Numerical modeling for parametrical study of fenestron geometry on the basis of energy analysis

Аuthors

Shydakov V. I.^*, Zavalov O. A.

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

*e-mail: k102@mai.ru

Abstract

The problem of selection of fenestron design parameters consists in finding such a combination of geometrical parameters of the fan, inlet and diffuser within the given overall dimension of the fenestron, which would provide the least possible power consumption of its fan at the given thrust value. The varied design parameters (parameters, which act as design variables) include the following: the fan radius R, the relative curvature radius of the inlet the relative diffuser length . The overall fenestron dimension is determined by the suction (low pressure) area on the intake surface. This dimension can thus be calculated according to the following formula .
The evaluation of the fenestron ability to generate thrust while consuming power is carried out according to the value of its energy conversion quality (power conversion factor) () [5]. This criterion was initially proposed by B. N. Yurev for determining the energy efficiency of the main helicopter rotor in hover mode.
The value of energy conversion quality of a fenestron in the International System of Units would be calculated according to the following formula   Here is the aerodynamic efficiency of the fenestron. shows by how many times the thrust of the «ducted fan» system is bigger than that of the open rotor. is the relative efficiency of the ducted fan.
The following equation is true for the hover mode of the helicopter at low altitude. Here is the specific thrust per unit of power (thrust efficiency). the specific thrust loading per unit of rotor disk area.
A fenestron with the following values of the design parameters was taken as an initial version for carrying out the numerical modeling: the fan hub radius , the number of blades 11, flat rectangular blades (with no twist) with NACA 23012 airfoil section, the gap between blade tips and duct walls 0,005R, the diffuser expansion angle 12°, the blade tip speed 220 m/sec.
The calculations resulted in the functional dependencies for the relative duct and fan thrust (which are taken relative to the total fenestron thrust ), the aerodynamic efficiency of the fenestron (), the relative efficiency of the ducted fan (), the energy conversion quality (power conversion factor) . All of the abovementioned characteristics were obtained as functions of the relative curvature radius of the inlet and relative diffuser length () . The calculations show that the rational values of  and  dimensions lie within the following ranges: .
Two fenestron design alternatives were considered: 1) a fan with the blades geometry, which provides equal speeds along the flow horizontal section at the diffuser exit; 2) a fan with flat rectangular blades and increased flow speed near duct walls. The calculations show the following: in the first case the fenestron fan has a higher relative efficiency () and lower aerodynamic efficiency due to the decreased induced power losses; in the second case the fenestron has a higher aerodynamic efficiency and lower value of due to an increased inlet (lip) thrust. At that the second alternative wins in terms of energy conversion quality and appears to be the preferable option. It was established that power losses on swirling the flow behind the fan amount to 7 8% of the power, which is consumed by the fenestron.
Thus it is recommended to install a flow-straightening device behind the fan.
The research demonstrates that the rational choice of the fenestron design parameters allows its power consumption, which is required for compensation of the torque reaction moment from the main helicopter rotor, to not exceed the power consumption of a conventional tail rotor by much.

Keywords:

fenestron, aerodynamic characteristics, energy analysis

References

1. Shaidakov V.I. Vestnik Moskovskogo aviatsionnogo instituta, 2013, vol. 20, no. 4, pp. 36-46.
2. Shaidakov V.I. Aerodinamika vinta v koltse (Shrouded rotor aerodynamics), Moscow, MAI, 1996, 88 p.
3. Shaidakov V.I., Zavalov O.A. Aerodinamicheskoe proektirovanie fenestrona (Fenestron aerodynamic design), Moscow, MAI, 1980, 66 p.
4. Russier M. The Fenestron Antitorque Concept. The Royal Aeronautical Society Conference on Helicopter Yaw Control Concepts, London, March 1990, pp. 182-194.
5. Yurev B.N. Aerodinamicheskii raschet vertoleta (Helicopter aerodynamic design), Moscow, AN SSSR, 1961, vol. 1, 551 p.