helicopter rest report

7/28/2019 helicopter rest Report

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Actuator ShortingPrevious testing with 2 blades on the hoverstand in November had issues with actuators

shorting. This has been solved. For one actuator, the flap mount cusp was too close to the side

of the actuator and so with many actuation cycles, it started to rub against the actuator causing

shorts between actuator layers. For another actuator, the clamp was shorted to the top actuatorlayer. It was thought this would not cause problems but it does as the voltage applied to the

actuator leaks to the whole blade. Also, flap actuation on the benchtop was done without the flap

covers. When on the hoverstand, the flap covers made contact with the actuators when theydeflected. So, now benchtop testing has the flap covers on to notice any problems. All these

problems developed over many cycles of the actuators. The problem can be reduced by lowering

the driving voltage. This was done so the voltage limit in the poling direction for the actuators isnow +270V instead of +360V.

A test was carried out to see the effect of this change. A 10-layer (same number as blade

actuators) PZT-5K4 actuator with 10 mil thick layers was constructed. The actuator was not for

the blade so the same level of precision was not used in constructing it as usual. So, the top and

bottom halves had slightly different dimensions. The length dimensions for the layers were .28/.39/.50/.625/.625/2mil brass shim/.66/.66/.53/.41/.29 inches. The width varied slightly from .7865 to .7740 inches. The actuator has superglue dabbed over the solder points. 0.25 inch wide

aluminum clamps are attached to the actuator with 2 layers of FM196 purple film adhesive. The

actuator besides the clamps is coated with one application of M-Coat A (top, bottom and sides).The actuator peak to peak deflection at 2Hz driving voltage was measured with the laser

height sensor. The driving voltage was increased until the depoling limit of -90V was reached.

The bias (the ratio of voltage in poled direction to depoled direction) was increased from 3:1 to

5.33:1 by putting two bias boxes in series. 5.33:1 would mean -90V to +480V. The results areshown in Figure 1.

1

2

3

4

5

6

7

8

9

0 100 200 300 400

Driving RMS Voltage (2Hz)

ActHeight(mil)

3:1 Bias

4:1 Bias

5:1 Bias

5.33:1 Bias

Figure 1: Actuator Amplitude vs. Driving Voltage


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The 5K4 material piezoelectric effect is non-linear so at higher voltages, the slope of

strain vs. electric field decreases. So, increasing the voltage from -80V to -90V has a greater

effect on strain than going from +200V to +220V. So, at the same RMS voltage, the lower biashas a higher negative voltage meaning more tip deflection. The results are summarized in the

table below.

Bias Change Improvement inMaximum Height

Amplitude

3:1 to 4:1 7.0%

4:1 to 5:1 6.0%

3:1 to 5:1 13.4%

5:1 to 5.33:1 0.0%

So, by changing to a 3:1 bias (-90V+270V) from 4:1 bias, the actuator loses 7% of it straincapability. This actuator was then tested for many cycles to get a hint as to whether the actuators

were shorting due to fatigue. The actuator was run from -90V to +360V at 33Hz for 247500

cycles (125 minutes) before a crack in the actuator developed which led to shorting. So, running

at -90V+270V should lengthen the actuator life. It is also seen the material strain saturates sothere is no noticeable increase by going from +450V to +480V (10 mil thick layers).

One last thing looked at was moisture causing actuator shorts. A 5K4 plate coated in M-

Coat A was left to soak in water overnight. The M-Coat A did not break down and so no waterwas on the plate edges to cause shorting. This was shown by measuring resistance and also

applying high voltage with the plate underwater. So, moisture is not suspected to be a problem.

So, it was suspected that shorting was due to some manufacturing errors causing some aluminumparts to rub against the actuators and the flap covers making contact with the actuators and these

were exacerbated by the high driving voltage which has now been reduced.

Flap Angle Measurement Method Adjustment

So, the shorted actuators were removed and new ones made and installed into the blades.All four Hall sensors were recalibrated using National Instruments data acquisition equipment.One SG24 module was used for two Hall sensors (one module for one blade). The module

provides 10V excitation for the Hall sensor. A resistor is added between this and the P+P- Hall

sensor terminals in order to lower the voltage so the current in the Hall sensor is correct. The

modules have two read channels which have a 100 gain. This is read in Labview 8.0 program.The modules have a nulling resistor to set the zero volt output of the Hall sensor. This setup

replaces the Vishay 2311 Signal Conditioning Amplifier boxes. These gave unreliable excitation

voltage, had the zero drift and gave a noisy signal. At small flap deflections (such as were seenabove 900RPM), the flap motion could not be seen. Instead the amplitude was determined using

FFT analysis of the signal. With the SG24 modules, these issues are eliminated. The zero does

not drift, the voltage excitation does not change and the noise is greatly reduced. Even at verysmall angles, (less than 0.4 degrees), the flap motion can be seen clearly in the raw unfiltered

data even when the blade is rotating.

Validation of PZT-5K4 strain rangeAfter replacing the two shorted actuators, the actuator tip deflection for all four flaps (2

blades) was measured. This shown in Figure 2.


