Wing Drop During the Ground Run
by Scott Lennon
During a CGC instructors panel meeting, the topic of wing drop during the ground run phase of launching was raised. Wing drops are caused by an uneven amount of lift created across the wingspan. I have learnt from experience in my Standard Libelle, which is particularly susceptible to this wing drop effect, so I feel qualified to offer some of the common causes and rate some of our gliders into categories for how they handle these situations. This compilation also includes some detailed thoughts from Dave Pietsch and Drew McKinnie.
Firstly I offer some comments on perspective. A wing drop is not uncommon for many glider types but the consequences can be quite serious. Once a wing tip contacts the ground, you enter a whole lot of extra variables into the glider motion equation. A small wing tip wheel or skid dragging on soft ground or long grass can create a large turning effect on the glider, which unless corrected can lead to loss of directional control and in the worst case a ground loop situation. The early recovery from this situation is obviously to pick up the wing with aileron as quickly as possible, with early and appropriate use of rudder to keep the glider straight on the runway behind the tug. The dynamics of the launch sequence are complex but a forward positioning of the tow hook on the glider will improve the directional stability and a higher power to weight ratio for the tug-glider combination will accelerate quicker reducing the potential time that the ailerons are ineffective for roll control.
The causes of wing drop apply to all gliders, but some types exaggerate the effects more than others. A good wing runner can assist by holding the wings level for as long as possible during the accelerating phase of the ground run so that the gap between letting the wing go and gaining full aileron authority is as short as possible. With calm conditions and light direct headwinds the natural inertia of the wing will tend to keep them level until the speed builds to a point where the ailerons have authority to counteract any external effects that may cause a wing to drop. Most instances of wing drop in these conditions are caused by small disturbances causing relatively slow rolling movements and the recovery time is dependent mainly on the glider characteristics and the rate of acceleration on tow. For my first example a Libelle behind a low powered tug in calm conditions often runs a wing tip lightly on the ground for some distance until the aileron can pick the wing up. This is usually not a cause for concern provided sufficient control can be maintained to keep the glider straight behind the tug. A more powerful tug or stronger direct headwind launch has similar characteristics except the time for aileron control to become effective is much quicker.
Different gliders will exhibit different characteristics depending on their design, wing geometry, wing section, wash-out, aileron size etc. All these design issues affect angle of attack of the wing and the ‘bite’ or effectiveness of the ailerons. The Puchacz and Blanik both have good aileron authority at low speed due to the large size of their ailerons. Other good features are the heavy wings with inherent inertia, large clearance under the wing tips, powerful rudders, and in the case of the Puchacz, a main wheel behind the CG that reduces the angle of attack of the wings and aids in ground tracking stability. These features help these gliders avoid wings being dropped in the first instance and assist recovery if it does occur. However the longer and heavier wings are harder to pick up once on the ground and create larger turning moments.
Pilots flying flapped gliders like the Open Libelle and ASW-20 use a negative flap setting during the ground roll to reduce the angle of attack on the wing. This reduces the lift generated, and promotes clean airflow over the wing, enabling the ailerons to more effectively overcome any asymmetric lifting forces. A glider like the Standard Libelle has relatively ineffective ailerons at low speed, light wings with very little inertia, and fixed angle of attack on the ground. At low speed early in the ground roll, very small disturbances or minor increases of lift on one wing only will produce a wing drop that often cannot be corrected with aileron until the speed builds up on tow. Other factors such as partially filled water ballast tanks can causing further roll instability, as water moves to the side of a dropping wing. This is why the Jantar manual prohibits partial water ballast to be used during take-off.
Cross winds provide a complex set of conditions that are more likely to cause violent wing drops and potentially a loss of directional control. There is the usual weather cocking effect, where the glider tends to turn into wind due to the crosswind acting on the fin. This should be counteracted by the initial use of “down wind” rudder to keep the glider tracking behind the tug on the ground. While rolling on the ground in a crosswind, there is a component of yaw provided by the crosswind, which has the same effects on a glider as yawing in flight. A small amount of into wind aileron may be required to prevent any secondary effects of yaw rolling the wings away from the wind and to maintain the into wind wing slightly lower than the downwind wing. This is more important in gliders with large dihedral angles because the dihedral angle has the effect of increasing the angle of attack of the into-wind wing, particularly if there are gusts. It is important to prevent the up wind wing getting too high, as the lift differential can cause loss of roll control.
