Anyone who has ever flown in an airplane knows this problem: You want to read a book or to snooze without being disturbed, but around you there is an annoying bucking and a humming noise. The sources of these disturbing events are usually wing or fuselage vibrations which are amongst others caused by wind gusts, turbulence or by engine operation. Do passengers have to keep on suffering from the negative consequences of such vibrations in the future? Or is there any prospect of improvement? In order to give a positive answer to this question, the Institute of Aeroelasticity is researching the tailored design of passive damping applications for vibration reduction.
Elastomers – The ultimate materials for damping design
Elastomeres - Changes of the material stiffness depending on temperature variation
-30 °C corresponds to a possible cruise temperature, while 20 °C corresponds to the temperature during take-off. The material stiffness is reduced to 3 % compared to the “cruise” stiffness.
So-called elastomers play an essential role in the development of passive damping treatments. Elastomers are viscoelastic rubber-like materials, possessing a high damping potential due to their chemical composition. But that's not all: Elastomers change their mechanical properties depending on various environmental influences such as temperature. For instance, at cruise flight temperature, such a material can be 100 times stiffer than at take-off from high temperatures. This special behavior is mathematically described by means of complex material models and results in a challenge for the design of damping treatments.
It’s not all about the material
Indeed, elastomers have the potential for high damping. However, this potential needs to be retrieved by appropriate mechanisms. The shear deformation of elastomers using the so-called Constrained Layer Damping principle has proven to be particularly effective. In this setup, an elastomeric core layer is constrained between the host structure and a stiff face layer. The core layer experiences shear deformation as a consequence of bending vibrations. Due to its special properties, the elastomer converts the vibrational energy into heat and contributes significantly to the vibration damping.
Bending mode of a Constrained Layer Damping beam
Right: Illustration of the occurring shear deformation in the black elastomeric layer. This deformation type is decisive for the vibration damping, since it “activates” the damping potential of the elastomer.
Within the scope of the DFG Excellence Strategy SE²A (Sustainable and Energy-Efficient Aviation) the application of Constrained Layer Damping treatments for load alleviation of wing vibrations is investigated. With the help of a developed optimization algorithm, the "sandwich" design is specifically tailored to selected vibration modes and temperature levels to achieve maximum damping. The results of a recent design optimization draw a positive picture: Depending on the vibration mode and acceptable weight penalty, damping ratios up to 10 % seem to be achievable. In this way, not only the decay time of vibrations is significantly shortened, also the peak values of the vibrations are reduced.
Deflection of the wing tip in dependence of frequency
The blue line indicates the deflection of the undamped wing, while the red line shows the deflection of the wing with an optimized damping for the 3rd bending mode. The optimized design leads to a significant decrease of the deflection peak (black rectangle).
Next to their application for wing vibrations, elastomers can also be used to improve the acoustic comfort for passengers in the cabin. By adding individual Constrained Layer Damping elements to the fuselage frame or fuselage skin, it is possible to attenuate the fuselage vibrations specifically responsible for sound radiation. A decisive criterion for the effectiveness of this treatment is the location of the elements that is currently being investigated by the Institute of Aeroelasticity.
