Experiments in Flight testing

- Mat Redsell

Instruments required:

Manometer to test airspeed instrument. This manometer is rather easy to construct and tells you a lot about your specific instrument. One of the Airspeed instruments I tested had a leak!

Software required:
Computer with Excel spreadsheet installed
Excell spreadsheet compiled by Mat Redsell
Instrument calibration
True airspeed calibration
Doug Gray's Calibrated airspeed calibration
Polar calculation.

Instruments needed during flight:
GPS to check airspeed using the airspeed spreadsheet
Flytech or other fai approved barograph, with zeroing altitude and chronometer
Palm pilot with Soar Pilot to trace flight
Outside temperature probe ( radio shack)
Tape recorder

Airspeed Calibration
If you are going to use a regular airspeed instrument you should check out its accuracy with a very simply constructed manometer seen above.

My feeling is that there are so many things that can adversely influence the airspeed of the aircraft that I calibrate the entire system with the use of a GPS. This process has gradually been refined and especially helped by the finding of a spreadsheet and formulas on the internet by Doug Gray of Austrailia.

The process I use is to fly at a set airspeed, say 40 mph record the direction in degrees and the GPS airspeed. Next I turn right at approximately 90 degrees to this previous track keeping a constant that 40 mph and record the direction of travel in degrees and the GPS ground speed. Lastly for the final 40 mph airspeed I turn right again about 90 degrees and then record( use of tape recorder makes it easy) the direction of travel in degrees and the GPS airspeed. Do this for all of the airspeeds from stall to VNE at say 10 mph increments.

From this testing I put the results in the spreadsheet developed by Doug Gray of Australia and you will have the true airspeed (TAS )for each indicated airspeed on the aircraft airspeed instrument. So if the instrument reads 40 mph and your testing shows that it is really 42.6 mph then that is your true airspeed at that pressure and temperature. Now you do also have to compensate for the temperature and pressure when calculating the performance for the standard temperature and pressure.

You can also calibrate any other airspeed indicator as well such as the electronic flytec airspeed. The Turbine of the Flytec does read true airspeed but it too may need some calibration to compensate for its location and instrument error.

Doug Grays spreadsheet to calculate calibrated airspeed from the GPS is most helpful. From doug Grays spreadsheet one can then calculate the performance using the spreadsheet form Dave Wells.

Performance calculation.
Do final performance testing In the middle of a high pressure with little wind and the altimeter set to 29.92 inches of mercury (just remember your landing altitude will be off). Tow to an altitude of at least 5000 ft agl. Record the outside temperatures as you climb at 500 ft intervals with a voice recorder..

Do all of your calculations at standard pressures. Remember to set the altimeter to 29.92 " on either of your instruments. My favorite is the Flytec.

On the Flytec, a commercially available FAI approved barograph instrument, one can set the standard pressure. You also have two altimeters and well as a chronometer and vario. The Barograph also proves very useful that can be downloaded onto a computer for varification.

To calculate actual performance you need to set an airspeed and then record the time to descend 500 feet. This is easily done with the flytek. Record on your tape recorder, the start altitude on Altimeter one, record the outside temperature then simply set your course by the GPS, zero altimeter 2 on the flytec, start the chronometer then watch the altimeter. When it reaches say a minus 500 ft stop the chronometer. Read the time and the altitude to the voice recorder.

Record the descents for all the calibrated airspeeds such as 40, 60, 70 and 80. That will use about 2500 ft

When you have finished your flight testing playback all the tape of the flight and put those numbers in the the Excel spreadsheet, developed by Dave Wells, can then calculate the L/D and the draw charts.

References:
Using GPS to accurately extablist True Airpspeed(TAS) - Kitplanes Feb 1995 page 49\
How to Calibrate your ASI -Jim Weit Kitplanes
Airspeed Calibration- Paul Bikle- Soaring Magazine Jan 1971
Technical soaring Vol II #1

Thanks to Dave Wells for his performance spreadsheet, Doug Gray for his very helpful method of Instrument calibration and Robert Mudd who dug out a number of articles to help my study.