Aerospace and Electronic Systems Magazine October 2017 - 22

Measurement of Wing Deflections in Flexible Aircraft

Figure 15.

Results from wind tunnel tests.

between the peak frequency and the wing deflection. The tip deflection once again increased from 0 to just under 4.5″. At the same
time, the peak frequency of the sensor was seen to decrease from
10.627 MHz to 10.605 MHz. The opposite relationship between
the left and right sensor is due to the fact that, on the left wing, the
electrode moves further away from the sensor with an increase in
wing bending, while on the right wing, the electrode moves nearer
to the sensor with an increase in wing bending. Overall, the results
from the static bench tests showed that a trend did indeed exist
between the amount of tip deflection and the shift in the frequency
of the resonant peak of the SansEC sensor.

WIND TUNNEL TESTING
For the left wing, the relationship between tip deflection and frequency response of the sensor is shown in Figure 15a. The square
22

symbols represent positive angles of attack, while the diamond
symbols represent negative angles of attack. The size of the
symbol is increased as the magnitude of the angle of attack is
increased.
From the above figure, a tight-fitting, positive correlation between the frequency of the sensor and the tip deflection is clearly
seen. As the deflection increased from -2.5″ to 3″, a linear increase
was seen in the peak frequency of the sensor, from 9.136 MHz to
9.163 MHz. Steeper angles of attack caused a greater amount of
upward deflection (up to a point), since a larger angle resulted in
more lifting force.
For the right wing, the relationship between tip deflection
and frequency response of the sensor is shown in Figure 15b.
In this case, a negative correlation between the frequency of the
sensor and the tip deflection was observed. As the deflection
increased from -2.5″ to 3″, a decrease was seen in the peak frequency of the sensor, from approximately 10.66 MHz to 10.62
MHz. While a linear relationship was still observed, the recorded
frequencies appeared to be more scattered in this case, and not
as tight-fitting as with the left wing/sensor. This was especially
so for the angles of attack with greater magnitudes. This behavior could perhaps be attributed to airflow issues over the wing.
With higher positive angles of attack, for example, where the
wing is brought close to the stall region, some amount of airflow
separation from the surface of the wing could have resulted. This
would have caused the flow to turn turbulent, and the graphite
electrode (which was mounted on the upper side of the wing
and was not shielded from the freestream) would have been subjected to higher vibrations. If such was the case, the distance
between the electrode and the sensor would have also fluctuated
with the vibrations, resulting in erroneous readings recorded by
the network analyzer. The left electrode, which was mounted on
the underside of the wing, may not have encountered this issue
at higher angles of attack, since flow tends to stay attached to
the bottom surface for a longer time. Further reinforcing this
notion is the fact that both measurements from the left and right
sensor looked very similar in the static bench test, where the airflow over the wing was effectively zero and the aircraft was not
subject to vibrations from the freestream. These irregularities
notwithstanding, the linear relationship between tip deflection
and frequency response is still seen to exist.

CONCLUSIONS
In this article, the effectiveness of the SansEC sensor for measuring
wing deflections in a flexible aircraft was investigated. The experimental setup comprised of SansEC sensors incorporating floating
electrodes mounted on the left and right wings of a model aircraft,
and performing static load tests and wind tunnel tests on the model.
In the wind tunnel, the model aircraft was subjected to various flow
speeds over a wide range of angles of attack, thereby causing the
wings to deflect over a range of displacements. The corresponding frequency responses of the SansEC sensors were measured.
Analysis of the results demonstrated a strong relationship between
the amount of wing deflection and the frequency response of the
SansEC. The results provide sufficient evidence to suggest that the

IEEE A&E SYSTEMS MAGAZINE

OCTOBER 2017

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