Torsion Coupling
5.2.3 Results
Several experiments were conducted during the work progress.
So, spar, wing, and RPV fi nal assembly were all tested. Th e experiments comprehended static and dynamic on passive and active solutions. Results obtained include sensor output analysis, fast Fourier transform (FFT), and damping calculations (see Figure 5.8).
5.2.3.1 Spar
Th e spar with the actuators and sensors was clamped on its root and an impact was given to the tip. Figure 5.8 shows the sensor output time evolution. Using the data attained, the fi rst natural frequency and damping coeffi cient were determined for both situations.
From the results shown, it is easily seen that the improvement is signifi cant. Up to 80% of damping increase can be achieved and up to 12%, fi rst natural frequency augment can be attained (see Table 5.2).
5.2.3.2 Wing Design and Fabrication
Aft er fi nishing the spar’s experiments, a wing was built with it to be tunnel tested. Th is wing was intended to be used in the future as a specimen capable to allow the implementation of SHM methods. Th us, the model was designed taking into consider- ation two SHM methods, namely vibration based and wave Amplifier Piezoelectric
actuator Signal
conditioning
Signal conditioning D/A
converter
A/D converter Bending
gain
Lowpass filter Controller
Piezoelectric sensor Active
wing
FIGURE 5.6 Proportional control scheme.
Filter on
Filter off
Input w/filt
Clear input
+ +
+ +
+ +
+ +
−
Test-output
Product Active_gain
X DAC
DAC
Active off
Active on
Test-gain
Multiport switch
Active switch
SwitchRec DS1401 flight recorder
DS1401 flight recorder
(single) (single)
X ++
Data type conversion2 Pulse
generator
Pulse generator1
Pulse offset
79944 .001 0.3999 79 DS1401
Flight recorder SensorRec
Sensor output1
Final output 1
Coarse gain 2.18
12:34
Digital clock Data type conversion3 TimeRec
Gain1
−1 (single)
Data type conversion1
0.5
0
0
0 0
1 1
−0.0068 ADC
V offset
Test-offset Offset_sensor
ADC_type1_M1_con1
DAC_type1_M1_C1
DAC_type1_M1_C2 1
RTI Data
FIGURE 5.7 SIMULINK control law model.
−1
−0.8
−0.6
−0.4
−0.2 0 0.2 0.4 0.6 0.8 1
0 0.05 0.1 0.15 0.2 Time (s) Dimensionalized
sensor signal
Active Passive
FIGURE 5.8 Passive and active responses.
propagation. One main feature of the wave propagation method application is that an attached wing spar rib or any kind of stiff - ener limits wave propagation. Th us, in order to have space to place the actuators and sensors and to allow the wave to propa- gate, the general structure of this small wing model for wind tunnel testing was divided only into two cells with a continuous wing spar. However, to assure the aerodynamic shape of the wing nonstructural ribs (not attached to the main spar) were positioned spanwise, 8 cm apart from each other. Beyond these two cells, there was a structural rib at the wing root and another one 4 cm apart spanwise, connected to each other by foam (in addition to the wing spar). Th is was intended to form a rein- forced section to guarantee the wing–fuselage connection. Also, the wing skin was not structural and could be made of a trans- parent fi lm, which allowed seeing the internal wing structure, actuators/sensors positioning, systems, etc. Figure 5.9 shows the wind tunnel testing apparatus.
For each test, a sensor signal amplitude output versus time sample was retrieved and an FFT analysis was done. For instance, Figure 5.10 represents the FFT results obtained for the 20 m/s velocity for the passive wing and active wing with a gain of 50%
max. A clear decrease in the vibration amplitude near for the fi rst natural frequency can be observed.
Maximum signal output amplitudes were retrieved and Figure 5.11 shows the results. As shown, the amplitude decreased from passive to active tests. Generally, this amplitude tends to be smaller when a higher gain is applied. Figure 5.12 shows the per- centage of maximum sensor signal output amplitude decrease: a
maximum 8.83% decrease is reached for 30 m/s of velocity and a gain of 50% max.
As regards the damping analysis, studies were concentrated around the fi rst natural frequency. Having the sensor signal output, FFT amplitude versus frequency graphs, damping was calculated, retrieving the frequency value for the peak ampli- tude and frequency values (around the previous) obtained for maximum amplitude divided by the square root of 2. With these TABLE 5.2 Passive and Active Damping Coeffi cients
and First Natural Frequencies
Coeffi cient Damping Frequency (Hz) First Natural Frequency
Passive spar 0.054 28.92
Active spar 0.1 32.46
Increase 80% 12%
FIGURE 5.9 Wind tunnel test setup.
three values, it was possible to determine a damping coeffi cient proportional value (not the damping coeffi cient itself) by sub- tracting the highest and lowest frequency values divided by the value of the central frequency.
With this algorithm, all the damping coeffi cients were calcu- lated and plotted on the same graph (Figure 5.13) along with the
respective quadratic polynomial trendlines. Th ese last ones, allowed us to predict fl utter speed (that corresponds to the veloc- ity for the zero damping coeffi cient, i.e., when the trendlines would ideally cross the x/velocity axis).
It is noted that the fl utter speed increases from passive to active tests, and it reaches its highest value for the 50% max gain.
