SAMPE Journal - May/June 2021 - 11

Deflection Measurements
Two string potentiometers were positioned along the torsion box to measure
vertical deflection. One was placed on the hydraulic actuator to measure
deflection at the point of load application, while the other one was placed near
the mid-span of the torsion box (2830mm from the root).
Four Qualisys motion tracking cameras were positioned around the
reaction frame to track the motion of markers (some are visible in Figure 5)
placed on the blade. The system was calibrated to determine the initial relative
distances between markers and their absolute positions in 3D space. To obtain
an accurate measurement, it was essential to place markers in the field of
view of as many cameras as possible, which is why four cameras were used.
Markers were also placed around the hydraulic actuator to measure the angle
of the ram during loading. This ensured that this angle did not deviate too
much from the desired loading angle as the load was increased.
RESULTS AND ANALYSIS
Mass and COG Measurements
The S-beam load cell was removed from the hydraulic actuator and placed
on a strap connected to the gantry crane located in the laboratory. A strap
wrapped around the torsion box was then connected to the load cell and used
to lift the blade. The strap was moved up and down the span until the blade
was stable when lifted from a single point, identified as the centre of gravity
(COG). The blade was measured to have a weight of 117.8 ± 0.5 kg and the COG
was measured at 1570 ± 30 mm from the root. The error on the COG location
was relatively high because the friction between the strap and the blade meant
it was difficult to find the exact COG location and the curvature of the blade
then made it difficult to measure the exact distance from the COG line to the
blade root.
Natural Frequency and Damping
Once attached and clamped to the test fixture, a free vibration analysis of the
blade was carried out before and after each load case. A 3-axis accelerometer
placed on the demonstrator was used to record data as the blade was pushed
either vertically (X-Direction) or horizontally (Y-Direction). A fast Fourier
transform (FFT) was carried out to obtain the frequency domain response of the
acceleration data. The natural frequency of the blade could then be measured
as the peak of the frequency domain response. A beating effect was noticed
during free vibration of the blade and this was confirmed in the frequency
domain by the presence of two peaks at 11.5 Hz in the X-direction and 12.2
Hz in Y- Direction Figure 6. This is caused by the fact that the demonstrator
is free to oscillate in both the horizontal and vertical directions which both
have slightly different natural frequencies - the blade is stiffer in the flapwise
direction. Free oscillation of the blade was carried out with the 73kg saddle
attached and the damping ratio was calculated using the average damping
ratio from cycles 30 to 100. The obtained damping ratio for the torsion box was
0.0031 ± 0.0006. Both the natural frequency and damping ratios of the torsion
box remained constant throughout the test program.
Strain Measurements
Using the rosette strain gauge data, the longitudinal, transverse and shear
strains were obtained at each gauge location. The average strain value was
then measured at the target test loads specified in Table 1. Figure 7 shows the
longitudinal strain values obtained for all three test runs of Load Case 1. The
black vertical lines in the plot represent the extremities of the transition region.
The strain values show very good consistency between the three runs with the

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May June 2021 Journal.indd 11

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SAMPE Journal - May/June 2021

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