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An Engineering Modification of the Blade Element Momentum Equation for Vertical Descent: An Autorotation Case Study

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An engineering modification of blade element/momentum theory is applied to describe the vertical autorotation of helicopter rotors. A full non‐linear aerodynamic model is considered for the airfoils, taking into account the dependence of lift and drag coefficients on both the angle of attack and the Reynolds number. The proposed model, which has been validated in previous work, has allowed the identification of different autorotation modes, which depend on the descent velocity and the twist of the rotor blades. These modes present different radial distributions of driven and driving blade regions, as well as different radial upwash/downwash patterns. The number of blade sections with zero tangential force, the existence of a downwash region in the rotor disk, the stability of the autorotation state, and the overall rotor autorotation efficiency, are all analyzed in terms of the flight velocity and the characteristics of the rotor. It is shown that, in vertical autorotation, larger blade twist leads to smaller values of descent velocity for a given thrust generated by the rotor in the autorotational state.
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Document Type: Research Article

Affiliations: IDR/UPM, E.T.S.I. Aerona´uticos, Universidad Polite´cnica de Madrid, Madrid, Spain

Publication date: October 1, 2006

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  • The Journal of the AHS is the world's only scientific journal dedicated to vertical flight technology. It is a peer-reviewed technical journal published quarterly by The Vertical Flight Society and presents innovative papers covering the state-of-the-art in all disciplines of VTOL design, research and development. (Please note that VFS members receive significant discounts on articles and subscriptions.)

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