Advancing our knowledge: Exploring sideslip in the Moyes CSX hang glider

November 3, 2005 edition
Steve Seibel
steve at aeroexperiments.org
www.aeroexperiments.org

 

 

The Moyes CSX hang glider is one of the earlier topless hang gliders and has a reputation for being prone to dramatic "sideslips" in certain situations.  (See for example this article from the Moyes website: "Moyes CSX--born and bred to race".)   Therefore it is a good test vehicle for shedding light on the ideas concerning the interaction between pitch "coordination" inputs and sideslip expressed elsewhere on this website. 

 

To explore this, you'll need to make a centerline-mounted yaw string as described elsewhere on the aeroexperiments website, or get in touch me and I'll loan you a suitable mounting bracket. 

 

A) With the yaw string in place, make an aggressive roll input to increase the bank angle (starting from wings-level).  For a given starting airspeed and a given roll rate, does the yaw string displace to the side any further or any less with any of these 3 techniques: 1) making a normal nose-up pitch "coordination" input to hold the pitch attitude and airspeed roughly constant as the bank angle increases; 2) making no nose-up pitch "coordination" input, so that the nose drops and the airspeed rises as the bank angle increases; 3) making a nose-down "anti-coordination " input to make the airspeed rise rapidly as the bank angle increases.  Explore this both VG off and VG on.  Begin your explorations of cases 1-3 by starting near trim speed, and then expand your explorations of cases 1-3 by starting with the bar well pulled-in (perhaps to mid-chest).  Consider the roll rate as well as the yaw string behavior: In case 1 where you made a "normal" pitch coordination input, did the roll rate tend to be dramatically higher when the maneuver was started at a high speed than when the maneuver was started near trim? What about with the VG loose?  What about with the VG tight?  What about in cases 2 (no pitch input) and 3 ("anti-coordination" pitch input)?  Gather as much information as you can about the way the glider responds in pitch, in roll, and in sideslip to various combinations of pitch and roll inputs, starting from various initial airspeeds, with the VG on and with the VG off.

 

(For comparison: every hang glider I've flown exhibits adverse yaw (sideslip) whenever the pilot initiates an increase in the bank angle, and this sideslip is not any less pronounced when the pilot makes a "normal" pitch coordination to hold the nose up, than when the pilot makes no pitch "coordination" input, allowing the nose to fall, or when the pilot actually pulls the nose lower as he rolls the glider into the bank.  Also, the adverse yaw or sideslip is no worse when the roll away from wings-level is initiated at a high airspeed, than when the roll away from wings-level is initiated at a low airspeed.  In no case have I ever found that pulling in the control bar while a glider is banked produces a sideslip.)

 

B) The CSX is said to slip dramatically if a pilot first rolls into a bank, and then pulls in the control bar, especially with the VG off.  If the pilot first pulls in the control bar, and then rolls into a bank, this tendency is said to be avoided.  Explore this, both with the VG on and with the VG off, paying careful attention to the yaw string.  Do you agree with the above description?  After you reach a target bank angle, pull in the bar and watch the yaw string.  Does it displace strongly to the side?  (This would signify a sideslip).  Or is the glider actually just entering a rapidly accelerating dive without slipping sideways through the air?  If the latter is true, if you maintain the new bar position, how long does it take for the glider to stabilize at a constant airspeed?  Did the airspeed initially rise to a much greater value before settling at the constant value?  Compared to other gliders you've flown, what aspects of the Moyes CSX's handling characteristics stand out?  How would you best describe the dynamics that other pilots have characterized as a tendency to sideslip when the glider is first rolled into a bank, and then the bar is pulled in?  Do you find that this particular maneuver does in fact involve a sideslip?  If so, what is the bank angle doing?  Could it be the case that the glider tends to increase its bank angle rapidly when the bar is pulled in (even in the absence of a pilot roll input), and that the sideslip is actually just due to adverse yaw such as would accompany any other change in bank angle?  If you force the bank angle to stay constant, then do you see a sideslip when you pull in the bar, starting from a banked attitude?