Porpoising is a general term that refers to pitch oscillations that can occur in gliders and aircraft in general. In most cases, pilots induce these oscillations through overcontrolling the glider as they attempt to stop the oscillations from occurring in the first place.
Pilot-Induced Oscillations (PIOs)
The instability of a glider’s attitude that arises when the pilot fails to recognize the lag time inherent in controlling the glider is known as a pilot-induced oscillation (PIO). Typically, PIOs occur when the glider fails to respond instantly to control input and the pilot quickly increases the pressure on the controls. By the time the pilot judges that the glider is responding satisfactorily, the extra control pressures have resulted in such a vigorous response that the glider overshoots the desired flight attitude. In an attempt to correct the situation, the pilot moves the controls rapidly in the opposite direction, overcompensating for the mistake. The undesired glider motion slows, stops for an instant, and then reverses. The alarmed pilot maintains significant control pressures to try to increase the rate of response. The glider, now in rapid motion in the desired direction in response to heavy-handed control inputs, again shoots past the desired attitude as the now thoroughly alarmed pilot jerks the flight controls in the opposite direction. Unless the pilot understands that these oscillations are the direct result of overcontrolling the glider, it is unlikely that the oscillations will cease. More likely, they increase in intensity until there is a complete loss of control.
Although PIOs can occur at anytime, these situations arise most commonly during primary training. Pitch instability is another result of center of gravity aft beyond limits. If a pilot encounters PIO, they should ensure the CG is within limits. Pitch instability tends to disappear as pilot experience grows because pilots gain familiarity with the lag time inherent in the flight controls. These types of oscillations may also occur when a pilot is making flights in unfamiliar types of gliders. For this reason, particular care must be taken when the pilot is preparing to fly a single-seat glider in which the pilot has no prior experience. When checking out a new make of single seat glider, the lag time of the flight controls must be learned without the obvious benefit of having an experienced glider flight instructor aboard during flight to offer advice or, if necessary, to intervene. While most PIO discussions are devoted to pitch oscillations, consideration should be given to roll-and-yaw induced oscillations.
The first step toward interrupting the PIO cycle is to recognize the lag time inherent in the glider’s response. Any change in glider flight attitude takes an appreciable amount of time to accomplish as the flight controls take effect, and the mass of the glider responds to the pilot’s control inputs. The second step is to modify control inputs to avoid overcontrolling the glider. The correct technique is to pressure the controls until the glider begins to respond in the desired direction, and then ease off the pressure. As the glider nears the desired attitude, center the appropriate flight control so that overshooting does not occur.
PIOs During Launch
PIOs are most likely to occur during launch because the glider’s lag time changes rapidly as the glider accelerates. During the first moments of the takeoff roll, aerodynamic control is poor, the control feel of the glider is very sluggish, and lag time is great. Flight controls require a wide range of movement or input to have an effect on the glider’s flightpath. As the glider gains speed, aerodynamic response improves, control feel becomes crisper, and lag time decreases. When the glider has acquired safe flying speed, lag time is short, the controls feel normal, and PIOs become much less likely.
Factors Influencing PIOs
The characteristics of the towhook/towline combination on pitch of the glider being flown may cause uncommanded pitch excursions if the pilot does not compensate for those effects. This can contribute to PIO during aerotow launch. In addition, the propwash and wing vortices of the towplane, through which the glider must pass if there is little or no crosswind, affect the flight attitude and control response of the glider, especially at low speeds during the start of the takeoff roll.
To minimize the influence of the towplane’s wake, use a towline of adequate length—200 feet is the minimum length for normal towing operations. A longer towline provides more isolation from towplane wake during aerotow launch. Short towlines, on the other hand, keep the glider closer to the towplane and its turbulent wake, complicating the problem of controlling the glider.
There are several techniques that reduce the likelihood and severity of PIOs during aerotow launch. A pilot should not try to lift off until confident that flying speed and good aerodynamic control has been achieved. Also, just after the moment of lift-off, allow the glider to rise several feet above the runway before stabilizing the altitude of the glider. Two to three feet is high enough that minor excursions in pitch attitude, if corrected promptly, do not result in glider contact with the runway surface, but not high enough to lose sight of the towplane below the nose of the glider. Caution should be exercised if attempting to stabilize the glider just a few inches above the ground because it provides little margin for error if a PIO occurs.
Improper Elevator Trim Setting
The elevator trim control position also contributes to PIO in pitch attitude. The takeoff checklist includes a check to confirm the proper takeoff elevator trim setting. Trim set properly for takeoff results in normal elevator pressures felt through the control stick is normal, and the likelihood of PIO is reduced. If the elevator trim is set incorrectly, however, abnormal elevator pressure is felt through the control stick and may contribute to PIO.
Excessively nose-down trim requires the pilot to hold back pressure on the control stick to achieve and maintain the desired pitch attitude during launch and climb-out. If the trim is set excessively nose up, the pilot needs to hold forward pressure. The more pressure is needed, the more likely it is that the pilot overcontrols the glider.
Although all gliders exhibit these tendencies if the trim is improperly set, the effect is most pronounced on those gliders with an aerodynamic elevator trim tab or an antiservo tab on the elevator. The effect usually is less pronounced on those glider fitted with a simple spring system elevator trim. Regardless of the type of elevator trim installed in the glider, error prevention is superior to error correction. Use a comprehensive pretakeoff checklist and set the elevator trim in the appropriate position prior to launch to help prevent PIO attributable to elevator trim misuse. [Figure 8-1]
Improper Wing Flaps Setting
The likelihood of PIOs increases if the wing flaps are not correctly set in the desired takeoff position. For the majority of flap-equipped gliders, most Glider Flight Manuals/Pilot Operating Handbooks (GFM/POH) recommend that flaps be set at 0° for takeoff. Pilots should review their GFM/POH for the manufacturer recommendation for takeoff settings.
If the flaps are incorrectly set for takeoff, a positive flap setting increases wing camber and wing lift, the glider tends to rise off the runway prematurely, perhaps even before the elevator control is sufficient to control the pitch attitude. Attempting to prevent the glider from ballooning high above the runway, the pilot may exert considerable forward pressure on the control stick. As the glider continues to accelerate, this forward pressure on the control stick exerts a rapidly increasing nose-down force on the glider due to the increasing airflow over the elevator. When the glider eventually pitches down, the pilot may exert considerable back pressure on the stick to arrest the descent. Severe pitching or PIOs are likely to result. If allowed to continue, hard contact with the runway surface may result in glider damage and personal injury.
If the wing flaps are incorrectly set to a negative flap setting, decreasing wing camber and wing lift, then takeoff may be delayed so long that the towplane lifts off and begins to climb out while the glider is still rolling down the runway, unable to get airborne. Excessive back pressure on the control stick may eventually assist the glider in leaving the runway, but the relatively high airspeed at lift-off translates into a very effective elevator, and ballooning may occur as a result of the extreme elevator position. Overcorrecting for the ballooning with excessive forward pressure on the control stick increases the magnitude of this pitch excursion. A series of PIOs may result. If this condition is allowed to continue, the launch could lead to a premature termination of the tow.
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