Also, damp improvements are observed from passive to active tests, and they are higher also for the 50% max gain. Overactuation and delays infl uence the active gain results for gains higher than 50%, resulting in poorer outcomes. Figure 5.14 shows the per- centage damping improvements: a maximum 93% improvement is reached for 45 m/s of velocity and using 50% maximum gain.
Th e maximum damp improvement, now obtained, is higher than the one obtained for the spar when its mechanical tests were performed (80% max improvement). Th is increase can be explained by the BTC, which had no infl uence on the spar behav- ior when its mechanical tests were done, but which is essential on wind tunnel tests. On these fi nal tests, this coupling aff ected the aeroelastic behavior of the wing. When the wing bends and con- sequently rotates along the spanwise axis due to the coupling, the aerodynamics of the wing will be infl uenced, i.e., aerodynamic
FFT 20 m/s
0 0.002 0.004 0.006 0.008 0.01 0.012
0 20 40 60 80 100
Hz
Amplitude
Pass Act G5
FIGURE 5.10 Th e 20 m/s passive and active wing FFT analysis.
Max amplitude
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
20 30 40 45
m/s
Signal amplitude
Pass G25 G50 G100
FIGURE 5.11 Wing maximum amplitude.
Max amplitude decrease
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
20 30 40 45
m/s
%
G25 G50 G100
FIGURE 5.12 Wing maximum amplitude decrease.
loads applied and their distribution is altered, aff ecting the wing structure, changing consequently the wing structure deformations.
5.2.3.2.1 0°/10° Wing Performance Comparison
In order to truly assess the fi ber misalignment behavior, a similar 0° fi ber-oriented spar was manufactured and tested. Figure 5.15 presents the amplitude variation for both spars at diff erent
velocities. It can be inferred that the misalignment on spars (to a certain extent) by itself can lead to an improvement in the wing’s behavior. Aft erwards, it can be clearly augmented by implement- ing active materials.
5.2.3.3 RPV: Flight Tests
Aft er the successful wind-tunnel testing, the wing was prepared to be installed on an RPV for fl ight testing. It does not compromise Damping
0 0.002 0.004 0.006 0.008 0.01 0.012
20 30 40 50 60 70
m/s
Damp
G25 G50 G100 Pass Poly. (G50) Poly. (G100) Poly. (G25) Poly. (Pass)
FIGURE 5.13 Damping for 25%, 50%, and 100% of maximum gain.
Damp improvement
0 10 20 30 40 50 60 70 80 90 100
20 30 40 45
m/s
%
G25 G50 G100
FIGURE 5.14 Damping improvement for 25%, 50%, and 100% of maximum gain.
Max amplitude
0 0.2 0.4 0.6 0.8 1 1.2
20 35 40 45
m/s
Signal amplitude
10º 0º
Max amplitude decrease
0 5 10 15 20 25 30 35
20 35 40 45
m/s
%
FIGURE 5.15 0° and 10° maximum amplitude and its comparison.
the desired behavior but induces some mass increase. Th e wing must also be equipped with an aileron and respective servo action mechanism (Figure 5.16). Its root had to be reinforced in order to receive the clamping pin provided by the fuselage.
To carry out the fl ight tests, the amplifi er also had to be changed. Th e one used on previous tests was adequate and was replaced by a small battery-powered one. Figures 5.17 and 5.18 show all the active equipment gear on board and in-ground.
During the ground equipment testing, an interference was detected between the engine and the sensor system. Due to this condition, it was opted to do gliding fl ight testing.
Th e procedure consisted in fl ying the RPV as high as possible, which was limited by the pilot’s observation capability.
Aft erwards the engine was cut off and a glide descent aimed to a speed of 20 m/s was performed. Th e airspeed was monitored in the PC using the Jet-Tronic soft ware, which received the pilot telemetry signal from onboard and the testing time was being correlated with speed, and both values registered. Th is operation revealed that it was very diffi cult to conduct, even with two assis- tants, because of the diffi culty in keeping constant conditions.
Two consecutive fl ights were carried out. Figure 5.19 shows the fl ight data recorded, including the altitude gain. Figure 5.20
is focused on the glide part of both fl ights. Th e ramp functions represent the fl ight time.
Flying the RPV must be done not very far from the pilot, and the aircraft performed a series of turns that infl uenced the results. In the turns, eff ective wing lift increased and the vibra- tions were higher. Th is may explain why sometimes when the control was ON (and was supposed to decrease vibrations), some very large amplitude peaks appeared. Due to this, no maximum or minimum comparison was done because they do not repre- sent the general wing’s behavior.
If one considers that a window represents a state of active or inactive, fl ight 1 had six windows and fl ight 2 had fi ve windows.
Selecting joined windows believed to represent the most equiva- lent fl ight conditions, we achieved the following FFT analysis, shown in Figures 5.21 and 5.22.
A clear improvement can be observed. Another interesting fact is that the improvement seems to be higher in the last part of both fl ights, the fi nal straight approach. Th is can be justifi ed by the fact that the wind conditions were more severe near the ground, thus implying more vibrations on the wing. Adding to this, since the airplane was aligned with the runway less pilots, inputs were necessary to guide the RPV to the landing.