Pilot-Induced Roll Oscillations During Launch
Pilot-induced roll oscillations occur primarily during launch, particularly via aerotow. As the tow pilot applies full power, the glider moves forward, balanced laterally on its main wheel by the wing-runner. After the wing-runner releases his or her balancing hold, aerodynamics or crosswind could cause a wing to drop. If a wingtip begins to drop toward the ground before the glider achieves significant speed, aileron control is marginal and considerable stick displacement must be applied to elicit a response from the glider’s ailerons. As the glider accelerates, the control response improves and the latency of response from the glider shortens. As acceleration continues, the pilot must recognize the increased responsiveness of the glider to avoid overcontrolling the glider. [Figure 8-2]
Although roll oscillations can develop during ground launch operations, they occur less often than during aerotow operations because excellent aerodynamic control of the glider is quickly achieved due to the rapid acceleration. Since control improves as acceleration increases, operations that use a strong winch or launch vehicle are less likely to be hampered by oscillations.
Wing mass also affects roll oscillations. During low speeds, if the wings do not stay level, the pilot applies considerable aileron pressure to return the wings to level attitude. Because of the large mass and considerable aerodynamic damping that long-winged gliders exhibit, there is a considerable lag time from the moment pressure is applied until the moment the wings are level again. Inexperienced pilots maintain considerable pressure on the ailerons until the wings are level before releasing control pressure. The wings continue their rolling moment due to their mass, length, and momentum about the longitudinal axis of the glider. The pilot senses this momentum too late, and applies considerable pressure in the opposite direction in another attempt to level the wings.
After a time, the wings respond and roll back to level, whereupon the pilot centers the ailerons once again. As before, the momentum of the wings about the longitudinal axis is considerable, and the wings continue their motion in roll. This series of PIOs may continue until one wingtip contacts the ground, possibly with considerable force, causing wing damage or a ground loop and an aborted launch. To reduce the likelihood of this type of roll oscillation, anticipate the momentum of the glider wings about the longitudinal axis and reduce aileron control pressure as the wings approach the level position.
Pilot-Induced Yaw Oscillations During Launch
Pilot-induced yaw oscillations are usually caused by overcontrolling the rudder. As with roll oscillations, the problem is the failure of the pilot to recognize that the glider is accelerating and has considerable momentum. If the glider veers away from the towplane, rudder application in the appropriate direction helps correct the situation. If the rudder pressure is held too long, the large yaw momentum of the glider wings and fuselage results in overshooting the desired yaw position and veering off in the opposite direction. Overcompensating for large yaw, the pilot applies considerable rudder pressure in the direction opposite from the original rudder pressure. The alarmed pilot now applies considerable rudder pressure in the direction opposite the original rudder pressure. As the glider continues to accelerate, the power of the rudder increases and the lag time decreases. In extreme cases, the glider may veer off the runway and collide with runway border markers, airport lights, parked glider, or other obstacles. The cure for this type of yaw oscillation is anticipating the momentum of the glider wings and fuselage about the vertical axis and reduce rudder pedal pressure when the nose of the glider begins to yaw in the desired direction in response to rudder inputs. [Figure 8-3]
When a glider’s wingtip contacts the ground during takeoff roll, an uncommanded yaw results. The drag of the wingtip on the ground induces a yaw in the direction of the grounded wingtip. The yaw usually is mild if the wingtip is on smooth pavement but much more vigorous if the wingtip is dragging through tall grass. If appropriate aileron pressure fails to raise the wingtip off the ground quickly, the only solution is to release the towline and abort the takeoff attempt before losing all control of the glider.
The greater the wing mass is and the longer the wingspan is, the more momentum the glider exhibits whenever roll or yaw oscillations arise. Some very high performance gliders feature remarkably long and heavy wings; once in motion, they tend to remain in motion for a considerable time. This is true not only of forward momentum, but yaw and roll momentum as well. The mass of the wings, coupled with the very long moment arm of large-span wings, results in substantial lag times in response to aileron and rudder inputs during the early portion of the takeoff roll and during the latter portion of the landing rollout. Even highly proficient glider pilots find takeoffs and landings in these gliders to be challenging. Many of these gliders are designed for racing or cross-country flights and have provisions for adding water ballast to the wings. Adding ballast increases mass, which results in an increase in lag time.
Low time pilots and pilots new to such high performance gliders should review the GFM/POH thoroughly prior to flight. It is also recommended to review normal procedures, emergency procedures, and the glider’s fight characteristics with a qualified pilot or instructor pilot before attempting flight in any high performance glider.