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0

2

4

6

8

10

12

14

0.0 50.0 100.0 150.0 200.0

RMS Driving Voltage, 2 Hz, 3:1 bias

HeighHalfpk-pk(mil)

Root F

Tip F

Root E

Tip E

Figure 2: Actuator Tip Deflection

Actuator

MaxDeflection

(mil)

% from

average

E inboard 12.35 8.6%

E outboard 10.7 5.9%

F inboard 11.2 1.5%

F outboard 11.25 1.1%

Average 11.38 -

Table 1: Maximum Actuator Deflection

This is done for -90V+270V which is what is used in hoverstand testing. The actuators

tip deflection is within 9% of the average of the 4 actuators as seen in Table 1. This can be used

to estimate the strain capabilities of PZT-5K4. Previous attempts at measuring this with straingages were inconclusive. The finite element code used to design the actuators uses material

strain range as an input. This can be adjusted until the predicted free deflection matches the

measured free tip deflection. The finite element code underpredicted free tip deflection by 15-20% for PZT-5H2 actuators. Taking this into account means that the 5K4 has a strain range at

-90V+270V of 1023 which is a 29.5% improvement over 5H2 which was driven at

-90V+360V. The 5K4 design was done assuming a 35% improvement. As previouslyexplained, a 5K4 actuator gets 8% more free tip deflection if it is driven at -90V+360V instead of

-90V+270V. So, this would be 135% strain range increase. So, it is proven that the 5K4

material provides at least a 29.5% improvement in strain range for the same material stiffness

over 5H2.

Better HingeThe method for setting the hinge distance has been adjusted. The actuators are bonded

into the flap anchors. The actuator is driven at full voltage. The voltage is turned off exactly at

0V at 0.01Hz. This is because the actuator neutral position after actuation is different than after


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it is cured and before it ever has an electric field applied to it. Then two strips of plastic are

placed in between the actuator rod to be bonded to the actuator tip and the actuator tip. The rod

is placed in the flap mount cusp and then the flap is taped at zero degrees. The rod is then slid upagainst the actuator tip (though separated slightly by the two plastic strips). Then a couple drops

of superglue are placed to bond the rod to the actuator. Then the hinge is measured by

measuring flap deflection and actuator tip deflection at various voltages. Then acetone is used todissolve the superglue and remove the rod. The actuator is sanded if it is found to be too long or

more plastic strips are used in between the rod and actuator tip if it is too short. Then the rod is

rebonded with superglue and the hinge is remeasured. This is repeated until satisfactory. Thenthe rod is bonded with 2-ton epoxy which cannot be undone. This process will change the

actuator length by up to 20 mil which will not significantly affect actuator performance.

The hinges for the flaps are shown below in Table 2. The hinge can now be set to within

1 mil as all but E outboard are close to the goal of 51 mil (slightly different than Figure 1 assome numbers changed after setting hinge which reduced optimum hinge to 48.4 mil).

Flap Hinge (mil) Flap Amplitude (deg)

E inboard 49.4 14.2

E outboard 29.1 20.6F inboard 51.1 12.5

F outboard 51.7 12.4

Table 2: Flap Hinges

The Blade E outboard flap was not replaced and so its hinge was attached using the older lessprecise method. Due to its shorter hinge, its free tip deflection is substantially larger than the

other three flaps. This means this flap is less stiff than the other three flaps. The hinge sets the

flap stiffness. This can be seen in Figure 3.

Figure 3: Effect of Flap Hinge

This plot shows the maximum flap angle that can be achieved with just the propeller momentrestricting the flap. The aerodynamic moment is not reliably estimated at this time. This simply

shows the advantage of impedance matching. The dots on the close-up inset represent the actual

measured hinges. The red line is the flap angle goal. So, all four hinges are acceptable though

At 2400RPM, max flap angle as function of hinge lever arm

10 layer (1.08/1.08/.9/.8/.5) 5K4 Actuator

maximum of curve at d= 48.4 mil,but only true for one If value (36% reduction from value used to match vacuum chamber results)

0.01.02.03.04.05.06.07.0

0 50 100 150 200 250

d, hinge lever arm (mil)

Flap

angle

(deg)

Close-Up

4.0

4.5

5.0

5.5

6.0

6.5

7.0

20 30 40 50 60 70 80


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the one not set with the new method will have lower performance. If the aerodynamic hinge

moment is known from experiment, then the optimum hinge distance can be better calculated.

Conclusions

Though the flap deflection was lower than desired, several important things have beenshown. First, the bond layer between the actuator and the flap anchor held up to 1800RPM.

This has not been the case for prior bond layers with this new piezoelectric material. Secondly,

it was not known if the flap covers that are flush with the blade would hold onto the blade withjust two small screws. They did so without any issue. With careful spanwise balancing, the

blades ran smoothly with relatively little noise at 1800 RPM. Finally, the blades worked as well

after being spun to 1800RPM as before so structurally the blade is fine to 1800RPM. If theactuator cantilever issue can be solved, the flaps should work.

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