Ph.D Thesis

Ph.D StudentEnciu Jacob
SubjectStability and Handling Qualities of a Helicopter Carrying
a Slung Load
DepartmentDepartment of Aerospace Engineering
Supervisor PROFESSOR EMERITUS Aviv Rosen
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


The development of models and methods suitable for investigating of the stability and control aspects of the combined helicopter-slung load system is described.

The dynamics and stability of a load underslung beneath a gimbal system in a wind tunnel is studied using a simulation, with results compared to observations from dynamic wind tunnel tests. Dynamical System Theory (DST) tools are then used in order to provide a comprehensive description of the load's stability characteristics as airspeed is varied. Theoretical DST results are compared with dynamic wind tunnel tests results and the comparisons exhibited good agreement between both. This proved the adequacy of the model for predicting the underslung load's dynamics and stability characteristics.

Next, an extended MATLAB/SIMULINK simulation is developed, representing a UH-60 Black Hawk helicopter carrying the fins stabilized underslung load that was studied earlier. A human pilot model is incorporated into the simulation to allow the study of the impact of pilot flying techniques on the entire system's dynamics and stability.

The simulation is validated by comparing its predictions with available flight test results for the combined helicopter-slung load system.

Analysis of the combined system is first performed using the simulation's time history predictions at various airspeeds. Then, Extended Bifurcation Analysis (EBA) is undertaken in order to provide the trimmed flight bifurcation curves for the system. The research shows that an underslung load stabilized by two rear mounted fins is a highly nonlinear dynamic system characterized by: multiple equilibrium points, trajectory solutions sensitivity to initial conditions, and system instabilities that are expressed as load limit cycle oscillations (LCO). Results also show that the disappearance of load instabilities above certain airspeed does not eliminate the possibility of their re-appearance at higher airspeeds.

The connection of the underslung load to a helicopter stabilizes the load, but at the same time increases the helicopter's inherent aeromechanical instability. Piloting technique has a dominant effect on the helicopter-slung load system. As long as the pilot refrains from issuing lateral stick commands for damping the helicopter roll oscillations, the system is practically stable. When the pilot becomes active in the roll control loop the system becomes unstable within significant parts of the flight envelope. Pilot pure time delay was found to be the prime parameter determining the degree of the system's instability. Since the instabilities nearly disappear when a low time delay is used, it is deduced that automatic stabilization of a helicopter carrying an underslung load is feasible as long as time delays in the control systems are kept low.