A Correlative Study on Aeroelastic Stability Improvement for Composite Hingeless Hub System by Comparing Analysis and Experimental Results

A recent study has shown that composite hingeless hub system can have a better aeroelastic stability rather than metallic hybrid hub system. In helicopter, composite materials have been used more and more due to their specific strength to weight ratio and high fatigue strength. The existing metallic hybrid flexure of hingeless hub system was replaced with composite flexure and the improvement of aeroelastic stability could be verified by analysis and experiment. In addition to the aeroelastic improvement of rotor system, applying composite materials to existing metallic parts could contribute to the weight reduction of rotor system, specially, the weight and size of lead-lag damper. To verify aeroelastic stability improvement, the composite hub system has been developed through the general design approach of rotor system. The analysis and experimental test were performed. Especially, the flexure, which is a core part of composite hub, was designed to match the sectional properties of existing metallic hybrid hub by using classical laminate theory (CLT) and CORDAS (Refs. 1, 2 & 3). Advanced blade was used for analysis and test that has a paddle-type tip shape. This blade was attached to both existing metallic hybrid hub and new developed composite hub. Two kinds of tests were performed, which were hover test in ground and forward flight test in a wind tunnel. The hover test was performed in General Small-scaled Rotor Test System (GSRTS) at KARI. The forward flight test was performed in the 3m x 4m Low Speed Wind Tunnel (LSWT) at KARI. The normal operational rotating speed (Ω) of rotor test (Froude scaled test) is about 780rpm. The 1st lag frequency and 1st flap frequency of the existing metallic hybrid rotor is around 0.68 Ω and 1.02 Ω from the experimental test results. In order to compare amount of damping improvement, the 1st lag frequencies and mode shapes of two different hub systems were almost similarly matched. The dynamic characteristics of these hingeless rotor systems are predicted by using FLIGHTLAB. Frequency diagram at each collective pitch was obtained. The hover test was performed at the condition of – 2, 0, 2, 4, 6 and 8 degree of collective pitch angle, and the forward flight test was performed at the condition of advance ratio (μ) 0.1 and 0.25, respectively. Analysis and experimental test results of frequencies were compared. The correlative comparison of analysis and experiment results was described in this paper. However, there occurred a little in the 1st torsional frequency. Second, the 1st lagwise damping value of composite hub system was measured by exciting swashplate and the results are presented. The improvement of aeroelastic stability for composite hub system could be verified and identified both by analysis and by experimental test.