Self-Launch Glider Oscillations During Powered Flight

Introduction to Glider Flying > Abnormal and Emergency Procedures > Self-Launch Glider Oscillations During Powered Flight

Gliders equipped with an extended pod engine and propeller located high above the glider’s longitudinal axis exhibit a complex relationship between power setting and pitch attitude. When power changes are made, the location of the thrust line of the propeller in this location has a noticeable effect on pitch attitude. The changing speed of the propwash over the elevator causes considerable variation in elevator effectiveness, modifying pitch attitude still further. Prior to flight, study the GFM/POH carefully to discover what these undesired effects are and how to counteract them. When throttle settings must be changed, it is good practice to move the throttle control smoothly and gradually. This gives the pilot time to recognize and counteract the effect the power setting change has on pitch attitude. In most self-launched gliders, the effect is greatest when flying at or near minimum control-lable airspeed (MCA). Self-launch glider pilots avoid slow flight when flying at low altitude under power. [Figure 8-5]

Figure 8-5. Pitch attitude power setting relationships for self-launch glider with engine pod.

Self-launch gliders may also be susceptible to PIOs during takeoff roll, particularly those with a pylon engine mounted high above the longitudinal axis. The high thrust line and the propeller wash influence on the air flow over the self-launch glider’s elevator may tend to cause considerable change in the pitch attitude of the glider when power changes are made.

During takeoff roll, the best way to avoid porpoising in a nosewheel-equipped glider is to use the elevator to lift the nosewheel off the runway as soon as practi-cable, then set the pitch attitude so the glider’s main wheel is the only wheel in contact with the ground. To avoid porpoising during landing, hold the glider off during the flare until the mainwheel and tailwheel touch simultaneously. During rollout, use the elevator to keep the nosewheel off the ground for as long as
possible.