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The FTDS-K analog board provides separate DC– and AC-coupled signal paths that allow low-level signals to be amplified by up to 7,500 V/V. The DC and frequency response behaviors of these systems were tested and found to provide consistent performance between 100 Hz and 15 kHz, depending on ADC sampling strategy used. Overall power consumption is on the same order as predicted in previous power supply calculations.

Table 3.23. Summary of performance parameters of the FTDS-K analog board

Parameter Value Units Notes

Length 96.7 [3.81] mm [in] Adds 63.5 mm [2.5”] of length to FTDS main board. Width 31.3 [1.23] mm [in]

Does not increase width of FTDS main board.

Height Does not increase height of FTDS main assembly

Mass 10

[0.35]

g [oz]

Does not include wiring harness or sensors.

Active power consumption 69 mW

Analog board circuitry consumes approx. 21 mA at 3.3V. Does not include power dissipated directly by sensors. For 2 sensors with 1.5 kΗΩ bridge resistance, add ~30 mW.

Estimated endurance 9 hrs

Assuming core module is run at a 100% duty cycle, and analog board is run at 50% duty cycle. Includes power

consumption for 2x sensors as described above, estimated power consumption for this mode is 208 mW.

Filter power consumption

(per channel) <3.3 mW <1 mA/filter

Maximum sample rate 200 kSPS For single-channel acquisition. 100 kSPS for dual-channel acquisition.

Maximum undistorted frequency

100-

15,000 Hz

Within +/- 5% of unity gain, per frequency response of inverting amplifier +

antialiasing filters + AC coupling. If sampling at 45.5 kSPS, upper limit is closer to 10kHz.

Maximum deviation of instrumentation amplifier

gain from nominal

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Parameter Value Units Notes

Maximum deviation of inverting amplifier gain

from nominal

-1.25 % Maximum deviation of AA

filter gain from nominal 0.57 %

Not used in overall calibration of device Maximum deviation of ADC

gain from nominal -0.45 % Maximum deviation of

sample rate from nominal 1.28 % Overall signal path gain (DC-

coupled) 50 V/V Nominal. Does not include PGA gain. Overall signal path gain (AC-

coupled) 7,500 V/V Nominal. Does not include PGA gain. Noise-free bits 7.5 bits For both DC– and AC-coupled signal

paths. Using 0.1% crest factor.

Maximum DC offset voltage -0.32 mV Measured at instrumentation amplifier output.

AA filter type Butterworth

Can be bypassed via optional jumpers on board.

AA filter order 4 ul

AA filter cutoff frequency 28 kHz AC-coupling cutoff

frequency 16 Hz

Location of first noise spike

in spectrum 19.5 kHz

The work described above provided assurances that the FTDS-K’s analog and digital

systems are capable of performing turbulence measurement missions. The following section will describe the test setup and analysis used to implement the FTDS-K to measure turbulent pressure fluctuations using Kulite sensors.

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4 MEASURING TURBULENCE WITH THE FTDS-K

While the FTDS-K analog board can be configured for any number of high-frequency measurement missions, the split DC– and AC-coupled signal paths make it particularly capable for measurement of low-level fluctuations on a comparatively much larger mean signal. This capability is intended to enable simultaneous measurements of mean absolute pressure and turbulent pressure fluctuations on the skin of an aircraft in flight, where full-scale range of the sensor cannot be closely matched to the signal level of fluctuations.

An estimate of the mean static pressure and freestream dynamic pressure experienced by a representative jet aircraft (a Boeing 737) are shown in Figure 4.1. This assumes a cruising speed of Mach 0.8 at an altitude of 40,000 ft.

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Figure 4.1. Projected static pressure (upper subplot) and dynamic pressure for a 737 in cruising flight

The root mean square (RMS) fluctuations in the static pressure level due to turbulence would, however, scale on roughly 1% of the dynamic pressure [34]. As an estimate, the expected peak-to-peak pressure fluctuations in normal flight conditions would be on the order of 200 Pa. Compared to the full-scale range of 101 kPa, this presents a wide range of pressures to resolve. While within the electrical capabilities of the FTDS-K, whether a sensor in a real flow environment can resolve these signals remains to be seen. Testing these capabilities on an aircraft in flight is impractical, motivating the use of wind tunnel testing instead. A flat plate model in a 110-mph wind tunnel was used as a test model, providing pressure fluctuations in the tens to hundreds of RMS Pa, representative of the level of pressure fluctuations that would be seen in flight.

Ambient pressure (kPa)

20 20 20 30 30 30 40 40 40 50 50 50 60 60 60 70 70 70 80 80 80 90 90 90 100 100 100 80 100 120 140 160 180 200 220 240 260 Speed (m/s) 0 10 20 30 40 A lt it u d e ( k ft )

Dynamic pressure (kPa)

5 5 10 10 10 15 15 20 20 25 30 35 80 100 120 140 160 180 200 220 240 260 Speed (m/s) 0 10 20 30 40 A lt it u d e ( k ft )

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Testing was approached by developing estimates for the expected level of pressure fluctuation, creating a basic model for measurement noise, and then evaluating the small- signal calibration of the pressure sensors. After this, wind tunnel tests were performed to gather data on pressure fluctuations in turbulent and laminar flows. This testing was divided into two stages: measurement of dynamic pressure fluctuations and measurement of static pressure fluctuations (Figure 4.2).

Figure 4.2. Comparison of dynamic (left) and static (right) pressure measurement methods.

Dynamic pressure fluctuations are larger in amplitude for a given airspeed, providing an opportunity to evaluate experimental and data-processing techniques with a higher AC- coupled signal level. Once these tests were completed, tests were run while measuring static pressure fluctuations. Across these tests, noise reduction techniques were evaluated for their effectiveness in isolating pressure fluctuations due to turbulence. Finally, data collected in these tests were compared to previous work.

Sensor Dynamic fluctuation measurement Static fluctuation measurement Sensor Boun dary layer Boun dary layer

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