Glider/Tow-plane upsets

Abstract

Several accidents have occurred in the U.K. and elsewhere, in which a towed glider has climbed above the towplane, pulling up its tail. The towplane pilot has lost control, resulting in a sharp nose-down pitch. If the upset takes place near the ground, the results are often fatal. Calculation by F.G. Irving and P.L. Bisgood have indicated dangerous combinations of glider flight path slope and towrope angle. Various gliders were considered, towed by a 180 hp Super Cub. Surging loads can induce upsets at quite modest towrope angles-about 15 degrees -particularly if the glider pilot allowsa nose-up pitch to develop. Flight tests confirm these findings. Various technical measures to reduce the likelihood of such upsets are mentioned. Tests have been made of towhooks designed to release the rope automatically when its upward angle reaches a predetermined angle. They have not worked particularly well and have confirmed the theoretical prediction that they would only deal with part of the problem. A more promising approach is a proposal for a hook which released automatically when the vertical component of load at the tail of the towplane reaches some pre-determined value.