Rotor loads reduction by dynamically extendable chord

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Han, D.
Barakos, G.N.
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Dynamically extendable blade chord sections show promise for reducing helicopter rotor loads. A rotor model based on elastic beam concept, and capable to predict helicopter power, is utilized. A four bladed rigid rotor with the shape similar to the UH-60A rotor, is used as baseline for comparisons. For the control of the 4/rev vertical hub force, it is not beneficial to actuate the extendable chord at hover and low speed flight. At a high speed of 270km/h, the extendable chord, with a width of 10% rotor radius and responded to 10% of chord length, obtained a maximum force reduction of 89.4%. The magnitude of the dynamic chord needs to be optimized according to the flight state. The performance can be enhanced by increasing the extension or width of the dynamic chord. The dynamically extendable chord was not suitable for reducing the 2/rev blade flapwise root bending moment. A 3/rev dynamic chord though showed great potential in reducing the 3/rev flapwise root bending moment and the 4/rev rotor rolling and pitching moments, simultaneously. The effectiveness of a 5/rev dynamic chord in reducing the 4/rev rotor rolling or pitching moment degraded significantly compared with a 3/rev actuated. To control the 4/rev target load originating from the 3/rev flapwise root bending moment, the phase difference for the maximum rotor rolling and pitching moment reduction was 180o for the 5/rev dynamic chord. Based on the analyses, it is recommended to use the 4/rev dynamically extendable chord to reduce the 4/rev vertical hub force, and use the 3/rev dynamic chord to reduce the 3/rev blade flapwise root bending moment and 4/rev rotor rolling and pitching moments.