Selection of fenestron design parameters

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 paper presents the analysis of the available power consumption by the fenestron in different flight modes as well as the analysis of the change of the single-rotor helicopter weight with the installation of the fenestron. The analysis aims at determining the values of the design parameters of the fenestron and evaluating the rationality of its use on helicopters of various weight categories.
The parameters of the fenestron, which require value selection, include the shroud duct parameters (parameters of the duct behind the fenestron rotor) and the fenestron rotor parameters. The weight of the cargo, which is transported by the helicopter with a fenestron over a given range, is used as the indicator of its effectiveness. It is also used as a criterion for the selection of the fenestron parameter values. The weight of the cargo, which is transported by the helicopter with a fenestron over a given range, is determined from the aircraft existence equation

Here  is the helicopter take-off weight;  the helicopter empty weight (it is mainly determined by the helicopter structure weight); is the weight of the fuel, which is required to carry out the flight; is the weight of the operational load
The fenestron parameters influence the helicopter engines power and thus the weight of fuel, which is required to carry out the flight, accordingly. In its turn the engines power has a substantial impact on the weight of the helicopter structure as a whole. Therefore it also influences the weight of the cargo, which the helicopter can transport.
The optimal value of the ratio between the fenestron rotor diameter and the main rotor diameter was determined as a result of the parametric studies. It was pointed out that the increase of the main rotor disc loading pнв corresponds to higher optimal values of . These values are equal to 0,07..0,08 for light helicopters and up to 0,09...0,11 for helicopters, which belong to the medium weight category.
The relative diffuser length , which is calculated in fractions of the fenestron rotor radius , proves itself in a similar way in the helicopter characteristics. On one hand, the increase of the relative diffuser length leads to the decrease of the power that the fenestron consumes in hover mode. On the other hand its increase results in the growth of the lateral fenestron dimension and thus leads to the rise of the helicopter parasitic drag in horizontal flight. It was established that the optimal relative length of the diffuser depends significantly on the requirements concerning the design flight range of the considered helicopter. The bigger is the design flight range the shorter the diffuser should be. Thus, for example, it is possible to make a relatively long diffuser for a light helicopter with the relatively short flight ranges (L = 200...400 km). Flying over longer ranges requires the decrease of  value down to 0,4...0,5.
The paper also presents the comparative analysis of the characteristics of a helicopter with a fenestron and a helicopter with a conventional tail rotor. The results of this analysis were used to obtain the dependencies, which allow the designer to evaluate the rationality of installation of the fenestron on helicopters of various weight categories. Thus, for example, the use of the fenestrons on light helicopters almost does not affect the ratios of their payload and fuel weight to take-off weight. The losses (decrease) of the ratio of the payload and fuel weight to take-off weight can amount up to 10% for helicopters of medium weight category with maximum take-off weight of 10000 kg and design flight range of 600 km. The use of fenestron is not rational for helicopters with bigger take-off weights due to even bigger losses in their ratios of the payload and fuel weight to take-off weight.

Keywords:

helicopter, fenestron, design parameters

References

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