So far, very careful investigations have been made regarding the flight properties, in particular the static and dynamic stability, of engine-propelled aircraft and of untowed gliders. In contrast, almost no investigations exist regarding the stability of airplanes towed by a towline. Thus, the following report will aim at investigating the directional stability of the towed airplane and, particularly, at determining what parameters of the flight attitude and what configuration properties affect the stability. The most important parameters of the flight attitude are the dynamic pressure, the aerodynamic coefficients of the flight attitude, and the climbing angle. Among the configuration properties, the following exert the greatest influence on the stability: the tow-cable length, the tow-cable attachment point, the ratio of the wing loadings of the towing and the towed airplanes, the moments of inertia, and the wing dihedral of the towed airplane. In addition, the size and shape of the towed airplane vertical tail, the vertical tail length, and the fuselage configuration are decisive factors in determining the yawing moment and side force due to sideslip, respectively.

A theoretical analysis of the lateral stability characteristics of a glider towed by two parallel towlines has been made and correlated with previously reported dynamic flight tests of a glider model in the NACA free-flight tunnel. Calculations were made for a range of effective dihedral angles from -6 to 14 degrees, a range of towline lengths from one to three glider spans, lift coefficients of 0.30 and 0.75, and a range of glider lift-drag ratios from 4.5 to 21.0. The results of the theoretical analysis indicated that a glider towed by twin parallel cables possesses sufficient stability for satisfactory pilotless towed flight.

Summary: A theoretical investigation has been made of the oscillations performed by suspended bodies of the type commonly used for trailing airspeed heads ans similar towed devices. The primary purpose of the investigation was to design an instrument that will remain stable as it is drawn up to a support underneath an airplane without attention on the part of the pilot. Flight tests of a model airspeed head were made to supplement the theoretical study. Unstable oscillations of the body at short cable lengths were predicted by the theory, but the rate of increase of amplitude of these oscillations was very small. In flight tests, more violent types of instability were believed to be caused by unsteady or nonuniform air flow in the region where the cable was lowered from the airplane. No practical method was found to provide large damping of the oscillations at short cable lengths, but the degree of stability present in a suitably designed suspended body was shown to be satisfactory if the body was lowered into a uniform air stream.

Chiefly translations from foreign aeronautical journals.