Aiaa 2001-4193 Glider Aerotow Training – Feedback Theory Interpretation and Pc-based Simulation

A simple feedback-control theory analysis of lateral control of a towed glider shows the importance of perceiving the lateral tug velocity from the rotation rate of the towrope. A bank component relative to the tug's bank angle that is proportional to the rotation rate should be applied by the pilot to damp lateral motion. Pilot-induced oscillations result from prime attention to the relative position of the tug. Tow rope forces are shown to be of less significance than pilot actions. A simple PC-based simulator is described that may be used to develop pilot skills in preparation for in-flight aerotow training. Introduction Pupil glider pilots who have no difficulty flying a glider in normal flight, frequently experience significant difficulty mastering aerotow. Pupils are often told they are over-correcting, and are shown that the glider flies more stably if the controls are released. However if excessive displacements occur due to tug banking, sudden lift, or inattention, the pupil must make corrections which can then trigger pilot-induced oscillation. Experienced power pilots often report difficulty when trying aerotow, typically describing the controls as very sensitive. Formation flying, station keeping during air-air photography, and air-air refueling are also found to be challenging control situations , and have much in common with aerotow. The observation that these problems are not unique to specific aircraft suggests that a generic explanation of the difficulty should be possible and useful. This paper applies feedback control theory to the simplist realistic model of the situation, and is able to explain the difficulties experienced in flight. The analysis shows that the pilot's control priorities should be to 1. Keep the glider bank angle closely parallel to the tug's wings to minimise lateral acceleration differences. 2. Add a bank (or aileron) deflection in the direction of relative velocity of the tug, irrespective of its position. This correction damps lateral motion of the glider, and is the critical action to prevent pilot-induced oscillation. The relative velocity may best be observed by judging the rotation rate of the tow rope relative to the tug. 3. With a lower importance, add an aileron or bank deflection towards the tug to reduce the positional error. The strategy of zeroing the rope rate to damp oscillation was not expounded to the author during his flight instruction, and would have been of considerable help. Instructors' comments of react faster caused the author to increase his control gain rather …