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).

Equations of motion are derived for general slung load systems. Systematic approaches have not previously been studied for deriving slung load dynamic models, hut an interest in simulation, analysis, and control for slung loads carried by two o r more helicopters motivates attention to this topic. Slung load systems a r e approximated as several rigid bodies connected by suspensions consisting of straight line cables and links which can be assumed elastic o r inelastic and which transmit force only along the link. Equations for the general system are obtained from the Newton-Euler equations with the introduction of generalized speeds. Several systematic formulations are given which generalize previous case-specific results from the slung load literature or occur in the existing literature on the dynamics of multibody systems. A novel formulation is derived for inelastic suspensions from the elastic suspension equations and in which the constraint forces appear explicitly. The result is computationally more ellicient for slung loads than the other formulations, is readily combined with the elastic suspension equations in a single simulation, and is readily applied to complex dual and multilift systems. The new formulation is applied to a Cbody dual lift system. Simulation equations are given along with a review of issues related to simulation of the dynamics, and numerical results. NotationA. B matrices from the kincmaticsv = Au +Bpoftheunconstrained system Al, L partition of A into the Jacobians dv/dul, dv/d .

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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

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