraft, must account for the involved disciplines and their interactions from the outset by non-linear relationships and equations (it can be safely assumed that future jet transport aircraft will be designed with high aspect ratio wings). This is one of the current research topics of the FMRA at the TU Berlin. The TU-Flex research UAV was designed as a flight demonstrator with modular construction that enables the application and testing of different sets of wings with individual stiffness properties. Its highly flexible wing leads to elastic structural frequencies which are already close to the natural frequencies of flight mechanic modes (especially the short period mode). This results in a pronounced coupling of flight mechanics and structural dynamics during flight. Such a behavior is not intended for a conventional transport aircraft, but it is deliberately taken into the design of the set of highly flexible wings of TU-Flex. Using modern flight control technologies, also developed at the FMRA at TU Berlin, a decoupling of the flight mechanic and elastic oscillations will be demonstrated in flight tests. This approach will finally help an aircraft with high aspect ratio wings to succeed.
Despite accurate analysis methods and numerical simulation tools, the results of the aircraft design must be critically verified - especially when it comes to highly flexible configurations, for which significantly less experience is on hand for the design by now. Typically, various components of the aircraft, primarily the wings, are tested in wind tunnel to minimize risks with the design. This procedure was also chosen for the wing of TU-Flex. Following the extensive development of the wing (structural design and optimization), it was built and equipped with complex sensor technology. Steady and unsteady wind tunnel tests at varying airspeeds and angles of attack were performed in the Seitenwindkanal Göttingen (SWG) of the DLR Institute of Aerodynamics and Flow Technology at the DLR site in Göttingen. Due to the dimensions of the wind tunnel the wing could be built in full size, i.e. no scaling was required (the wingspan is 1.8 meters), which means that the same airspeeds and flow parameters (density, pressure, temperature) can be set in the wind tunnel as they will prevail in the real flight testing of TU-Flex. The model was built according to the specifications of TU Berlin by Weberschock Development in negative molds using fiber composite materials. A high-strength foam core with a spar was used to effectively avoid buckling, glass and carbon fiber layers with particular orientation (determined in the wing design) were used as skins.
A range of sensors for the measurement of strains, accelerations and rotational rates was installed at selected positions along the span of the wing. Hence a large amount of measurement data is available for validation with numerical simulation models following the wind tunnel tests. Furthermore, the wing has three trailing edge flaps which are controlled by powerful and precise actuators.
Following the assembly of the wing, its structural properties were analyzed in detail. The goal is to identify deviations of the real model from the theoretical (finite element) model in terms of stiffness and mass properties. Simplifying assumptions were made in the structural design and optimization, the impacts of which can be quantified on the real model. The structural dynamic investigations - which were carried out in the wind tunnel at rest - consist of a static and a dynamic test, both are equally important. The stiffness properties of the model are identified by the static test. Therefore, the wing is loaded at selected locations along the span by weights and the resulting structural deformations are measured. A dynamic test (the modal analysis) was performed subsequently to determine the modes of vibration and associated frequencies. An impact hammer was used to excite the structure, the resulting accelerations were measured by accelerometers that were densely distributed on the surface of the wing.
Finally, steady and unsteady wind tunnel tests of the TU-Flex model were carried out in the SWG at the DLR Institute of Aerodynamics and Flow Technology in Göttingen. The SWG is the perfect choice for this test: A large cross section (width 2.4 m, height 2 m), a low freestream turbulence level, and an appropriate velocity or Reynolds number range. The measurement campaign began with steady tests in which polars of the wing were recorded for a variety of angles of attack and Reynolds numbers. The pronounced flexibility of the wing became apparent at high dynamic pressures and large angles of attack. Besides the three control surfaces at the trailing edge, a high performance pitch actuator was used for the transient excitation of the model which rotates the clamping section of the wing according to a prescribed function of time. In particular, time-dependent pitch angles in the form of "1-cos" profiles were specified to approximate a discrete gust - the same approach is used for loads analysis of transport aircraft. The unsteady experimental results are particularly valuable, especially for the validation of aeroelastic simulations of highly flexible wings that require state of the art, complex simulation methods and tools.
Selected results of the wind tunnel tests as well as initial comparisons with simulation results have been presented at international conferences [1]. A further report about this activity can also be found on the web page of FMRA.
[1] Gonzalez, Pedro and Barbosa, Guilherme and Quesada, Álvaro and Stavorinus, Gerrit and Silvestre, Flavio J. and Hilger, Jonathan and Hanke, Charlotte and Voß, Arne and Krüger, Wolf R. (2024) Wind tunnel testing and modal validation of TU-Flex's high aspect-ratio wings. International Forum on Aeroelasticity and Structural Dynamics IFASD, Den Haag, Niederlande, 2024.
Markus Ritter, Load Analysis and Design Department, DLR Institute of Aeroelasticity
