M.Sc Thesis

M.Sc StudentVertzberger Omer
SubjectDevelopment and Investigation of a Theoretical Model of
a Slung Load in a Wind Tunnel which is Attached to
a Moving Point
DepartmentDepartment of Mechanical Engineering
Supervisor PROFESSOR EMERITUS Aviv Rosen
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


For many years the method of carrying external loads attached to the helicopter by cables (slung loads) has been used to transport large and heavy loads to locations where access is difficult or practically impossible for ground-based vehicles. The pendulum motion of the load, which appears during flight, affects the dynamic characteristics of the helicopter and may cause system instability. Understanding the dynamic and aerodynamic phenomena that develop during the flight is necessary for the derivation of a reliable computational model that, together with wind tunnel tests, will reduce the expensive and risky flight tests, which are currently used to authorize the flight of a certain load by a certain helicopter.

The model that has been developed in the current research represents a box-like cargo that has rear stabilizing fins. The load's upper four corners are connected, by four cables, to a single suspension point. The suspension point performs a general movement in all three axes. The aerodynamic loads that act on the load are calculated using an empirical-aerodynamic model that was developed and validated in a previous research. The suspension point movement represents the motion of the helicopter.

First the model was validated for the case of a fixed suspension point. The results of the current model were compared with results of simulations performed for the same case in a previous research. The results of these previous simulations were validated by comparisons to the results of wind tunnel tests. The comparison between the previous simulation and the current simulation exhibited an excellent match.

Then the model was validated for the case of a moving suspension point. The basis for this validation was a previous simulation of a helicopter flight with a slung load. This simulation was validated by a comparison with flight test results.  After the validation of the model, the load's dynamic behavior was investigated at 45 different wind speeds, in the range of 10-120 kts.

In order to further study the system's dynamic characteristics, frequency sweeps were run and the effects of various parameters on the system response were studied. These parameters included: nonlinear effects, wind speed (flight speed), and the amplitude of the excitations. Each frequency sweep was comprised of two hundred simulations: Each one of these simulations was conducted with constant excitation amplitude, a constant wind speed, and frequencies in the range of 0.01 Hz to 1 Hz. For each simulation Fourier transforms of the excitation signal and the load response were carried out and Bode diagrams were plotted. The coherence, which measures the correlation between the input and output signals, was used as a measure of the significance of nonlinear effects in the system's response.