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2. R. F. Ganiev and I. G. Pavlov, ?Theory of the ground resonance of helicopters,? Prikl. Mekh.,9, No. 5 (1973).
3. M. L. Mil', et al., Helicopters, Calculation and Design, Part 2 [in Russian], Izd. Mashinostroenie (1966).
4. A. A. Kharkevich, Self-Oscillations [in Russian], Gostekhizdat, Moscow (1954).