Tail rotors with various solidity values characteristics in hover mode while helicopter rotation computational studies

Аuthors

Animitsa V. A.^1*, Krymskii V. S.^2**, Leont'ev V. A.^1*

1. Central Aerohydrodynamic Institute named after N.E. Zhukovsky (TsAGI), 1, Zhukovsky str., Zhukovsky, Moscow Region, 140180, Russia
2. Moscow Aviation Institute (National Research University), 4, Volokolamskoe shosse, Moscow, А-80, GSP-3, 125993, Russia

*e-mail: spintest@tsagi.ru
**e-mail: vovan5490@yandex.ru

Abstract

A helicopter uncontrolled left rotation mode (for the main rotor clockwise rotation) is one of the worst modes for the helicopters of classic structure. This phenomenon was repeatedly observed at the stages of a helicopter hovering, take-off and landing, requiring increased values of engines’ apparent power at low flight speeds.

The main cause of helicopter’s entering the uncontrolled rotation mode is the main rotor operation special feature, associated, in the first place, with the flow induced by the main rotor impact on the tail rotor under a certain air speed direction [1]. There are other contributing factors, which may cause helicopters’ entering uncontrolled rotation mode, such as maximum gross weight; high ambient air temperature; engine low power margin; main rotor reduced rotational speed, blustery wind, landing site blanket created by buildings and constructions capable of generating wind flow vortices, or its directivity and velocity variations. The takeoff or the sideslip landing can be considered as contributing factors as well.

At the uncontrolled rotation mode itself, the conditions of tail rotor flow conditions altering. However, this alternation does not facilitate the dangerous mode quitting. Moreover, the angular velocity progressively increases.

In the course of this work, the characteristics of various tail rotors have been calculated under rotation conditions with the left angular velocities of ω_у=0; 30; 60; 120°/s, when hovering out of air-cushion effect. The computational studies have been carried out by using the helicopter mathematical model, developed in TsAGI [2]. A relationship between the tail rotors relative thrust coefficient and the angular velocities С_t/σ(ω_у) has been obtained.

The computational results of studies of the tail rotor with the solidity σ = 0,12 are shown in Figure 1. Under minor tail rotor pitch φ_tr, the coefficient C_t/σ is increasing insignificantly, and the blades rotate under pre-stall mode. When the φ_tr is increasing up to 21° and more, the coefficient C_t/σ is sharply decreasing. The tail rotor thrust is not increasing, but is sharply decreasing.

Figure 1. Relationships С_t/σ = f(ω_у) for σ = 0,12

The angular velocities ω_у influence on the tail rotor operating conditions has been analyzed. The distributions of the lift force coefficients С_у and the tail rotor blade angle of attack α, on the r = 0,75R blade section, vs the blade azimuth and the angular velocity have been obtained. The changes of the tail rotor blade lift force coefficient С_у on the φ_tr = 25° are shown in Figure 2. The Graph shows, that when the ω_у increases, the lift force coefficient С_у decreases.

Figure 2. Relationships C_y = f (ψ) for σ = 0,12, φtr 25º

Keywords:

helicopter, hover, rotation, tail rotor, stall

References

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