Submitted by Ken Kopp to the BD-4 mailing list August 27, 2001.
A simple yet effective flight test technique to
truly determine if the airplane is statically stable in the lateral and
directional axis is to perform a Steady Heading Sideslip or SHSS. Here is what
you do:
A. Pick a prominent landmark on the horizon and fly toward it.
B. With the ball in the center note your rudder and yoke positions.
C. Note your heading and add rudder (either direction) and opposite aileron as required to maintain your track toward the landmark. Your initial change in heading is roughly your sideslip. Keep your sideslip angles small at first (5 degrees is a good start).
D. Once you are stabilized in your SHSS note your rudder and yoke positions.
If you need to keep rudder pedal away from sideslip (example: it takes more left pedal to generate right side slip: wind in the right ear) and yoke into the sideslip then the airplane has positive Directional Stability and Positive Dihedral Effect. In other words, positive directional stability is evidenced by the requirement to add additional opposite rudder with increasing sideslip. Positive dihedral effect is evidenced by the need to increase aileron into an increasing sideslip. Try increasing the sideslip a few more degrees. Be careful not to add too much sideslip at high speeds. You only need a few data points for left and right sideslips to develop a good plot. As with any flight test. Be very careful and build up from easy to more difficult maneuvers. Thoroughly plan your flight and make sure it is a nice day! You can record your pedal and yoke positions by connecting a fabric tape measure to a fixed reference point and using it to determine the change in pedal position. Data you would record would be:
- IAS
- Pressure Altitude
- Initial heading
- Trimmed control positions
- Heading after stabilized in SHSS
- Control positions once stabilized
- Change in IAS:
- Change in VSI.
Plot control positions versus sideslip to get the sign of your airplanes stability. Keep in mind though that this test only tells you if your plane is statically stable or unstable not how strong the stability is. Contact me for any other information if you like. Hope this helps! I can send examples of data cards and plots if anyone is really interested in flight test and analysis.
Ken about himself:
"I'm an active duty Navy pilot just completing a three year Test Pilot
program. I have completed many many hours of detailed flight test on my BD-4 in
accordance with standard Naval Flight Test Maneuvers. My BD-4 is fully
instrumented with a data collection device for capturing real-time data. I
designed and built it myself as part of my Aeronautical Engineering Masters. I
would be very happy to share what I have learned about my airplane and how to
interested builders can test their own airplanes correctly and safely."