SPECIAL PROBLEMS: SAILPLANE TUCK UNDER

Radio controlled sailplanes sometimes run away out of control in a dive, which steepens rapidly until, despite full up elevator the model is vertical and even beyond. Quick thinking by the pilot can sometimes save the situation by pushing the stick forward and so helping the model through the ‘bunt’, to emerge at high speed in level flight, but inverted. The strains of such a manoevre may cause structural failure, but if not the model may be saved.

The cause is almost certainly lack of static margin. That is to say, the centre of gravity should be moved forward and the tailplane re-trimmed to improve static stability. This seems difficult for some model fliers to grasp, since they tend to equate a nose-down pitch with too great a weight in the nose of the model. The foregoing discussion of balance should disabuse them. If the model is balanced in straight and level flight, by suitable tail trim angle, it will still be balanced in a dive, but if the c. g. is too far aft it may lack stability and tuck under with elevators up.

Another very likely cause is structural flexibility. The models which exhibit this tendency are often lightly built and have strongly cambered wings. The camber increases the negative pitching moment and if the tailplane and rear fuselage are somewhat flexible, or if the control rods and linkages are sloppy, there may be enough distortion to reduce the stability of the model to a dangerous extent when it is flying fast, as in a shallow dive. The wings twist also, as the pitching forces increase, and they may break or flutter. The discussion of stability above, it should be remembered, assumed a rigid structure (12.14).

A method of trimming a model sailplane which has been widely advocated is the so-called ‘dive test’. This is not to be recommended although some pilots evidently like the feel of models which have been set up in this way. Following the dictates of the dive test generally increases tail drag and so tends to spoil the all round performance of the model slightly, though probably not enough for this to be apparent to the pilot. More importantly, it may reduce die inherent stability of the model to die point where a runaway ‘tuck under’ is more likely. Some models have in fact been written off in ’tuck under’ accidents during attempts to follow the dive test procedure.

In brief, the dive test, as described in some publications, requires the model flier to put the sailplane into a steep dive of about 60 degrees and hold it there for several seconds to allow the airspeed to build up. Obviously this has to be done when the sailplane is at a considerable height. The controls are then returned to neutral and the model is observed to see how it responds. That is, the elevator is first set for diving and held for a count of five to ten seconds, then it is returned to the position for level flight.

The question is whether or not the sailplane will obey the controls. Advocates of the dive test evidently prefer a model which does not respond normally. What they seek is a model which continues in the steep dive even when the elevators are in the neutral position. To achieve this they progressively move the centre of gravity aft, reducing the stability of the model until this result is arrived at. A model which does in fact behave this way is on the verge of tucking under.

A stable model will respond to the elevator in the normal way. That is, when the elevator is moved from the diving position to the level flight trim, the model will obey and pull out of the dive. Because of the excess airspeed of course the model will not return instantly to level flight but will over-correct — the nose will rise beyond the horizontal, followed by the usual stable oscillating, nose up, nose down, response which the pilot should have no difficulty in smoothing out to restore level flight. Such a response is perfectly normal and safe.

• The model which does not pull itself out of a steep dive with the controls central, is neutrally stable and in a very dangerous condition. Such a trim is not the trim for least drag (see section 12.8 above).

As explained above (sections 12.12 and 12.20), the stability of a model is entirely under the control of the operator and can be adjusted by moving the centre of gravity, i. e., by adding or removing ballast from the nose. Such changes have an immediate effect on the sensitivity of the elevator. Some pilots prefer a docile model which does not require constant attention, once trimmed for a particular airspeed. Others prefer more sensitivity and may move the c. g. aft slightly to achieve this. But to move the c. g. so far aft that the model no longer pulls itself out of a steep dive with neutral controls, is asking for trouble.