STRATOSPHERE S.A develops Adaptive Flutter Suppression technologies

During the last decade, aircraft manufacturers have strongly fostered green innovations to reimagine the future of commercial aviation. The objective is clear, to seed a culture of innovation and take the lead in this frenetic race against carbonization with zero-emission aircraft. Just like the advent of electric mobility will utterly transform great cities and the way we commute, future zero-emission aircraft will give rise to far reaching changes in how we travel, potentially leading to a metamorphosis of the world’s economy. However, in contrast to the automotive industry, air travel faces huger technical challenges to reach the zero-emission objective. It’s not an easy endeavour to make the safest and fastest means of transportation while becoming one of the least polluting in the world.

Since the late 60s, the aviation industry has been constantly moving forward to improve airliners’ fuel efficiency.On average, the fuel burn per passenger and mile has decreased constantly with an annual improvement of nearly 1.3% per year.

Many of these efficiency improvements have been a direct consequence of the following innovations:

  • Advanced extreme bypass ratio turbofans (a large ratio between the amount of air that bypasses the combustion chambers and the one that goes through the fan).
  • Enhanced aerodynamic wing design: Integrating Computational Fluid Dynamics (CFD) optimization with computer-aided design (CAD) has allowed improving the aerodynamic performance of new airliners (e.g. new-generation winglets).
  • Active lift distribution control via flight control surfaces
  • Composite materials

However, one of the “most apparent” ways of improving an airliner’s fuel efficiency has not been included yet in this list of breakthroughs: to increase the wing’s slenderness, or, technically speaking, the wing’s aspect ratio. The wing’s aspect ratio is one of the fundamental parameters that can improve an aircraft’s induced drag. So, a priori, increasing the wing’s aspect ratio sounds like a straightforward solution to the energy efficiency issue. It turns out that this solution, albeit the one having the greatest impact on fuel efficiency, entails huge technical challenges.

The higher the wing aspect ratio, the lower the aerodynamic induced drag, and thus the higher the aircraft’s fuel efficiency. Theoretically, one could design a light composite wing with an ultra-high aspect ratio (AR > 15) to minimize the induced drag in cruise conditions. However, presently, few airliners have a wing aspect ratio greater than AR = 10. The increase of AR has been already solved with the relatively new strut-braced designs. It turns out that there’s a physical phenomenon that’s still nowadays preventing the aerospace giants from designing airliners with lighter ultra-high aspect ratio wings.

The phenomenon, known as flutter, is a well-known aeroelastic instability, an auto-sustained oscillatory motion induced by the interaction between aerodynamic, inertial, and elastic forces. This destructive instability can be onset when the energy injected by nonsteady aerodynamic forces is higher than the energy the structure can dissipate via hysteresis and plasticity mechanisms.At the point where the injected energy equals the dissipated energy, a limit cycle oscillation (LCO) develops. This is known as the “flutter point”.

If the airspeed increases beyond the flutter point  the structure’s oscillations could either remain bounded (LCOs) because of non-linear effects, or suddenly diverge leading to a catastrophic failure.

It’s customary for control engineers to base their designs around high-fidelity mathematical models that match wind tunnel and ground vibration test data. Still, no matter how many millions of dollars the aerospace industry invest in simulation tools and expensive wind tunnel test campaigns, there will always be a gap between the mathematical models and the actual characteristics of the aircraft’s structure and aerodynamics. Model inaccuracies and uncertainties due to non-modeled non-linear dynamics or even due to the aging of the structure can render an a priori perfect active flutter suppression system to be completely unsuccessful,leading to catastrophic consequences. That’s why the robustness of an active flutter suppression system is of paramount importance to cope with any uncertainty in the aircraft’s mass, aerodynamic and elastic characteristics.

Here’s where Adaptive Flutter Suppression (AFS) systems fit can be important. Just like our brain does, an adaptive system has a sort of plasticity, as it can modify itself on the fly based on current and previous experiences. The adaptive approach has the inherent advantage of minimizing both the design time frames and the upload of embedded flight control software patches to correct possible model-mismatch deficiencies discovered late in the design of non-adaptive control architectures.

Moreover, AFS systems’ adaptation capability could be a real game-changer for air travel, as it could help to cope with the problem of aging structures and undetected in-flight failures.

Stratosphere S.A. Has a beyond state technology fully aligned with the major tech trends. The company developed over the last 10 years health management systems and technology for SHM, WHM and IVHM and AFS. Our technology is one of the most advanced frameworks in the Industry.

Stratosphere S.A. is a global provider of integrated health management solutions. The robustness and integration of its solutions are the pinnacle of SHM systems.