Aerodynamic characteristics of the shrouded rotor with an inlet and a short diffuser at the stationary hover operation mode

Аuthors

Shydakov V. I.

Moscow Aviation Institute (National Research University), 4, Volokolamskoe shosse, Moscow, А-80, GSP-3, 125993, Russia

e-mail: k102@mai.ru

Abstract

The paper is focused on the research of the influence of the diffuser length on the aerodynamic characteristics of the «shrouded rotor» system. «Shrouded rotor» is used as lifting and propulsive system on many types of aircraft. Single-rotor helicopter fenestron with a limited diffuser length, which is built into the tail boom fin, is an individual version of such «shrouded rotor» system. The early works of the author adduce the methods of calculation of aerodynamic characteristics of the shrouded rotor with a diffuser, which is long enough to provide the full expansion of the exit air flow stream. However, the fenestron exit flow cannot fully expand due to the limited length of its diffusor. Thus there is a contraction in the exit flow stream of the fenestron, which affects its aerodynamic characteristics.
The adduced calculation method considers the «shrouded rotor» system as a complex of elements, which create hydraulic drag in the passing air flow. The complex consists of an inlet and a short diffuser. These elements compose the duct where the rotor and the internal devices, which support the rotor and create local pressure losses within the complex, are installed.
The rotor itself is modeled according to the scheme of the rotor disk theory with taking into account the additional induced flow, which is created by the shroud. It was established that the relative curvature radius of the inlet and relative diffuser length (where R is the shroud radius) influence significantly the improvement of the fenestron aerodynamic characteristics, which is attained due to the aerodynamic force performance of the inlet at the duct entrance. Shroud lift effectiveness is determined relative to the total system thrust .
Calculations show that to attain the best shroud lift effectiveness its design parameters should be sel ected fr om within the following ranges: ; .

In this case the shroud lift effectiveness would be ~ 35% of the total system thrust. It can be increased by installing a diverging diffuser with the expansion angle of no more than 12°.
The presented  values were obtained based on the assumption that the speed of the diffuser exit air flow is constant along the horizontal section of the stream (the air flow speed is constant along the radius of the blade). However, the shroud lift effectiveness can be increased substantially, if the flow is organized in such a way that the speed near the duct walls is higher.
Therefore it is reasonable to design a rotor with rectangular shape and small twist of the blades. It is also reasonable to give the geometrical shroud parameters their values as follows: inlet lip spherical radius diffuser length .

Calculation results were confirmed by the experiment, which was carried out at TsAGI. The specific values of  should be determined based on the flow energy analysis.

Keywords:

shrouded rotor, aerodynamic characteristics, calculation method

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