How much does a wing deform in launch? What do the different construction methods bring?
We have been looking into this more deeply and measured the torsional rigidity of our old wings and the new ones with lattice fabric. Oscillation test were performed therefore.
The measurements showed respectable results (see chart below, plotted is D_r at a distance of 570 mm from center line). The UHM carbon lattice increases stiffness by 35% at only 3 g additional weight. Specific stiffness has increased by almost 40%, as the new wings are lighter than the old ones.
Without airlerons cut (Stephan's wing) stiffness is visibly greater, even this difference is perfectly resolved by the measurement.
Just as a brief outline: From the periodic time of a torsion pendulum (1) D_r can be determined with known I_0 and T_0.
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(1) |
A bar with two end masses was fixed to one wing half with negatives made from Styrodur at a distance of 570 mm from the center. After excitation the periodic time was determined as average of 20 cycles (about 11,6 resp. 13,5 s). Standard deviation of 3 measurements was 1,5%.
The moment of inertia of the bar was measured by putting it on a steel wire of 4 mm diameter in similar arrangement (I_0=0,119 kgm²). The stiffness of the wire was calculated from shear modulus, length and polar moment of inertia.