SIMPLE CRITERIA FOR TOWING HOOK LOCATION

Abstract

The problem of the correct behavior of a glider in towed flight is very often discussed among the designers and groups responsible for safety. During take-off the gliders, especially those of a great span the inertia moments of which are high, suffer troubles due to disturbance of the take-off path when catching the wing tip on the grass or other obstacle. In such a situation a recovery moment of the towing cable is necessary to regain the initial direction of the ground run. So for good control there must be a satisfactory arm of the towing cable force about the glider c.g. In other words the towing hook location should be as close to the fuselage nose as possible. This requirement, however, is adverse for performance reasons, where the designers attempt to preserve the laminar flow on the fuselage front part as far as possible. The towing hook creates a source of air-stream perturbation and leads to turbulent flow, decreasing the low-drag portion of the fuselage wetted area. The designers thus have to balance the two things - performance and safety in towed flight. Since the problems are complex it is not possible to give a full and simple criterion for the towing hook location. Nevertheless some data based on statistics as well as the test pilots opinion can be helpful in making the choice. The gliders: SZD-31,'ZEFIR 4', SZD-32 'FOKA 5',SZD-42 "JANTAR 2", and SZD-42-2 'JANTAR 2B' are reported by the pilots to be rather difficult in aerotowing especially during the ground run for take-off. Some troubles are met also with gliders: SZD-29 'ZEFIF.S', SZD-40X "HALNY," SZM-3 "ORION" and SZD-SFI. Comparison of the pilot opinion with the calculation results shows that: - In the vertical plane /pitching/ the criterion value of ky <0,8 for the front c.g. limit and ky <1.,0 for the rear c.g. limit are within the area of poor towing behavior. For the values of ky <0,7 for front c.g. limit and ky <0,8 for rear c.g. liinit the severe difficulty can be met especially at the beginning of the ground run for take-off. - In the horizontal plane /yawing/ for the criterion values kz <0,25 for the front c.g. limitand kz<0,20 for the rear c.g. limit the gliders are rather difficult to
control on aerotowing and ground run as well.