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This sensor is primarily designed for use with a drag probe for measuring wing section profile drag. It provides a voltage output that is proportional to differential pressure. An optional, built-in LCD voltmeter indicates measurements with accuracies that cannot be achieved using traditional airspeed indicators.

The heart of this device is an ultra-low pressure, solid state, piezoresistive, differential pressure sensor that is both amplified and compensated to give stable readings over varying battery voltages and temperatures. A 9V alkaline battery powers the device for 20 hours. Assuming 15 minutes of data collection time per test flight, this is good for about 80 test flights.

The sensor measures pressures from -2.0 to +2.5 inches water column. The positive side is perfect for measuring wing profile drag of sailplane wings as the difference between aircraft Pitot and the drag probe pressures. Used to measure airspeed, this instrument is good to 62 kts on the positive side. Voltage readings have an offset of about 2.27 Volts that is subtracted out when the data are processed, in a computer spreadsheet for example. The actual offset depends on the instrument and can be obtained simply by reading the instrument without motion in calm air.

Each DPS is calibrated to a best fit straight line, in a computer spreadsheet, for converting voltage to pressure. As indicated in the graphs on the right, nonlinearity errors are less than 2%. The horizontal axis of this graph is labeled Differential Airspeed. This is the aircraft Pitot/drag probe differential pressure converted to airspeed. It should not be confused with a true difference in airspeeds. Nevertheless, it is intuitively useful and is often used for reporting profile drag data because traditionally measurements have been taken with an ASI.

This instrument is very stable and gives repeatable results to within 1% as indicated in the graph below. This kind of performance is not obtainable using mechanical airspeed indicators.

Specifications

dimensions	case: 2.4" x 4.15" x .85" LCD digit height: 0.5"
power	9V alkaline battery
battery life	with LCD Meter: 20 hours (80 test flights) without LCD Meter: 70 hours (280 test flights)
pressure range	-2.25" to +2.50" H2O
sensitivity	1V per inch H2O
pressure resolution	0.01" H2O
linearity error	2% of reading, typically
repeatability error	1% of reading
maximum overpressure	100 in H2O
tube connections	0.19" barbed ports
operating temperatures	32 to 122ºF (0 to 50ºC)

Measurement Techniques
For comparing data between different aircraft, the airspeed system of the aircraft needs to be calibrated, not just the ASI. This is not a trivial problem. However, for testing differences in the same aircraft, uncalibrated airspeeds are fine.

Even very slight deviations in airspeed cause significant changes in the drag reading, so it is important to take drag readings only when the airspeed is truly on target and stable. This requires reasonably smooth air and skill in reaching and holding target airspeeds. Vertical air movement does not matter since sink rates are not being measured.

A vibrator on the ASI takes a lot of the tedium out of the process, reduces the time and altitude needed to take the data and improves accuracy. One can be constructed from a 3V DC hobby motor by mounting a penny, that has been drilled 1/32" off center, on the shaft. Drive it with 1.5V to reduce the vibration frequency, reduce wear and tear on the motor and increase battery life. Some double sided tape and a cable tie are sufficient to hold the motor onto the ASI.

Depending on pilot proficiency, 4000 to 5000 feet of smooth air, usually above the inversion, will be needed to get data for a set of ten airspeeds. Five to ten readings should be taken at each airspeed and averaged. Points that are obviously out of line should be discarded. One method is to routinely discard the high and low points. To eliminate bias, note the first reading seen when shifting your focus from the ASI to the drag meter.

The hard part is holding the airspeed absolutely constant while jotting down the data. If the airspeed drifts, even a little, time will be lost getting it back on target and the pilot may be tempted to take readings before the ASI is stabilized. Since the only delay in the drag meter is in the connecting tubes, it runs ahead of the ASI; so if the pilot takes a reading while overshooting the target airspeed, it will be in error.

One way to get better data is to video tape the panel during test flights. The flight can then be reviewed later to pick the best data points.

Another method, that we, will be testing, is to use a logging voltmeter. The drag sensor has test points for connection to an external voltmeter. The Fluke 189 is logging meter that can be used for this purpose. Recorded data can be read out manually, or uploaded to your computer using software that comes with the Fluke 189. When using a logging device, the pilot should notch the data by changing airspeeds significantly between speed runs. Then the data can be plotted in a spreadsheet to reveal points where the airspeed changed.

About Drag Probes

Johnson Drag Rake

Drag probes mount on the trailing edge of the wing and average stagnation (Pitot) pressure readings in the boundary layer at selected heights above the wing surface. Since the boundary layer leaving the wing is slowed by profile drag, it produces less pressure at the drag rake than at the aircraft Pitot. The drag sensor connects between these, and reads the difference. More difference means more drag; the theoretical limit is where the drag probe sees no airspeed at all and effectively becomes a static port causing the drag meter to indicate the aircraft airspeed. The theoretical lower limit would be for a wing that has no profile drag at all, in which case the drag probe sees the same stagnation pressure as the aircraft ASI causing the drag meter to indicate zero (in the case of this sensor, the offset voltage).

For a description of Dick Johnson's drag rake, see the article A Flight Test Evaluation of the Grob 103c Twin III (Richard H. Johnson, Soaring Magazine, March 1990). Click the image on the right for an enlarged drawing.

D. Althaus has designed an improved drag probe that takes into account the greatly differing boundary layer airspeeds on the upper and lower wing surfaces. He points out that Johnson style probes do not correctly indicate two sided results and should not be used for determining optimal flap settings. The Althaus probe averages each surface separately and then averages the two surfaces in a second stage.

For an interesting illustration of using a drag rake to determine flap settings, see the web article Wing Drag Meter by Jim Phoenix. Of course, Althaus argues that the results of this simple probe are misleading.

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