Another effect that is not documented in the Basic Gliding Knowledge or Instructor’s Handbook is the effect of the tug prop wash over the wings of the glider. Imagine the initial stage of the launch, when the glider is stopped or moving only slowly, as the tug pilot increases power. If you have a moderate cross wind from your right during a launch, the tug prop wash tends to drift down wind and, by the time it arrives at the glider on the end of the tow rope, most of this prop blast passes under your left wing. The different airspeed experienced by the left and right wings during the initial ground roll can be significant. There are many variables involving the type of tug, wind speed, crosswind component, and rope length that will modify the actual effect on the glider. Here is an example of an average launch at Bunyan in my Standard Libelle. With a 15 knot wind at 30 degrees off the strip, I could expect a sudden into wind wing drop, 1-2 seconds after the wing runner lets go. At this point the ailerons are not effective to recover the situation for a further 1-2 seconds. This is a very long time to wait while holding full aileron to pick up the wing and full rudder to counteract the combined effects of weather cocking and drag of the wingtip on the ground. The effects in cross winds have proved to be entirely predictable in similar conditions but even with anticipation, the Standard Libelle ailerons are usually not powerful enough at low speed to prevent some degree of wing drop from occurring. Unlike the previous example of the Libelle in calm conditions, in the crosswind prop blast case I have experienced very high rolling forces which require additional speed before the ailerons are effective enough to lift the wing. This situation is much worse than having the wing tip run lightly over the ground, as the tip tends to be heavily loaded and prone to digging the small tip wheel into softer ground. I currently use techniques covered below to overcome this handling characteristic.
In terms of some glider types operating at our club, I have observed the Standard Libelle and SZD-55 as almost equal in their lack of low speed aileron authority. Judicious use of airbrake early in the ground roll is sometimes employed to help the ailerons bite quickly. The technique I now use is to do my normal CHAOTIC then during take up slack, I open the brakes fully, and leave my hand on the spoilers. As soon as I can feel aileron authority on the ground roll, I smoothly close the spoilers and lock them. There are various schools of thought to explain this effect. Some say the airbrakes shift the lift distribution of the wing towards the tips, and improve aileron effectiveness, while I believe the reduction in lift around the spoilers allows the ailerons to overcome the reduced asymmetric forces generated by the rest of the wing. I spent five years dealing with the Libelle wing drops before adopting this practice and a significant improvement is evident. I must stress that this technique involves a higher cockpit workload on take-off, and improves some gliders more than others. Using this method requires your left hand to be on the spoiler handle rather than near the release during the early ground run, and demands a higher degree of familiarity with the type being flown. Having said that I wish I had been introduced to the technique from the very first launch in the Libelle as it would have made many of my earlier launches much safer, and less stressful.
At the next level I would include the Astir and flapped gliders like Open Libelle, and ASW-20. These all have fairly good aileron control almost as the wing runner lets go, but can be disturbed by external influences and late or inappropriate use of controls. The situations I have described in the Standard Libelle will be evident but less extreme in these gliders. It is however these gliders that require the most diligence as a bad wing drop may not have been experienced before and could come as a complete surprise to an inexperienced pilot. Use of the spoilers during the ground roll on some of these gliders might be prudent to assist in extreme conditions. The Jantar is a little better, whilst the Discus and two seaters all have excellent control at low speed and are very resilient to external influences.
There are three main messages arising from this wing drop issue. Firstly, be aware of the factors that are about to affect your next launch, including cross-wind, tug prop wash, blanketing effects from obstacles like hangars, potential turbulence and launch failure options. These should be assessed at the O check in CHAOTIC. Secondly, know the limitations and characteristics of the gliders you fly and be prepared to counter any of these external influences. Have a worst case plan. Third, If you are unfortunate enough to get in a situation that you are not sure you can control, release earlier while your speed is low rather than later. Holding your fingertips close to the release handle (not on the handle) is recommended.
