Glider-Induced Oscillations

Pitch Influence of the Glider Towhook Position

The location of the glider’s aerotow towhook influences pitch attitude control of the glider during aerotow operations. During these operations, the towline is under considerable tension. If the towline is connected to a glider towhook located more or less directly on the longitudinal axis of the glider, the towline tension has little effect on the pitch attitude of the glider.

On many gliders, the tow hook is located below the cockpit or just forward of the landing gear. Many European gliders have the towhook located on the belly of the glider, just forward of the main landing gear and below the longitudinal axis of the glider. The glider’s center of mass is above the location of the towhook in this position. In fact, virtually all of the glider’s mass is above the towhook. The mass of the glider has inertia and resists acceleration when the towline tension increases. In these tow hook configurations, an increase in tension on the towline causes an uncommanded pitch-up of the glider nose as shown in Figure 8-4. Decrease in towline tension results in an uncommanded pitch-down.

Rapid changes in towline tension, most likely to occur during aerotow in turbulent air, cause these effects in alternation. Naturally, on days when good lift is available, the aerotow is conducted in turbulent air. The potential for inducing pitch oscillations is obvious, as rapid alternations in towline tension induce rapid changes in the pitch attitude of the glider. To maintain a steady pitch attitude during aerotow, the pilot must be alert to variations in towline tension and adjust pressure on the flight controls to counteract the pitch effect of variations in towline tension.

Self-Launching Glider Oscillations During Powered Flight

In some self-launching gliders and gliders equipped with a sustainer engine, the engine is extended above the glider just behind the cockpit. The engine’s propeller thrust vector is located above the glider’s longitudinal axis and center of mass. [Figure 8-5] This combination of engine and airframe exhibits a complex relationship between power setting and pitch attitude. When power changes are made, the propeller’s thrust line vector has a noticeable effect on the glider’s pitch attitude. This includes the effects of propeller wash over the elevator causing variations in elevator effectiveness and adding to the complexities of flight. 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, coordinating with proper flight control input. This gives the pilot time to recognize and counteract the effect the power setting change has on pitch attitude. In most self-launching gliders, the effect is greatest when flying at or near minimum controllable airspeed (VMCA). Self-launching glider pilots should avoid slow flight when flying at low altitude under power. [Figure 8-5]

Self-launching gliders may also be susceptible to PIOs during takeoff roll, particularly those with a pylon engine mounted high above the longitudinal axis. [Figure 8-5] The high thrust vector and the propeller wash influence on the air flow over the self-launching glider’s elevator may tend to cause considerable change in the pitch attitude of the glider when power changes are made.

Nosewheel Glider Oscillations During Launches and Landings

Many tandem two-seat fiberglass gliders, and some singleseat fiberglass gliders, feature a three-wheel landing gear configuration. The main wheel is equipped with a traditional large pneumatic tire; the tailwheel and the nosewheel are equipped with smaller pneumatic tires. During ground operations, if the pneumatic nosewheel remains in contact with the ground, any bump compresses the nosewheel tire. When the pneumatic nosewheel tire rebounds, an uncommanded pitch-up occurs. If the pitch-up is sufficient, as is likely to be the case after hitting a bump at fast taxi speeds, the tailwheel contacts the runway, compresses, and rebounds. This can result in porpoising, as the nosewheel and tailwheel alternate in hitting the runway, compressing, and rebounding. In extreme cases, the fuselage of the glider may be heavily damaged. 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 practicable, 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 main wheel and tailwheel touch simultaneously. During roll out, use the elevator to keep the nosewheel off the ground for as long as possible.

Tailwheel/Tailskid Equipped Glider Oscillations During Launches and Landings

Most gliders have a tailwheel. When loaded and ready for flight, these gliders have the main wheel and the tailwheel or tailskid in contact with the ground. In these gliders, the center of gravity is aft of the main wheel(s). Because of this, any upward thrust on the main landing gear tends to pitch the nose of the glider upward unless the tailwheel or tailskid is in contact with the ground and prevents the change in pitch attitude.

Upward thrust on the main landing gear can occur in numerous circumstances. One cause is a bump in the runway surface during takeoff or landing roll. If the resultant pitchup is vigorous enough, it is likely that the glider leaves the ground momentarily. If airspeed is slow, the elevator control is marginal. As the pilot reacts to the unexpected bounce or launch, overcontrolling the elevator results in a PIO. [Figure 8-6]

Improper landing technique in a tailwheel glider also can lead to upward thrust on the main landing gear and subsequent PIOs. Landing a tailwheel glider in a nose-down attitude, or even in a level pitch attitude, can lead to trouble. If the main wheel contacts the ground before the tailwheel or tailskid, the compression of the pneumatic tire and its inevitable rebound provides significant upward thrust. The glider nose may pitch up, the angle of attack increases, and the glider becomes airborne. As before, overcontrol of the elevator leads to PIOs.

To prevent this type of PIO, do not allow the glider to settle onto the landing surface with a nose-down attitude or with excess airspeed. During the landing flare, hold the glider a few inches above the ground with gentle backpressure on the control stick as necessary. The speed decays and the pitch attitude gradually changes to a slightly nose-up pitch attitude. The ideal touchdown is simultaneous gentle contact of main wheel and tailwheel or tailskid. Delaying the touchdown just a small amount results in the tailwheel or tailskid contacting the landing surface an instant before the mainwheel. This type of landing may be acceptable and desirable for many tailwheel gliders because it makes a rebound into the air very unlikely. Consult the GFM/POH for the glider being flown for further information about recommended procedure for touchdown.

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