Drag measurement instrumentation
For the Standard Cirrus wing at the 53" station, the Pitot hole separation should be 0.163" and the outermost hole should be 0.652" from the surface. However, due to an error in calculation, the actual drag rake was given 0.125" between holes, placing the outermost hole 0.5" from the surface. This is not a problem as long as comparative testing is done using the same drag rake. With these dimensions, this rake is best suited for the 163" span station where the chord length is 25".
The pressure difference between the ASI Pitot and the drag rake is measured by a Honeywell DC002NDR4 piezoresistive, differential pressure sensor (Fig. 5) with a range of ±2" H2O. This device is amplified and temperature compensated, so as to give consistent voltage outputs with wide variations of supply voltage and ambient temperature. The sensor was mounted in a plastic project box (Fig. 6) with a 9V battery, a power switch and a miniature multimeter for reading the sensor voltage. Holes were drilled into an instrument-hole blank to accommodate the pneumatic connections and the two mounting screws (Fig. 6) to hold the device onto the blank. Fig. 9 shows the finished installation.
A flight test was run on 6/21/2003 to verify the functioning of the first drag rake and the pressure sensor, and to see of how difficult it would be to hold constant airspeeds while taking accurate readings of the pressure sensor display. The equipment worked flawlessly, but holding the airspeed while noting the readouts proved to be quite challenging. To improve this situation, a small video camera (Fig. 10) was mounted behind the pilot's right shoulder to record the entire test sequence. Later, the video was transferred to VHS tape for careful review and noting of the data points and for archival purposes. This made it easier to match take pressure readings only when the ASI was dead on the target airspeed and stable. With practice, however, we found that eyeball readings taken in flight were very close to those taken from the VHS tape. One advantage of using the video camera is that the L-NAV temperature display can be recorded during the test sequence.
The Althaus drag probe
D. Althaus has performed extensive testing of drag probes and concluded that because of the considerable difference in velocities between the upper and lower surface boundary layers, Johnson style probes give results that are not representative of total wing section drag, although they are useful in pointing out relative differences for changes made to a given wing section. Althaus has designed and tested a probe that averages the upper and lower boundary layer velocities independently and then averages the two in a second stage. This gives results that correspond nicely to wind tunnel data taken with a more sophisticated drag mouse. See An Instrument for Drag Measurement in Flight for Althaus' full report. For this reason, my tests with the probes described above are performed on one wing surface at a time.
Airspeeds shown in graphs are instrument calibrated. The aircraft airspeed system is not calibrated. Errors in the Standard Cirrus static/Pitot system bias the data towards higher speeds. This makes polars seem better than they really are. However, this is not an issue when the purpose is only to show comparitive data on the same glider.
|© Copyright 2003-2012 Jim Hendrix