In forward flight helicopters carrying slung loads frequently encounter load instability problems that reduce their speed envelope to well below the power limit of the helicopter/slung-load system. The paper presents a procedure for the development and flight test verification of passive stabilizers designed to increase the maximum flight speed of the system. Most of the development is carried out during wind tunnel tests. A scale model is suspended from the tunnel ceiling by a gimbaled setup that simulates the hook-sling attachment. The model is free to perform lateral and longitudinal pendulum motions, as well as yaw rotation. All three motions are recorded as functions of time. The model dynamics are studied as a function of the wind tunnel speed. Various techniques for stabilizing the load can be investigated by wind tunnel tests, which are much cheaper, faster, and less risky than equivalent flight tests. The present study investigates the use of passive vertical fins to stabilize the 6 × 6 × 8 ft CONEX cargo container. The optimal geometry and location of the fins are determined in wind tunnel tests. Later on this optimal configuration is built and tested in full-scale flight tests. The dynamic behavior during these flight tests is compared with the wind tunnel results. Good agreement between both can result in a significant reduction in the number and duration of the flight tests that are required to certify the stabilization method. By using the above-described technique, the maximum flight speed of a UH-60/CONEX system is increased from 60 kt (the operational limit for the unstabilized CONEX) to 110 kt (the power limit of the system).

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The first new load is the TRIO container that can be flown in three configurations with different heights. The second new load is a ribbon bridge interior bay section. Wind tunnel results with models of the new loads exhibit in general good agreement with flight-test results. The results of the new loads strengthen further the approach of using wind tunnel tests to accelerate slung-load clearance, as well as reduce risk and cost. NomenclatureA φc , A θc , A ψ amplitudes of the load roll, pitch, and yaw oscillations, deg h distance between the load cg position and the suspension point, m h floor height above the wind tunnel floor, m I xx , I yy , I zz moment of inertia components, kg-m 2 n model scale factor S area of the slung load, m 2 V velocity, kt or m/s ω φc , ω θc , ω ψ load roll and pitch pendulum frequencies and yaw oscillation frequency, rad/s Subscripts FS full scale FT flight test WT wind tunnel

The stabilization of “difficult” loads that become aerodynamically unstable at airspeeds well below the power-limited speed of the helicopter-load configuration has been studied since the 1960s. This paper looks at the possibility of stabilizing slung loads in forward flight by imposing a slow steady rotation in yaw (spin stabilization). Slow rotations of 100–150 deg/s suffice to suppress the pendulum motions of the load. A swivel is required at the hook, and only a few foot-pounds of yaw moment are needed to overcome swivel friction and impose the desired yaw rate. The approach is limited to single-point suspensions. A stabilizer design consisting of a one-shaft anemometer-like device with hemispherical cups at the ends was developed in wind tunnel tests. The shaft angle can be controlled to vary the applied yaw moment and allow feedback regulation of the load yaw rate. Flight tests with two cargo containers demonstrated that a simple linear control law with fixed gains was effective in maintaining the desired yaw rate in forward flight over the range of configurations of the test loads. Wind tunnel data were obtained at all stages of the development and testing and proved to be an accurate source of design data and an accurate predictor of performance in flight.

Previously, the authors showed that dynamic wind tunnel tests of a suspended CONEX cargo container model exhibited encouraging levels of success in predicting the stability characteristics and speed envelope of the full-scale load. The present study includes further use of the UH-60/CONEX system to investigate effects that were observed previously, but not fully addressed. These effects include the influence of pilot inputs and helicopter motions on the coupled pilot/helicopter/slungload dynamics, the influence of center of gravity offset of the slung load, and the behavior of a load when a yaw swivel is not used in the suspension. It is shown that all three effects are important and affect the slung-load dynamics. The capability of wind tunnel tests to predict the behavior of slung loads in flight is shown for these effects. NomenclatureA () amplitudes of () cg center of gravity D aerodynamic drag force, N PF principal frequency, rad/s SPF simple pendulum frequency, rad/s W weight, N Y aerodynamic lateral force, N φ c , θ c roll (lateral) and pitch (longitudinal) cable angles, deg. ω ψ yaw oscillation angular frequency, rad/s * Corresponding author;