Gust-Induced Oscillations

Gusty headwinds can induce pitch oscillations because the effectiveness of the elevator varies due to changes in the speed of the airflow over the elevator. Crosswinds also can induce yaw and roll oscillations. In gusty crosswinds, the effects on glider control change rapidly depending on the speed and rate of the crosswind component. A crosswind from the right, for instance, tends to weathervane the glider into the wind, causing an uncommanded yaw to the right. Crosswinds tend to lift the upwind wing of the glider and push the tail downwind.

Local terrain can have a considerable effect on the wind. Wind blowing over and around obstacles can be gusty and chaotic. Nearby obstacles, such as hangars, groves or lines of trees, hills, and ridges can have a pronounced effect on low altitude winds, particularly on the downwind side of the obstruction. In general, the effect of an upwind obstacle is to induce additional turbulence and gustiness in the wind. These conditions are usually found from the surface to an altitude of 300 feet or more. If flight in these conditions cannot be avoided, the general rule during takeoff is to achieve a faster-than-normal speed prior to lift-off.

The additional speed increases the responsiveness of the controls and simplifies the problem of correcting for turbulence and gusts. This provides a measure of protection against PIOs. The additional speed also provides a safer margin above stall airspeed. This is very desirable on gusty days because variations in the headwind component have a considerable effect on indicated airspeed.

Caution: Do not exceed the glider’s tow speed limitations when adding safety speed margins for takeoff in windy conditions.

Vertical Gusts During High-Speed Cruise

Although PIOs occur most commonly during launch, they can occur during cruising flight, even when cruising at high speed. Turbulence usually plays a role in this type of PIO, as does the elasticity and flexibility of the glider structure. An example is an encounter with an abrupt updraft during wings-level high-speed cruise. The upward-blowing gust increases the angle of attack of the wings, which bend upward very quickly, storing elastic energy in the wing spars. For a moment, the G-loading in the cabin is significantly greater than one G. Like a compressed coil spring seeking release, the wing spars reflex downward, lofting the fuselage higher. When the fuselage reaches the top of this motion, the wing spars are storing elastic energy in the downward direction, and the fuselage is sprung downward in response to the release of elastic energy in the wing spars. The pilot then experiences reduced G-load, accompanied perhaps by a head bang against the top of the canopy if the seat belt and shoulder harness are loosely fastened.

During these excursions, the weight of the pilot’s hand and arm on the control stick may cause the control stick to move a significant distance forward or aft. During positive G-loading, the increased apparent weight of the pilot’s arm tends to move the control stick aft, further increasing the angle of attack of the wing and G-load factor. During negative G-loading, the reduced apparent weight of the pilot’s arm tends to result in forward stick motion, reducing the angle of attack and reducing the G-load factor still further. In short, this rapid cycle of induced flight control input affects load factor and increases the intensity of vertical gusts on the glider’s airframe and the pilot. One protection against this is to reduce speed when cruising through turbulent air. Another protection is to brace both arms and use both hands on the control stick when cruising through turbulent air at high speed. It is worth noting that some glider designs incorporate a parallelogram control stick linkage to reduce the tendency toward PIO during high-speed cruise.

Pilot-Induced Pitch Oscillations During Landing

Instances of PIO may occur during the landing approach in turbulent air for the same reasons previously stated. Landing the glider involves interacting with ground effect during the flare and keeping precise control of the glider even as airspeed decays and control authority declines. A pilot can cause a PIO by overcontrolling the elevator during the flare, causing the glider to balloon well above the landing surface even as airspeed is decreasing. If the pilot reacts by pushing the stick well forward, the glider will quickly dive for the ground with a fairly rapid rate of descent. If the pilot pulls the control stick back to arrest this descent while still in possession of considerable airspeed, the glider balloons again and the PIO cycle continues. If airspeed is low when the pilot pulls back on the stick to avoid a hard landing, there will probably be insufficient lift available to arrest the descent. A hard or a nose-first landing may result.

To reduce ballooning during the flare, stabilize the glider at an altitude of 3 or 4 feet, and then begin the flare anew. Do not try to force the nose of the glider down on to the runway. If airspeed during the ballooning is low and the ballooning takes the glider higher than a normal flare altitude, it may be necessary to reduce the extension of the spoilers/dive brakes in order to moderate the descent rate of the glider. Care must be taken to avoid abrupt changes. Partial retraction of the spoilers/dive brakes allows the wing to provide a bit more lift despite decaying airspeed.

Another source of PIOs during the approach to landing is overly abrupt adjustment of the spoilers/dive brakes setting. The spoilers/dive brakes on most modern gliders provide a very large amount of drag when fully deployed, and they reduce the lift of the wing considerably. Excessive use of the spoilers/dive brakes during the approach to land can easily lead to oscillations in pitch attitude and airspeed changes. The easiest way to guard against these oscillations is to make smooth adjustments in the spoilers/dive brakes setting whenever spoilers/dive brakes adjustment is necessary. This becomes particularly important during the landing flare just prior to touchdown. A sudden increase in spoilers/dive brakes extension results in a high sink rate and possible hard contact with the runway. This can lead to a rebound into the air, setting the stage for a series of PIOs. As before, the cure is to stabilize the glider, then resume the flare. If the spoilers/ dive brakes are retracted abruptly during the flare, the glider will probably balloon into the air because of the increased lift provided by the wings. Remember, spoiler/dive brakes provide drag and reduce lift. This extra lift and pilot reaction may result in overcontrolling and PIOs. The use of spoiler/ dive brakes should be determined by flight conditions. If a wind gust has caused the glider to balloon, then spoiler/dive brake use is an option for the pilot to reestablish the flare attitude. If the spoilers/dive brakes must be adjusted, do so with a smooth, gentle motion.

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