Torsion Coupling
5.2.4 Concluding Remarks
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.
FIGURE 5.18 Flight test setup on runway.
00 0.2 0.4 0.6 0.8 1 1.2 1.4
1 2 3
Time
Sensor output (V)
⫻105
4 5 6
FIGURE 5.19 Flight test sensor pattern.
00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
1 2 3 4 5
Time
Sensor output (V)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Sensor output (V)
⫻104
6 7 8 0 1 2 3 4 5
Time ⫻104
6 7
FIGURE 5.20 First and second gliding fl ights sensor output.
an angle with respect to the spanwise direction. On a planview, these fi bers must start in the trailing edge and end in the leading edge such that the leading edge decreases its displacement when the wing experiences a lift load. For a simple constant rectangular cross-section spar with an overall constant thickness and manu- factured with one material, this is the only way of achieving the desired BTC.
Next, the issue was to decide the fi nal dimensions and the right misalignment of the fi bers. On one hand, a small misalign- ment could result in a poor BTC but good vertical stiff ness and high natural bending modes frequencies. On the other hand, a high misalignment would present a better coupling but poor stiff ness and low natural bending frequencies. To determine the best solution, an optimization problem was defi ned and solved.
Th e use of active materials was tested right way at this very early stage of work. Sensors and actuators positions were care- fully studied. Having in mind future objectives such as structural health monitoring, other than vibration reduction and fl utter suppression, two pairs of actuators were placed, one at the root and the other at the midspan. In each case, one actuator was glued on the upper and the other on the lower part of the spar.
Two pairs of sensors were also glued to the spar, but for this work, only one was necessary. Its position is in the midspan and both upper and lower positions were suitable for its task.
Experimental active tests were then run and compared to passive ones. Th e active methodology consisted of reading the sensors strain and feed them back to the actuators, via a propor- tional control system, to oppose the movement. Substantial damping and torsional natural frequency increases were FFT
0 0.001 0.002 0.003 0.004 0.005 0.006
10 20 30 40 50
Hz
Amplitude
Passive Active
FFT
−0.001 0 0.001 0.002 0.003 0.004 0.005 0.006
10 20 30 40 50
Hz
Amplitude
Passive Active
FIGURE 5.21 Second/third and fi ft h/sixth windows passive/active FFT analysis.
0 0.001 0.002 0.003 0.004 0.005 0.006
10 20 30 40 50
Hz
Amplitude Amplitude
Passive Active FFT FFT
0 0.001 0.002 0.003 0.004 0.005 0.006
10 20 30 40 50
Hz
Passive Active
FIGURE 5.22 Second/third and fourth/fi ft h windows passive/active FFT analysis.
attained. Th is presented a good motivation for continuing the path followed so far.
At this stage, the next milestone was identifi ed. A wing was built with one of the manufactured spars. Since this spar was intended to be used for future SHM methods testing, a wing composed of two main cells was designed. One of these methods was based on wave propagation. Starting from the root, this wing had a reinforcement box for clamping in wind tunnel and the RPV fuselage. Th en it had two cells. Each one was defi ned by two structural ribs (attached to the spar). Between these two, there were “form” ribs, which were not attached and were placed to guarantee the airfoil shape and transmit the aerodynamic loads to the edges. Th ese were then transmitted to the spar by the attached structural ribs. Th is way, mechanical waves generated by the actuators could progress through the spar in each cell.
Next, wind tunnel tests were carried out. Th ese were run at several stabilized velocities, starting from 20 m/s to a maximum near the fl utter speed initiation behavior, approximately around 50 m/s. A similar wing was built at the same time, using a 0° spar to test and compare with the native behavior. Th e results showed that the native performance is clearly improved when applying a misalignment to the fi bers. An approximately 30% reduction in the maximum vertical displacement amplitude was found for a test run at 45 m/s. It also presented a stable vertical displacement behavior as the velocity was increased. Th e 0° spar tended to increase this parameter as the velocity rose.
When applying active control using the piezo actuators, this aeroelastic performance of the wing was enhanced. A structural damping increase around 90% was attained for 45 m/s. Th e fl ut- ter was predicted to be delayed by 8 m/s, which corresponds to a 16% increase. Th e best results were found when using a 50% gain increase in the proportional feedback control system.
At this stage, the second milestone was completed and the main objective of the work satisfi ed, since the wind tunnel tests closely reproduced the structure’s response to the real airfl ow inputs. Nevertheless, fl ight testing can push the limits and test other aspects and act as a determinant to assess the systems via- bility. So, the fi nal milestone was to adapt this wing to the RPV and test it in fl ight. To do so, the control equipment had to be put onboard. Th e energy source was changed to batteries and an RPV, which indicated airspeed measuring equipment, was installed. Th e RPV was manually piloted from the ground. Th e control system was preprogrammed with the control law and set to save all the fl ight data for future postprocessing. During the fl ight, the active system was automatically set on and off in
intervals of 12 s. Prefl ight tests revealed engine interferences of an undetermined source. Gliding fl ights option was taken and spectral frequency analyses were carried out. Th e results showed a signifi cant vibration reduction despite the reduced available amplifi cation provided by the internal amplifi er.
Th is fi nal work stage closed the loop of the spar development program and on the passive and active system implementation.
As a fi nal conclusion, it can be said that both the passive and active multicell cross-section spars proved to be a viable solu- tion in active aeroelastic control solutions in adaptive fl ight vehicles.
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