Wind Tunnel and Flight Evaluation of Passive Stabilization of a Cargo Container Slung Load

Abstract

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).

Using Wind Tunnel Tests for Slung-Load Clearance, Part 1: The CONEX Cargo Container

“…Initial tests indicated that the dynamic behavior of the wind tunnel model is not sensitive to these details (Ref. 3). Thus, the wind tunnel model was a simple box without corrugations, indented corners, or skids.…”

“…3) that PIO played a role in the system dynamics at airspeeds where the system was marginally stable. Thus, at those flight speeds, after the pilot introduced a lateral excitation, two modes of the system response were studied: a "lateral stick fixed" (SF) mode and a mode where the pilot remained active in the lateral loop (denoted as "pilot in the loop" (PIL)) and tried to minimize the helicopter roll oscillations.…”

“…Data were recorded at 100 Hz (for more details, see Refs. 2,3,12). Of special importance was the load instrumentation package that measured the load motions using a medium-accuracy military-grade GPS/INU plus load cells inserted in the sling legs at the lift points to measure sling leg tensions.…”

“…It was shown (Ref. 3) that elevated stabilizing fins, which were identified during wind tunnel tests as the configuration with the best potential to stabilize the system and extend its flight envelope, indeed extended the maximum flight speed to more than 110 kt, which was more than 80% higher than the unstabilized configuration. Furthermore, wind tunnel test results, which predicted increasing stability at high speeds, were crucial for the decision to continue the flight envelope expansion and reaching a power limited flight speed, in spite of moderate instability problems at low/medium flight speeds.…”

“…3), a new integrated procedure for developing methods to stabilize problematic slung loads and extend their flight envelope was presented. The procedure starts with wind tunnel tests of a scaled model of the load, suspended from the wind tunnel ceiling.…”